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Curly Horse Genetics, More Than One Gene?

Curly Horse Genetics, More Than One Gene?
24 avril 2018 | CURLY RESEARCH

Thank you to Dr. Karen Zierler and Dr. Mitch Wilkinson


Since the early 1800s, American Curly Horses have always been a part of the romantic lore of

the Wild West.  Legends of curly horses have traditionally been associated with the wild horse herds found in the American Northern Plains, Great Basin, and central plains of Canada.  These are American horses that have a curly coat and pass this trait on through a dominant mode of transmission to their offspring. As their numbers were few, and they were originally found within roaming mustang herds, the cowboys, Indians, and early breeders of the curly horse told mysterious tales about their origins.


There was an unsubstantiated legend about an 1890s Nevada horse breeder, Tom Dixon, who was said to have imported several curly coated horses from India.  In Nevada, Dixon range bred the curly horses to his remuda of several thousand horses. There was also speculation that curly horses came to North America from Siberia with early Russian colonists, but there was no documented evidence.  Stories were told about the Skjonsberg family being rustlers and bringing curly horses to Canada. The tale related that the Skonsbergs stole a curly horse at a water hole from an “Indian”. All these stories and many more made for colorful tales, but factual evidence was non-existent. An old newspaper cartoon from the 1930s suggested (falsely) that curly horses were Bashkir horses from Russia. In the 1950s, a Quarter Horse breeder in South Dakota bought his curly horses from an original source, the Lakota Sioux (the Bad Warrior line) who considered curly horses sacred. In the end, modern breeders caught up in the tales, spent hours reflecting on curly origins and trying to sort out fact from fiction, but all agreed the goal was the production of quality horses.


This goal of producing quality horses was not an easy task. In the 1970s when the first registry was founded, most breeders thought that curly horses were all the same, and all curly horses came from the same source. And, so, they put all the horses together in one registry, The American Bashkir Curly Registry. Horses of different coat types, backgrounds, confirmations, and geographical locations were all combined together and bred. Research into the origin of the curly horses led breeders to contact horse registries in Russia and South America with little concrete results.

A second registry was later founded in the 1990s, The International Curly Horse Organization. Many curly horse breeders had observed that the horses had different conformations, different curls types, and different origins. The founders wanted more investigation into the backgrounds of the curly horse and breed types. This began a long quest to isolate the gene responsible for curl. The investigation was begun and funded by ICHO in 1991.

The problem was, the curl wasn't the same for all horses. Some curly horses had “microcurls” which were very tight and looked like wire hair. These horses had long manes and tails which did not shed during the summer months. Many of this line of curly horses were also gaited and had been bred with the American Missouri Fox Trotter breed.  Other curly coated horses came from a family in Nevada, the Dameles. The Damele horses were small Mustangs and later more Morab in build, with soft curls, and in the summer months often rubbed out manes and tails in the suspected homozygous state. The curly gene in these horses came from the wild horse herds found near the Damele ranch. While the curly horses in the Dakotas from the Lakota Sioux had soft curls, but a stockhorse conformation.


As it turned out, the curly horse is also hypoallergenic (allergy sufferers react less to a curly than a normal horse breed), and there were differences in how allergy sufferers responded to different curly horse bloodlines. Some allergy sufferers were less allergic to some bloodlines of curly horses than another. One line of curly horses might be better for particular allergy sufferer, while another line might trigger horse allergies. These phenotypes, or coat types suggested that there was something more involved and that the differences in bloodlines and individual horses was critical to the hypoallergenic reactions allergy sufferers observed with certain coat types and lines in curly horses.


On November 16, 2017, after 26 years of research projects, a French and American research collaboration published their discovery of a Curly gene. Dr. Schibler and colleagues with Dr. Gus Cothran at Texas A&M University in their paper, “A missense Variant in the Coil1A Domain of the Keratin 25 gene is Associated with the Dominant Curly Hair Trait (Crd) in Horse.”  The scientists were able to demonstrate that a mutation in a keratin gene on chromosome 11 was responsible for the curly coat in a particular horse. Dr. Schibler’s experimental model was based on 51 curly coated horses and 19 straight haired horses from 13 paternal families.  Samples of horses from both France and North America were included. To help identify candidate genes, a whole genome sequence was obtained from a presumed heterozygous stallion, BFC Spartacular Splashes, and his straight-haired son, Alias Splash. Dr. Schibler identified a missense mutation at KRT25:p.R89H as responsible for the dominant curly coat trait. The keratin gene, KRT25 has known association with hair structure.


This was a major sensation! A single base pair mutation was responsible. In the resulting keratin protein, an arginine was changed to a histamine. This small substitution of one amino acid for another in the protein caused a structural change in the resulting hair shaft. This change resulted in curly coat hairs.


Having once identified the gene and the mutation, the scientists wanted to show that all curly horses share these genetics. However, the missense, point mutation found in the KRT25 gene was only found to be present in a little over half of the curly coated horses that were subsequently tested. The other curly horses were thought to have a different mutation or genetic basis for their curly coats.

Curly horses have been identified by bloodlines for years. They are generally identified by their founding breeders, such as the “Damele line” which the Damele family bred in the high mountains of Nevada, or the “Bad Warrior line” which were bred in South Dakota and were originally obtained from the Lakota Sioux breeder, Eli Bad Warrior, and so on. The other bloodlines then, with unknown, curly hair producing gene mutations were given designations based on founding their sires Curly Jim, a curly sire of unknown origin who was bred to American Missouri Fox Trotters, founding breeders (Cook), mustang herd origins (Sulphur and Spanish Mustang) or location (Patagonian, Mongolian, or Siberian).


What does this mean? It means that because the registry in the early days put all the horses together, some horses carry the KRT25 mutation only, while other curly horses carry another genetic mutation and do not have the KRT25 mutation. Interestingly, many curly horses are suspected of having a mixture of both the KRT25 and other mutations as all the curly horses were pooled together. The KRT25 mutation has a specific phenotype, or how the curls are expressed. The other phenotypes are also distinctly different.


In the homozygous state, where a horse has two copies (from sire and dam) of the KRT25 mutation, the horses have the tendency to produce sparse mane and tail hair. Although shorter in general, there is individual variation in mane and tail growth. The mane and tail hair of these horses is curly, but also more subject to summer shedding. These hairs seem to be more easily broken or brittle than normal horse mane and tail hair. The body coat of homozygous KRT25 horses is curly during winter. It has a soft texture and can vary from a wave like appearance to tight curls, but not the wire like curls found in the Curly Jim line. 

Breeding of a heterozygous curly horse with a heterozygous mate can also produce a straight coated horse (one without curl) ¼ of the time. However, some allergy sufferers do not react to these horses, also suggesting that there are more aspects to the inheritance and genetics involved. Straight offspring from curly horses will often have a bunny hair like coat, softer than a normal horse.


Both groups of horses (homozygous and heterozygous for the KRT25 mutation) shed curly guard hairs and one layer of undercoat during the summer months. Curly hairs are retained on the tail, mane, and ears during all seasons. In summer, the Curly can look almost as if it was sheered, keeping a very short straight looking coat.


What about the other curly horses that do not have the KRT25 mutation? One prominent line of horses that also produce curly coats that are not due to the KRT25 mutation is known as the Curly Jim line. This line of horses is also associated with gaited Missouri Fox Trotters.  The popular Curly Jim Line is named after a founding sire named Curly Jim. Most of these horses also carry the DMRT3 mutation which produces gait. Intensive research is underway to identify the genetic mutation which produces the unique coat of the Curly Jim line. Interestingly, the phenotype in this line is not only tight curly, wire hair in the homozygous inheritance, but in the heterozygous state the body curl is observed to be soft curling wave like curls while mane and tail hairs are not brittle and grow to a long luxurious length.



In addition, research work is being done to identify mutations in Sulphur and Spanish Mustangs in which curls have been observed, as well as the Cook line of Curly horses. Curly Horses are also noted in Patagonia, Mongolia and Siberia. Research in the future will compare the genetics of these curly horses to that of the North American horses to give a clearer picture as to whether the KRT25 mutation was spontaneous and originated in America, or if some of the American curly horses may be related to curly horses from other parts of the world.


The discovery of the KRT25 mutation and the fact that different bloodlines have unique phenotypes caused by different mutations, assists breeding associations in making decisions with respect to matching stallions to mares. Historically, conformation and function are different between a Stockhorse and an American Missouri Fox Trotter. They have different standards and uses. It is now clear, that the curly gene also follows a similar pattern. The registries will now have to decide if mixing or matching curly genes in the various bloodlines is desired.


All in all, whether the curl in an individual horse comes from KRT25 or not, the curly horse can compete in many different disciplines, has excellent stamina, and is blessed with dense bone stemming from years of range-selected breeding. The curly horse either on the prairie, selected for its gait, or it’s hypoallergenic properties comes from registries that have worked hard to maintain a people-oriented character along with curl. As the researchers get closer to the discovery of the origins of the curly horse, riders with or without allergies, can enjoy a horse with curl.