Curly question answered: Second gene behind curly horse coats identified

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Research into the genetics of curly coats in horses is continuing apace. Pictured is Blackie, a wild-born curly horse from Nevada.
Research into the genetics of curly coats in horses is continuing apace. Pictured is Blackie, a wild-born curly horse from Nevada.

A second gene responsible for curly coats in horses has been identified by researchers, pinpointing the cause of the so-called “Curly Jim” mutation seen in some horses.

Researchers from Germany’s University of Veterinary Medicine Hannover Foundation identified a missense mutation in the SP6 gene. A missense mutation is where the change means it codes for a different amino acid. In effect, the gene is misspelled, but is still viable. The new version produces a slightly different protein.

A mutation in the KRT25 gene has already been identified as responsible for curly coats in most horses that exhibit a curly coat pattern. However, scientists quickly established that this was not the only gene responsible, as some horses with a curly coat have tested negative for the KRT25 mutation.

“Our study clearly demonstrated the independent effects of KRT25 and SP6 on curly coat development in horses,” the study team reported.

Annika Thomer and her colleagues, writing in the journal Scientific Reports, said curly hair was an extraordinary coat type seen in some horses, particularly in American Bashkir Curly Horses and Missouri Foxtrotters.

Some horses with a curly coat also had sparse hair or a coat that was not as thick or long as normally expected – a condition known as hypotrichosis.

Curly Jim, photographed after his arrival in Mountainview, Missouri, by train in 1956. The coat in the prominent Curly Jim line is not due to the KRT25 mutation, but a mutation in the SP6 gene recently identified by German researchers.
Curly Jim, photographed after his arrival in Mountainview, Missouri, by train in 1956. The coat in the prominent Curly Jim line is not due to the KRT25 mutation, but a mutation in the SP6 gene recently identified by German researchers.

The researchers set out to investigate the genetic background of curly coats with and without hypotrichosis through DNA analysis of 216 horses.

Their analysis led them to a mutation on chromosome 11, in the SP6 gene.

The study team suggested that mutations within KRT25 and SP6 could explain all hair types found in curly horses.

Horses with only the KRT25 mutation – heterozygous or homozygous – showed both a curly coat and varying degrees of hypotrichosis, whereas horses with only the SP6 variant had a full curly coat – that is, with no hypotrichosis.

Horses carrying both variants developed both curly hair and hypotrichosis. Thus, the mutant KRT25 gene was effectively masking the “full coat” effect of SP6, the researchers said. This phenomenon is known in classical genetics as an epistatic relationship between two independent gene mutations. The KRT25 mutation masks the effects of the SP6 and is therefore epistatic to the SP6 mutation.

“All horses with KRT25 variant are additionally hypotrichotic due to the KRT25 epistatic effect on SP6,” they said.

This means that the KRT25 will effectively override SP6 when a horse carries both mutations, resulting in hypotrichosis, in the same way that a gene in a person for total baldness would mask the fact that they carry the gene for brown hair.

Dr Mitch Wilkinson, who is chairman of the Curly Mustang Association and vice-chairman of the Research Department of the International Curly Horse Organization, welcomed the findings.

He said they would prove invaluable in learning more about some lines of curly horses, and research will be undertaken in coming weeks.

Wilkinson said the researchers implied that all curly horse mysteries were solved by this isolation of the SP6 gene mutation in addition to the KRT25 mutation, but he doubted that was the case.

He believes further testing will show that there are several additional curly gene mutations yet to be identified.

“So, as Sherlock Holmes would say, ‘The game is still afoot!’ ”

An epistatic effect of KRT25 on SP6 is involved in curly coat in horses
Annika Thomer, Maren Gottschalk, Anna Christmann, Fanny Naccache, Klaus Jung, Marion Hewicker-Trautwein, Ottmar Distl & Julia Metzger
Scientific Reports volume 8, Article number: 6374 (2018) doi:10.1038/s41598-018-24865-3

The study, published under a Creative Commons License, can be read here.

Earlier Horsetalk reports:
Curious twists in the genetics of horses with curly coats
Curly coats on horses are caused by multiple gene mutations
Unraveling each genetic piece in the curly horse puzzle

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