Findings challenge long-held assumptions about the Arabian horse

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The majestic Arabian horse has swept the globe, but their desert cousins still harbor a genetic diversity not seen elsewhere in the world, according to researchers.

Arabian horses are renowned for their athleticism, endurance and natural beauty — qualities which resulted in breeding stock being exported over the centuries to virtually every corner of the equestrian globe.

Now, a major new study has delved into the genetic diversity of the historic breed, shedding new light on its origins.

The findings include surprising revelations, including the likelihood that the founding “Arabians” that gave rise to the Thoroughbred were Oriental breeds, but probably not Arabians.

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They also found that while the global genetic diversity of the breed was good, that was not necessarily the case within important subgroups, in which breeding for certain traits has likely taken a genetic toll.

They also found evidence suggesting some modern outcrossing of registered racing-type Arabians to the Thoroughbred may have occurred.

The study team, writing in the journal Scientific Reports, suggest that only a small proportion of total genetic diversity within the breed left the Middle East when Arabian horses were imported to Europe and the United States over the last 200 years.

“We identified registered Arabian horses resident in the Middle East that clustered with the Arabian breed, but that carried expanded genetic and phenotypic diversity,” University of Florida researcher Samantha Brooks and her colleagues reported.

In particular, the Syrian/Tunisia/Bahrain and Iranian subgroups examined in their research displayed what they described as a high degree of genetic variation and complex ancestry.

Such “desert-bred” Arabians, they said, often lack deep written pedigrees and have a diversity of physical characteristics typical of a domesticated, locally adapted, traditional variety of a species that has developed over time. This includes the breed’s ability to thrive in a hot, dry environment.

Yet they still clearly cluster genetically with other modern Arabians. The increased diversity seen in these subgroups is consistent with a Middle Eastern origin for the modern Arabian horse, the researchers said.

“Unfortunately, documentable ancestral populations may no longer exist for the Arabian, as for most domesticated horses, and populations of unregistered horses are difficult to identify and sample in this region,” they said.

The Arabian horse is one of the world’s oldest domestic breeds of any animal, with credible documentation and pictorial representations extending back at least 2000 years, which places the development of the breed in the Middle East region.

Genetic evidence supports the contribution of ancient Persian lineages of the Arabian during the early formation of modern European horse breeds around 1100 to 1300 years ago.

“The horse of the desert further expanded alongside the rise of the nomadic Bedouin tribes who valued these horses as a cultural symbol, source of wealth, and a military resource,” the study team said.

“Today, although surpassed in absolute numbers by the American Quarter Horse, the Arabian breed is still the most widespread around the world, with pedigree registries in at least 82 countries.”

The authors note that the modern Arabian horse has a particular look that includes a dish-shaped facial profile, wide-set eyes, an arched neck, and high tail carriage.

“However, Arabian horses in photographs made in the late 1800s and early 1900s often show less pronounced facial dishing and lower tail carriage, suggesting that these traits may be under strong selection by modern Arabian breeders, particularly for lines of horses used primarily for non-ridden show competitions.”

Arabian horses have been exported from their ancestral homeland for several centuries. “However, the exported populations usually had small numbers of founder animals, and consequently, now may have limited genetic diversity.”

This, together with breeder selection for certain conformational features, suggests the likelihood of harmful inbreeding in Arabians should be high, they said.

Arabian Horse Society National Championships
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“It is not surprising that several important autosomal recessive inherited diseases have been identified in Arabians,” Brooks and her fellow researchers added.

“In contrast, the few studies of diversity of Arabian horses in the Middle East have shown higher levels of variation in these horses compared to the progeny of exported Arabians in other parts of the world.

“Despite the evidence for the antiquity of the Arabian breed, there is relatively little solid documentation for the various strains and maternal lineages of Arabian horses that are maintained by Arabian horse fanciers and breeders.”

For more than 100 years the influence of Arabian horses in “improving” other horse breeds has been generally accepted among horsemen, the study team said.

“The best-documented example of such influence is in the pedigree of the Thoroughbred breed, which has been maintained as a Stud Book since 1791.

In a pedigree-based analysis of founder lines of the Thoroughbred, three stallions imported to England from the Middle East around the turn of the 18th century remain major contributors to the modern-day Thoroughbred gene pool: the Godolphin Arabian, estimated to contribute 13.53% of the modern gene pool by pedigree analysis, as well as the Darley Arabian, and the Byerley Turk.

Recently however, Y chromosome analysis has indicated that the Y haplotype of the Darley Arabian actually originated from the Turkoman horse, an ancient breed from the Middle East and Central Asia that is like the Arabian Horse — also an “Oriental” type breed.

“This calls into question the role of the Arabian as a founder of the Thoroughbred breed, and more generally, to its influence on other horse breeds,” the authors said.

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For their research, the study team undertook genetic testing of 378 Arabian horses from 12 countries — what they termed a large global sampling. They compared the resulting data with comparable data in the public domain from 18 other breeds.

“We identified a high degree of genetic variation and complex ancestry in Arabian horses from the Middle East region.

“Also, contrary to popular belief, we could detect no significant genomic contribution of the Arabian breed to the Thoroughbred racehorse, including Y chromosome ancestry.

“However, we found strong evidence for recent interbreeding of Thoroughbreds with Arabians used for flat-racing competitions.”

They found evidence for genes for combating oxidative damage during exercise (likely a contributor to the breed’s endurance) and, within the “Straight Egyptian” subgroup, genes that shape the subgroup’s facial features.

Discussing their findings, the researcher said the Arabian horse presents a paradox within equestrian culture.

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“To those who admire the breed, the gracefully shaped head with dished forehead and wide-set eyes are the iconic representation of the Arabian horse. Furthermore, virtually every horse fancier can recite the story of the influence of Arabian stallions in founding the modern Thoroughbred breed.

“To its detractors, the Arabian represents an overly inbred horse breed with a high incidence of inherited autosomal recessive diseases.

“Our results challenge long-held assumptions about the Arabian horse.

“Despite having been dispersed widely across the globe by humans, the breed as a whole maintains a unique genetic identity.”

They found significant differences between all Arabian subgroups examined in the study.

Three subgroups of Arabian horses segregate uniquely, they said: the Straight Egyptian, the Polish Arabians, and the horses from Saudi Arabia.

“This finding agrees with the written histories of these groups, characterized by closely controlled breeding of these lineages over the past 200 years.”

Within the Arabian breed, they found evidence of relatively high inbreeding within individuals, especially those belonging to the Straight Egyptian sub-group, which represents 3-5% of all Arabians. While it is a small subgroup, it is highly prized by owners who compete in non-ridden horse show competitions.

“This subgroup may be subject to relatively intense selection for the specific conformation types preferred in the show ring.

“Despite relatively diverse pedigrees, the high inbreeding values were also observed within individual horses of the multi-origin category.

“This may reflect population bottlenecks that occurred during exportation of these horses from the Middle East to individual stud farms in the USA and Europe, followed by modern breeding practices that are often driven by a popular sire effect.”

The researchers said they identified registered Arabian horses resident in the Middle East that clustered with the Arabian breed, but carried expanded genetic and phenotypic diversity.

“In particular, the Syrian/Tunisia/Bahrain and Iranian subgroups examined here displayed complex ancestry.”

They noted that earlier work by other researchers involving more than 600 Arabian horses from several locations in the Middle East, Europe, and the US had also found higher genetic diversity in Arabian horses from the Middle East region than in horses elsewhere.

“Thus, although the global population of Arabian horses is diverse, loss of diversity within some subgroups like the Straight Egyptian may be reaching levels sufficient to impact animal health.

“Indeed at least three recessive genetic diseases segregate within the Arabian horse, and are already of particular concern within some subgroups.

“This finding highlights the need for use of genomic tools to manage inbreeding within these populations as pedigree-based calculations may not accurately measure loss of diversity due to historic events.

“Likewise, identity-by-state guided breeding decisions could assist in maintaining rare alleles and heterozygosity in endangered populations, both in the Middle East and abroad.”

Historical evidence suggests that the Arabian has been selectively bred for 3000 years, they said.

Selection for endurance and iconic conformational traits such as the dished facial profile has likely left “signatures” detectable in the genomes of modern Arabians.

“Finally, we identified undocumented relationships between the Thoroughbred breed and the modern Arabian that are contrary to breed registry regulations and dispute long-held myths.

“Although celebrated in many historical accounts, the three ‘Arabian’ sires recorded as the main male founders of the Thoroughbred breed (the Darley Arabian, Godolphin Arabian and Byerley Turk) were likely individuals of other Oriental horse populations, and the Arabian breed appears to have contributed little to the autosomal genomic content of the modern Thoroughbred.

The Godolphin Arabian
The Godolphin Arabian

“This disagreement may stem from a simple confusion surrounding the naming of these horses.

“For example, the Darley Arabian was certainly a stallion purchased by Thomas Darley and shipped from within Arabia, but its breed was likely of yet unknown genetic origin.”

However, they detected evidence of modern outcrossing of registered racing-type Arabians to the Thoroughbred, a practice that is prohibited by Arabian horse registries.

“This finding was confirmed by examination of Y-chromosome haplotypes, where several racing Arabians possessed the ‘Whalebone’ haplotype specific to modern Thoroughbred lineages.”

The study team concluded: “Taken together, our observations lead us to hypothesize that only a small proportion of total genetic diversity left the Middle East when Arabian horses were imported to Europe and the USA over the past 200 years.”

The diversity seen within the Arabian horse breed in the Middle East is indicative of a high long-term effective population size, and also reflects the overall robust genetic health of this population, they said.

“The genetic history of the Arabian thus holds greater interest and fascination than the myths that have surrounded this charismatic breed of horse for over 200 years.”

They say the application of modern breeding techniques, such as artificial insemination, is producing an international pedigree of modern Arabian horses marked by genetic homogenization, and in some cases, severe inbreeding and pedigree errors.

“This emphasizes the critical need for more detailed studies of genomic diversity in native Arabian horses in order to enable conservation efforts and manage inbreeding in at-risk subgroups.

“The Middle Eastern subgroups examined here (Iranian, Bahraini, Tunisian, Syrian and others) may represent refugia of genetic diversity crucial to the future of the Arabian horse.”

The full study team Elissa Cosgrove, Raheleh Sadeghi, Florencia Schlamp, Heather Holl, Mohammad Moradi-Shahrbabak, Seyed Reza Miraei-Ashtiani, Salma Abdalla, Ben Shykind, Mats Troedsson, Monika Stefaniuk-Szmukier, Anil Prabhu, Stefania Bucca, Monika Bugno-Poniewierska, Barbara Wallner, Joel Malek, Donald Miller, Andrew Clark, Douglas Antczak and Brooks.

The researchers are variously affiliated with the University of Florida; Cornell University in New York; the University of Tehran in Iran; Weill Cornell Medical College in Qatar; the University of Kentucky; the University of Agriculture in Kraków, Poland; the Equine Hospital at Sha Tin Racecourse, Hong Kong; the Equine Veterinary Medical Center in Qatar; and the University of Veterinary Medicine Vienna.

Cosgrove, E.J., Sadeghi, R., Schlamp, F. et al. Genome Diversity and the Origin of the Arabian Horse. Sci Rep 10, 9702 (2020). https://doi.org/10.1038/s41598-020-66232-1

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

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