Genetic testing has shown that the paternal lines of nearly all modern horses trace back to stallions brought to Europe from the Orient over the last 700 years.
The key contributors for male lines in most major horse breeds are the Arabian horse, which originated on the Arabian peninsula, and the Turkoman horse, from the steppes of Central Asia.
English Thoroughbred founder stallions can be traced back to Turkoman origin, according to the researchers, who say that English Thoroughbred sires went on to be largely responsible for the predominance of this genetic line in modern horses.
The English Thoroughbred has had a closed studbook since 1793 and was founded by an earlier introduction of Oriental stallions, bred to local mares.
This breeding history – and the dominance of the breed – has led to a situation where only a handful of founder lineages remain within modern horse breeds, and the breeding success of imported bloodlines might have resulted in the complete replacement of some Y chromosome variants.
Researchers from the University of Veterinary Medicine Vienna based their conclusions on their investigation of the Y chromosomes from more than 50 horses representing 21 breeds.
Their findings, reported this week in the journal Current Biology, reveal the overwhelming influence of breeding schemes driven by strong selection on males.
With the genetic lines-of-descent in hand, it is now possible to elucidate the origin and relationship of any stallion line in detail, the researchers say.
“Apart from stallion lines in Northern European breeds, all stallion lines detected in other modern breeds derive from more recently introduced Oriental ancestors,” says Barbara Wallner, from the Austrian university.
The information gathered in the study illuminates the enormous impact modern horse-breeding strategies —characterized by strong selection of males and the import of Oriental stallions — during the past few hundred years has had on Y chromosome diversity, she says.
Y chromosomes are passed down from fathers to sons. This inheritance pattern makes the Y chromosome a good place to look for clues about the unique history of the males in a species.
In the study, the researchers focused on a portion of the Y chromosome that is passed down from one generation to another faithfully. Any changes to that portion of the Y chromosome are the result of new mutations.
“Since random mutations accumulate over time, males who originate from a common patrilineal ancestor will share a particular collection of Y chromosome mutations,” Wallner explains, forming what’s known as a haplogroup.
It had previously been difficult to reconstruct the history of stallions because there is extremely low diversity in the Y chromosomes of modern horses to start with.
The researchers got around that problem by using deep, next-generation DNA sequencing, allowing them to pick up on even the smallest changes.
Their analysis of the 52 Y chromosomes showed that the paternal lineages of various modern horses split much more recently than the domestication of the species, which goes back more than 5000 years.
Apart from a few private Northern European haplotypes, all modern horse breeds included in the study clustered into a roughly 700-year-old haplogroup, transmitted to Europe by the import of Oriental stallions, they report.
The haplogroup includes two major subgroups (or clades): the Original Arabian lineage from the Arabian Peninsula and the Turkoman horse lineage from the steppes of Central Asia.
By linking the Y chromosome lineages with genealogical information derived from written records, the researchers say it’s now possible to define Y haplotypes for certain founder stallions.
Using this approach, they unraveled the origin of English Thoroughbreds, tracing them to Turkoman founder stallions.
“Our results pave the way for a fine-scaled genetic characterization of stallion lines, which should become routine in the near future,” Wallner says.
The researchers say they now plan to create a global Y chromosome phylogeny to include stallion lineages from more rural horses, which most likely lack any recent Oriental influence.
“Y Chromosome Uncovers the Recent Oriental Origin of Modern Stallions
Barbara Wallner, Nicola Palmieri, Claus Vogl, Doris Rigler, Elif Bozlak, Thomas Druml, Vidhya Jagannathan, Tosso Leeb, Ruedi Fries, Jens Tetens, Georg Thaller, Julia Metzger, Ottmar Distl, Gabriella Lindgren, Carl-Johan Rubin, Leif Andersson, Robert Schaefer, Molly McCue, Markus Neuditschko, Stefan Rieder, Christian Schlötterer and Gottfried Brem.
Current Biology, June 29, 2017, http://www.cell.com/current-biology/fulltext/S0960-9822(17)30694-2
The study can be read here.