marker-assisted selection in wheat - ictsd

Chapter 17 – Marker-assisted selection in fish and shellfish breeding schemes 351Figure 5Relative efficiency of combined GAS (for different family sizes [full-sibs]) and a known QTL,explaining 10 percent of the genetic variance) versus an index using information from full-sibsonly for different values of the overall heritability (h²)80Relative efficiency of combined MAS (full-sibs plus aQTL) versus full-sib information only(percentage)706050403020100.1 0.2 0.3 0.400 10 20 30 40 50Family sizeThe data were obtained from the ratio of square root of the variance of the indices. Selection indexformulae were derived from Lande and Thompson (1990).the same rate of inbreeding (Pong-Wonget al., 2002). This is because the accuracyof predicting QTL effects using markersis always smaller than when the QTLeffects are known, as in GAS schemes. Inreality, it is likely that MAS will be carriedout using information from manymarkers to predict the allelic effects of morethan one QTL simultaneously whereas, inGAS schemes, only a limited number ofpolymorphisms are likely to be available.Therefore, on the whole, MAS schemesmay yield greater genetic response becausea greater proportion of the genetic variationis marked and used. Still, more markergenotyping is required for MAS schemes,which means that the additional proportionof the variance typed should pay for theincrease in the cost of many markers typedsimultaneously.Due to the biology of many speciesin aquaculture, large family sizes can beused in a breeding programme. Followingthe deterministic model of Lande andThompson (1990), Figure 5 describes theeffect of family size and amount of polygenicvariation on the relative efficiency ofaccuracy estimates for an index using differentnumbers of full-sibs measured forthe trait, versus an index also includinginformation on candidates for selection genotypedat loci targeted for GAS schemes (V.Martinez, unpublished results). For a singleQTL explaining 10 percent of the geneticvariance, when the heritability is relativelylarge, family size has a small impact on theaccuracy. On the other hand, when the heritabilityof the trait is small, selection for aknown QTL has a major impact on relativeefficiency, particularly when the family size