marker-assisted selection in wheat - ictsd

Chapter 16 – Possibilities for marker-assisted selection in aquaculture breeding schemes 317Figure 1Two-generation marker evaluation and selection scheme for aquaculture speciesParental generation: with genotypic record♂ ( M1 M2 ) x ( M3 M4 )♀Progeny generation:Group 1: Group 2:Test- progeny with phenotypicand genotypic records, usedto estimate marker effectsBroodstock candidateprogeny with genotypic recordonlyM1 M3SelectM1 M4 Perhaps select someM2 M3Perhaps select someM2 M4 Select noneIndividuals with marker genotypes shown in bold are selected. The proportion of each genotype selected will vary withselection intensity.Thompson (1990) showed that the efficiencyof MAS relative to conventionalselection alone depends upon the heritabilityof the trait under selection, theproportion of genetic variance associatedwith marker loci and the particular selectionscheme at issue.Stochastic simulations of two-generationwithin-family MAS schemes in a typicalaquaculture breeding programme werecarried out by Sonesson (2006) for selectionfor one trait. Selection was truncatedbased upon best linear unbiased prediction(BLUP) breeding values including informationof the genetic marker (Fernando andGrossman, 1989). The heritability, h 2 , ofthe trait under selection was 0.06 or 0.12,and 20 percent of the genetic variance wasaccounted for by the QTL.Genetic gain was 0.202 for MAS and0.176 for conventional breeding, after selectionin generation 1 (Table 3), i.e. MAS had15 percent higher genetic gain than conventionalbreeding. After selection in generation2, genetic gain was 68 percent higher forMAS than conventional breeding. The performance-increasingQTL allele was thenalmost fixed (i.e. its frequency approached1). The increase in genetic gain is mainlydue to the increased frequency of the positiveQTL allele for the MAS scheme, wherea higher QTL frequency implies more