marker-assisted selection in wheat - ictsd

Chapter 11 – Marker-assisted selection in poultry 193SNPs in the near future. However, the finemapping and characterization of identifiedQTL remain costly and time-consumingprocesses and are often restricted to themost promising QTL, resulting in hundredsof QTL that will never make it past thestage of mapping to a 30 cM confidenceinterval.While current research and developmentsin poultry functional genomics are relevantto all four possible applications of MAS tolivestock, poultry breeders need to decideat what level they want to exploit molecularinformation and for which traits.The emerging picture is that breedersare more comfortable with known genemutations as this provides an easy route toimplementation as well as knowledge aboutthe underlying biology. Furthermore, thereis concern that the marker-trait linkagewill break down over a relatively few generationsof selection in large commercialflocks. While candidate gene studies wouldprovide the quickest route to implementation,fine mapping and characterization ofQTL (e.g. using expression studies) mayreveal gene variants that are not obviouscandidate genes for quantitative traits.Potential for MAS in poultry indeveloping countriesOwing to the relatively low value ofsingle animals, the high reproductive ratein poultry and good portability of eggsor day-old hatchlings, the concentrationof resources is very high in the poultrybreeding industry and all poultry breedingis privately owned. Fifty years ago therewere many primary breeders in each andevery industrialized country, but not solong ago there were only 20 breeding companiesworldwide. Today, three groups ofprimary breeders dominate the internationallayer market. Equally, in the chickenmeat industry, there are four major playersin broiler breeding worldwide (Flockand Preisinger, 2002). The concentrationprocess is probably now complete, and thepresent players are sufficient to meet theglobal supply for 700 000 million eggs asfinal products. A similar trend is expectedin the pig industry, where internationalbreeding companies of hybrid products areincreasing their market share (Preisinger,2004). For large-scale farming of broilersand layers in developing countries thereare additional challenges with regard toheat stress and potential disease pressure.With increasing poultry productionin developing countries, breeding companiesmay give priority to using breedingand molecular tools to address these additionalchallenges. While chickens are veryefficient in converting grain into valuablemeat and egg protein, and smallholderchicken production can be valuable forsustaining the livelihoods of farmers in thedeveloping world, this type of poultry productionwould require robust dual-purpose(meat and egg) birds, rather than specializedbroiler and layer lines. It is unlikelythat the commercial breeders will developsuch lines but there may be scope fornational or international research organizationsto do so. Any MAS would haveto be done at the institutional level wherethe line is developed and would necessitateprior knowledge of trait-marker associationsat the farm level. The implementationof whole genome SNP approaches to farmlevel recording might facilitate progress inthis area but the challenges, both practicaland theoretical, are formidable.Concluding remarksAmong livestock species, chicken haveby far the most comprehensive genomictoolbox. However, uptake of MAS will