Researchers investigate colour variations in bay horses

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Quarter horses illustrating the diversity in shade of bay coat colour, quantified as a ranking within the 129 sampled individuals.
Quarter horses illustrating the diversity in shade of bay coat colour, quantified as a ranking within the 129 sampled individuals. The bay shade rank number for each horse is (A) 3, (B) 27, (C) 64, and (D) 101 and (E) 120. (Corbin, Pope, et al)

What causes the wide variation seen in the bay coat colour of horses?

Scientists don’t have all the answers, but research just published in the journal Genes has pinpointed the region responsible.

Genetically, the base coat colour of horses is controlled by two principal genes, MC1R and ASIP.

These genes direct the type of pigment produced, either red pheomelanin in the case of MC1R, or black eumelanin in the case of ASIP, as well as the relative concentration and the distribution of melanin pigment deposits in the skin and hair coat.

Laura Corbin and her fellow researchers carried out a genome-wide association study in a bid to identify new regions involved in the determination of the shade of bay.

In total, 126 horses from five different breeds were ranked according to the extent of the distribution of eumelanin. The horses had coat variations that ranged from black restricted only to the extremities, to the presence of some black pigment across nearly all of the body surface.

They pinpointed what they described as a significant region on Chromosome 22 involved in this shading process.

The candidate region encompasses about 1.3MB and contains 21 annotated genes. Of these, RALY and ASIP are the only genes previously implicated in pigment phenotypes.

They said the lead single nucleotide polymorphism (SNP) in this area, dubbed AX-103117105, is unlikely to encode for actual coat colour. Rather, it is likely that it is tagging a form of variation that has an effect on the coat colour — in other words, the shade.

It lies right between the ASIP and RALY genes.

“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,” the researchers wrote.

The study team comprised Corbin and Jessica Pope, with the University of Bristol in England; Jacqueline Sanson and Samantha Brooks, with the University of Florida; and Douglas Antczak, Donald Miller and Raheleh Sadeghi, with Cornell University in New York State.

Corbin, L.J.; Pope, J.; Sanson, J.; Antczak, D.F.; Miller, D.; Sadeghi, R.; Brooks, S.A. An Independent Locus Upstream of ASIP Controls Variation in the Shade of the Bay Coat Colour in Horses. Genes 2020, 11, 606. https://doi.org/10.3390/genes11060606

The study, published under a Creative Commons License, can be read here

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