marker-assisted selection in wheat - ictsd

352Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishis relatively small. Hence, this approach canbe important for traits that are expensive ordifficult to measure such as carcass quality,disease resistance or antibody response.Given the research efforts carried out atdiverse laboratories worldwide, it is likelythat direct tests will be available in the nearfuture for GAS schemes for different traits.With an increasing amount of data on ESTs,together with a greater understanding ofthe function of known genes in aquaculturespecies and new gene discovery, thereis a possibility of more rapidly identifyingand subsequently using polymorphismsthat are within coding regions. However,the research effort required to develop testsfor polymorphisms explaining allelic effectscannot be underestimated, and the factorsinfluencing the profitability of GAS willinclude:• the amount of variation explained by thetest and the number of tests (genes) availablefor explaining the phenotype;• the frequency of the favourable allele in,and the presence of the direct test (e.g.SNPs), for the target population;• the interaction between the polymorphismand the background genome andpossible pleiotropic effects on fitness;• the trade-off between the marginal returngiven by the additional genetic gainobtained through the non-linear changesin the allele frequency of the favourableallele until fixation;• fixed costs of implementing genotypingand patenting.MAS in populations in LDUsing information from dense markermaps, it is possible to make use of LDbetween the markers and the beneficialmutations influencing the quantitative traitsacross the population. Under this scenario,there are two possible ways to use the LDin MAS programmes i.e. using informationon a single haplotype effect in LD with thebeneficial polymorphism across the population,or predicting the total genetic valueusing genome-wide, dense marker maps(genome-wide marker-assisted selection,or G-MAS) (Lande and Thompson, 1990;Meuwissen, Hayes and Goddard, 2001).The effectiveness of each scenario islargely dependent on the actual magnitudeof the effects associated with the polymorphism,either across the whole genome orat specific genes. It is likely that, in thenear future, high-throughput SNP technologywill make dense marker maps costeffective for selective breeding purposes inaquaculture. Thus, it can be expected thatLD-MAS will be implemented over thewhole genome, basically using markers tounravel the genetic architecture of quantitativetraits. Information from multiple traitsjointly and for multiple genes (and theirinteractions within and between loci) willbe used, rather than first relying on mappingQTL in experimental populations andthen implementing this information in MASprogrammes. A profit analysis includingmultiple traits (e.g. to study undesirablepleiotropic effects on the breeding goal)will be needed on a case-by-case basis todetermine whether the use of a single ormultiple haplotypes simultaneously is mostprofitable and which method of LD-MASbetter suits the population under selection.Specific genes are not being evaluatedwhen LD is used across the population;rather, haplotype effects on the phenotypeare being estimated. As this is done on asingle generation across the whole genome,it would be possible to use these haplotypeeffects for selecting candidates some generationsafter the initial estimation withoutrelying on phenotypes (Meuwissen, Hayesand Goddard, 2001). Recombination will