MFS VARIABLE INSURANCE TRUST
497, 1997-05-05
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<PAGE>
 
<TABLE>
<S>                          <C>
MFS-REGISTERED TRADEMARK-
RESEARCH SERIES
MFS-REGISTERED TRADEMARK-    PROSPECTUS
UTILITIES SERIES             May 1, 1997
</TABLE>
 
- --------------------------------------------------------------------------------
 
<TABLE>
<S>                                        <C>
MFS-Registered Trademark- VARIABLE
INSURANCE TRUST-SM-
</TABLE>
 
MFS Variable Insurance Trust (the "Trust") is an open-end management investment
company offering insurance company separate accounts a selection of investment
vehicles for variable annuity and variable life insurance contracts (the
"Contracts"). Currently the Trust offers shares of beneficial interest of 12
separate mutual fund series (individually or collectively hereinafter referred
to as a "Series" or the "Series"), two of which are offered pursuant to this
Prospectus:
 
- -- MFS RESEARCH SERIES (the "Research Series"), which seeks to provide long-term
   growth of capital and future income; and
 
- -- MFS UTILITIES SERIES (the "Utilities Series"), which seeks capital growth and
   current income (income above that available from a portfolio invested
   entirely in equity securities).
                              -------------------
 
THE RESEARCH SERIES IS INTENDED FOR INVESTORS WHO UNDERSTAND AND ARE WILLING TO
ACCEPT THE RISKS ENTAILED IN SEEKING LONG-TERM GROWTH OF CAPITAL. BECAUSE OF
THEIR INVESTMENT POLICIES PERMITTING INVESTMENT IN FOREIGN SECURITIES,
INVESTMENTS IN EACH SERIES MAY BE SUBJECT TO A GREATER DEGREE OF RISK THAN
INVESTMENTS IN OTHER INVESTMENT COMPANIES WHICH INVEST ENTIRELY IN DOMESTIC
SECURITIES.
                              -------------------
 
  THESE SECURITIES HAVE NOT BEEN APPROVED OR DISAPPROVED BY THE SECURITIES AND
    EXCHANGE COMMISSION OR ANY STATE SECURITIES COMMISSION NOR HAS THE
     SECURITIES AND EXCHANGE COMMISSION OR ANY STATE SECURITIES COMMISSION
       PASSED UPON THE ACCURACY OR ADEQUACY OF THIS PROSPECTUS. ANY
             REPRESENTATION TO THE CONTRARY IS A CRIMINAL OFFENSE.
                              -------------------
 
SHARES OF THE TRUST ARE AVAILABLE AND ARE BEING MARKETED AS A POOLED FUNDING
VEHICLE FOR LIFE INSURANCE COMPANIES WRITING ALL TYPES OF CONTRACTS.
 
This Prospectus sets forth concisely the information about each Series that a
prospective investor should know before applying for the Contracts offered by
the separate accounts of certain insurance companies ("Participating Insurance
Companies"). Investors are advised to read this Prospectus and the applicable
Contract prospectus carefully and retain them for future reference. If you
require more detailed information, a Statement of Additional Information dated
May 1, 1997, as amended or supplemented from time to time (the "SAI"), is
available upon request without charge and may be obtained by calling or by
writing to the Shareholder Servicing Agent (see back cover for address and phone
number). The SAI, which is incorporated by reference into this Prospectus, has
been filed with the Securities and Exchange Commission (the "SEC"). The SEC
maintains an Internet World Wide Web site that contains the SAI, materials that
are incorporated by reference into this Prospectus and the SAI, and other
information regarding the Series. This Prospectus is available on the Adviser's
Internet World Wide Web site at http://www.mfs.com.
 
   INVESTORS SHOULD READ THIS PROSPECTUS AND RETAIN IT FOR FUTURE REFERENCE.
<PAGE>
TABLE OF CONTENTS
 
<TABLE>
<CAPTION>
                                                                                                       PAGE
                                                                                                       ---
  <S>                                                                                                  <C>
  1.  Expense Summary................................................................................   3
  2.  Investment Concept of the Trust................................................................   3
  3.  Condensed Financial Information................................................................   4
  4.  Investment Objectives and Policies.............................................................   6
      MFS Research Series............................................................................   6
      MFS Utilities Series...........................................................................   6
  5.  Investment Techniques..........................................................................   8
  6.  Additional Risk Factors........................................................................  12
  7.  Management of the Series.......................................................................  15
  8.  Information Concerning Shares of Each Series...................................................  16
      Purchases and Redemptions......................................................................  16
      Net Asset Value................................................................................  17
      Distributions..................................................................................  17
      Tax Status.....................................................................................  17
      Description of Shares, Voting Rights and Liabilities...........................................  18
      Performance Information........................................................................  18
      Expenses.......................................................................................  18
      Shareholder Communications.....................................................................  19
  Appendix A -- Description of Bond Ratings..........................................................  A-1
  Appendix B -- Principal Sectors of the Utilities Industry..........................................  B-1
</TABLE>
 
                                       2
<PAGE>
1.  EXPENSE SUMMARY
 
ANNUAL OPERATING EXPENSES (AS A PERCENTAGE OF AVERAGE NET ASSETS):
 
<TABLE>
<CAPTION>
                                                                MFS
                                                              RESEARCH     MFS
                                                               SERIES    UTILITIES
                                                              --------   --------
<S>                                                           <C>        <C>
Management Fee..............................................  0.75%      0.75%
Other Expenses (after expense limitation)(1)(2).............  0.25%      0.25%
                                                              --------    ---
Total Operating Expenses (after expense limitation)(2)......  1.00%      1.00%
<FN>
- ------------------------
(1)        Each  Series has an expense offset  arrangement which reduces the Series' custodian  fee based upon the amount of
           cash maintained by the Series  with its custodian and  dividend disbursing agent, and  may enter into other  such
           arrangements  and directed  brokerage arrangements  (which would  also have  the effect  of reducing  the Series'
           expenses). Any such fee reductions are not reflected under "Other Expenses."
(2)        The Adviser has agreed to bear expenses for each Series, subject to reimbursement by each Series, such that  each
           Series'  "Other Expenses" shall not exceed 0.25% of the average daily net assets of the Series during the current
           fiscal year. See "Information Concerning Shares of  Each Series-- Expenses." Otherwise, "Other Expenses" for  the
           Research  Series and the Utilities Series would be  0.73% and 2.00%, respectively, and "Total Operating Expenses"
           would be 1.48% and 2.75%, respectively, for these Series.
</TABLE>
 
    The  purpose  of  the  expense  table  above  is  to  assist  investors   in
understanding  the various costs  and expenses that a  shareholder of the Series
will bear directly or indirectly. The  Series' annual operating expenses do  not
reflect  expenses  imposed  by  separate  accounts  of  Participating  Insurance
Companies through  which an  investment in  a Series  is made  or their  related
Contracts. A separate account's expenses are disclosed in the prospectus through
which the Contract relating to that separate account is offered for sale.
 
2.  INVESTMENT CONCEPT OF THE TRUST
 
    The Trust is an open-end, registered management investment company comprised
of  the following twelve series: Emerging  Growth Series, Value Series, Research
Series, Growth With Income Series,  Total Return Series, Utilities Series,  High
Income  Series, World  Governments Series,  Strategic Fixed  Income Series, Bond
Series, Limited  Maturity Series  and  Money Market  Series.  Each Series  is  a
segregated,  separately  managed portfolio  of  securities. All  of  the Series,
except the Utilities Series, World Governments Series and Strategic Fixed Income
Series, are diversified. Additional series may be created from time to time. The
Trust was organized as a  business trust under the  laws of The Commonwealth  of
Massachusetts by a Declaration of Trust dated February 1, 1994.
 
    The  Trust  currently  offers shares  of  each Series  to  insurance company
separate accounts that fund Contracts. Separate accounts may purchase or  redeem
shares  at net  asset value  without any  sales or  redemption charge.  Fees and
charges imposed by a separate account, however, will affect the actual return to
the  holder  of  a  Contract.  A  separate  account  may  also  impose   certain
restrictions  or limitations on the allocation  of purchase payments or Contract
value to one or more Series, and  not all Series may be available in  connection
with  a particular Contract. Prospective investors should consult the applicable
Contract prospectus for information regarding fees and expenses of the  Contract
and  separate account and any applicable  restrictions or limitations. The Trust
assumes no responsibility for such prospectuses.
 
    Shares of the Series are offered  to the separate accounts of  Participating
Insurance  Companies  that are  affiliated  or unaffiliated  ("shared funding").
Shares of the Series may serve  as the underlying investments for both  variable
annuity  and  variable  life  insurance  contracts  ("mixed  funding").  Due  to
differences in tax treatment or  other considerations, the interests of  various
Contract  owners might at some time be in conflict. The Trust currently does not
foresee any such conflict. Nevertheless, the Trust's Trustees intend to  monitor
events  in order  to identify  any material  irreconcilable conflicts  which may
possibly arise and to determine what action, if any, should be taken in response
thereto. If such a conflict were to occur, one or more separate accounts of  the
Participating  Insurance Companies might be required to withdraw its investments
in one one  or more  Series. This  might force a  Series to  sell securities  at
disadvantageous prices.
 
    Individual Contract holders are not the "shareholders" of the Trust. Rather,
the  Participating  Insurance  Companies  and their  separate  accounts  are the
shareholders or  investors,  although such  companies  may pass  through  voting
rights to their Contract holders.
 
                                       3
<PAGE>
    The Trust's Board of Trustees provides broad supervision over the affairs of
the  Trust and the Series. Massachusetts  Financial Services Company, a Delaware
corporation ("MFS" or the "Adviser"), is the investment adviser to each  Series.
A majority of the Trustees of the Trust are not affiliated with the Adviser. The
Adviser  is responsible for the management of  the assets of each Series and the
officers of the Trust  are responsible for the  operations. The Adviser  manages
the  Series'  portfolios  from day  to  day  in accordance  with  the investment
objectives and policies of each Series. The selection of investments and the way
they are managed  depend on the  conditions and  trends in the  economy and  the
financial marketplaces.
 
3.  CONDENSED FINANCIAL INFORMATION
 
The  following financial information has been  audited since the commencement of
investment operations of such Series and should be read in conjunction with  the
financial  statements included  in the  Series' Annual  Reports to Shareholders.
These financial  statements  are  incorporated  by reference  into  the  SAI  in
reliance  upon the report  of the Series' independent  auditors given upon their
authority as experts in accounting and auditing. The Series' current independent
auditors are Deloitte & Touche LLP.
                                RESEARCH SERIES
 
<TABLE>
<CAPTION>
                                                                       YEAR ENDED           PERIOD ENDED
                                                                    DECEMBER 31, 1996    DECEMBER 31, 1995*
                                                                   -------------------   ------------------
<S>                                                                <C>                   <C>
Per share data (for a share outstanding throughout each period):
Net asset value--beginning of period.............................        $ 10.89               $10.00
                                                                         -------               ------
Income from investment operations#--
  Net investment incomeSection...................................        $  0.06               $ 0.05
  Net realized and unrealized gain on investments and foreign
    currency transactions........................................           2.37                 1.01
                                                                         -------               ------
    Total from investment operations.............................        $  2.43               $ 1.06
                                                                         -------               ------
Less distributions declared to shareholders--
  From net investment income.....................................        $ (0.02)              $(0.03)
  From net realized gain on investments and foreign currency
    transactions.................................................          (0.16)               (0.14)
  In excess of net realized gain on investments and foreign
    currency transactions........................................          (0.01)                  --
                                                                         -------               ------
    Total distributions declared to shareholders.................        $ (0.19)              $(0.17)
                                                                         -------               ------
Net asset value--end of period...................................        $ 13.13               $10.89
                                                                         -------               ------
                                                                         -------               ------
Total return.....................................................          22.33%               10.62%++
Ratios (to average net assets)/Supplemental dataSection:
  Expenses.......................................................           1.00%                1.00%+
  Net investment income..........................................           0.47%                1.15%+
Portfolio turnover...............................................             56%                  28%
Average commission rate###.......................................        $0.0295                   --
Net assets at end of period (000 omitted)........................        $35,710               $2,530
- ---------
        *  For the period from the commencement of investment operations, July 26, 1995 to December 31, 1995.
        +  Annualized.
       ++  Not annualized.
        #  Per share data is based on average shares outstanding.
      ###  Average commission rate is calculated for funds with fiscal years beginning on or after September 1, 1995.
  Section  The Adviser voluntarily agreed to maintain the expenses of the Series at not more than 1.00% of average daily
           net assets. To the extent actual expenses were over these limitations, the net investment loss per share  and
           the ratios would have been:
 
Net investment loss..............................................               --           $(0.08)
Ratios (to average net assets):
  Expenses.......................................................             1.48%            3.90%+
  Net investment loss............................................               --            (1.73)%+
</TABLE>
 
                                       4
<PAGE>
                                UTILITIES SERIES
 
<TABLE>
<CAPTION>
                                                                       YEAR ENDED           PERIOD ENDED
                                                                    DECEMBER 31, 1996    DECEMBER 31, 1995*
                                                                   -------------------   ------------------
<S>                                                                <C>                   <C>
Per share data (for a share outstanding throughout each period):
Net asset value--beginning of period.............................        $ 12.57               $10.00
                                                                         -------               ------
Income from investment operations#--
  Net investment incomeSection...................................        $  0.55               $ 0.39
  Net realized and unrealized gain on investments and foreign
    currency transactions........................................           1.78                 3.00
                                                                         -------               ------
    Total from investment operations.............................        $  2.33               $ 3.39
                                                                         -------               ------
Less distributions declared to shareholders--
  From net investment income.....................................        $ (0.35)              $(0.24)
  From net realized gain on investments and foreign currency
    transactions.................................................          (0.88)               (0.58)
  In excess of realized gain on investments and foreign currency
    transactions.................................................          (0.01)                  --
                                                                         -------               ------
    Total distributions declared to shareholders.................        $ (1.24)              $(0.82)
                                                                         -------               ------
Net asset value--end of period...................................        $ 13.66               $12.57
                                                                         -------               ------
                                                                         -------               ------
Total return.....................................................          18.51%               33.94%++
Ratios (to average net assets)/Supplemental dataSection:
  Expenses.......................................................           1.00%                1.00%+
  Net investment income..........................................           4.19%                3.66%+
Portfolio turnover...............................................            121%                  94%
Average commission rate###.......................................        $0.0416                   --
Net assets at end of period (000 omitted)........................        $ 9,572               $2,373
- ---------
        * For the period from the commencement of investment operations, January 3, 1995 to December 31, 1995.
        +  Annualized.
       ++  Not annualized.
        #  Per share data is based on average shares outstanding.
      ###  Average commission rate is calculated for funds with fiscal years beginning on or after September 1, 1995.
  Section  The Adviser voluntarily agreed to maintain the expenses of the Series at not more than 1.00% of average daily
           net  assets. To the extent actual  expenses were over these limitations,  the net investment income per share
           and the ratios would have been:
 
Net investment income............................................            $0.32             $0.17
Ratios (to average net assets):
  Expenses.......................................................             2.75%             3.08%+
  Net investment income..........................................             2.44%             1.62%+
</TABLE>
 
                                       5
<PAGE>
4.  INVESTMENT OBJECTIVES AND POLICIES
 
    Each Series has  different investment  objectives which  it pursues  through
separate  investment policies, as described below. The differences in objectives
and policies among the Series can be expected to affect the degree of market and
financial risk to which each  Series is subject and  the return of each  Series.
The  investment objectives  and policies  of each  Series may,  unless otherwise
specifically stated, be changed by the Trustees  of the Trust without a vote  of
the  shareholders. Any investment  involves risk and there  is no assurance that
the objectives of any Series will be achieved.
 
    In addition  to  the specific  investment  practices described  below,  each
Series  may also engage in certain  investment techniques as described under the
caption  "Investment  Techniques"  below  and  in  the  SAI  under  the  caption
"Investment  Techniques." The Series' investments  are subject to certain risks,
as described in the above-referenced sections of this Prospectus and the SAI and
as described below under the caption "Additional Risk Factors."
 
MFS RESEARCH SERIES -- The Research  Series' investment objective is to  provide
long-term growth of capital and future income.
 
    The  portfolio securities of the Research Series are selected by a committee
of investment  research analysts.  This committee  includes investment  analysts
employed not only by the Adviser but also by MFS International (U.K.) Limited, a
wholly  owned  subsidiary  of  MFS.  The  Series'  assets  are  allocated  among
industries by the analysts acting together  as a group. Individual analysts  are
then  responsible for selecting what they view  as the securities best suited to
meet  the   Series'  investment   objective  within   their  assigned   industry
responsibility.
 
    The  Research Series'  policy is to  invest a substantial  proportion of its
assets in equity securities of companies believed to possess better than average
prospects for long-term growth. Equity securities in which the Series may invest
include the following:  common stocks, preferred  stocks and preference  stocks;
securities  such as bonds, warrants or  rights that are convertible into stocks;
and depositary receipts for those securities. These securities may be listed  on
securities  exchanges,  traded in  various over-the-counter  markets or  have no
organized markets. A smaller proportion of the assets may be invested in  bonds,
short-term  obligations,  preferred  stocks  or  common  stocks  whose principal
characteristic is income production rather than growth. Such securities may also
offer opportunities for growth of capital as well as income. In the case of both
growth stocks  and  income  issues,  emphasis is  placed  on  the  selection  of
progressive,   well-managed   companies.   The   Series'   non-convertible  debt
investments, if any, may consist of "investment grade" securities (rated Baa  or
better  by Moody's or BBB or better by S&P or by Fitch), and, with respect to no
more than  10%  of  the  Series'  net assets,  securities  in  the  lower  rated
categories  (rated Ba or lower by Moody's or BB  or lower by S&P or by Fitch) or
securities which the Adviser  believes to be of  similar quality to these  lower
rated  securities (commonly  known as "junk  bonds"). For a  description of bond
ratings, see Appendix A to this Prospectus.
 
    Consistent with its investment objective  and policies described above,  the
Series  may  also invest  up  to 20%  of its  net  assets in  foreign securities
(including emerging market securities) which are not traded on a U.S. exchange.
 
MFS UTILITIES SERIES --  The Utilities Series' investment  objective is to  seek
capital  growth and current income (income above that available from a portfolio
invested entirely in equity securities).
 
    The Utilities Series will seek to achieve its objective by investing,  under
normal  circumstances, at  least 65% (but  up to  100% at the  discretion of the
Adviser) of  its assets  in equity  and  debt securities  of both  domestic  and
foreign  companies in  the utilities  industry. Equity  securities in  which the
Series may invest  include the  following: common stocks;  preferred stocks  and
preference  stocks;  securities  such  as bonds,  warrants  or  rights  that are
convertible into stocks;  and depositary  receipts for  those securities.  These
securities   may  be   listed  on   securities  exchanges,   traded  in  various
over-the-counter markets  or have  no organized  markets. At  least 80%  of  the
non-convertible  fixed income securities held by the Series will be rated at the
time of investment at least Baa by Moody's or BBB by S&P or by Fitch or will  be
of  comparable  quality  as  determined by  the  Adviser  (see  "Additional Risk
Factors" below). See Appendix  A to this Prospectus  for a description of  these
ratings.  The Series may also invest in debt and equity securities of issuers in
other industries, as  discussed below, although  under normal circumstances  not
more than 35% of the Series' assets will be so invested. In addition, the Series
may hold a portion of its assets in cash and money market instruments.
 
    Companies  in the  utilities industry include  (i) companies  engaged in the
manufacture, production,  generation,  transmission,  sale  or  distribution  of
electric,  gas or other  types of energy,  water or other  sanitary services and
 
                                       6
<PAGE>
(ii) companies  engaged  in telecommunications,  including  telephone,  cellular
telephones,   telegraph,  satellite,  microwave,   cable  television  and  other
communications media (but  not companies  engaged in  public broadcasting).  The
Adviser  deems a particular company  to be in the  utilities industry if, at the
time of investment, the  Adviser determines that at  least 50% of the  company's
assets or revenues are derived from one or more of those industries. The portion
of  the Utilities Series' assets invested in a particular type of utility and in
equity or  debt securities  will vary  in light  of changes  in interest  rates,
market  conditions  and  economic  conditions  and  other  factors.  For further
information on the  principal sectors  of the  utilities industry  in which  the
Series may invest, see Appendix B to this Prospectus.
 
    Consistent  with its investment objective  and policies described above, the
Series may  also invest  up  to 35%  of its  net  assets in  foreign  securities
(including emerging market securities and Brady Bonds) which are not traded on a
U.S. exchange.
 
    Since   the  Utilities  Series'  investments  are  concentrated  in  utility
securities, the  value of  the Series'  shares will  be especially  affected  by
factors  peculiar to the utilities industry,  and may fluctuate more widely than
the value of shares of a fund that invests in a broader range of industries. The
rates many  utility  companies may  charge  their customers  are  controlled  by
governmental  regulatory commissions which may result  in a delay in the utility
company passing along increases in costs to its customers. Furthermore, there is
no assurance  that  regulatory  authorities  will, in  the  future,  grant  rate
increases  or that  such increases  will be  adequate to  permit the  payment of
dividends on common stocks. Many utility companies, especially electric and  gas
and   other   energy  related   utility  companies,   are  subject   to  various
uncertainties, including: risks of increases in fuel and other operating  costs;
the  high cost of borrowing to  finance capital construction during inflationary
periods; difficulty  obtaining adequate  returns on  invested capital,  even  if
frequent rate increases are approved by public service commissions; restrictions
on  operations and increased costs  and delays as a  result of environmental and
nuclear safety regulations; securing  financing for large construction  projects
during  an inflationary period; difficulties of the capital markets in absorbing
utility debt and equity  securities; difficulty in  raising capital in  adequate
amounts  on reasonable terms in periods  of high inflation and unsettled capital
markets; technological innovations which  may render existing plants,  equipment
or  products obsolete;  the potential impact  of natural  or man-made disasters;
difficulties in obtaining natural gas for resale or fuel for electric generation
at reasonable prices; coping  with the general  effects of energy  conservation,
particularly  in light of changing policies  regarding energy; and special risks
associated with  the  construction and  operation  of nuclear  power  generating
facilities,  including  technical factors  and costs,  and the  possibility that
federal, state and municipal government authorities may from time to time review
existing  requirements  and  impose  additional  requirements.  Certain  utility
companies,  especially gas and telephone utility companies, have in recent years
been affected  by  increased  competition,  which  could  adversely  affect  the
profitability  of such  utility companies. Furthermore,  there are uncertainties
resulting from certain  telecommunications companies'  diversification into  new
domestic  and  international  businesses  as well  as  agreements  by  many such
companies linking  future  rate increases  to  inflation or  other  factors  not
directly related to the active operating profits of the enterprise.
 
    Foreign  utility  companies are  also subject  to regulation,  although such
regulations may or may not be comparable to those in the United States.  Foreign
utility  companies may be more heavily regulated by their respective governments
than utilities in the U.S. and, as  in the U.S., generally are required to  seek
government  approval  for  rate  increases.  In  addition,  since  many  foreign
utilities use fuel that causes more pollution than those used in the U.S.,  such
utilities  may be required to invest in  pollution control equipment to meet any
proposed pollution restrictions. Foreign regulatory systems vary from country to
country and may evolve in ways different from regulation in the U.S.
 
    The Utilities Series is  permitted to invest in  securities of issuers  that
are outside the utilities industry, although under normal circumstances not more
than 35% of the Series' assets will be so invested. Such investments may include
common  stocks,  debt  securities  (including  municipal  debt  securities)  and
preferred stocks and will be selected  to meet the Series' investment  objective
of  both capital growth  and current income.  These securities may  be issued by
either U.S. or non-U.S.  companies. Some of these  issuers may be in  industries
related  to the  utilities industry  and, therefore,  may be  subject to similar
risks.
 
    Investments outside the utilities industry may also include U.S.  Government
Securities,   as  that  term   is  defined  under   "Investment  Objectives  and
Policies--MFS Total Return Series" above. When and if available, U.S. Government
Securities may be purchased at a  discount from face value. However, the  Series
does not intend to hold such securities to maturity for the purpose of achieving
potential  capital  gains,  unless  current  yields  on  the  securities  remain
attractive.
 
                                       7
<PAGE>
5.  INVESTMENT TECHNIQUES
 
    LENDING OF PORTFOLIO SECURITIES: Each Series may seek to increase its income
by lending portfolio securities. Such loans will usually be made to member firms
(and subsidiaries thereof) of the New  York Stock Exchange (the "Exchange")  and
to  member banks  of the  Federal Reserve  System, and  would be  required to be
secured continuously  by collateral  in  cash, U.S.  Treasury securities  or  an
irrevocable letter of credit maintained on a current basis at an amount at least
equal to the market value of the securities loaned. If the Adviser determines to
make  securities loans, it is  intended that the value  of the securities loaned
would not exceed 10%  of the value of  the net assets of  the Series making  the
loans.
 
    EMERGING  MARKET  SECURITIES: Consistent  with their  respective objectives,
each Series may invest in securities  of issuers whose principal activities  are
located  in  emerging market  countries. Emerging  market countries  include any
country determined by  the Adviser to  have an emerging  market economy,  taking
into  account a number of  factors, including whether the  country has a low- to
middle- income economy  according to the  International Bank for  Reconstruction
and  Development, the country's foreign currency  debt rating, its political and
economic stability and the development of its financial and capital markets. The
Adviser determines whether an  issuer's principal activities  are located in  an
emerging   market  country  by  considering  such  factors  as  its  country  of
organization, the principal trading market for its securities and the source  of
its  revenues and assets. The issuer's principal activities generally are deemed
to be  located  in a  particular  country if:  (a)  the security  is  issued  or
guaranteed by the government of that country or any of its agencies, authorities
or  instrumentalities;  (b)  the issuer  is  organized  under the  laws  of, and
maintains a principal office in that  country; (c) the issuer has its  principal
securities trading market in that country; (d) the issuer derives 50% or more of
its total revenues from goods sold or services performed in that country; or (e)
the issuer has 50% or more of its assets in that country.
 
    BRADY  BONDS:  The Utilities  Series may  invest in  Brady Bonds,  which are
securities created through  the exchange  of existing commercial  bank loans  to
public  and  private  entities in  certain  emerging  markets for  new  bonds in
connection with debt restructurings under  a debt restructuring plan  introduced
by  former U.S. Secretary of the Treasury, Nicholas F. Brady (the "Brady Plan").
Brady Plan  debt restructurings  have  been implemented  to date  in  Argentina,
Brazil,  Bulgaria,  Costa  Rica, Dominican  Republic,  Ecuador,  Jordan, Mexico,
Nigeria, Panama, the  Philippines, Poland,  Uruguay and  Venezuela. Brady  Bonds
have  been issued only recently, and for that  reason do not have a long payment
history. Brady Bonds may  be collateralized or  uncollateralized, are issued  in
various  currencies (but primarily  the U.S. dollar) and  are actively traded in
over-the-counter  secondary  markets.  U.S.  dollar-denominated,  collateralized
Brady Bonds, which may be fixed-rate bonds or floating-rate bonds, are generally
collateralized in full as to principal by U.S. Treasury zero coupon bonds having
the  same maturity as the bonds. Brady Bonds are often viewed as having three or
four valuation components:  the collateralized repayment  of principal at  final
maturity;  the collateralized  interest payments;  the uncollateralized interest
payments; and any  uncollateralized repayment  of principal  at maturity  (these
uncollateralized  amounts  constituting the  "residual risk").  In light  of the
residual risk of Brady  Bonds and the history  of defaults of countries  issuing
Brady  Bonds  with  respect  to  commercial bank  loans  by  public  and private
entities, investments in Brady Bonds may be viewed as speculative.
 
    REPURCHASE AGREEMENTS: Each Series may  enter into repurchase agreements  in
order  to earn  income on  available cash or  as a  temporary defensive measure.
Under a  repurchase  agreement, a  Series  acquires securities  subject  to  the
seller's  agreement to repurchase at  a specified time and  price. If the seller
becomes subject to  a proceeding  under the bankruptcy  laws or  its assets  are
otherwise subject to a stay order, the Series' right to liquidate the securities
may be restricted (during which time the value of the securities could decline).
As  discussed in the SAI, each Series has adopted certain procedures intended to
minimize risk.
 
    "WHEN-ISSUED" SECURITIES: The Utilities Series may purchase securities on  a
"when-issued"  or on a "forward delivery" basis, which means that the securities
will be  delivered to  the Series  at  a future  date usually  beyond  customary
settlement time. The commitment to purchase a security for which payment will be
made  on a future date may be deemed a separate security. In general, the Series
does not pay  for such  securities until received,  and does  not start  earning
interest on the securities until the contractual settlement date. While awaiting
delivery  of securities purchased on such bases, the Series will normally invest
in liquid assets.
 
                                       8
<PAGE>
    MORTGAGE "DOLLAR ROLL"  TRANSACTIONS: The  Utilities Series  may enter  into
mortgage  "dollar  roll"  transactions with  selected  banks  and broker-dealers
pursuant to which the  Series sells mortgage-backed  securities for delivery  in
the future (generally within 30 days) and simultaneously contracts to repurchase
substantially similar (same type, coupon and maturity) securities on a specified
future  date.  The  Series  record  these  transactions  as  sale  and  purchase
transactions, rather than as borrowing transactions. The Series will only  enter
into covered rolls. A "covered roll" is a specific type of dollar roll for which
there  is an  offsetting cash  position or  a cash  equivalent security position
which matures  on or  before the  forward  settlement date  of the  dollar  roll
transaction.  In the  event that  the party  with whom  the Series  contracts to
replace substantially similar securities on a future date fails to deliver  such
securities,  the Series may not  be able to obtain  such securities at the price
specified in such contract and thus may not benefit from the price  differential
between the current sales price and the repurchase price.
 
    RESTRICTED  SECURITIES:  Each Series  may purchase  securities that  are not
registered under  the  Securities Act  of  1933 (the  "1933  Act")  ("restricted
securities"),  including  those  that  can be  offered  and  sold  to "qualified
institutional  buyers"  under  Rule  144A   under  the  1933  Act  ("Rule   144A
securities").  A determination  is made  based upon  a continuing  review of the
trading markets for  a specific  Rule 144A  security, whether  such security  is
liquid and thus not subject to the Series' limitation on investing not more than
15% of its net assets in illiquid investments. The Board of Trustees has adopted
guidelines and delegated to MFS the daily function of determining and monitoring
the  liquidity of Rule  144A securities. The  Board, however, retains oversight,
focusing  on  factors   such  as  valuation,   liquidity  and  availability   of
information.  Investing  in  Rule  144A  securities  could  have  the  effect of
decreasing the  level of  liquidity in  a Series  to the  extent that  qualified
institutional  buyers become  for a  time uninterested  in purchasing  Rule 144A
securities held in the Series' portfolio.
 
    CORPORATE ASSET-BACKED  SECURITIES:  The  Utilities  Series  may  invest  in
corporate  asset-backed  securities.  These  securities,  issued  by  trusts and
special purpose corporations,  are backed by  a pool of  assets, such as  credit
card  and automobile loan receivables, representing  the obligations of a number
of different parties.
 
    Corporate asset-backed securities  present certain risks.  For instance,  in
the  case of credit card receivables, these  securities may not have the benefit
of any security interest in the related collateral. Credit card receivables  are
generally  unsecured and the debtors are entitled  to the protection of a number
of state and federal consumer credit laws,  many of which give such debtors  the
right  to set off certain amounts owed on the credit cards, thereby reducing the
balance due.  Most issuers  of automobile  receivables permit  the servicers  to
retain  possession of the  underlying obligations. If the  servicer were to sell
these obligations to  another party, there  is a risk  that the purchaser  would
acquire  an interest superior to  that of the holders  of the related automobile
receivables. In addition, because of the large number of vehicles involved in  a
typical  issuance and technical  requirements under state  laws, the trustee for
the holders  of  the automobile  receivables  may  not have  a  proper  security
interest in all of the obligations backing such receivables. Therefore, there is
the  possibility  that recoveries  on repossessed  collateral  may not,  in some
cases, be  available to  support payments  on these  securities. The  underlying
assets  (E.G.,  loans)  are  also  subject  to  prepayments  which  shorten  the
securities' weighted average life and may lower their return.
 
    Corporate asset-backed  securities are  often  backed by  a pool  of  assets
representing  the obligations  of a number  of different parties.  To lessen the
effect of  failures by  obligors  on underlying  assets  to make  payments,  the
securities   may  contain  elements  of  credit  support  which  fall  into  two
categories:  (i)  liquidity  protection;  and  (ii)  protection  against  losses
resulting  from  ultimate  default  by  an  obligor  on  the  underlying assets.
Liquidity protection  refers to  the  provision of  advances, generally  by  the
entity  administering the pool of assets, to ensure that the receipt of payments
on the underlying  pool occurs in  a timely fashion.  Protection against  losses
resulting  from ultimate default  ensures payment through  insurance policies or
letters of credit  obtained by  the issuer or  sponsor from  third parties.  The
Series  will not  pay any  additional or separate  fees for  credit support. The
degree of  credit  support  provided  for  each  issue  is  generally  based  on
historical  information respecting the level of  credit risk associated with the
underlying assets. Delinquency or loss in excess of that anticipated or  failure
of the credit support could adversely affect the return on an investment in such
a security.
 
    ZERO  COUPON BONDS,  DEFERRED INTEREST  BONDS AND  PIK BONDS:  The Utilities
Series may invest in zero coupon  bonds. Zero coupon bonds are debt  obligations
which  are issued or  purchased at a  significant discount from  face value. The
discount approximates the  total amount of  interest the bonds  will accrue  and
compound  over the period until maturity or the first interest payment date at a
rate of interest  reflecting the  market rate  of the  security at  the time  of
issuance. Zero
 
                                       9
<PAGE>
coupon  bonds do not require the  periodic payment of interest. Such investments
benefit the issuer by  mitigating its need  for cash to  meet debt service,  but
also  require a higher  rate of return  to attract investors  who are willing to
defer receipt of such cash.  Such investments may experience greater  volatility
in  market value due  to changes in  interest rates than  debt obligations which
make regular  payments  of interest.  The  Series  will accrue  income  on  such
investments for tax and accounting purposes, as required, which is distributable
to  shareholders and which, because no cash  is received at the time of accrual,
may require the liquidation of other portfolio securities to satisfy the Series'
distribution obligations.
 
    COLLATERALIZED MORTGAGE OBLIGATIONS AND MULTICLASS PASS-THROUGH  SECURITIES:
The  Utilities  Series may  invest  a portion  of  its assets  in collateralized
mortgage obligations or  "CMOs," which  are debt  obligations collateralized  by
mortgage   loans  or  mortgage  pass-through  securities.  Typically,  CMOs  are
collateralized by certificates  issued by  GNMA, the  Federal National  Mortgage
Association  ("FNMA") or the  Federal Home Loan  Mortgage Corporation ("FHLMC"),
but also may be collateralized by  whole loans or private mortgage  pass-through
securities  (such collateral collectively referred to as "Mortgage Assets"). The
Series may  also invest  a  portion of  its  assets in  multiclass  pass-through
securities  which are  interests in  a trust  composed of  Mortgage Assets. CMOs
(which include multiclass  pass-through securities) may  be issued by  agencies,
authorities   or  instrumentalities  of  the   U.S.  Government  or  by  private
originators of,  or investors  in, mortgage  loans, including  savings and  loan
associations,  mortgage banks,  commercial banks,  investment banks  and special
purpose subsidiaries of the foregoing. Payments of principal of and interest  on
the  Mortgage Assets, and any reinvestment  income thereon, provide the funds to
pay debt service on the CMOs  or make scheduled distributions on the  multiclass
pass-through securities. In a CMO, a series of bonds or certificates are usually
issued  in multiple classes with different maturities. Each class of CMOs, often
referred to as a  "tranche," is issued  at a specific  fixed or floating  coupon
rate and has a stated maturity or final distribution date. Principal prepayments
on  the Mortgage Assets may  cause the CMOs to  be retired substantially earlier
than their stated maturities or final distribution dates, resulting in a loss of
all or part of the  premium if any has been  paid. Certain classes of CMOs  have
priority  over  others  with  respect  to  the  receipt  of  prepayments  on the
mortgages. Therefore, depending on the type of CMOs in which the Series invests,
the investment may be subject  to a greater or  lesser risk of prepayments  than
other types of mortgage-related securities.
 
    The  Utilities  Series may  also  invest in  parallel  pay CMOs  and Planned
Amortization Class  CMOs ("PAC  Bonds").  Parallel pay  CMOs are  structured  to
provide  payments of principal on each payment  date to more than one class. PAC
Bonds generally require  payments of  a specified  amount of  principal on  each
payment date. PAC Bonds are always parallel pay CMOs with the required principal
payment  on such securities having the  highest priority after interest has been
paid to all classes. For  a further description of  CMOs, parallel pay CMOs  and
PAC Bonds and the risks related to transactions therein, see the SAI.
 
    INDEXED  SECURITIES: The Utilities  Series may invest  in indexed securities
whose value  is  linked  to foreign  currencies,  interest  rates,  commodities,
indices  or other  financial indicators.  Most indexed  securities are  short to
intermediate term  fixed  income  securities whose  values  at  maturity  (I.E.,
principal  value) and/or interest rates rise or  fall according to the change in
one  or  more  specified  underlying  instruments.  Indexed  securities  may  be
positively  or negatively indexed (I.E., their principal value or interest rates
may increase or decrease if the underlying instrument appreciates), and may have
return  characteristics  similar  to   direct  investments  in  the   underlying
instrument  or  to one  or more  options on  the underlying  instrument. Indexed
securities may be more volatile than the underlying instrument itself.
 
    OPTIONS ON STOCK INDICES: The Utilities Series may write (sell) covered call
and put options and purchase call and  put options on stock indices. The  Series
may  write options  on stock  indices for  the purpose  of increasing  its gross
income and to protect its portfolio against declines in the value of  securities
it  owns or increases in the value of securities to be acquired. When the Series
writes an option on a stock index, and the value of the index moves adversely to
the holder's position,  the option will  not be exercised,  and the Series  will
either  close out the option at a profit  or allow it to expire unexercised. The
Series will thereby retain the amount  of the premium, less related  transaction
costs, which will increase its gross income and offset part of the reduced value
of portfolio securities or the increased cost of securities to be acquired. Such
transactions, however, will constitute only partial hedges against adverse price
fluctuations,  since any such fluctuations will be  offset only to the extent of
the premium received by the Series for  the writing of the option, less  related
transaction  costs.  In addition,  if  the value  of  an underlying  index moves
adversely to the Series' option position,  the option may be exercised, and  the
Series  will experience a loss which may  only be partially offset by the amount
of the premium received.
 
                                       10
<PAGE>
    The Utilities Series may also purchase put or call options on stock  indices
in  order, respectively, to hedge its investments  against a decline in value or
to attempt to reduce the risk of missing a market or industry segment advance. A
Series' possible loss in either case will be limited to the premium paid for the
option, plus related transaction costs.
 
    FUTURES CONTRACTS AND OPTIONS ON FUTURES CONTRACTS: The Utilities Series may
purchase and  sell  futures  contracts  on  foreign  or  domestic  fixed  income
securities  or indices of such securities,  including municipal bond indices and
any other indices of foreign or domestic fixed income securities that may become
available for  trading  ("Futures Contracts").  The  Utilities Series  may  also
purchase  and  write  options on  such  Futures Contracts  ("Options  on Futures
Contracts"), and may purchase and  sell Futures Contracts on foreign  currencies
or indices of foreign currencies. The Series may also purchase and write Options
on such Futures Contracts.
 
    Such  transactions  will  be  entered  into  for  hedging  purposes  or  for
non-hedging purposes to the extent permitted by applicable law. The Series  will
incur  brokerage fees when it purchases and sells Futures Contracts, and will be
required to  maintain margin  deposits. In  addition, Futures  Contracts  entail
risks. Although the Adviser believes that use of such Contracts will benefit the
Series, if its investment judgment about the general direction of exchange rates
or  the stock market is incorrect, the Series' overall performance may be poorer
than if it had not entered into any  such contract and the Series may realize  a
loss.  The  Series  will not  enter  into  any Futures  Contract  if immediately
thereafter the value  of securities  and other obligations  underlying all  such
Futures  Contracts held by the Series would exceed 50% of the value of its total
assets.
 
    Purchases of Options on Futures Contracts  may present less risk in  hedging
the  Series'  portfolio than  the  purchase or  sale  of the  underlying Futures
Contracts since the potential loss is limited to the amount of the premium  plus
related  transaction costs, although it may  be necessary to exercise the option
to realize any profit, which results in the establishment of a futures position.
The writing of Options on Futures Contracts, however, does not present less risk
than the trading of Futures Contracts and will constitute only a partial  hedge,
up  to  the  amount  of the  premium  received.  In addition,  if  an  option is
exercised, the Series may suffer a loss on the transaction.
 
    Futures Contracts and Options on Futures Contracts that are entered into  by
the Series will be traded on U.S. and foreign exchanges.
 
    FORWARD  CONTRACTS:  Each Series  may  enter into  forward  foreign currency
exchange contracts for the  purchase or sale  of a fixed  quantity of a  foreign
currency  at a  future date  ("Forward Contracts").  Each Series  may enter into
Forward Contracts  for  hedging purposes  and  for non-hedging  purposes  (I.E.,
speculative  purposes). By entering  into transactions in  Forward Contracts for
hedging  purposes,  a  Series  may  be  required  to  forego  the  benefits   of
advantageous  changes in  exchange rates and,  in the case  of Forward Contracts
entered into for non-hedging  purposes, a Series may  sustain losses which  will
reduce  its  gross income.  Such  transactions, therefore,  could  be considered
speculative. Forward Contracts are traded over-the-counter and not on  organized
commodities or securities exchanges. As a result, Forward Contracts operate in a
manner distinct from exchange-traded instruments, and their use involves certain
risks  beyond those associated with transactions in Futures Contracts or options
traded on exchanges. A Series may choose to, or be required to, receive delivery
of the  foreign currencies  underlying Forward  Contracts it  has entered  into.
Under  certain  circumstances,  such  as where  the  Adviser  believes  that the
applicable exchange rate is unfavorable at the time the currencies are  received
or  the Adviser anticipates, for  any other reason, that  the exchange rate will
improve, the Series may hold such currencies for an indefinite period of time. A
Series may also enter into a Forward  Contract on one currency to hedge  against
risk  of  loss arising  from  fluctuations in  the  value of  a  second currency
(referred to  as  a  "cross hedge")  if,  in  the judgment  of  the  Adviser,  a
reasonable degree of correlation can be expected between movements in the values
of  the  two  currencies.  Each  of  these  Series  has  established  procedures
consistent with statements of the SEC and its staff regarding the use of Forward
Contracts by registered investment companies,  which requires use of  segregated
assets or "cover" in connection with the purchase and sale of such contracts.
 
                                       11
<PAGE>
    OPTIONS  ON FOREIGN CURRENCIES: The Utilities  Series may purchase and write
options on foreign currencies ("Options on Foreign Currencies") for the  purpose
of  protecting against declines in the  dollar value of portfolio securities and
against increases in the  dollar cost of  securities to be  acquired. As in  the
case  of other types  of options, however,  the writing of  an Option on Foreign
Currency will constitute only a partial hedge,  up to the amount of the  premium
received,  and the Series may be required to purchase or sell foreign currencies
at disadvantageous exchange rates, thereby incurring losses. The purchase of  an
Option   on  Foreign  Currency   may  constitute  an   effective  hedge  against
fluctuations in exchange rates although, in the event of rate movements  adverse
to  the Series' position, it  may forfeit the entire  amount of the premium paid
for the option plus related transaction costs. The Series may also choose to, or
be required to, receive delivery of the foreign currencies underlying Options on
Foreign Currencies it  has entered  into. Under certain  circumstances, such  as
where  the Adviser believes that the  applicable exchange rate is unfavorable at
the time the currencies are received  or the Adviser anticipates, for any  other
reason, that the exchange rate will improve, the Series may hold such currencies
for an indefinite period of time.
 
6.  ADDITIONAL RISK FACTORS
 
    OPTIONS,  FUTURES CONTRACTS AND FORWARD CONTRACTS: Although each Series will
enter into  transactions  in  options, Futures  Contracts,  Options  on  Futures
Contracts,  Forward  Contracts and  Options  on Foreign  Currencies  for hedging
purposes, such transactions nevertheless involve  certain risks. For example,  a
lack  of  correlation between  the instrument  underlying  an option  or Futures
Contract and the  assets being  hedged, or unexpected  adverse price  movements,
could render a Series' hedging strategy unsuccessful and could result in losses.
Certain  Series also may enter into  transactions in options, Futures Contracts,
Options on  Futures  Contracts and  Forward  Contracts for  other  than  hedging
purposes,  which  involves greater  risk. In  particular, such  transactions may
result in losses for a Series which  are not offset by gains on other  portfolio
positions,  thereby reducing gross income. In addition, foreign currency markets
may be extremely volatile from time to time. There also can be no assurance that
a liquid secondary market will exist for  any contract purchased or sold, and  a
Series  may be  required to  maintain a  position until  exercise or expiration,
which could result in losses. The SAI  contains a description of the nature  and
trading  mechanics of options, Futures  Contracts, Options on Futures Contracts,
Forward Contracts and Options on  Foreign Currencies, and includes a  discussion
of the risks related to transactions therein.
 
    Transactions   in  Forward  Contracts  may  be  entered  into  only  in  the
over-the-counter market. Futures Contracts and Options on Futures Contracts  may
be  entered into  on U.S. exchanges  regulated by the  Commodity Futures Trading
Commission and on  foreign exchanges.  In addition, the  securities and  indexes
underlying options, Futures Contracts and Options on Futures Contracts traded by
the Series will include both domestic and foreign securities.
 
    LOWER  RATED BONDS: Each Series may  invest in fixed income securities rated
Baa by Moody's or BBB by S&P  or Fitch and comparable unrated securities.  These
securities,  while  normally  exhibiting  adequate  protection  parameters, have
speculative  characteristics  and  changes  in  economic  conditions  or   other
circumstances  are more likely to lead to  a weakened capacity to make principal
and interest payments than in the case of higher grade securities.
 
    Each Series may also invest in securities rated Ba or lower by Moody's or BB
or lower by S&P  or Fitch and comparable  unrated securities (commonly known  as
"junk  bonds") to the extent described above.  See Appendix A to this Prospectus
for a description of these ratings. These securities are considered  speculative
and,  while generally providing greater income  than investments in higher rated
securities, will involve  greater risk  of principal and  income (including  the
possibility  of default or bankruptcy of the issuers of such securities) and may
involve greater  volatility  of price  (especially  during periods  of  economic
uncertainty or change) than securities in the higher rating categories. However,
since  yields vary over time,  no specific level of  income can ever be assured.
These lower  rated  high yielding  fixed  income securities  generally  tend  to
reflect economic changes and short-term corporate and industry developments to a
greater   extent  than  higher   rated  securities  which   react  primarily  to
fluctuations in the general level of interest rates (although these lower  rated
fixed  income securities  are also  affected by  changes in  interest rates, the
market's perception  of  their credit  quality,  and the  outlook  for  economic
growth).  In the past, economic downturns or an increase in interest rates have,
under certain circumstances, caused a higher incidence of default by the issuers
of these securities  and may  do so  in the future,  especially in  the case  of
highly leveraged issuers. During certain periods, the higher yields on a Series'
lower  rated high yielding fixed income securities are paid primarily because of
the increased risk of loss of principal and income, arising from such factors as
the heightened  possibility of  default or  bankruptcy of  the issuers  of  such
securities. Due to the fixed
 
                                       12
<PAGE>
income  payments of  these securities,  a Series may  continue to  earn the same
level of interest  income while its  net asset value  declines due to  portfolio
losses,  which could  result in  an increase  in the  Series' yield  despite the
actual loss  of  principal.  The  market for  these  lower  rated  fixed  income
securities  may be less liquid than the market for investment grade fixed income
securities, and  judgment may  at times  play a  greater role  in valuing  these
securities than in the case of investment grade fixed income securities. Changes
in  the value of securities subsequent to their acquisition will not affect cash
income or yield to maturity to a Series  but will be reflected in the net  asset
value  of shares of the Series. See the  SAI for more information on lower rated
securities.
 
    FOREIGN  SECURITIES:  Each  Series  may  invest  in  dollar-denominated  and
non-dollar-denominated  foreign securities.  Investing in  securities of foreign
issuers generally involves  risks not  ordinarily associated  with investing  in
securities  of  domestic  issuers.  These  include  changes  in  currency rates,
exchange  control  regulations,  governmental  administration  or  economic   or
monetary  policy (in  the U.S. or  abroad) or circumstances  in dealings between
nations. Costs may be  incurred in connection  with conversions between  various
currencies.  Special considerations  may also  include more  limited information
about foreign issuers,  higher brokerage costs,  different accounting  standards
and thinner trading markets. Foreign securities markets may also be less liquid,
more  volatile and  less subject  to government  supervision than  in the United
States. Investments  in foreign  countries could  be affected  by other  factors
including  expropriation,  confiscatory taxation  and potential  difficulties in
enforcing contractual obligations  and could be  subject to extended  settlement
periods.  Each  Series may  hold foreign  currency  received in  connection with
investments in foreign securities when, in the judgment of the Adviser, it would
be beneficial to convert such currency into U.S. dollars at a later date,  based
on  anticipated changes in the relevant exchange rate. Such Series may also hold
foreign currency in anticipation of  purchasing foreign securities. See the  SAI
for  further  discussion  of  foreign  securities  and  the  holding  of foreign
currency, as well as the associated risks.
 
    AMERICAN DEPOSITARY  RECEIPTS: Each  Series  may invest  in ADRs  which  are
certificates  issued  by a  U.S.  depository (usually  a  bank) and  represent a
specified quantity of shares of an  underlying non-U.S. stock on deposit with  a
custodian  bank as collateral. Because ADRs  trade on U.S. securities exchanges,
the Adviser does not treat them as foreign securities. However, they are subject
to many of the risks of foreign securities such as changes in exchange rates and
more limited information about foreign issuers.
 
    EMERGING MARKET SECURITIES: Each Series  may invest in emerging markets.  In
addition to the general risks of investing in foreign securities, investments in
emerging  markets involve special risks. Securities  of many issuers in emerging
markets may  be less  liquid and  more volatile  than securities  of  comparable
domestic  issuers.  These securities  may be  considered speculative  and, while
generally offering higher income and the potential for capital appreciation, may
present  significantly  greater  risk.  Emerging  markets  may  have   different
clearance  and settlement  procedures, and  in certain  markets there  have been
times when  settlements  have  been unable  to  keep  pace with  the  volume  of
securities  transactions,  making  it difficult  to  conduct  such transactions.
Delays in settlement  could result in  temporary periods when  a portion of  the
assets  of a Series is uninvested and no return is earned thereon. The inability
of a Series to make intended security purchases due to settlement problems could
cause a Series to miss attractive investment opportunities. Inability to dispose
of portfolio securities due to settlement  problems could result in losses to  a
Series  due  to subsequent  declines  in value  of  the portfolio  securities, a
decrease in the level of  liquidity in a Series' portfolio,  or if a Series  has
entered  into  a  contract  to  sell the  security,  possible  liability  to the
purchaser. Certain markets may require  payment for securities before  delivery,
and  in such  markets a Series  bears the risk  that the securities  will not be
delivered and that the Series' payments will not be returned. Securities  prices
in  emerging  markets  can  be  significantly more  volatile  than  in  the more
developed  nations  of  the  world,  reflecting  the  greater  uncertainties  of
investing  in less established  markets and economies.  In particular, countries
with emerging markets may have relatively unstable governments, present the risk
of  nationalization  of  businesses,  restrictions  on  foreign  ownership,   or
prohibitions of repatriation of assets, and may have less protection of property
rights  than more developed countries. The  economies of countries with emerging
markets may  be predominantly  based on  only a  few industries,  may be  highly
vulnerable  to changes in local or global  trade conditions, and may suffer from
extreme and volatile debt burdens  or inflation rates. Local securities  markets
may  trade a small number of securities and may be unable to respond effectively
to increases  in  trading  volume,  potentially  making  prompt  liquidation  of
substantial  holdings difficult  or impossible  at times.  Securities of issuers
located in countries with  emerging markets may  have limited marketability  and
may be subject to more abrupt or erratic movements.
 
                                       13
<PAGE>
    Certain   emerging  markets  may  require   governmental  approval  for  the
repatriation  of  investment  income,  capital  or  the  proceeds  of  sales  of
securities  by foreign investors.  In addition, if a  deterioration occurs in an
emerging market's balance  of payments  or for  other reasons,  a country  could
impose  temporary restrictions on foreign capital remittances. A Series could be
adversely  affected  by  delays  in,  or  a  refusal  to  grant,  any   required
governmental approval for repatriation of capital, as well as by the application
to the Series of any restrictions on investments.
 
    Investment  in  certain  foreign  emerging market  debt  obligations  may be
restricted or controlled to varying degrees. These restrictions or controls  may
at times preclude investment in certain foreign emerging market debt obligations
and increase the expenses of a Series.
 
    NON-DIVERSIFICATION: The Utilities Series is "non-diversified," as that term
is  defined in the Investment Company Act  of 1940 (the "1940 Act"), but intends
to qualify as a  "regulated investment company" ("RIC")  for federal income  tax
purposes.  This means,  in general,  that although more  than 5%  of the Series'
total assets  may be  invested in  the  securities of  one issuer  (including  a
foreign  government),  at the  close of  each  quarter of  its taxable  year the
aggregate amount of such holdings may not  exceed 50% of the value of its  total
assets, and no more than 25% of the value of its total assets may be invested in
the  securities of a single issuer. To  the extent that a non-diversified Series
at times may hold the securities of a smaller number of issuers than if it  were
"diversified"  (as defined in  the 1940 Act),  the Series will  at such times be
subject to greater  risk with respect  to its portfolio  securities than a  fund
that  invests in a broader range of securities, because changes in the financial
condition or market assessment of a single issuer may cause greater fluctuations
in the Series' total return and the net asset value of its shares.
                              -------------------
 
SHORT-TERM INVESTMENTS FOR  TEMPORARY DEFENSIVE  PURPOSES --  During periods  of
unusual market conditions when the Adviser believes that investing for temporary
defensive  purposes is appropriate,  or in order  to meet anticipated redemption
requests, a large portion or all of the assets of each Series may be invested in
cash (including foreign currency) or cash equivalents including, but not limited
to,  obligations  of   banks  (including  certificates   of  deposit,   bankers'
acceptances,   time  deposits  and  repurchase  agreements),  commercial  paper,
short-term notes, U.S. Government Securities and related repurchase agreements.
 
PORTFOLIO TRADING
 
    Each  Series  intends  to  manage  its  portfolio  by  buying  and   selling
securities,  as  well as  holding  securities to  maturity,  to help  attain its
investment objectives and policies.
 
    Each Series will engage in portfolio  trading if it believes a  transaction,
net  of  costs  (including  custodian  charges),  will  help  in  attaining  its
investment objectives. In trading portfolio  securities, a Series seeks to  take
advantage of market
developments,  yield  disparities  and  variations  in  the  creditworthiness of
issuers. For a description of the strategies which may be used by the Series  in
trading   portfolio  securities,  see   "Portfolio  Transactions  and  Brokerage
Commissions" in the SAI.
 
    Because the Utilities Series is expected  to have a portfolio turnover  rate
of over 100%, transactions costs incurred by the Series and the realized capital
gains  and losses of the Series may be greater than that of a fund with a lesser
portfolio turnover rate.
 
    The primary consideration  in placing portfolio  security transactions  with
broker-dealers  for execution  is to obtain,  and maintain  the availability of,
execution at  the  most  favorable  prices and  in  the  most  effective  manner
possible. Consistent with the foregoing primary consideration, the Rules of Fair
Practice  of the National  Association of Securities  Dealers, Inc. (the "NASD")
and such other policies as the Trustees of the Trust may determine, the  Adviser
may  consider sales of  Contracts for which  the Trust is  an investment option,
together with sales of  shares of other investment  company clients of MFS  Fund
Distributors, Inc., the distributor of shares of the Trust and of the MFS Family
of Funds, as a factor in the selection of broker-dealers to execute each Series'
portfolio  transactions.  From  time  to time  the  Adviser  may  direct certain
portfolio transactions to broker-dealer firms which, in turn, have agreed to pay
a portion of the Series' operating expenses (e.g., fees charged by the custodian
of the Series' assets). For a  further discussion of portfolio trading, see  the
SAI.
                              -------------------
 
                                       14
<PAGE>
    The  SAI includes a  discussion of other investment  policies and listing of
specific investment restrictions  which govern the  investment policies of  each
Series.  The specific investment  restrictions listed in the  SAI may be changed
without shareholder  approval  unless indicated  otherwise  (see the  SAI).  The
Series'  investment limitations, policies and rating standards are adhered to at
the  time  of  purchase  or  utilization  of  assets;  a  subsequent  change  in
circumstances will not be considered to result in a violation of policy.
 
7.  MANAGEMENT OF THE SERIES
 
    The   Trust's  Board  of  Trustees,  as   part  of  its  overall  management
responsibility, oversees  various  organizations responsible  for  each  Series'
day-to-day management.
 
INVESTMENT ADVISER -- MFS manages each Series pursuant to an Investment Advisory
Agreement  with the  Trust on behalf  of each  Series dated April  14, 1994 (the
"Advisory Agreement"). Under  the Advisory  Agreement, MFS  provides the  Series
with  overall  investment advisory  services. Subject  to  such policies  as the
Trustees may determine, MFS makes investment decisions for each Series. For  its
services  and  facilities,  MFS receives  a  management fee,  computed  and paid
monthly, in an amount equal to the  following annual rates of the average  daily
net assets of each Series:
 
<TABLE>
<CAPTION>
                                                                                              PERCENTAGE OF THE
                                                                                              AVERAGE DAILY NET
                                                                                                    ASSETS
SERIES                                                                                          OF EACH SERIES
- ------------------------------------------------------------------------------------------  ----------------------
<S>                                                                                         <C>
Research Series...........................................................................             0.75%
Utilities Series..........................................................................             0.75%
</TABLE>
 
    MFS  or its  affiliates will  pay a  fee to  Jefferson Pilot  Life Insurance
Company equal, on an annualized basis, to  0.15% of the aggregate net assets  of
each Series, attributable to Contracts offered by separate accounts of Jefferson
Pilot  Life Insurance Company or their affiliates. Such fees will not be paid by
the Series, their shareholders, or by the Contract holders.
 
    For the fiscal  year ended  December 31,  1996, MFS  received the  following
management  fees from  the Series under  the Advisory Agreement  and assumed the
following amounts of the Series' expenses (see "Expenses" below);
 
<TABLE>
<CAPTION>
                                                                                 MANAGEMENT FEE  EXPENSES ASSUMED
SERIES                                                                            PAID TO MFS         BY MFS
- -------------------------------------------------------------------------------  --------------  ----------------
<S>                                                                              <C>             <C>
Research Series................................................................    $   92,348       $   56,859
Utilities Series...............................................................        39,863           91,877
</TABLE>
 
    The identity and background of the portfolio managers for each Series is set
forth below. Unless  indicated otherwise,  each portfolio manager  has acted  in
that capacity since the commencement of investment operations of each Series.
 
<TABLE>
<CAPTION>
SERIES                                                         PORTFOLIO MANAGERS
- -----------------------------  -----------------------------------------------------------------------------------
<S>                            <C>
Research Series                The  Series is  currently managed  by a  committee comprised  of various investment
                               research analysts.
Utilities Series               Maura A. Shaughnessy, a  Vice President of  the Adviser, has  been employed by  the
                               Adviser as a portfolio manager since 1991.
</TABLE>
 
    MFS  also serves as investment adviser to each of the other funds in the MFS
Family of Funds  (the "MFS  Funds") and to  MFS-Registered Trademark-  Municipal
Income Trust, MFS Multimarket Income Trust, MFS Government Markets Income Trust,
MFS  Intermediate  Income Trust,  MFS Charter  Income  Trust, MFS  Special Value
Trust, MFS Institutional Trust,  MFS Union Standard  Trust, MFS/Sun Life  Series
Trust,  and seven  variable accounts, each  of which is  a registered investment
company established by Sun Life Assurance Company of Canada (U.S.) ("Sun Life of
Canada (U.S.)") in connection  with the sale  of various fixed/variable  annuity
contracts.  MFS  and its  wholly owned  subsidiary, MFS  Institutional Advisers,
Inc., provide investment advice to substantial private clients.
 
    MFS is America's oldest  mutual fund organization.  MFS and its  predecessor
organizations  have  a history  of  money management  dating  from 1924  and the
founding of the first mutual fund in the United States, Massachusetts  Investors
Trust.   Net  assets  under   the  management  of   the  MFS  organization  were
approximately $52.8  billion on  behalf of  approximately 2.3  million  investor
accounts  as of February 28, 1997. As of such date, the MFS organization managed
approximately  $28.9  billion  of  assets  invested  in  equity  securities  and
approximately $19.9 billion of assets invested in
 
                                       15
<PAGE>
fixed income securities. Approximately $4.0 billion of the assets managed by MFS
are  invested in securities  of foreign issuers  and non-U.S. dollar-denominated
securities of U.S. issuers. MFS  is a subsidiary of  Sun Life of Canada  (U.S.),
which  in turn  is a wholly  owned subsidiary  of Sun Life  Assurance Company of
Canada ("Sun  Life"). The  Directors of  MFS are  A. Keith  Brodkin, Jeffrey  L.
Shames,  Arnold D. Scott, John  D. McNeil and Donald  A. Stewart. Mr. Brodkin is
the Chairman, Mr. Shames is the President  and Mr. Scott is the Secretary and  a
Senior  Executive  Vice President  of MFS.  Messrs. McNeil  and Stewart  are the
Chairman and  President, respectively,  of Sun  Life. Sun  Life, a  mutual  life
insurance  company, is one of the largest international life insurance companies
and has  been  operating  in  the  United  States  since  1895,  establishing  a
headquarters  office here in 1973.  The executive officers of  MFS report to the
Chairman of Sun Life.
 
    A. Keith Brodkin, the Chairman  and a Director of  MFS, is the Chairman  and
President  and a Trustee of the Trust. W. Thomas London, Stephen E. Cavan, James
R. Bordewick, Jr.,  and James  O. Yost,  all of whom  are officers  of MFS,  are
officers of the Trust.
 
    MFS  has established a strategic alliance with Foreign & Colonial Management
Ltd. ("Foreign & Colonial"). Foreign  & Colonial is a  subsidiary of two of  the
world's  oldest  financial  services institutions,  the  London-based  Foreign &
Colonial Investment Trust PLC, which pioneered the idea of investment management
in 1868, and HYPO-BANK (Bayerische Hypotheken-und Weschsel-Bank AG), the  oldest
publicly  listed bank in Germany, founded in 1835. As part of this alliance, the
portfolio managers and investment analysts of  MFS and Foreign & Colonial  share
their  views on  a variety  of investment related  issues, such  as the economy,
securities  markets,  portfolio   securities  and   their  issuers,   investment
recommendations,  strategies and  techniques, risk  analysis, trading strategies
and other  portfolio  management  matters.  MFS  has  access  to  the  extensive
international  equity investment expertise of Foreign  & Colonial, and Foreign &
Colonial has access to  the extensive U.S. equity  investment expertise of  MFS.
MFS  and Foreign Colonial each have investment personnel working in each other's
offices in Boston and London, respectively.
 
    In certain  instances there  may  be securities  which  are suitable  for  a
Series'  portfolio as well as for portfolios  of other clients of MFS or clients
of Foreign  &  Colonial.  Some simultaneous  transactions  are  inevitable  when
several  clients  receive investment  advice from  MFS  and Foreign  & Colonial,
particularly when the same security is suitable for more than one client.  While
in  some cases this arrangement could have  a detrimental effect on the price or
availability of the security as  far as a Series  is concerned, in other  cases,
however, it may produce increased investment opportunities for the Series.
 
    From  time to time, the Adviser may  purchase, redeem and exchange shares of
any Series. The  purchase by  the Adviser  of shares of  a Series  may have  the
effect  of  lowering that  Series' expense  ratio, while  the redemption  by the
Adviser of shares of  a Series may  have the effect  of increasing that  Series'
expense ratio.
 
    DISTRIBUTOR   --  MFS  Fund  Distributors,  Inc.  ("MFD"),  a  wholly  owned
subsidiary of MFS, is the distributor of  shares of each Series and also  serves
as distributor for certain of the other mutual funds managed by MFS.
 
    ADMINISTRATOR  --  MFS  provides  the  Series  with  certain  administrative
services pursuant to a Master  Administrative Services Agreement dated March  1,
1997.  Under this  Agreement, MFS  provides the  Series with  certain financial,
legal, compliance, shareholder communications and other administrative services.
As a partial reimbursement for the cost of providing these services, the  Series
pays  MFS an administrative  fee up to  0.015% per annum  of the Series' average
daily net assets,  provided that  the administrative fee  is not  assessed on  a
Series' assets that exceed $3 billion.
 
    SHAREHOLDER  SERVICING AGENT --  MFS Service Center,  Inc. (the "Shareholder
Servicing Agent"), a wholly owned  subsidiary of MFS, performs transfer  agency,
certain dividend disbursing agency and other services for each Series.
 
8.  INFORMATION CONCERNING SHARES OF EACH SERIES
 
PURCHASES AND REDEMPTIONS
 
    The  separate accounts of the Participating Insurance Companies place orders
to purchase and redeem shares of each  Series based on, among other things,  the
amount of premium payments to be invested and surrender and transfer requests to
be  effected on that day pursuant to Contracts. Orders received by the Trust are
effected on days on which the Exchange is open for trading. For orders  received
by  the Trust before  the close of  regular trading on  the Exchange (normally 4
p.m. eastern time), such purchases and redemptions of the shares of each  Series
are  effected at the respective net asset  values per share determined as of the
close   of   regular   trading   on    the   Exchange   on   that   same    day.
 
                                       16
<PAGE>
Participating Insurance Companies shall be the designee of the Trust for receipt
of  purchase and  redemption orders  from Contract  holders and  receipt by such
designee shall constitute receipt by the Trust; provided that the Trust receives
notice of such  order by 9:30  a.m. eastern time  on the next  following day  on
which  the Exchange is open for trading.  Payment for shares shall be by federal
funds transmitted by wire and must be received by 2:00 p.m. eastern time on  the
next  following day on which the Exchange is open for trading after the purchase
order is received. Redemption proceeds shall be by federal funds transmitted  by
wire  and shall be sent by  2:00 p.m. eastern time on  the next following day on
which the Exchange is open for  trading after the redemption order is  received.
No fee is charged the shareholders when they redeem Series shares.
 
    The  offering of shares of any Series may  be suspended for a period of time
and each  Series reserves  the  right to  refuse  any specific  purchase  order.
Purchase  orders may be refused if, in the Adviser's opinion, they are of a size
that would disrupt the management of a  Series. The Trust may suspend the  right
of  redemption of shares of any Series  and may postpone payment for any period:
(i) during which the Exchange is closed other than customary weekend and holiday
closings or during which  trading on the Exchange  is restricted; (ii) when  the
SEC  determines  that a  state of  emergency  exists which  may make  payment or
transfer not reasonably practicable;  (iii) as the SEC  may by order permit  for
the  protection of the security  holders of the Trust; or  (iv) at any time when
the Trust may, under applicable laws, rules and regulations, suspend payment  on
the redemption of its shares.
 
    Should  any conflict between Contract holders arise which would require that
a substantial  amount  of net  assets  be  withdrawn from  any  Series,  orderly
portfolio  management  could be  disrupted to  the  potential detriment  of such
Contract.
 
NET ASSET VALUE
 
    The net asset value per share of  each Series is determined each day  during
which  the Exchange is open for trading.  This determination is made once during
each such day as of  the close of regular trading  on the Exchange by  deducting
the  amount of the Series' liabilities from  the value of the Series' assets and
dividing the  difference by  the number  of shares  of the  Series  outstanding.
Values  of assets in  a Series' portfolio  are determined on  the basis of their
market or other fair value as described in the SAI. All investments, assets  and
liabilities  are expressed in U.S. dollars  based upon current currency exchange
rates.
 
DISTRIBUTIONS
 
    Substantially all of  each Series'  net investment income  for any  calendar
year  is declared as dividends  and paid to its  shareholders as dividends on an
annual basis. In addition, each Series may make one or more distributions during
the calendar year to its shareholders from any long-term capital gains, and  may
also  make one or more distributions to its shareholders from short-term capital
gains. In determining the  net investment income  available for distribution,  a
Series  may rely on projections of  its anticipated net investment income (which
may include  short-term capital  gains from  the sales  of securities  or  other
assets,  and, if  allowed by  a Series'  investment restrictions,  premiums from
options written), over  a longer  term, rather  than its  actual net  investment
income for the period.
 
    Shareholders of any of the Series may elect to receive dividends and capital
gain distributions in either cash or additional shares.
 
TAX STATUS
 
    Each  Series of the Trust is treated as a separate entity for federal income
tax purposes. In  order to  minimize the taxes  each Series  would otherwise  be
required  to  pay, each  Series intends  to  qualify each  year as  a "regulated
investment company" under Subchapter M of the Internal Revenue Code of 1986,  as
amended  ("the Code"). Because each Series intends  to distribute all of its net
investment income and net capital gains  to its shareholders in accordance  with
the  timing requirements imposed by the Code, it is not expected that any of the
Series will be required to pay entity level federal income or excise taxes.
 
    Shares of  the  Series  are  offered only  to  the  Participating  Insurance
Companies'  separate accounts that  fund Contracts. See  the applicable Contract
prospectus for  a discussion  of the  federal income  tax treatment  of (1)  the
separate  accounts that purchase and  hold Series shares and  (2) the holders of
the Contracts  that  are funded  through  those  accounts. In  addition  to  the
diversification  requirements  of Subchapter  M of  the  Code, each  Series also
intends to diversify its assets as  required by Code Section 817(h)(1), and  the
regulations thereunder. See also "Tax Status" in the SAI.
 
                                       17
<PAGE>
DESCRIPTION OF SHARES, VOTING RIGHTS AND LIABILITIES
 
    Each Series currently has one class of shares, entitled Shares of Beneficial
Interest  (without par value).  The Trust has  reserved the right  to create and
issue additional  classes and  series of  shares, in  which case  each class  of
shares  of a  series would  participate equally  in the  earnings, dividends and
assets attributable to that  class of that  particular series. Shareholders  are
entitled to one vote for each share held, and shares of each Series are entitled
to  vote  separately to  approve investment  advisory  agreements or  changes in
investment restrictions with respect  to that Series, but  shares of all  Series
vote  together  in  the  election  of  Trustees  and  selection  of accountants.
Additionally, each Series will vote separately on any other matter that  affects
solely  that Series, but will  otherwise vote together with  all other Series on
all other  matters.  The  Trust  does not  intend  to  hold  annual  shareholder
meetings.  The Declaration of Trust provides that  a Trustee may be removed from
office in  certain instances.  See  "Description of  Shares, Voting  Rights  and
Liabilities" in the SAI.
 
    Each  share of  a Series represents  an equal proportionate  interest in the
Series with each  share, subject to  the liabilities of  the particular  Series.
Shares  have no pre-emptive or conversion rights. Shares are fully paid and non-
assessable. Should a Series  be liquidated, shareholders  are entitled to  share
PRO  RATA in the  net assets available for  distribution to shareholders. Shares
will remain on  deposit with  the Shareholder Servicing  Agent and  certificates
will not be issued.
 
    The  Trust  is an  entity of  the  type commonly  known as  a "Massachusetts
business trust."  Under Massachusetts  law, shareholders  of such  a trust  may,
under  certain  circumstances, be  held personally  liable  as partners  for its
obligations. However,  the risk  of a  shareholder incurring  financial loss  on
account  of  shareholder liability  is limited  to  circumstances in  which both
inadequate insurance existed (E.G., fidelity bonding and omission insurance) and
the Trust itself was unable to meet its obligations.
 
    United of  Omaha Life  Insurance  Company, Omaha,  NE,  owns 50.75%  of  the
Research  Series'  shares,  and,  therefore, controls  the  Series;  CG Variable
Annuity--Separate Account II, Hartford, CT, owns 29.08% of the Utilities Series'
shares, and,  therefore,  controls  the  Series;  and  Ameritas  Life  Insurance
Company--Separate  Account  VA-2  (Annuity),  Lincoln, NE,  owns  56.16%  of the
Utilities Series' shares, and, therefore, controls the Series.
 
PERFORMANCE INFORMATION
 
    Each Series' performance may be quoted in advertising in terms of yield  and
total  return. Performance is based on historical results and is not intended to
indicate future performance. Performance quoted for a Series includes the effect
of deducting that  Series' expenses, but  may not include  charges and  expenses
attributable  to any particular insurance  product. Excluding these charges from
quotations of a Series' performance has the effect of increasing the performance
quoted. Performance for a Series will vary based on, among other things, changes
in market conditions, the level of interest  rates and the level of the  Series'
expenses.  For further information about the  Series' performance for the fiscal
year ended December 31, 1996, please see  the Series' Annual Reports. A copy  of
these   Annual  Reports  may  be  obtained  without  charge  by  contacting  the
Shareholder Servicing Agent (see back cover for address and phone number).
 
    From time to time,  quotations of a  Series' total return  and yield may  be
included  in  advertisements, sales  literature  or reports  to  shareholders or
prospective investors. The total return of a Series refers to return assuming an
investment has been  held in the  Series for one  year and for  the life of  the
Series  (the ending date of  which will be stated).  The total return quotations
may be expressed in terms  of average annual or  cumulative rates of return  for
all  periods quoted.  Average annual total  return refers to  the average annual
compound rate of return  of an investment in  a Series. Cumulative total  return
represents  the cumulative change  in value of  an investment in  a Series. Both
will assume that all dividends and capital gains distributions were  reinvested.
The yield of a Series refers to net investment income generated by a Series over
a specified 30-day (or one month) period. This income is then "annualized." That
is,  the amount  of income generated  by the  Series during that  30-day (or one
month) period is assumed to be generated over a 12-month period and is shown  as
a percentage of net asset value.
 
EXPENSES
 
    The  Trust pays the compensation of the Trustees who are not officers of MFS
and all expenses of each Series (other than those assumed by MFS) including  but
not  limited to: governmental fees; interest  charges; taxes; membership dues in
the Investment Company Institute allocable to each Series; fees and expenses  of
independent auditors, of legal counsel,
 
                                       18
<PAGE>
and  of  any transfer  agent,  registrar or  dividend  disbursing agent  of each
Series; expenses of repurchasing and redeeming shares and servicing  shareholder
accounts;  expenses of  preparing, printing  and mailing  prospectuses, periodic
reports, notices  and  proxy  statements to  shareholders  and  to  governmental
officers  and  commissions;  brokerage  and other  expenses  connected  with the
execution,  recording  and  settlement   of  portfolio  security   transactions;
insurance  premiums; fees  and expenses of  Investors Bank &  Trust Company, the
Trust's Custodian, for  all services  to each Series,  including safekeeping  of
funds  and securities and  maintaining required books  and accounts; expenses of
calculating the  net asset  value of  shares  of each  Series; and  expenses  of
shareholder  meetings.  Expenses  relating  to  the  issuance,  registration and
qualification of shares of each Series and the preparation, printing and mailing
of prospectuses are borne by each Series except that the Distribution  Agreement
with  MFD requires  MFD to pay  for prospectuses that  are to be  used for sales
purposes. Expenses of the Trust which are not attributable to a specific  Series
are allocated between the Series in a manner believed by management of the Trust
to be fair and equitable.
 
    Subject  to termination or revision  at the sole discretion  of MFS, MFS has
agreed to bear expenses of each of  the Series such that the respective  Series'
"Other Expenses," which are defined to include all expenses of the Series except
for  management fees, do not exceed 0.25% of the average daily net assets of the
Series (the "Maximum Percentage"). The obligation of MFS to bear these  expenses
for  a Series terminates on the last day of the Series' fiscal year in which its
"Other Expenses" are less than or equal to the Maximum Percentage. The  payments
made  by MFS  on behalf  of each  Series under  this arrangement  are subject to
reimbursement by the Series to MFS, which will be accomplished by the payment of
an expense reimbursement fee by the Series to MFS computed and paid monthly at a
percentage of the Series' average daily  net assets for its then current  fiscal
year,  with a limitation that immediately  after such payment the Series' "Other
Expenses" will not exceed the Maximum Percentage. This expense reimbursement  by
each  Series to MFS terminates on the earlier of the date on which payments made
by the Series equal the  prior payment of such  reimbursable expenses by MFS  or
December 31, 2004.
 
SHAREHOLDER COMMUNICATIONS
 
    Owners  of Contracts issued  by Participating Insurance  Companies for which
shares of one or more  Series are the investment  vehicle will receive from  the
Participating  Insurance Companies semi-annual  financial statements and audited
year-end financial  statements certified  by the  Trust's independent  certified
public accountants. Each report will show the investments owned by the Trust and
the  valuations thereof  as determined  by the  Trustees and  will provide other
information about the Trust and its operations.
 
    Participating Insurance  Companies with  inquiries regarding  the Trust  may
call  the Trust's Shareholder  Servicing Agent. (See back  cover for address and
phone number.)
 
                              -------------------
 
    The SAI for the Trust,  dated May 1, 1997,  as amended or supplemented  from
time  to  time, contains  more detailed  information about  each of  the Series,
including information related to: (i)  the investment policies and  restrictions
of each Series; (ii) the Trustees, officers and investment adviser of the Trust;
(iii)  portfolio transactions; (iv) the shares  of each Series, including rights
and liabilities of  shareholders; (v)  the method  used to  calculate yield  and
total  rate of return quotations  of each Series; (vi)  the determination of net
asset value  of  shares of  each  Series; and  (vii)  certain voting  rights  of
shareholders of each Series.
 
                                       19
<PAGE>
                                                                      APPENDIX A
 
                          DESCRIPTION OF BOND RATINGS
 
The ratings of Moody's, S&P and Fitch represent their opinions as to the quality
of  various debt instruments. It should be emphasized, however, that ratings are
not absolute standards of quality. Consequently, debt instruments with the  same
maturity,  coupon and rating may have different yields while debt instruments of
the same maturity and coupon with different ratings may have the same yield.
 
                        MOODY'S INVESTORS SERVICE, INC.
 
    AAA: Bonds which are rated  Aaa are judged to be  of the best quality.  They
carry  the smallest degree of  investment risk and are  generally referred to as
"gilt edged." Interest payments are protected by a large or by an  exceptionally
stable margin and principal is secure. While the various protective elements are
likely  to change, such changes as can be visualized are most unlikely to impair
the fundamentally strong position of such issues.
 
    AA: Bonds  which are  rated Aa  are  judged to  be of  high quality  by  all
standards. Together with the Aaa group they comprise what are generally known as
high  grade bonds. They are  rated lower than the  best bonds because margins of
protection may  not  be  as  large  as in  Aaa  securities  or  fluctuations  of
protective  elements may be of greater amplitude  or there may be other elements
present which  make the  long-term  risks appear  somewhat  larger than  in  Aaa
securities.
 
    A:  Bonds which are rated A possess many favorable investment attributes and
are to be considered as upper-medium-grade obligations. Factors giving  security
to  principal and interest are considered  adequate, but elements may be present
which suggest a susceptibility to impairment sometime in the future.
 
    BAA: Bonds which are  rated Baa are  considered as medium-grade  obligations
(I.E.,  they are neither highly protected nor poorly secured). Interest payments
and principal security appear  adequate for the  present but certain  protective
elements  may be lacking or may  be characteristically unreliable over any great
length of time. Such  bonds lack outstanding  investment characteristics and  in
fact have speculative characteristics as well.
 
    BA:  Bonds which are rated Ba are judged to have speculative elements; their
future cannot be considered  as well-assured. Often  the protection of  interest
and  principal payments  may be very  moderate and thereby  not well safeguarded
during both  good  and  bad  times over  the  future.  Uncertainty  of  position
characterizes bonds in this class.
 
    B:  Bonds which are rated B  generally lack characteristics of the desirable
investment. Assurance of interest  and principal payments  or of maintenance  of
other terms of the contract over any long period of time may be small.
 
    CAA:  Bonds which are rated Caa are of  poor standing. Such issues may be in
default or there may be present elements of danger with respect to principal  or
interest.
 
    CA:  Bonds which are rated Ca represent obligations which are speculative in
a  high  degree.  Such  issues  are  often  in  default  or  have  other  marked
shortcomings.
 
    C: Bonds which are rated C are the lowest rated class of bonds and issues so
rated  can be regarded as having extremely  poor prospects of ever attaining any
real investment standing.
 
    ABSENCE OF RATING: Where no rating has  been assigned or where a rating  has
been  suspended or withdrawn, it may be  for reasons unrelated to the quality of
the issue.
 
    Should no rating be assigned, the reason may be one of the following:
 
    1.  an application for rating was not received or accepted;
 
    2.  the issue or issuer belongs  to a group of securities or companies  that
       are not rated as a matter of policy;
 
    3.  there is a lack of essential data pertaining to the issue or issuer; or
 
    4.    the  issue was  privately  placed, in  which  case the  rating  is not
       published in Moody's publications.
 
    Suspension or withdrawal may occur if new and material circumstances  arise,
the  effects  of which  preclude satisfactory  analysis; if  there is  no longer
available reasonable up-to-date  data to permit  a judgment to  be formed; if  a
bond is called for redemption; or for other reasons.
 
                                      A-1
<PAGE>
                       STANDARD & POOR'S RATINGS SERVICES
 
    AAA:  Debt rated AAA has the highest rating assigned by S&P. Capacity to pay
interest and repay principal is extremely strong.
 
    AA: Debt rated  AA has  a very  strong capacity  to pay  interest and  repay
principal and differs from the highest rated issues only in small degree.
 
    A:  Debt rated A has a strong  capacity to pay interest and repay principal,
although it is somewhat  more susceptible to the  adverse effects of changes  in
circumstances and economic conditions than debt in higher-rated categories.
 
    BBB:  Debt  rated BBB  is regarded  as  having an  adequate capacity  to pay
interest and repay principal. Whereas  it normally exhibits adequate  protection
parameters,  adverse  economic  conditions or  changing  circumstances  are more
likely to lead to a  weakened capacity to pay  interest and repay principal  for
debt in this category than in higher-rated categories.
 
    BB:  Debt rated  BB has less  near-term vulnerability to  default than other
speculative issues. However, it faces major ongoing uncertainties or exposure to
adverse  business,  financial,  or  economic  conditions  which  could  lead  to
inadequate  capacity  to meet  timely interest  and  principal payments.  The BB
rating category  is also  used for  debt  subordinated to  senior debt  that  is
assigned an actual or implied BBB- rating.
 
    B: Debt rated B has a greater vulnerability to default but currently has the
capacity  to meet interest payments  and principal repayments. Adverse business,
financial or economic conditions will  likely impair capacity or willingness  to
pay  interest and repay principal.  The B rating category  is also used for debt
subordinated to senior  debt that is  assigned an  actual or implied  BB or  BB-
rating.
 
    CCC:  Debt rated CCC has a  currently identifiable vulnerability to default,
and is dependent upon favorable  business, financial and economic conditions  to
meet  timely payment  of interest  and repayment of  principal. In  the event of
adverse business, financial, or  economic conditions, it is  not likely to  have
the  capacity to pay  interest and repay  principal. The CCC  rating category is
also used for debt  subordinated to senior  debt that is  assigned an actual  or
implied B or B- rating.
 
    CC:  The rating CC is typically applied  to debt subordinated to senior debt
that is assigned an actual or implied CCC rating.
 
    C: The rating  C is typically  applied to debt  subordinated to senior  debt
which  is assigned an  actual or implied CCC-  debt rating. The  C rating may be
used to cover a situation where a  bankruptcy petition has been filed, but  debt
service payments are continued.
 
    CI: The rating CI is reserved for income bonds on which no interest is being
paid.
 
    D:  Debt rated D is  in payment default. The D  rating category is used when
interest payments or principal payments  are not made on  the date due, even  if
the  applicable  grace period  has not  expired, unless  S&P believes  that such
payments will be made during such grace  period. The D rating also will be  used
upon  the  filing  of  a  bankruptcy  petition  if  debt  service  payments  are
jeopardized.
 
    PLUS (+) OR MINUS  (-): The ratings from  AA to CCC may  be modified by  the
addition  of a  plus or minus  sign to  show relative standing  within the major
rating categories.
 
    NR: Indicates  that no  public  rating has  been  requested, that  there  is
insufficient  information on which to base a rating, or that S&P does not rate a
particular type of obligation as a matter of policy.
 
A-1 AND P-1 COMMERCIAL PAPER RATINGS
 
Description of S&P, Fitch and Moody's highest commercial paper ratings:
 
    The rating "A" is  the highest commercial paper  rating assigned by S&P  and
Fitch,  and issues  so rated  are regarded as  having the  greatest capacity for
timely payment. Issues in the "A" category are delineated with the numbers 1,  2
and  3 to indicate the relative degree  of safety. The A-1 designation indicates
that the degree  of safety regarding  timely payment is  either overwhelming  or
very  strong.  Those  A-1  issues  determined  to  possess  overwhelming  safety
characteristics will be denoted with a plus (+) sign designation.
 
                                      A-2
<PAGE>
    The rating P-1 is the highest  commercial paper rating assigned by  Moody's.
Issuers  rated P-1 have a superior ability for repayment. P-1 repayment capacity
will normally be evidenced by the following characteristics: (1) leading  market
positions  in well  established industries;  (2) high  rates of  return on funds
employed; (3) conservative  capitalization structure with  moderate reliance  on
debt and ample asset protection; (4) broad margins in earnings coverage of fixed
financial  charges and high  internal cash generation;  and (5) well established
access to  a  range  of  financial markets  and  assured  sources  of  alternate
liquidity.
 
                         FITCH INVESTORS SERVICE, INC.
 
    AAA:  Bonds  considered to  be investment  grade and  of the  highest credit
quality. The obligor  has an exceptionally  strong ability to  pay interest  and
prepay  principal, which  is unlikely to  be affected  by reasonably foreseeable
events.
 
    AA: Bonds considered to be investment grade and of very high credit quality.
The obligor's  ability to  pay  interest and  repay  principal is  very  strong,
although  not quite as strong  as bonds rated 'AAA'.  Because bonds rated in the
'AAA' and 'AA' categories are not significantly vulnerable to foreseeable future
developments, short-term debt of these issuers is generally rated 'F-1+'.
 
    A: Bonds considered to be investment grade and of very high credit  quality.
The  obligor's ability to pay  interest and repay principal  is considered to be
strong, but may be more vulnerable to adverse changes in economic conditions and
circumstances than bonds with higher ratings.
 
    BBB: Bonds  considered to  be investment  grade and  of satisfactory  credit
quality. The obligor's ability to pay interest and repay principal is considered
to  be  adequate.  Adverse  changes in  economic  conditions  and circumstances,
however, are more likely to have  adverse impact on these bonds and,  therefore,
impair  timely payment. The likelihood that the ratings of these bonds will fall
below investment grade is higher than for bonds with higher ratings.
 
    BB: Bonds are considered speculative. The obligor's ability to pay  interest
and  repay  principal may  be affected  over time  by adverse  economic changes.
However, business  and  financial alternatives  can  be identified  which  could
assist the obligor in satisfying its debt service requirements.
 
    B:  Bonds are considered  highly speculative. While bonds  in this class are
currently meeting debt service requirements, the probability of continued timely
payment of  principal and  interest  reflects the  obligor's limited  margin  of
safety and the need for reasonable business and economic activity throughout the
life of the issue.
 
    CCC: Bonds have certain identifiable characteristics which, if not remedied,
may  lead to default.  The ability to meet  obligations requires an advantageous
business and economic environment.
 
    CC: Bonds are  minimally protected.  Default in payment  of interest  and/or
principal seems probable over time.
 
    C: Bonds are in imminent default in payment of interest of principal.
 
    PLUS(+)  MINUS(-): Plus  and minus  signs are used  with a  rating symbol to
indicate the relative position of a  credit within the rated category. Plus  and
minus signs, however, are not used in the 'AAA' category.
 
    NR: indicates that Fitch does not rate the specific issue.
 
    CONDITIONAL:  A conditional rating is  premised on the successful completion
of a project or the occurrence of a specific event.
 
    SUSPENDED: A rating is suspended when Fitch deems the amount of  information
available from the issuer to be inadequate for rating purposes.
 
    WITHDRAWN:  A rating will be withdrawn when an issue matures or is called or
refinanced, and, at Fitch's discretion, when  an issuer fails to furnish  proper
and timely information.
 
    FITCHALERT:  Ratings  are placed  on FitchAlert  to  notify investors  of an
occurrence that is likely to result in a rating change and the likely  direction
of  such  change.  These are  designated  a "Positive,"  indicating  a potential
upgrade, "Negative," for potential downgrade,  or "Evolving," where ratings  may
be lowered. FitchAlert is relatively short-term and should be resolved within 12
months.
 
                                      A-3
<PAGE>
                        DUFF & PHELPS CREDIT RATING CO.
 
    AAA:  Bonds  considered to  be investment  grade and  of the  highest credit
quality. The obligor  has an exceptionally  strong ability to  pay interest  and
repay  principal, which  is unlikely  to be  affected by  reasonably foreseeable
events.
 
    AA: Bonds considered to be investment grade and or very high credit quality.
The obligor's  ability to  pay  interest and  repay  principal is  very  strong,
although  not quite as strong  as bonds rated 'AAA'.  Because bonds rated in the
'AAA' and 'AA' categories are not significantly vulnerable to foreseeable future
developments, short-term debt of these issuers is generally rated 'D-1+'.
 
    A: Bonds considered to be investment  grade and of high credit quality.  The
obligor's  ability  to pay  interest  and repay  principal  is considered  to be
strong, but may be more vulnerable to adverse changes in economic conditions and
circumstances than bonds with higher ratings.
 
    BBB: Bonds  considered to  be investment  grade and  of satisfactory  credit
quality. The obligor's ability to pay interest and repay principal is considered
to  be  adequate.  Adverse  changes in  economic  conditions  and circumstances,
however, are more likely  to have adverse impact  on these bonds, and  therefore
impair  timely payment. The likelihood that the ratings of these bonds will fall
below investment grade is higher than for bonds with higher ratings.
 
    BB: Bonds are considered speculative. The obligor's ability to pay  interest
and  repay  principal may  be affected  over time  by adverse  economic changes.
However, business,  and financial  alternatives can  be identified  which  could
assist the obligor in satisfying its debt service requirements.
 
    B:  Bonds are considered  highly speculative. While bonds  in this class are
currently meeting debt service requirements, the probability of continued timely
payment of  principal and  interest  reflects the  obligor's limited  margin  of
safety and the need for reasonable business and economic activity throughout the
life of the issue.
 
    CCC: Bonds have certain identifiable characteristics which, if not remedied,
may  lead to default.  The ability to meet  obligations requires an advantageous
business and economic environment.
 
    PLUS (+) OR MINUS (-): Plus and minus signs are used with a rating symbol to
indicate the relative position  of a credit within  a rating category. Plus  and
minus signs, however, are not used in the 'AAA' category.
 
    NR: Indicates that Duff & Phelps does not rate the specific issue.
 
                        DUFF & PHELPS SHORT-TERM RATINGS
 
    D-1+:  Highest certainty of timely  payment. Short-term liquidity, including
internal operation factors  and/or access  to alternative sources  of funds,  is
outstanding  and  safety  is  just  below  risk-free  U.S.  Treasury  short-term
obligations.
 
    D-1: Very high certainty of timely payment. Liquidity factors are  excellent
and supported by good fundamental protection factors. Risk factors are minor.
 
    D-1-:  High certainty  of timely payment.  Liquidity factors  are strong and
supported by good fundamental protection factors. Risk factors are very small.
 
    D-2: Good  certainty  of  timely  payment.  Liquidity  factors  and  company
fundamentals  are  sound.  Although  ongoing  funding  needs  may  enlarge total
financing requirements,  access to  capital markets  is good.  Risk factors  are
small.
 
    D-3:  Satisfactory liquidity and other  protection factors qualify issues as
to investment grade.  Risk factors  are larger  and subject  to more  variation.
Nevertheless, timely payment is expected.
 
    D-4:  Speculative investment characteristics. Liquidity is not sufficient to
insure against disruption in debt  service. Operating factors and market  access
may be subject to a high degree of variation.
 
    D-5: Issuer failed to meet scheduled principal and/or interest payments.
 
                                      A-4
<PAGE>
                                                                      APPENDIX B
 
                  PRINCIPAL SECTORS OF THE UTILITIES INDUSTRY
 
The  principal sectors of the utility industry in which the Utilities Series may
invest are discussed below.
 
ELECTRIC -- The electric utility industry consists of companies that are engaged
principally in  the  generation,  transmission  and  sale  of  electric  energy,
although  many  also provide  other  energy-related services.  Domestic electric
utility companies, in general,  recently have been  favorably affected by  lower
fuel  and financing costs and the full  or near completion of major construction
programs. In  addition, many  of these  companies recently  have generated  cash
flows  in excess  of current  operating expenses  and construction expenditures,
permitting some  degree of  diversification  into unregulated  businesses.  Some
electric  utilities have also taken advantage of the right to sell power outside
of their traditional geographic  areas. Electric utility companies  historically
have  been subject  to the  risks associated  with increases  in fuel  and other
operating  costs,  high  interest  costs   on  borrowings  needed  for   capital
construction  programs, costs associated with  compliance with environmental and
safety regulations and changes in the regulatory climate.
 
    In the U.S., the construction and  operation of nuclear power facilities  is
subject  to  increased scrutiny  by, and  evolving  regulations of,  the Nuclear
Regulatory  Commission  and  state  agencies  having  comparable   jurisdiction.
Increased  scrutiny might  result in higher  operating costs  and higher capital
expenditures, with the risk that the regulators may disallow inclusion of  these
costs  in rate authorizations or the risk that a company may not be permitted to
operate or  complete  construction of  a  facility. In  addition,  operators  of
nuclear power plants may be subject to significant costs for disposal of nuclear
fuel and for the de-commissioning of such plants.
 
TELECOMMUNICATIONS  -- The telephone industry  is large and highly concentrated.
Companies that distribute telephone services and provide access to the telephone
networks comprise the greatest portion  of this segment. Telephone companies  in
the U.S. are still experiencing the effects of the breakup of American Telephone
&  Telegraph Company, which  occurred in 1984. Since  1984, companies engaged in
telephone communication services  have expanded  their non-regulated  activities
into  other businesses, including cellular  telephone services, data processing,
equipment retailing,  computer software  and  hardware services,  and  financial
services.  This  expansion has  provided  significant opportunities  for certain
telephone companies to  increase their  earnings and dividends  at faster  rates
than   had  been  allowed  in  traditionally  regulated  businesses.  Increasing
competition, technological innovations  and other  structural changes,  however,
could adversely affect the profitability of such utilities.
 
GAS  --  Gas  transmission companies  and  gas distribution  companies  are also
undergoing significant changes. In  the U.S., interstate transmission  companies
are regulated by the Federal Energy Regulatory Commission, which is reducing its
regulation  of the  industry. Many companies  have diversified into  oil and gas
exploration and development, making returns more sensitive to energy prices.  In
the  recent  decade,  gas  utility companies  have  been  adversely  affected by
disruptions in  the  oil industry  and  have  also been  affected  by  increased
concentration   and  competition.  In  the  opinion  of  the  Adviser,  however,
environmental considerations  could  improve the  gas  industry outlook  in  the
future.  For example, natural gas is the  cleanest of the hydrocarbon fuels, and
this may result in incremental shifts in fuel consumption toward natural gas and
away from oil and coal.
 
WATER -- Water supply utilities  are companies that collect, purify,  distribute
and  sell  water. In  the  U.S. and  around the  world,  the industry  is highly
fragmented because  most  of  the  supplies  are  owned  by  local  authorities.
Companies  in this industry are generally  mature and are experiencing little or
no per capita volume growth.
 
                              -------------------
 
    There can  be  no assurance  that  the positive  developments  noted  above,
including those relating to changing regulation, will occur or that risk factors
other than those noted above will not develop in the future.
 
                                      B-1
<PAGE>
INVESTMENT ADVISER
Massachusetts Financial Services Company
500 Boylston Street, Boston, MA 02116
(617) 954-5000
(800) 637-8730
 
DISTRIBUTOR
MFS Fund Distributors, Inc.
500 Boylston Street, Boston, MA 02116
(617) 954-5000
CUSTODIAN
Investors Bank & Trust Company
89 South Street, Boston, MA 02111
 
DIVIDEND DISBURSING AGENT
State Street Bank and Trust Company
225 Franklin Street, Boston, MA 02110
 
SHAREHOLDER SERVICING AGENT
MFS Service Center, Inc.
500 Boylston Street, Boston, MA 02116
Toll free: (800) 343-2829, ext. 3500
MAILING ADDRESS:
P.O. Box 1400, Boston, MA 02104-9985
INDEPENDENT AUDITORS
Deloitte & Touche LLP
125 Summer Street, Boston, MA 02110
 
                      ------------------------------------
                           MFS-REGISTERED TRADEMARK-
                                RESEARCH SERIES
                           MFS-REGISTERED TRADEMARK-
                                UTILITIES SERIES
 
                                     [LOGO]
 
                                   PROSPECTUS
 
                                  MAY 1, 1997
 
                                     [LOGO]
 
                   MFS-REGISTERED TRADEMARK- RESEARCH SERIES
                   MFS-REGISTERED TRADEMARK- UTILITIES SERIES
                     500 Boylston Street, Boston, MA 02116
 
                            ------------------------


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