The genetic mechanism responsible for the horse coloration pattern known as Dun is reported in a paper published online this week in Nature Genetics. The study suggests that Dun may have been important for camouflage in wild horses, and a similar mechanism may underlie zebra stripes.
Dun coloration in horses is characterized by a few dark-colored markings, such as a stripe along the animal’s back, and a relatively lighter, or ‘dilute’, appearance of the rest of the coat. Dun is presumed to be the ancestral coloration in horses.
Leif Andersson, Gregory Barsh and colleagues show that non-dun horses carry one of two mutations in a gene called TBX3 that cause the gene to be expressed at lower levels in the skin than in Dun horses but do not affect the function of TBX3 in other tissues where it is critical for normal development. In Dun horses, TBX3 protein is expressed asymmetrically in the hair bulb, where it blocks pigment production, leading to hairs that are pigmented only on one side of the hair shaft. This causes the dilute appearance of the Dun horse’s coat. By comparing modern horse to ancient horse genomes (more than 40,000 years old), the authors find that one of the non-dun mutations was already present in ancient horses and was likely selected by humans during domestication.