192 VA Fasou D.A.Fasoul/Field Crops Research 75(02)191-209 The obiective of the r sent paper is to examine these tion is the development of high yielding.stable,and factors and discuss the principles underlying selection density indenendent cultivars These cultivars are efficiency in breeding programs.Our focus is on three favored by farmers because their yield is optimized main areas:(1)elucidation of the genetic basis of three over a wide range of plant densities. important phenomena related to heritability,i.e. To effectively select for the three components of vigor,degeneration,anc response to selection;(2) crop yield potential,a number of requirements need to that optimize response (3)tra -trait evaluation fulfilled:(1)comparisons on the basis genome phenotypi c evaluation sho not ch have the insights progeny the ha the thre oonents should be applied acr ross the plant-to-plant interference with the equal sharing of target area of adantation from the earliest generation growth resources caused by genetic and acquired and during all stages of the breeding program:(3) differences.and the clarification of its negative role honeycomb designs should be utilized to sample on two important aspects:(1)crop yield and (2) effectively for environmental diversity and exploit efficiency of sel he genotype-by-environment interaction for the xplains the ons that led to th ce the or h line hybrids.or The scientific reaso in t is essential to ark that“g otential underlyins the superiority of inbred lines is discussed concems the yielding capacity of a cultivar under extensively.The negative effect of competition on production environments that may range from stress- selection efficiency explains why the honeycomb ful to optimal. breeding advocates selection at the level of the indi where the nd the The phenotypic iority on which selection is link between single-plant performance and crop yield, based.may be due to hybrid vigor,inbred vigor,or through the partition of crop yield potential into both.Hybrid vigor is the superiority of the F over the genetic components that can be accurately assessed two inbred parents and results from heterozygosity at the single plant level,from the earliest gene rations whereas inbred vigor is the superiority of the recom of a breeding program(Fasoula and Fasoula,2000) binant inbreds over the two parents s and the Fi anc The firs ent of con P These gene、 e gene as sugges 74),and refer h plant dens ra The second compo cenes con 'inbred vigor" .will be used pr ntially.as it p trolling tolerance to the biotic and abiotic stresses to the cause of the phenomenon whereas "transgres These genes are assessed by the genotype's progeny sive vigor"is a descriptive term.Inbred vigor is due to standardized mean and expand the upper limit of the recombinants that outperform the hybrid,and by plant density range for optimal productivity.The third definition is superior to hybrid vigor.Inbred vigor represents the condition where the homozygote out forms the heterozygote and generates response to shows a plex locus nsible for the rotein.The two closely # >       $           (      ?      , -./                        ! -/       *     ! -/     +    &+     4             (     # #   -  0   -    1   0     (       &    , -./    -/ (   #     +     $                  @              # (            $  #        ( $ &           +    &      &     $*      $      (   -4/   +  * #   &+       % &  +                                        -        / # (                 #         A     $  &          #      +           #         A   *   $               #         +      #         + A   *    $  &   #        &+    +            +    #            *   &        #               '      (, -./       &+       $           &    ! -/                                   ! -/       *         $  ++           1  +    &    +  (             %  BB  AA                   +               #    &                 )       .   &       *   &        +     &    .     *   #  BB  AA &       -.6" .6"9/          BB    AA   1 -.65/ )&   BB  AA &              & BB  +   AA     3              (      3       &  *   +   *           . &  $            CD  # &   .6 2'' 3 
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