marker-assisted selection in wheat - ictsd

236Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishEwe fitness and adaptation are often usedas the main argument for the existenceof genotype x environment interaction inwool production, inhibiting the exchangeof genetic material among regions. Carrick(2005) found moderate to high genetic correlationsbetween wool production traitsin flock groups differentiated by their phenotypicmeans for a range of productiontraits. Discovering QTL for fitness andsurvival traits in different environmentswould be useful, but these are unlikely tobe found unless the traits themselves areclearly defined and measured.Dairy sheepDairy sheep are predominantly found inthe Mediterranean region with both milkand meat production being economicallyrelevant traits to farmers. A great varietyof breeds are being targeted in selectionprogrammes for the improvement of milkyield and milk composition but the importanceof functional traits such as uddercharacteristics and mastitis susceptibility isincreasing (Barillet, 1997; Barillet, Arranzand Carta, 2005). Genetic improvement fordairy traits, being sex-limited and measuredafter the first offspring are born, wouldparticularly benefit from MAS.GoatsMost goat farming systems focus on meatproduction (about 80 percent), with moreemphasis in developed countries on dairygoat production (Olivier et al., 2005) andfibre production (cashmere, mohair). Indairy goat breeding, the most developedbreeding programmes are found in Franceand are based on a strong goat cheesemarket. Based on AI and milk recording,Caprigene France runs selection schemesfor the Saanen and Alpine breeds, with300 000 goats in 2 500 herds being recordedfor milk traits. Dairy goat production isalso recorded on smaller scales in Italy,Norway and Spain, with no more than afew thousand animals recorded in othercountries (Montaldo and Manfredi, 2002).The main traits in dairy goat production aremilk yield and protein and fat content ofmilk. Being sex-limited and measured onlyafter first production of progeny, thesetraits would benefit from MAS.Goat meat production is widely spreadthroughout the developing world but thereare few breeding programmes of any significance.Genetic evaluation for Boer goatsand other meat breeds is taking place inAustralia and South Africa with weaningweight usually being the main trait measured.Ultrasound measurement of fat andmuscle traits is less common in goats, whilereproductive traits have had less attention,possibly because of their low heritabilityand multiparous nature. There are fewstudies concerning resistance to internalparasites in goats (Olayemi et al., 2002),but these seem to indicate that faecal WECscould be a similar selection trait as in sheep.However, the trait is hard to measure andthere is no systematic recording and evaluationin breeding programmes.Development of sheep and goatgenome mapsSeveral key publications have reportedprogress on the linkage map of the sheepgenome based on an international mappingflock developed in New Zealand (Crawfordet al., 1995; Maddox et al., 2001). The latestsheep linkage map (version 4.3) comprises1 256 gene markers mapped to unique locations(Maddox, 2004) and most genomicregions are well covered with a maximumgap of 20 cM. However, there are quitea number of markers of low quality, so atypical genome scan would leave a number