Linkage of LHB and MAG to GPI on sheep chromosome 14

Molecular markers associated with genes of economic importance have the potential to significantly enhance animal breeding by increasing the speed and efficiency of selection programs. Recently, genes affecting production characteristics have been mapped by linkage analysis in both pigs and sheep. In pigs, an autosomal recessive mutation in the calcium release channel (CRC) gene, responsible for halothane-induced malignant hyperthermia, maps to Chromosome (Chr) SSC6. In sheep, the Booroola fecundity gene (FecB) responsible for a marked increase in ovulation rate and litter size, maps to sheep Chr OOV6. Once the chromosomal locations of important genes are identified, more detailed maps are developed in these regions to facilitate fine mapping and positional cloning. The region of pig Chr SSC6 that contains the CRC locus has now been studied extensively. Genes on SSC6 show a high degree of conservation with genes on human Chr HSA19q and HSA1p. This conservation of synteny also extends to the Bovidae. One of the genes from this region, glucose phosphate isomerase (GPI), has been mapped by in situ hybridization to the q22-q24 segment of Chrs BBO18, OOV14, and CHI18 in cattle, sheep, and goats respectively. The banding homology of these chromosomes in the three species of Bovidae is highly conserved, and additional genes from HSA19 including apolipoprotein E (APOE), ryanodine receptor 1 (RYR1), the homolog of the CRC locus in pigs, and luteinizing hormone beta polypeptide (LHB) have been mapped to BBO18. During the genome search for the FecB locus, we identified polymorphisms associated with the LHB gene in sheep. LHB maps to human Chr HSA19q13.3 and encodes the beta subunit of the pituitary glycoprotein luteinizing hormone (LH). LH is secreted by the pituitary gland and controls gonadal function. A distinctive feature of this hormone is that it is released in discrete pulses. The frequency of pulses drives the maturation of ovarian follicles in the final stages before ovulation. Linkage analysis of LHB together with other genes in the Booroola pedigrees identified a linkage group including LHB, ILSTS002, myelin-associated glycoprotein (MAG), and GPI.*