marker-assisted selection in wheat - ictsd

Chapter 13 – Marker-assisted selection in sheep and goats 231IntroductionThe benefits of marker-assisted selection(MAS) to sheep and goat breedingprogrammes depend on a number ofconditions that are relevant for mostbreeding programmes across species. Theseconditions include the existence of a genotypetest predicting phenotypic differences,the economic value of these differencesand the value of the genotypic informationwithin the breeding programme. The valueof genetic information will depend heavilyon the socio-economic context of thebreeding programme and the productionsystem. In a technical sense, the value ofthis information is basically driven by theincrease in selection accuracy resulting fromknowledge of genotypes, which in turn willdiffer between animals from different ageclasses. In particular, the relative increase inselection accuracy of the youngest selectioncandidates will be critical to the valueof MAS. However, technical argumentsabout increased selection accuracy are oflittle value if these selection criteria arepoorly developed or accepted within theproduction system.The application of new technologies suchas MAS in animal breeding programmestherefore depends not only on a number oftechnical aspects associated with increasedrates of genetic improvement, but also onthe commercial structures of the industry.For example, the uptake of MAS in breedingprogrammes depends on the willingness ofbreeders to invest in genotypic information,and their ability to turn this intoknowledge that helps them improve theircommercial breeding activities. A basicunderstanding of breeding programmecharacteristics, the possible role of geneticinformation within these programmes, andthe commercial relationships among thedifferent players are needed to assess thevalue and predict the application of MASin breeding programmes. These commercialrelationships are distinctly differentin sheep and goat breeding programmesfrom those in the more intensive animalindustries, and the application of MAS willtherefore be different. For example, 96 percentof the world goat population is keptby smallholders in developing countries,and genetic improvement programmes arerare (Olivier et al., 2005).The purpose of this chapter is to describethe use of MAS in breeding programmesfor sheep and goats and the likely rate ofuptake of this technology in these species. Itbegins by characterizing such programmesand describing and comparing existing programmes.MAS is most useful for traits thatcannot be improved easily by phenotypicselection, either because they are difficultto measure on young animals (before sexualreproduction), or because of low heritability.Therefore, breeding objectives arediscussed in general terms and the traits thatare particularly suitable for MAS are identified.Based on some general well-knownadvantages of MAS, its possible role withinbreeding programmes can be predicted andexamples of these are provided. Examplesof marked genes are then described andan overview given of the status of “genediscovery” and gene mapping projects insheep and goats. The chapter concludesby describing cases of using this informationin actual breeding programmes. Somegene tests are based on actual functionalmutations, many of which do not affectquantitative traits that are generally targetedin breeding programmes. Although,the term “MAS” should be replaced insome cases by “genotype assisted selection”(GAS), the term MAS is used looselyto refer to all selection based on genotypicinformation. It will become clear that