An independent locus upstream of ASIP controls variation in the shade of the bay coat colour in horses

Novel coat colour phenotypes often emerge during domestication, and there is strong evidence of genetic selection for the two main genes that control base coat colour in horses: ASIP and MC1R. These genes direct the type of pigment produced, red pheomelanin or black eumelanin, as well as the relative concentration and temporal-spatial distribution of melanin pigment deposits in the skin and hair coat. Here we describe a genome-wide association study (GWAS) to identify novel genic regions involved in the determination of the shade of bay. 126 horses from five different breeds were ranked according to the extent of the distribution of eumelanin: spanning variation in phenotype from black colour restricted only to the extremities to the presence of some black pigment across nearly all the body surface. We identified a single region associated with the shade of bay ranking spanning approximately 0.5 MB on ECA22, just upstream of the ASIP gene (P = 9.76e-15). This candidate region encompasses the distal 5’ end of the ASIP transcript (as predicted from other species) as well as the RALY gene. Both loci are viable candidates based on the presence of similar alleles in other species. These results contribute to the growing understanding of coat colour genetics in the horse and to the mapping of genetic determinants of pigmentation on a molecular level. Given pleiotropic phenotypes in behaviour and obesity for ASIP alleles, especially those in the 5’ regulatory region, improved understanding of this new Shade allele may have implications for health management in the horse.