Analysis of agouti signaling protein (ASIP) gene polymorphisms and association with coat color in Tibetan sheep (Ovis aries)

Tibetan sheep, an indigenous breed, have a wide variety of phenotypes and a colorful coat, which make this breed an interesting model for evaluating the effects of coat-color gene mutations on this phenotypic trait. The agouti signaling protein (ASIP) gene is a positional candidate gene, as was inferred based on previous study. In our research, ASIP gene copy numbers in genomic DNA were detected using a novel approach, and the exon 2 g.100-104 mutation and copy number variation (CNV) of ASIP were associated with coat color in 256 sheep collected from eight populations with different coat colors by high-resolution melting curve assay. We found that the relative copy numbers of ASIP ranged from one to eight in Tibetan sheep. All of the g.100-104 genotypes in the populations were in Hardy-Weinberg equilibrium, and there was no relationship between the g.100-104 genotype and coat color (P > 0.05). The single ASIP CNV allele was found to be almost entirely associated with solid-black coat color; however, not all solid-black sheep displayed the putative single ASIP CNV genotype. From our study, we speculate that the ASIP CNV is under great selective pressure and the single ASIP CNV allows selection for black coat color in Tibetan sheep, but this does not explain all black phenotypes in Tibetan sheep.