Curious twists in the genetics of horses with curly coats

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Lucky Jack is a Canadian born curly horse. He is the author's main mountain horse.
Lucky Jack is a Canadian-born curly horse. He is the author’s main mountain horse.

Dr. Mitch Wilkinson, in the first of a three-part series, unravels the mysteries of the curly coats found on some horses. Wilkinson, a lifelong horse enthusiast with a post-doctoral master’s degree from Baylor University in biology, as well as dental and chemistry degrees, is chairman of the Curly Mustang Association and is vice-chairman of the Research Department of the International Curly Horse Organization (ICHO). The series is published courtesy of the ICHO Gazette. 

A great unsolved mystery in the equine world is the origin of curly haired horses in both the domestic and wild horse populations, not only in the Americas, but also parts of Asia.

The present histories of curly horse introductions into North and South America are based on oral legends, dubious speculations, and suppositions based on incomplete information that have been repeatedly stated over several decades to become something that masquerades as truth, but in fact, has little factual basis.

A good example is the incorrect term, “Bashkir”, which refers to a region of Russia and is incorporated into the name of one of the three curly horse registries. The speculation of North American curly horses coming from the Bashkir region of Russia came from a 1930’s newspaper cartoon that was noticed by early Nevada curly horse breeders. [2]

The legend of an Irish immigrant, horse breeder named Tom Dixon bringing curly horses to Northern Nevada via India in the late nineteenth century has documented evidence in the Nevada State Archives, but the actual evidence is based on a second-hand testimonial that was preserved in the 1950s.

There is a possibility that this account may well be factual, but this curly horse introduction could only account for a couple of mutations that contribute to the production of curly coats in North American equines. [2] [5] [18] [4]

For most of the history of curly coated horses in North America, it was thought there was only one coat mutation that imparted a curly coat to horses.

Curly enthusiasts spent many hours speculating about a mythical, “old world” breed that somehow found its way to the North American continent and was the direct ancestor of North American curly horses.

We now know that there are many possible mutations that can cause curly coats in horses.

The fact that curly horses have a wide variation in coat appearance is an indication of the several different types of curly coat patterns, each thought to be caused by a different gene mutation. [1]

The isolation of one gene mutation that causes curly coats in horses has led to the realization that there are other causative mutations within the population. Because of the lack of documented historical evidence, the relationships of one curly coated horse population to another can only be determined by genetic analysis. [1]

The following narrative not only describes a mutation that causes curly coats in a percentage of the curly horse population, but also describes other possible mutations and their effects.

Part One

Curly Genes – Genotypes and Phenotypes

One of several gene mutations that are the causative factors in curly coated horses has been isolated by Dr. Schibler and colleagues in France in collaboration with Dr. Gus Cothran at Texas A&M University.

A scientific paper was published on November 16,2017 in the journal, Genetics Selection Evolution reporting the results of genetic studies conducted almost four years prior to the date of publication. [1] https://gsejournal.biomedcentral.com/articles The article was titled, A missense Variant in the Coil1A Domain of the Keratin 25 gene is Associated with the Dominant Curly Hair Trait (Crd) in Horse.

Dr. Schibler’s genome sequencing with an Illumina Equine SNP50 Bead Chip used test subjects which included a combination of 70 straight-haired and curly haired horses. Horses that exhibited a curly coat type were compared to horses with straight hair from a variety of breeds. The results were used to isolate a mutation that was found to be on a Keratin gene on the 11th equine chromosome pair (ECA 11). This mutation was unique to many of the curly coated horses used as test subjects. The location within the 11th chromosome where the mutation was found is known to code for type I keratins which are the building blocks of hair. [1]

BFC Spartacular Splashes, owned by Aline & Jeroen Verschuren.
BFC Spartacular Splashes, owned by Aline & Jeroen Verschuren.

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 (pictured) 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 KRT25 gene has known association with hair. [1]

Explanation of some basic genetic concepts as they relate to the KRT25 mutation

A substitution of a single base pair within the site of a known gene called KRT25 seems to be the causative factor in a large percentage of curly coated horses tested. This mutation has been designated a numerical location on the DNA chain of the 11th chromosome and is mapped as p.R89H. The substitution of a single base in a gene sequence, or SNP (Single Nucleotide Polymorphism), is called a point mutation when it affects the organism in some way. [9]

Point mutations can, and often do, affect the selection of alternate amino acids which are the building blocks of proteins. There are 20 different amino acids that make up the proteins of all living things. [3] An individual protein can consist of between 50 and 1,000 amino acids that are bonded together in a specific order to create a polypeptide chain. Each polypeptide chain or protein has a specific shape which corresponds to its function. The addition, substitution, or deletion of even a single amino acid in the sequence can profoundly affect the resulting protein and consequently affect the resulting tissues that the protein either makes or regulates. [19] [9] [7] [8]

In the case of the mutation isolated within the gene, KRT25, the triplet DNA code for the amino acid, Arginine was changed to the code for another amino acid, Histidine. The code change happened due to the substitution of a Guanine base in the DNA sequence to an Adenine base. This change of a single base in the DNA sequence, or SNP, caused an alternate amino acid to be incorporated into the polypeptide chain. The resulting protein’s shape was then slightly altered. The change in shape modified a critical protein function that is essential for the proper assembly of keratin type I and type II complexes in the hair follicle.  Keratin complexes are the building blocks in hair formation.  A mutation which results in the substitution of one base for another in the DNA sequence and consequently the coding for an alternative amino acid is known as a missense mutation. [9] [19]

The missense, point mutation in KRT25 was found to be present in a little more than half of the curly coated horses that have been tested. [1] [10]

The remaining curly horse test subjects that did not have the KRT25 point mutation present were grouped together by pedigree, location, and physical appearance. These horses were suspected of carrying different gene mutations that resulted in curly coats. [10]

The other, unknown, curly hair producing gene mutations were given designations based on founding sires (Curly Jim), founding breeders (Cook), type (Sulphur and Spanish Mustang) or location (Patagonian, Mongolian, or Siberian). Currently, genome sequencing is being conducted by Dr. Cothran and his team at Texas A&M University to isolate other suspected mutations that result in curly coats on horses. This project has been funded by The International Curly Horse Organization on an ongoing basis since 2003. The late Sandy Hendrickson, one of the original founders of the organization, started the project to place the registry on sound scientific principles.

The phenotype, or appearance, of the horses that carry the KRT25 mutation is dependent on whether they carry a single copy of the mutation (heterozygous) or two copies of the mutation (homozygous) on chromosome 11 which consists of two DNA strands. One strand is inherited from the dam while the other is inherited from the sire. [19] [8] [7]

The KRT25 mutation has a dominant mode of inheritance. This means that an individual horse only needs one copy of the mutated gene to show traits. [3] Most of the curly coat traits are thought to have a dominant mode of transmission. [11]

Each suspected curly-coat-producing mutation results in horses that have distinctive coat characteristics or phenotypes.

Genes come in different versions, called alleles. Alleles determine the phenotype or appearance of the animal. The combination of all the alleles in all the various genes of a horse constitute that horse’s genotype. [9] [19] [7] [8]

In the language of genetics, the different alleles are assigned a short hand description representing the changes in the triplet code for a gene. Each chromosome in the pair is assigned a nucleotide abbreviation based on the type of base which was either the original or the substituted base.

The short hand version for the different alleles in the case of the KRT25 mutation are represented by: (AA) or homozygous for the mutation, (AG) heterozygous for the mutation, or (GG) for a horse that does not have the mutation. [9] [19]

Examples of horses suspected of carrying only one type of curly mutation – KRT25 – are shown below:

Homozygous

Horses carrying two copies of the KRT25 mutation (homozygous) are known to have the tendency to produce sparse mane and tail hair. Although shorter in general, there is individual variation in mane and tail growth, as can be seen from the KRT25 homozygous horses in the pictures below. 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.

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. [11] [12]

Duchess is a domestic curly horse who tested homozygous (AA). SHe is owned by Donna Hedicke.
Duchess is a domestic curly horse who tested homozygous (AA). SHe is owned by Donna Hedicke.
Spartacus is presumed homozygous (AA). He is a domestic founding sire owned by Sandy Hendrickson.
Spartacus is presumed homozygous (AA). He is a domestic founding sire owned by Sandy Hendrickson.
Jondra Moonwalk, also AA, is owned by the late Andrea Shcaap. The horse shows a sparse mane and tail.
Jondra Moonwalk, also AA, is owned by the late Andrea Shcaap. The horse shows a sparse mane and tail.

Heterozygous KRT25

A horse born with only one copy of the KRT25 mutation (heterozygous) normally has a much fuller mane and tail. As can be seen from the horses in the pictures below, a full mane and tail are present. The mane and tail hair is subject to shedding in the summer months, but is far less likely to shed completely. Brittleness of hair is still a problem, but the hair seems less brittle than in homozygous KRT25 horses. [11] [12]

O‘Sparky, owned by Angie Gaines, is a heterozygous (AG) wild-born curly horse from Nevada.
O‘Sparky, owned by Angie Gaines, is a heterozygous (AG) wild-born curly horse from Nevada.
Sparlock shows off his summer coat. He is a founding sire of domestic curly horses, who was tested AG by his owner, Donna Hedicke.
Sparlock shows off his summer coat. He is a founding sire of domestic curly horses, who was tested AG by his owner, Donna Hedicke.
Blackie, owned by the author, is a heterozygous (AG) wild-born curly horse from Nevada. He is pictured with his winter coat.
Blackie, owned by the author, is a heterozygous (AG) wild-born curly horse from Nevada. He is pictured with his winter coat.

Both groups of horses 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. [6] [3] [11]

Blackie in his summer coat.
Blackie in his summer coat.

“Discordant Horses”

Horses whose curly coats are not due to 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 of unknown origin named Curly Jim (pictured below). Most of these horses also carry the DMRT3 mutation which produces gait. [11] [16]

Curly Jim

Curly Jim arrived in Mountainview, Missouri, by train in 1956.
Curly Jim arrived in Mountainview, Missouri, by train in 1956.

According to Sandy Phipps, whose family has long been associated with the Curly Jim line of horses, Curly Jim arrived by train with another curly to Mountain View, Missouri in 1956. Both horses were bought at an auction in Tennessee, and no information was available about their backgrounds. The second horse was subsequently killed in a barbed wire fence.  Curly Jim was trained by a young Johnny Brooks who later became Sandy Phipps brother-in-law. The sorrel stallion was about 3 years old at the time. [14]

Curly Jim was bred to many local mares, but the popular gaited line of curly horses came from the breeding of Curly Jim to a non-curly, grade mare known as the “Bradford Mare”. Little information is available about the Bradford Mare. The curly coated foal that was produced from the breeding was a filly named Blaze.

Blaze inherited the non-KRT25 mutation for a curly coat from her father, Curly Jim. When mature, Blaze was bred to a popular Missouri Fox Trotter stallion named, Walker’s Marry Lad. Walker’s Marry Lad did not have a curly coat, but the colt that was produced from the mating did. This colt was named, Walker’s Prince T.  As a mature stallion, Walker’s Prince T, pictured below, was the most widely used stallion in the gaited curly line. [11]

Walker's Prince T was a widely used curly stallion.
Walker’s Prince T was a widely used curly stallion.

Current research on the Curly Jim mutation

An intensive scientific investigation is currently underway by Dr. Gus Cothran and his team at Texas A&M University. The study is funded by the International Curly Horse Organization. Its purpose is the isolation of the gene mutation which is responsible for producing curly coats in the descendants of Curly Jim.

A total of ten horses from the Curly Jim line were chosen for the study after their pedigrees were checked by Earlene “Bunny” Reveglia of the International Curly Horse Organization. These chosen horses did not have other lines of curly coated horses in their backgrounds. Blood and hair samples were provided by the owners for the study. [11]

Two horses that were found not to have KRT25 (discordant horses) in Dr. Laurent Schibler’s investigation had whole-genome sequencing completed in France. A subsequent study of the pedigree of the heterozygous, curly stallion, Drakvallmons Ite O Maguzu, and his straight-haired colt, Jak Boreal Maguzu, showed three lines of Curly Jim and one line of Damele. [1] The probability was very high that the stallion’s curly coat came from the Curly Jim lines, since the KRT25 mutation was not found in the curly stallion’s DNA. This pedigree information was provided to Dr. Cothran’s team by ICHO in a collaborative effort to isolate the Curly Jim gene mutation with Dr. Schibler. [11] [13]

 

Coat phenotypes (appearance) of Curly Jim Line horses

By studying the pedigrees of Curly Jim horses, some of the phenotypes or coat traits can be deduced for homozygous and heterozygous inheritance, even though the actual gene mutation that produces the curly coats has not yet been isolated.

Homozygous Curly Jim Mutation

Horses that carry the Curly Jim mutation that are strongly suspected of being homozygous are known for having tight curls on the body guard hairs. This is known as the “brillo-pad coat”. The tight body curls also known as “micro curls” resemble a brillo pad. Manes and tails of these horses have shorter hair, but not sparse hair. It also does not tend to shed in the summer months. [11] [12]

Pictured below is a horse called Kreskin, owned by Jackie Richardson. Kreskin was believed to be homozygous for the Curly Jim mutation. His coat appearance, or phenotype, exhibits the tight body curls characteristic of homozygous horses with Curly Jim ancestry. Kreskin’s mane and tail hair is thick and full which is common in mature Curly Jim mutation horses.

“Kreskin” suspected homozygous Curly Jim mutation. These three photos show micro body curls on the young stallion and the full mane and tail of the mature stallion which is associated with the Curly Jim Mutation
“Kreskin” is suspected to be homozygous Curly Jim mutation. These three photos show micro body curls on the young stallion and the full mane and tail of the mature stallion which is associated with the Curly Jim Mutation
This image of Kreskin, and one above, show the show micro body curls on the young stallion, whereas the image below shows him with the full mane and tail of the mature stallion, which is associated with the Curly Jim Mutation
This image of Kreskin, and one above, show the show micro body curls on the young stallion, whereas the image below shows him with the full mane and tail of the mature stallion, which is associated with the Curly Jim Mutation.
The mature Kreskin with his full mane.
The mature Kreskin with his full mane.
Micro curls, or "Brillo Pad" curls are tight, short, and course. They make up the body coat in horses with Curly Jim homozygous inheritance.
Micro curls, or “Brillo Pad” curls are tight, short, and course. They make up the body coat in horses with Curly Jim homozygous inheritance.

Heterozygous Curly Jim Mutation

Horses that are suspected of being heterozygous for the Curly Jim mutation have full manes and tails and body coats that are curly, but not the tight curls found on homozygous horses. [11] [12]. An example is Sir Patrick, pictured below.

Sir Patrick, owned by Jackie Richardson, is pictured above and below in his winter and summer coats.
Sir Patrick, owned by Jackie Richardson, is pictured above and below in his winter and summer coats.
Sir Patrick has a full mane and tail, which is common in Curly Jim mutation heterozygous horses.
Sir Patrick has a full mane and tail, which is common in Curly Jim mutation heterozygous horses.
Shooter, owned by Terry Schmidt, shows the soft, circular curl typical of the winter coat in horses that are heterozygous for the Curly Jim mutation.
Shooter, owned by Terry Schmidt, shows the soft, circular curl typical of the winter coat in horses that are heterozygous for the Curly Jim mutation.

In Part 2, more types of curly coated horses are presented which do not have the KRT25 mutation. Like the Curly Jim mutation, efforts are under way to discover the genetic mutations that produce these curly coats.

Part 2 can be read here.

Part 3 can be read here.

A statement from The President of the International Curly Horse Organization:
“The International Curly Horse Organization, ICHO, looks to scientific evidence to define a Curly horse. ICHO is a forward-thinking registry where all Curly horses are welcome. The unique traits of a Curly horse not only deserve thoughtful consideration, but deserve truth. ICHO, since its inception, is distinguished by its use of scientific research and is committed to support and fund Curly horse gene research today and will continue to do so in the future. Currently underway, ground breaking Curly horse gene research makes ICHO unique, and scientific research is a priority to support defining evidence of these marvelous horses. It’s with great pleasure to see the support of ICHO materialize in the article by Dr. Mitch Wilkinson. ICHO looks forward to sharing this work with continued support and great anticipation of what is yet to be revealed.” − Joan Duesbout Henning, ICHO President

References

  1. Schibler, Laurent; Cothran, Gus; Juras, Rytis; Morgenthaler, Caroline; et al. A Missense Variant in the coil1A domain of the Keratin 25 gene is Associated with the Dominant Curly Hair Coat Trait (Crd) in Horse. Genetics Selection Evolution 2017 49:85. Published on: November 15, 2017. GSE Journal. Biomedcentral.com http://www.gsejournal.biomedcentral.com
  2. Thomas, Shan. Myth and Mystery: The Curly Horse in America, New York: C.S. Fund Inc.: 1989.
  3. Scott, DW. Skin of the Neck, Mane, and Tail of the Curly Horse. Equine Vet Educ. 2004; 16:201 -6.
  4. Amaral, Anthony. Mustang – Life and Legends of Nevada’s Wild Horse. Univ. of Nevada Press. 1977.
  5. Woolley, Dale E. The Dameles and the American Curly Horse. Limited Publication 1993.
  6. Sponenberg, DP. Dominant Curly Coat in Horses. Genet Sel Evol. 1990;22:257-60.
  7. Bowling, Ann. Horse Genetics. CABI Publishing, 1996.
  8. Bailey, E. and Brooks, S. Horse Genetics -2nd Edition. CAB International, 2013.
  9. Willett, Edward. Genetics Demystified. McGraw-Hill, 2006.
  10. Reveglia, Earlene. Genetic Data Base of the International Curly Horse Organization.
  11. ICHO registry site. Curly Traits: including inheritance and curly hair types. and History of the Curly Horse.   org.
  12. American Bashkir Curly Registry records. abcregistry.org
  13. Curly Horse Pedigree Database. curlyhorses.info ]
  14. Phipps, Sandy. “Lakeview Curly Missouri Fox Trotters” E- mail correspondence. 2017.
  15. Reveglia, Earlene, et.al. History of Curly Jim Line. ICHO, investigation: yrs. 2000 – 2005.
  16. 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
  17. Zierler, Karen. The North American Curly Horse – Buffalo Pony to Modern Working Horse. 2015.
  18. Laughlin, Mike. Great Basin Curly Horse Buckaroos: The Dameles. Cowboy Showcase. 2004.
  19. Pierce, Benjamin. Genetics: A Conceptual Approach, 5th Macmillan Pub., 2013.
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