Scientists explore white markings in horses

Coat colours and pattern variations in animals are often the result of relatively simple genetic coding, but that is not the case for white face and leg markings in horses, scientists say.

Researchers from Switzerland, the United States and Australia, in a study published recently in the open-access peer-reviewed journal, PLoS ONE, described white face and leg markings as examples of complex traits in horses where little was known of the underlying genetics.

In their study, a genome-wide association study was performed for several white patterning traits in 1077 Franches-Montagnes horses, 749 of which were bay and 328 were chestnut.

Beforehand, the horses were scored for the occurrence of white facial and leg markings using a standardized scoring system.

Seven key loci (gene locations) were identified that affected the white markings.

Three loci – for the MC1R gene and the known white spotting genes, KIT and MITF – were identified as the major players underlying the extent of white patterning in this breed.

Together, the seven loci explained 54 percent of the genetic variance in total white marking score, while the MITF and KIT genes alone accounted for 26 percent.

“Although MITF and KIT are the major loci controlling white patterning, their influence varies according to the basic coat color of the horse and the specific body location of the white patterning,” the researchers said.

The researcher said analysis of complex traits was a major challenge.

The long history of artificial selection in domestic animal populations had created unique model populations that aided the study of complex traits, they said.

“Coat colors in animals have been popular model traits in genetics as the phenotypes are relatively easy to assess, and of broad public interest.”

There appeared to have been a steady rise in the occurrence of de-pigmentation and colour variation phenotypes in domestic animals, presumably as a result of domestication.

“It is believed that white markings and de-pigmentation patterns were favoured as a means of both identifying owned individuals and distinguishing them from their wild relatives.”

In many domestic species, it was known that de-pigmentation patterns are under the control of several known genes, typically studied as functional candidate genes by researchers studying white patterning.

“While it is known that these genes and others affect pigmentation in many mammalian species, it is interesting to consider whether as yet unidentified loci might also play a role in the distribution of white markings in the equine and other species.

“Various independent studies have shown that the extent of white markings in horses is highly heritable and it has been previously demonstrated that the horse’s basic coat colour has a significant impact on the expression of white.”

Previous research has shown that white markings exhibit a complex mode of inheritance and that environmental factors contribute to the occurrence of white markings in horses.

The researchers said the average extent of white markings had steadily increased in the Franches-Montagnes horse population over the past thirty years, despite a breed standard that called for a horse with little or no white markings.

Their genetic analysis enabled them to identified gene traits behind white markings, some of them linked to the base coat colour of the animal.

White markings on the heads appeared to be influenced by MITF and MC1R, rather than by the KIT gene, the found.

“MITF accounts for 23 percent of the genetic variance in white head markings in all horses, while KIT accounts only for 10 percent of the genetic variance in this trait,” they wrote.

In chestnut horses, the MITF gene accounted for 41 percent of the variance in white head markings, while the KIT gene accounted for 22 percent of the genetic variance.

Leg markings in the breed were driven predominantly by ECA3, with the KIT and MC1R genes explaining 10 percent of the variation, respectively.

“MITF explains on average 5 percent of variation in leg markings across all horses, but considerably more in bay horses.”

The researchers noted that both the KIT and MITF genes have been investigated intensively in conjunction with de-pigmentation patterns.

“It has been shown in many studies that they are crucial for melanocyte development and pigment synthesis. Several independent studies have demonstrated that mutations in MITF and KIT cause a wide range of de-pigmentation phenotypes with varying phenotypic expressions.”

The researcher said that their approach had advanced the understanding of some quite complex regulatory mechanisms for coating markings.

“We are able to explain a large proportion of the genotypic variation in total white markings score in the Franches-Montagnes breed and we show that MITF and KIT haplotypes act in an additive manner.”

The researchers were Bianca Haase, Heidi Signer-Hasler, Matthew Binns, Gabriela Obexer-Ruff, Regula Hauswirth, Rebecca Bellone, Dominik Burger, Stefan Rieder, Claire Wade, and Tosso Leeb.

The study was funded by the Swiss National Science Foundation.

Haase B, Signer-Hasler H, Binns MM, Obexer-Ruff G, Hauswirth R, et al. (2013) Accumulating Mutations in Series of Haplotypes at the KIT and MITF Loci Are Major Determinants of White Markings in Franches-Montagnes Horses. PLoS ONE 8(9): e75071. doi:10.1371/journal.pone.0075071

The full study can be read here.