marker-assisted selection in wheat - ictsd

240Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishprobable that a number of QTL identifiedhave not been published. It is likely thatsome of these wool QTL will be confirmedand available for gene testing over thenext few years. Polymorphisms associatedwith candidate genes for the wool proteinskeratin and sulphur have been described(Rogers, Hickford and Bickerstaffe, 1994;McLaren et al., 1997) and seem to beassociated significantly with fibre diameterand staple strength.The genetic regulation of some formsof pigmented wool fibres has oftenbeen associated with the Agouti gene(chromosome 13) but this has proven tobe a complex pattern of inheritance withseveral mutations seemingly involved(Smith et al., 2002). More specifically, thereappear to be two Agouti loci and at leasttwo different polymorphisms (deletions).Currently, a genetic test for self colouredblack wool is not yet available. Otherpigmented phenotypes such as badger faceand piebald also have a distinct Mendelianinheritance pattern (Sponenberg, 1997) butthe molecular basis of these phenotypicvariations has not been found.Dairy traitsResearch in dairy sheep has mainly focusedon milk protein polymorphisms, in particularαs1-casein and β-lactoglobulin, butresults have been inconclusive, unlike thosein goats. Together with unfavourable allelefrequencies, these results make it unlikelythat these polymorphisms will be veryuseful in a MAS programme. Further QTLmapping work is under way, focusing onproduction and functional traits (Barillet,Arranz and Carta, 2005).OtherThe Horns gene has been found in sheepas described by Montgomery et al. (1996),allowing improved selection efficiency forpolled sheep.GoatsTwo goat genes have been well studied.Substantial mapping work has been dedicatedtowards finding a gene associated withPolled Intersex Syndrome (PIS), and theactual mutation for PIS has been described(Pailhoux et al., 2005). Furthermore, theeffects of the αs1-casein gene on milk solidscontent (protein, fat, casein, casein/proteinratio) have been described in Frenchdairy goat breeds (Barbieri et al., 1995) andthe molecular basis has been unravelled(Yahyaoui et al., 2003).Examples of sheep and goat MASbreeding programmesThere is little formal literature aboutactual applications of MAS in breedingprogrammes for any livestock species, letalone for sheep and goats. In fact, genetesting and MAS in sheep and goats haveonly very recently been introduced, andtherefore the information compiled inthis section is based mainly on informationobtained from communication withcolleagues in a number of countries (seeAcknowledgements).There are currently two types of MASprogrammes. One is the use of genemarkers in selection programmes withinresearch projects. Usually the genotypingis subsidized and the purpose of the projectis to create additional data for confirmatorystudies of the QTL effect, or simply toobtain “proof of concept” where predictionsbased on simulation and modelling arebeing verified based on real data. In theother type of application, commercialgene testing is used. This is the scenariorequired for long-term and sustaineduse of the technology, but there are few