Unravelling the genetics of grey horses

All grey horses carry exactly the same mutation, inherited from a common ancestor that lived thousands of years ago.

The grey horse has had a huge impact on human culture across the world.

The mutation that caused this spectacular trait was identified in 2008 by an international team led by researchers at Sweden’s Uppsala University.

The great majority of white horses carry the dominant mutation for greying with age.

A grey horse is born coloured (black, brown or chestnut), but the greying process starts already during its first year and they are normally completely white by six to eight years of age, but the skin remains pigmented. Thus, the process resembles greying in humans, but the process is fast in these horses.

Their research showed that that all grey horses carry exactly the same mutation which must have been inherited from a common ancestor that lived thousands of years ago.

“It is a fascinating thought that once upon a time a horse was born that turned grey and subsequently white and the people that observed it were so fascinated by its spectacular appearance that they used the horse for breeding so that the mutation could be transmitted from generation to generation,” says Leif Andersson who led the study.

Today about one horse in 10 carries the mutation for greying with age.

Humans across the world greatly valued these grey horses, as documented by the rich collection of stories and paintings featuring white horses.

The grey horse is also interesting from a medical standpoint since the mutation also predisposes for development of melanoma.

About 75 per cent of grey horses aged over 15 years have a benign form of melanoma that in some cases develops into a malignant melanoma.

“We propose that the Grey mutation stimulates growth of melanocytes and that this leads to a premature loss of the melanocyte stem cells needed for hair pigmentation whereas the mutation promotes an expansion of some of the melanocytes causing skin pigmentation,” Andersson said.

The grey horse was a beautiful illustration of the importance of regulatory mutations as a major underlying mechanism for phenotypic diversity within and between species, he said.

The grey mutation does not change any protein structure, but it affects the genetic regulation of two genes. The researchers found that grey horses carried an extra copy of a DNA segment located in one of these genes.

“It is very likely that regulatory mutations like the one we found in these white horses constitute the dominating class of mutations explaining differences between breeds of domestic animals as well as between species like humans and chimpanzee,” Andersson said.

The findings of the research was published in July 2008 on the website of Nature Genetics.

 

‘A cis-acting regulatory mutation causes premature hair graying and susceptibility to melanoma in the horse’; Gerli Rosengren Pielberg, Anna Golovko, Elisabeth Sundstrom, Ino Curik, Johan Lennartsson, Monika H Seltenhammer, Thomas Drum, Matthew Binns, Carolyn Fitzsimmons, Gabriella Lindgren, Kaj Sandberg, Roswitha Baumung, Monika Vetterlein, Sara Stromberg, Manfred Grabherr, Claire Wade, Kerstin Lindblad-Toh, Fredrik Ponten, Carl-Henrik Heldin, Johann Solkner, and Leif Andersson.
doi:10.1038/ng.185