marker-assisted selection in wheat

348Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishdate, this analysis relies upon the use ofsegregating populations (of known origin)such as recombinant inbred lines (Carlborget al., 2005), and the analysis of outbredpopulations poses greater challenges (Pérez-Enciso, 2004). Still, aquaculture species canprovide sufficient information due to thelarge family sizes needed to unravel complexregulatory gene networks. How allthis information can be included in MASprogrammes is yet unclear.Incorporating molecular markersinto breeding programmes forfish and shellfishGeneral aspects of incorporatingmolecular information in breedingprogrammesThe response to selection ∆G is estimatedas:∆G =iσ H rwhere i = the intensity of selection, r = thecorrelation between the breeding objectiveand the selection criteria (i.e. accuracy), andσ H = the additive genetic standard deviationfor the breeding objective. As the majorimpact of incorporating information frommolecular markers will be on accuracyestimates, improvement of the response toselection will be higher for traits that haverelatively small accuracy than for traits ofrelatively large accuracy. Thus, breedingprogrammes for traits with low heritabilityand relatively few records per trait measuredsuch as carcass and disease resistanceare those most benefiting from incorporatingmarker information (Meuwissen,2003).The relative increase in accuracy dependson the amount of variation explained bymarkers, which in turn depends on thenumber of QTL identified and used in MASor GAS schemes (Lande and Thompson,1990). QTL experiments in other specieshave shown that the effects of markedgenes have a leptokurtic distribution, witha small number of genes having large effectsand polygenes (Hayes and Goddard, 2001),which is likely to be the case in aquaculturespecies (Martinez et al., 2005). Hence, it isexpected that more than a single markedgene will be needed for MAS schemes tobe efficient.Due to the biology of many fish andshellfish species, multistage selectionwill likely prove useful in MAS or GASschemes. Basically, a first stage of selectioncan be applied for traits expressedearly in the life cycle (e.g. body weight),and a second stage of selection will incorporateinformation from relatives plusmarked QTL. Optimization will be neededto determine the intensity of selection thatshould be applied at each stage to maximizeprofit while reducing the cost and labourof keeping individuals until later stages(Martinez et al., 2006b).Health and carcass traits are difficultto select for in fish and shellfish becausephenotypic records are obtained from relativesand not from candidates for selection(Gjoen and Bentsen, 1997). Sib or pedigreeevaluation has many disadvantages inrelation to the amount of genetic progressthat can be realized within a selection programmeusing only pedigree informationto predict breeding values using an animalmodel. First, selection accuracy using sibinformation is lower than when predictingbreeding values based on an individual’sown information (Falconer and Mackay,1996). Second, there is no variation ofestimated breeding value for polygeniceffects. Thus, variation of Mendelian samplingeffects within a family cannot be usedand consequently there may be a limitedscope for constraining rates of inbreeding