marker-assisted selection in wheat

Chapter 13 – Marker-assisted selection in sheep and goats 237of gaps. Most of the markers are microsatellites.The total number of sheep locilisted in the ARKdb database (http://iowa.thearkdb.org) contains more than 2 000markers, but many of these are not onthe linkage map. The development of thesheep genome map runs somewhat behinddevelopments for other livestock speciesbecause of substantially lower investments.Nevertheless, at the DNA level where thesequence can be aligned, there is a ~90 percenthomology with the cattle sequence andthrough gene coding regions ~96 percent,and the sequencing of the cattle genomewill therefore greatly enhance the developmentof the genome map in sheep. There isgenerally good agreement between sheepand cattle maps, with 598 mainly anonymouscommon microsatellite loci, i.e. genemarkers can be linked to a comparativemap. Based on sequence information inother mammals (mainly cattle) and sheepGeneBank sequences, comparative mappingcan be used to construct a predictedsheep map. This can be accessed fromthe Australian Gene Mapping Web site(Maddox, 2005a). The number of singlenucleotide polymorphism (SNP) markersin sheep is still very low, but with the cattlesequence known and with an internationalcollaborative sheep bacterial artificial chromosome(BAC)-end sequencing projectunder way, it is expected that there will bea large number (~16 000) of SNPs availablefor sheep towards the end of 2006. Thiswill form a set of markers that would allowhigh-density genome-wide scans.The goat map is more sparse thanthe sheep map and contains about halfthe number of markers known in sheep:731 loci with 271 genes and 423 microsatellites(http://locus.jouy.inra.fr/). The lastpub-lished linkage map for goats containsonly 307 markers (Schibler et al., 1998),with coverage of the whole goat genomebeing far from complete. Although thesparsity of the sheep map makes it difficultto develop a good homology between themaps, about two-thirds of the mapped goatmarkers can also be linked to the sheep map(Maddox, 2005b).QTL and gene mappingAn excellent overview of mapping experimentsin sheep can be found on the AustralianGene Mapping Web site (Maddox, 2005a),including references to identified QTL andgenes. Successfully identified genes andQTL are related mainly to fecundity, diseaseresistance and meat quality.FecundityTwo genetic mutations have been reportedfor fecundity: the Booroola mutation: FecBon chromosome 6 (Wilson et al., 2001;Mulsant et al., 2001; Souza et al., 2001) andthe Inverdale gene: FecX on the X chromosome(Galloway et al., 2000). The Booroolagene has a substantial additive effect onovulation rate with each copy increasingthis by about 1.5 eggs (i.e. scanned foetuses).The additional allelic effect of theBooroola mutation on litter size is about0.8 to 0.9 lambs (Davis et al., 1982; Piperand Bindon, 1982; Gootwine et al., 2003)whereas a second copy of the mutation hasa slightly smaller effect (0.4–0.6 lambs). Theeffect on number of lambs weaned is somewhatlower. The effect of the Booroolagene is often perceived as too large and thesurvival of twin and triplet lambs decreasessubstantially in extensive and harsh conditions,typical for many sheep flocks.For example, in the Australian Merinoindustry, the Booroola mutation is notseen as a desirable characteristic. However,the Booroola gene has been introduced inmany sheep populations around the world.