An update on sheep and goat linkage maps and other genomic resources

Abstract

The linkage maps of sheep and goats are strikingly disparate in terms of comprehensiveness. Since its inception in 1995, the sheep linkage map has been regularly updated with the most recent version (4.6) released in June 2006. The sex averaged sheep linkage map has essentially complete coverage of the sheep genome, spans approximately 3580 cM, and is comprised of 1374 markers representing 1333 loci. The sheep is unusual among mammals in that the male autosomal map of 3817 cM is significantly longer than the female autosomal map of 3146 cM. About 50% of the male map expansion occurs at the centromeric and telomeric ends of chromosomes. In contrast to the situation for sheep, the most recent version of the goat linkage map, with the exception of the map for chromosome 1, was published in 1998 and is available only as a male map. The goat linkage map spans 2737 cM and is comprised of 307 loci, although many of the chromosomes contain two or more linkage groups. The sheep and goat linkage maps are primarily comprised of anonymous microsatellite markers, derived from sheep or cattle sequences. Most markers on the goat map have also been positioned on the sheep map, and many of the markers on the sheep and goat maps have also been positioned on the bovine linkage maps so the maps from all three species can be readily aligned. Several sheep and goat partial linkage maps have been developed as byproducts of QTL studies but many of these partial linkage maps are not publicly available. Several genomics resources are available for sheep and to a lesser degree, for goats, including genomic and cDNA libraries, a large collection of sheep EST sequences, sheep bacterial artificial chromosome (BAC) end sequences from a library with 12-fold genome coverage, mitochondrial sequence maps for sheep and goats, sheep microarrays, and web sites containing genomics and genetics information for both sheep and goats. In addition, two radiation hybrid panels are available for sheep, and a large-scale public single nucleotide polymorphism (SNP) project will lead to a 20 K sheep SNP chip and a haplotype map. Finally, a large number of trait mapping experiments have been or are currently being conducted in sheep and goats. The high degree of sequence and chromosome conservation between sheep, goats and cattle means that many of the cattle genomic resources will also be highly useful for sheep and goats.