The storied history of the Thoroughbred goes back 300 years, giving rise to a breed of great economic importance.
All modern Thoroughbreds, whose breeding histories are invariably well documented, can trace their lineage back to three founding stallions imported into England in the late 17th and early 18th centuries.
The Byerley Turk (1680s), the Darley Arabian (1704), and the Godolphin Arabian (1729) are the founding fathers of the racing breed. Books have been written about them, documenting the great influence they exerted on the breed.
They created a line of horses that is at the centre of a multibillion-dollar industry.
The Thoroughbred also led to improvements in other breeds, thanks to crossbreeding.
But what of the maternal origins of the mighty Thoroughbred?
It takes two to tango, as they say, and much less is known about the contributions of mares to the origins of the Thoroughbred.
Now, researchers from Seoul National University in South Korea have delved into the mitochondrial DNA of the breed, which is passed down only the maternal line, to learn more about the contribution of mares to the Thoroughbred.
For their research, Sook Hee Yoon and her colleagues sequenced the complete mitochondrial genome (the mitogenome) of 14 Thoroughbreds and two Przewalski’s horses.
These sequences were analyzed and compared with 151 previously published horse mitochondrial genomes from a range of breeds across the globe.
The study team, writing in the open-access journal PLOS ONE, revealed that the Thoroughbred had multiple maternal origins, being closely related to horses from one Asian, two Middle Eastern, and five European breeds.
The sequencing of the Thoroughbred’s mitogenomes showed that the breed’s maternal lines were not monophyletic, meaning they were not all descendants of a common maternal ancestor, so were divided into four different groupings on that basis.
Thoroughbreds within Group 1 were found to be closely related to one of the two different European and one Asian breeds: the Italian Maremmano horse, the Norwegian Fjord and Jeju horse from Korea.
Those in Group 2 were clustered with breeds from three different European, one Middle East, and one Asian breed: Shire, Italian, Norwegian Fjord, Syrian, and the Jeju horse.
The next group was linked to an Iranian breed from the Middle East and the Kinsky horse breed from Central Europe; while the final group specimen was clustered with an Iranian breed from the Middle East.
The researchers found that the Thoroughbred horse breed was not directly related to the Przewalski’s horse, the only true wild horse species left in the world.
The most recent common ancestor of the Thoroughbred lived about 8100 to 111,500 years ago, which was significantly younger than the most recent common ancestor of modern horses.
The study team’s worked revealed that the population expansion of modern horses occurred about 5500 to 11,000 years ago, which coincided with the start of domestication.
The study team said their findings supported what is known as the multiple origin hypothesis for the maternal lineages of the Thoroughbred.
“In terms of the genetic relationship of Thoroughbred horses and the global horses, our results of phylogenomic analysis revealed that the Thoroughbreds were closely related to some horses from eight breeds: one (Jeju) from Asia, two (Syrian and Iranian) from Middle East, and five (Maremmano, Norwegian Fjord, Shire, Italian, and Kinsky) from Europe.”
However, there were contradicting studies on this topic, they acknowledged.
They said the information gathered in their study on Thoroughbred mitogenomes should provide useful information for future horse improvement projects as well as for studies investigating horse genomics, conservation, and geographical distribution.
The full study team comprised Sook Hee Yoon, Wonseok Lee, Hyeonju Ahn, Kelsey Caetano-Anolles and Heebal Kim, all from Seoul National University; and Kyoung-Do Park, from Chonbuk National University in South Korea.
Yoon SH, Lee W, Ahn H, Caetano-Anolles K, Park K-D, Kim H (2018) Origin and spread of Thoroughbred racehorses inferred from complete mitochondrial genome sequences: Phylogenomic and Bayesian coalescent perspectives. PLoS ONE 13(9): e0203917. https://doi.org/10.1371/journal.pone.0203917