200 VA.Fasoula,D.A.Fasoula/Field Crops Research 75(202)191-09 has increased dramatically as plant density increased from 3 to 7.9 plants/m2.Evidently,maize breeders have successfully expanded the upper limit of the opti mum plant density range by selecting for increasec tolerance to r,by not selecting to increased f the POP UL AT IONS hybrids. Tokatlidis et al.(2001)showed that combined selection in the absence of competition for high yield ASEXUAL HYBRIDS potential per plant and tolerance to stresses accelerates the development of density-independent maize Fig.4.The four basi categories of cultivars are depicted a the odf s bo on progeny tion.The ines are c by ther standardized mean CTokatlidis et al (adapted from Fasoulas.198). 1998.1999).Twelve inbred lines were chosen to develop six hybrids that were in essence "recycled" recombination (Thuriaux.1977:Hulbert.1998) since they were derived from PR-3183.The recycled Regarding the removal of deleterious genes and the hybrids were found to be les lensity-dependent than ixation of desirable genes,the ideal system of sexua the original hybrid PR-318 because they reproduction is self-fertilization.Under cross-fertili ow plar caton,gen I through lar equa at ine trolle (2)application ve stre lants 4 and stability of performance of maize hybrids in the nlant-to.plant acings that exclude inte future is the development of density-independent tition and facilitate self-pollination:(5)selection for hybrids.A prerequisite for developing density reduced anthesis-to-silking interval to facilitate self independent cultivars is the application of whole fertilization (only maize). genome phenotypic evaluation,1.e.,joint selection The evolutionary course for the thr nts of crop yield sible only domes ton is depicte when applie id othing:(1 3.2.Maximizing genetic variance the population requred to lish a c tain level The genetic variation,essential for both inbred of success.For example,the size of the sampled vigor and advance through selection,is enhanced by population for an autogamous crop (i.e..tomato) sexual reproduction.Sexual reproduction allows the differs greatly from the one required for an asexua repulsion into coupling vigo hybrid (i.e..potato).Usually,a tomato F2 populatior recombination.and of coupling gh gene hxation (Fig.1). ng t hybrid to ha th fre of the ared to the freque ency of interge imcrease to 1.000.000 et al 1992).This "   C                                 "6  <  0 *      $      +                  )&            &    $  &               +   #%   - ./ &                           + *   3   & $        + 
+.:                          *  -#%   .66: .666/ #&   &     $   &    BBAA   &  
+.: #   &       +     
+.:         &     5  <  '           :  < -#%    ./ 0  ( &          *           +    '     +       &+      >            &        '"' )=   #                 $    1$    &                             ($ -  ./ #       %     '            '      -#  $ .6""! )  .66:/          ($           $    +* N  ++ *  ($     , -./  +! -/        ! -/      -   $   /! -9/ ++   $   +    +! -5/      ++ %    + * - */ #               9       #               &  , -./          ! -/  *     '            $  *            - /      '     $   - / N                            -H  .656/ )&         *           .   -#   .66/ #   9 #                #                       #      *              -      .6::/  2'' 3 
'' 3  4 3   % $* 5" "6 78" 7