Leopard complex spotting has been maintained in the horse gene pool for at least 17,000 years, research suggests.
An international group of researchers have been exploring the patterning in horses, caused by an incompletely dominant gene (LP). Horses that are homozygous – having inherited the LP gene from both parents – are also affected with congenital stationary night blindness.
The researchers, whose findings have been published in the open-access journal, PLoS ONE, found strong evidence that the insertion of a long terminal repeat (LTR) of an endogenous retrovirus into the candidate gene, TRPM1, was behind both traits.
They said that while harmful element insertions should be quickly selected against in breeding, the identification of this insertion in three ancient DNA samples suggested it has been maintained in the horse gene pool for at least 17,000 years.
The researchers said domestic mammals were selectively bred for variability in coat color and pattern, making pigmentation an attractive system to study. Mutant genes controlling pigmentation in both domestic and experimental mammals have been shown to affect the likes of vision and hearing, and have become important means to study human disorders.
“Complete congenital stationary night blindness is one such example, where discovery of causal mutations in humans was aided by pigmentation studies in the horse.”
Leopard complex spotting is found in several breeds of horse and is characterized by the absence of pigment (white spotting) in the coat and associated pigmentation characteristics.
These white spotting patterns tended to be symmetrical and centered over the hips, the researchers said. “However, the amount of white on the coat varies and can range from a few white flecks on the rump to horses that are almost completely white.”
Other characteristics associated with the expression of the gene include white sclera, striped hooves, mottling, and progressive roaning, known as varnish roan.
“While these LP-associated characteristics all appear to result from a reduction in pigmentation, work is ongoing to determine if these characteristics are caused by a reduction in melanocyte number or loss of melanocyte function.”
Leopard complex spotting is inherited as a single incompletely dominant trait, where homozygotes in addition to being night blind have few or no spots of pigment known as leopard spots.
“While the precise role that TPRM1 plays in LP hypopigmentation is unclear, the irregularly shaped melanosomes in LP/LP melanocytes suggest that pigmentation in LP horses could be disrupted by improper melanosome maturation.
“This may stimulate melanocytosis resulting in white patterned areas during development and progressive roaning during the life of the animal.
“Alternatively, the LTR insertion could be acting in trans, potentially producing a non-coding RNA or short peptide that affects another pigmentation gene.”
As part of the research, DNA was extracted from eight ancient samples at the Humboldt-University of Berlin, in Germany, and at the University of York, in Britain.
In addition to the ancient samples, two modern samples were included to serve as positive controls for the enrichment strategy. It was this aspect of the research that revealed the LP gene had been maintained in the equine gene pool for at least 17,000 years.
Bellone RR, Holl H, Setaluri V, Devi S, Maddodi N, et al. (2013) Evidence for a Retroviral Insertion in TRPM1 as the Cause of Congenital Stationary Night Blindness and Leopard Complex Spotting in the Horse. PLoS ONE 8(10): e78280. doi:10.1371/journal.pone.0078280
The full study can be read here.