Unraveling the Mystery of Curly Coats in the Horse World
by Loretta L. Nielsen, Ph.D.
Copyright 2018. All rights reserved.


Horse enthusiasts have long wondered why some horses are born with curly hair instead of the more common straight hair. At long last, the great mystery of curly horses is starting to be revealed!

Recently, two scientific papers were published proving the existence of at least 2 genes linked to curly hair in horses.1,2 Both of these curly genes are inherited in a dominant manner. By "dominant", geneticists mean that a curly-coated foal only needs to inherit the curly gene from one parent (sire or dam). This foal is called "heterozygous" for the curly gene and has a 50% chance of passing it on to each of their offspring. A foal who inherits the curly gene from both parents is called "homozygous" and all of their offspring will be curly horses.

To greatly over-simplify the findings, among North American Curly Horses one of the newly identified dominant curly genes (KRT25 variant) was found primarily in descendants of wild mustangs captured and bred by ranchers in North America. The other dominant curly gene (SP6 variant) was found in some of these curly horses plus in descendents of a single stallion of unknown origin who produced curly-haired gaited offspring; some of whom were registered in the Missouri Fox Trotting Horse Breed Association (MBTHA).

Curly horses display many degrees of hair curliness from soft, relaxed waves all the way to tightly wrapped, brillo-like curls. Various degrees of curliness are also obvious in each individual in their eye lashes, ear hairs, mane, tail, and body coat. Breeders have long postulated the influence of many genes on the primary dominant curly gene to explain why curly horses have such a wide diversity of looks (phenotype). For example, I have two brothers from the same parents. One is black with a brillo-type body coat that gets much shorter during the hot summer months, but remains consistent year-round. His full brother is a cremello with very soft and loose waves of curly hair on his main body. The curls in his body coat get tighter and longer during the winter, but are almost none existent in his very short, summer coat.

Over the past few years, scientists from France, Germany and the U.S.A. set out to find the specific piece(s) of horse DNA responsible for putting the curl into horse hair. Their focus was the dominant curly gene, as well as another possible gene that seems to cause some curly horse bloodlines to develop various degrees of hair loss and bald patches; common among American Bashkir Curly horses. They used a genetic technique call genome-wide association to identify pieces of DNA that were present in curly haired horses and absent in straight haired relatives. This then allowed them to sequence those specific pieces of the horse genome. In other words, from all the DNA in a horse, they were able to pull out the pieces they wanted to study and then determine the exact sequence of molecular parts that made each piece unique.

One study used DNA samples from 51 curly- and 19 straight-haired French and North American horses descended from 13 different sires.
A single, strong signal associated with curly hair was found on equine chromosome 11, in a region that includes the type I keratin gene cluster with its 36 genes. Keratin is one of the primary components of mammalian hair. Therefore, a mutation (change) in the DNA serving as a template for keratin, and ultimately hair, production makes sense. In order to pinpoint the exact culprit, the scientists did an exhaustive comparison of the DNA in a heterozygous curly-haired stallion and his straight-haired son. Only one gene in the type I keratin gene cluster, the KRT25 gene, was sufficiently different between these two horses to warrant further study. The KRT25 variant was then confirmed to be a dominant curly gene using DNA from 353 additional horses. However, there were 5 other horses who had curly hair, but no DNA changes in the KRT25 gene. Closer examination of the DNA sequence of the KRT25 gene in two of these horses did not find any missed variations of the KRT25 gene. These curly horses had wild-type KRT25. These data provided strong evidence that some horses have a different dominant curly gene controlling the curliness of their hair. In other words, there is more than one dominant curly gene being passed down in North American curly horse bloodlines. Needless to say, this finding is extremely important for all curly horse breeders to know. Thus, the International Curly Horse Association (ICHO) is currently facilitating testing for the genes KRT25 and SP6 (see below): wild-type versus variant.

Another study used DNA samples from 216 curly- and straight-haired horses with varying degrees of hair loss or no abnormal hair loss at all. Using genetic analysis techniques similar to those described above, this group of scientists identified 2 genes on horse chromosome 11 associated with curly hair. They found genetic variants (areas of gene mutation compared with wild-type) for the
KRT25 and SP6 genes. Horses who had inherited the KRT25 variant from one or both parents had both curly hair and varying degrees of hair loss (from negligible to extensive). In contrast, horses who inherited the normal KRT25 gene with the SP6 variant had curly hair, but no bald patches or hair loss. Horses who inherited the mutated genes for both KRT25 and SP6 had both curly hair and areas of hair loss. Thus, the KRT25 variant gene "overpowers" the normalizing effects of the SP6 gene with regards to hair loss. This demonstrates how two genes can influence each other to change how a horse looks. It also appears to be the explanation for why registered curly Missouri Fox Trotters consistently have curly hair but no hair loss. They all inherited the SP6 variant from a single founding stallion along with the same wild-type KRT25 gene found in straight-haired Missouri Fox Trotters. Future research will probably identify more mutations in genes with primary control over the extent of hair curliness in different horse breeds and a multitude of secondary genes influencing their expression. Of note, mutant KRT25 and SP6 genes have also been associated with curly hair in humans, rats, mice, and cattle.

For more than a decade, ICHO members have collected DNA samples and photographs from curly-coated horses and their straight-haired relatives in order to create a genetic storehouse that was used by scientists in some of this research.
The figures below illustrate how difficult it is to distinguish between the phenotypes (hair appearance) of horses carrying the KRT25 variant or the SP6 variant. Only genotyping (DNA testing) can separate the types. However, there does seem to be an observable difference in curl tightness for heterozygotes (1 copy of a variant) versus homozygotes (2 copies of a variant and a tighter curl). Of most interest is the last figure showing a curly horse who carries one copy of each variant. According to his owner, his hair loss has always been minimal and decreased as he got older. However, his general appearance gives no clue as to his actual genotype. Recent testing of DNA from other North American Curly Horses in the ICHO storehouse has tentatively identified some curly horses without either variant (neither KRT25 nor SP6). [As yet unpublished and exciting data for future studies!]

Figure 1. Curly hair coat of a Fish Creek, Nevada, wild mustang stallion (Fishy Boy) proven by genetic testing to be homozygous for the KRT25 variant dominant curly gene.
Fishy_Boy AA

Figure 2. Curly hair coat of a gray American Bashkir Curly Horse gelding (Lilly's Moon Man) proven by genetic testing to be
heterozygous for the KRT25 variant dominant curly gene. The photo shows his soft, wavy coat and was taken in April in southern California.
Wally April 2016 copy

Figure 3. Close-up of the curly coat of a black Missouri Fox Trotter gelding (WDR Diego's Wizard) proven by genetic testing to be
homozygous for the SP6 variant dominant curly gene. The photo shows the brillo-hair coat in the neck and shoulder area, and was taken in November in southern California.
Wizard coat1

Figure 4. Close-up of the curly coat of a cremello Missouri Fox Trotter gelding (WDR Dragonsmoke) proven by genetic testing to be
heterozygous for the SP6 variant dominant curly gene. This horse is a full brother of the homozygous SP6 variant gelding shown in the previous figure, but has a more relaxed and softer curly coat. The photo shows the hair coat in the neck and shoulder area and was taken in November in southern California.
Dragon coat2

Figure 5. Curly hair coat of a bay North American Curly Horse stallion (Frostfire's Xeque) proven by genetic testing to be
heterozygous for the KRT25 variant plus heterozygous for the SP6 variant dominant curly genes. This horse is descended from both dominant curly North American mustangs and dominant curly Missouri Fox Trotters.

XequeHeadXeque photo 7.30.18


Undoubtedly, further exploration of the genetics controlling how horses look and perform will yield other exciting revelations in the coming years. This information is especially important for horse breeders to understand.
We now have genetic tests that allow breeding along either the KRT25 variant or SP6 variant curly bloodlines.

No doubt, in the future, geneticists will find other genes that affect the curliness or straightness of horse hair. Some of these genes might also cause health problems that should be eliminated from all bloodlines. For example, some Missouri Fox Trotter foals are reportedly born with extensive health problems, such as ulcers in the lining of the gastrointestinal tract - anywhere from the mouth to the anus, in addition to a curly coat. However, based on available evidence, these horses have inherited a not-yet-identified recessive gene; meaning the health problems only occur when they inherit the disease gene from both parents, never just one. So-called "silent carriers" with only one copy of the disease gene are outwardly healthy. This allows the recessive disease gene to remain hidden for long periods of time until two silent carriers are bred together; with a 25% chance of producing an unhealthy foal with 2 copies of the disease gene. Elimination of this not-yet-identified recessive gene is an important goal for improving the Missouri Fox Trotter breed. However, because these foals are so rare and usually don't reach adulthood, it will require a long-term, serious effort on the part of MFTHBA members to document and collect DNA from affected bloodlines for scientists to analyze. The American Quarter Horse Association (AQHA) has already set a good example of how a breed association, horse owners/breeders, and scientists can work together to eliminate devastating diseases from bloodlines once genetic tests are available (see the AQHA website).

References

1. Morgenthaler C, Diribarne M, Capitan A, Legendre R, Saintilan R, Gilles M, Esquerré D, Juras R, Khanshour A, Schibler L, Cothran G. A missense variant in the coil1A domain of the keratin 25 gene is associated with the dominant curly hair coat trait (Crd) in horse. Genet Sel Evol 49:8,2017. DOI 10.1186/s12711-017-0359-5
2. Thomer A, Gottschalk M, Christmann A, Naccache F, Jung K, Hewicker-Trautwein M, Distl O, Metzger J. An epistatic effect of KRT25 on SP6 is involved in curly coat in horses. www.nature.com/scientificreports 8:6374, 2018. DOI:10.1038/s41598-018-24865-3

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Basic equine genetics for nonscientists...
First, some basic concepts:
1) Mendelian theory defined the 'dominant' and 'recessive' labels for genes based on inheritance patterns long before modern molecular biology existed.
2) Every horse has 2 copies of every gene.
3) Every foal inherits one copy of each gene from their sire and the complementary copy from their dam.
4) A gene is dominant if one copy from either parent is enough to cause a specific trait to be expressed, for example brown eyes.
5) A gene is recessive if two identical copies, one from each parent, are needed to cause the expression of a specific trait, for example blue eyes.

The existence of a 'dominant curly gene' has been well established by a dominant inheritance pattern in hundreds of pedigrees. In other words, you can breed a curly-coated horse carrying only one copy of the curly gene to a straight-coated horse and have a 50% chance of getting a curly foal...Mendelian genetics. If one parent has 2 copies of the dominant curly gene and the other is straight-coated, then all offspring will be curly, having inherited one copy of the dominant curly gene and one copy of the corresponding straight-coated gene.

Just to make life more interesting, the expression of every gene (the traits you can see in an offspring, such as brown eyes) can be modified (changed) by other genes in the DNA make-up of each individual. This can lead to false assumptions that the underlying Mendelian inheritance isn't happening. There are hundreds of genes that affect the look and feel of a horse's coat, not just one or two.

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Gene Linked to Gaitedness in Horses

In August 2012 the cover story for the science journal Nature was the discovery of the first gene conclusively linked to gaiting in horses [1]. As described in the publication, all horses can walk, trot, and canter/gallop, but some horses have the ability to perform extra gaits, such as the pace (moving the two legs on the same side of the body in unison). The genetic basis of 'gaitedness' in horses was explored in one of the oldest and most inbred horse breeds in the world, the Icelandic. Because Icelandics have many genes in common, it is easier to find genes that cause differences between horses in this breed than it would be in more genetically diverse horse breeds.

An international group of equine genomics experts spearheaded by Swedish scientists identified a link between a premature stop codon in the
DMRT3 gene (in other words a shorter-than-normal version of the DMRT3 protein is produced) and the ability to perform alternative gaits. This mutant gene was found to be permissive for 'gaitedness' in horses [1,2]. This means, there's no guarantee a horse will be able to gait (pace, fox trot, running walk, etc.) when they carry this mutant gene, but it does appear to be a necessary condition for 'gaitedness' in the breeds studied so far.

In Icelandics [1], the horses with 2 copies of the mutant
DMRT3 gene (homozygous, A/A) were 5-gaited, while most 4-gaited Icelandics were heterozygous (1 copy of the mutant DMRT3 gene and 1 copy of the wild-type DMRT3 gene, C/A). The authors concluded that A/A is required for pacing in this breed. Almost all horse DNA samples from the other gaited breeds studied had at least one copy of the mutant DMRT3 gene (Kentucky mountain saddle horse, Missouri fox trotter, Paso fino, Peruvian paso, Rocky mountain horse, Tennessee walking horse). In contrast, none of the 8 non-gaited breeds carried A.

Mouse studies showed that the wild-type (full length)
DMRT3 gene controls development of the nervous system in the fetus in such a way as to produce coordinated limb movements [1]. In contrast, mice born entirely without the DMRT3 gene had normal muscle coordination and balance, but had alterations in how they used their legs to move.

In 2013, ICHO arranged to have a variety of DNA samples from Curlies tested by the Swedish research group. Early results showed that all 6 of the curly Missouri fox trotters on my ranch at the time that were tested and one straight-coated offspring of a curly Missouri-fox trotter sire were homozygous for the mutant
DMRT3 gene (A/A). In contrast, non-gaited Sporthorse DNA samples from another ICHO member were either heterozygous (C/A) or homozygous wild-type (C/C). The Swedish scientists have indicated that the majority of horses with C/A that they tested are non-gaited or somewhat gaited. Commercial testing for this gene has recently become available.

One thing to remember is that this gene discovery is probably only the tip of the iceberg. Certainly, Icelandic horses gait differently than Missouri fox trotters or Tennessee Walkers do. Just look at the differences in conformation among the different gaited breeds and it's clear there must be other genes controlling gaits besides this one. We don't know how important the
DMRT3 gene mutation is in the overall scheme of things, but it's still an exciting discovery and sheds light on an area we knew nothing about (genetically-speaking) before this discovery.

References
1. Andersson LS,
et al. Mutations in DMRT3 affect locomotion in horses and
spinal circuit function in mice. Nature Vol. 488, August 2012, pp. 642-646. Free for downloading at http://www.nature.com/nature/journal/v488/n7413/full/nature11399.html
2. Petersen JL,
et al. Genome-wide analysis reveals selection for important traits in domestic horse breeds. PLOS Genetics Jan 2013 vol 9, issue 1. Free for downloading
at
http://www.plosgenetics.org
For readers with an ongoing interest in this topic, I recommend periodic searches of the scientific literature at PubMed (http://www.ncbi.nlm.nih.gov/pubmed/). PubMed is a public portal that allows anyone with an internet connection to search the medical/scientific literature in a (U.S.) national database called MedLine that's maintained by the National Library of Congress and the National Institutes of Health. It concentrates on human medical articles, but also captures many animal studies pertaining to human health subjects, like gene mapping projects.