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Who's Your Mommy?

Sleek, speedy, and spirited, thoroughbred horses arose from Arabian stallions more than 3 centuries ago. But who were the mares that birthed these noble steeds? A new genetic analysis suggests that thoroughbred foremothers hailed from Ireland and Britain.

In the late 1600s and early 1700s, three stallions imported by British aristocrats became the famous forefathers of today's thoroughbreds: the Godolphin Arabian, Darley Arabian, and Byerley Turk. The three originated in the Middle East region but came to England through different paths: one was purchased in France, one acquired in Italy, and the third captured from a Turkish officer at the Battle of Buda in 1686. "We know their names and we have paintings of them," says Mim Bower, an archaeogeneticist at the University of Cambridge in the United Kingdom. Breeders also know a lot about the horses' descendants, as they carefully recorded the lineages over the centuries.

But no one kept track of the moms. "Nobody gave a shit about the females," says Greger Larson, an evolutionary biologist at Durham University in the United Kingdom who was not involved in the study. In the early days, breeders thought the important parent was the stallion, says Bower; any old mare would do as a mother. Many breeders assumed that the original mothers were native British horses. But in the early part of the 20th century, some breeders came to believe that thoroughbreds' mothers were also Arabians—probably because this idea seemed more aristocratic. Bower and her colleagues set out to settle the debate with genetics.

They focused on DNA housed within mitochondria—the cell's power plants. Mitochondrial DNA is passed down only by the mother, allowing researchers to trace maternal lineages, and it can be linked to particular geographical regions. The researchers analyzed DNA from about 300 thoroughbreds and nearly 2000 other horses of different breeds from across Europe and Asia. Most of the sequences came from a genetic database, but the researchers also wrote letters and made phone calls to horse people, asking for samples. "We ask people to pull hair," says Bower. Most owners go for a clump of mane hair—horses don't take too kindly to having their tails pulled. The team then sequenced short stretches of DNA extracted from the tissue attached to the hair.

The thoroughbreds' mitochondrial DNA sequences were closest to those of native Irish and British breeds, like the Connemara. There was a hint of other ancestries—including Arabian—but thoroughbred moms most likely hailed from the British Isles, the researchers will report online tomorrow in Biology Letters.

The fact that breeders ignored the contribution of females reveals the prejudices of society at the time, says Larson. "Just how fantastic is it that we're learning about human social conventions through a study of domestic animals?" Larson adds, however, that the geographic origins of horses are particularly tricky to pinpoint through genetics, so he'd like to see more data than the relatively short stretch of mitochondrial DNA used in this study. Using new high-throughput genetic techniques to analyze a lot more DNA "would nail this shut," he says.