Most horses today are treasured for their ability to run, work, or be ridden, but have lost their wild-type camouflage: pale hair with zebra-like dark stripes known as the Dun pattern. Now an international team of scientists has discovered what causes the Dun pattern and why it is lost in most horses. The results, published today in Nature Genetics, reveal a new mechanism of skin and hair biology, and provide new insight into horse domestication.

The work is an international collaboration led by groups at SciLifeLab/Uppsala University, and the Huntsville Institute of Biotechnology, Huntsville, Alabama, USA.

Pale hair colour in Dun horses provides camouflage as it makes a horse in the wild less conspicuous. In contrast, domestic horses, as well as many other domestic animals, have been selected over many generations to be more conspicuous, more appealing or simply different than the wild type. The pale hair colour in Dun horses does not affect all parts of the body; most Dun horses have a dark stripe along their back, and often show zebra-like leg stripes. However, the majority of domestic horses are non-dun and show a more intense pigmentation that is uniformly distributed

“Dun is clearly one of the most interesting coat colour variants in domestic animals because it does not just change the colour but the colour pattern”, states Leif Andersson, Faculty at SciLifeLab whose group led the genetic analysis. “We were really curious to understand the underlying molecular mechanism why Dun pigment dilution did not affect all parts of the body”.

The research team started by analysing the distribution of pigment in individual hairs. Genetic analysis and DNA sequencing revealed that Dun versus non-dun colour is determined by a single gene that codes for the T-box 3 (TBX3) transcription factor. In humans, inactivation of the TBX3 gene causes a constellation of birth defects known as Ulnar-Mammary Syndrome. But in horses that have lost their Dun colour, TBX3 mutations do not inactivate TBX3 protein function and instead only affect where the gene is expressed in the growing hair.

The results of the present study indicates that the non-dun2 variant occurred recently most likely after domestication. In contrast, both the Dun and non-dun1 variants predate domestication, which is evident from the observation that ancient DNA from a horse that lived about 43,000 years ago, long before horses were domesticated, carried both Dun and non-dun1 variants.

“This demonstrates that horse domestication involved two different colour morphs (Dun and non-dun1) and future studies of ancient DNA will be able to reveal the geographic distribution and the abundance of the two morphs”, said Leif Andersson.

Dun horses have very variable primitive markings, these photographs show some different variants of dark facial masks, dorsal stripes, shoulder crosses and zebra-like leg stripes. Photos: Freyja Imsland and Páll Imsland.