The last of the true wild horses—a stocky, short-maned species known as Przewalski’s horse—is much more distantly related to the domestic horse than researchers have previously thought, according to a new study led by Kateryna Makova, associate professor of biology at Penn State.
Przewalski’s horse—named for the Russian explorer who first encountered it on the steppes of Central Asia—is considered the closest relative of today’s domestic horse; indeed, some have argued it’s the direct progenitor. But DNA studies of the wild species have been sparse, as Przewalski’s horse was driven nearly to extinction during the middle of the last century by hunting and loss of habitat. “By the late 1950s,” Makova says, “only 12 individual horses remained.” Strenuous efforts by conservationists have raised the present-day population to 2,000 individuals living in wildlife reserves and zoos.
Using massively parallel sequencing technology, Makova and her team analyzed the complete mitochondrial genomes (the portion of the genome passed exclusively from mother to offspring) of four surviving female lineages within the Przewalski’s population. They first determined that the genomes of two of those lineages were identical, thus narrowing the genetic pool to three lineages. Then, they tested their data against the prevailing hypotheses about the genetic history of the species.
They concluded that, although previous scientists had assumed that Przewalski’s horse (Equus przewalskii) and the domestic horse (Equus caballus) had diverged around the time that horses were domesticated—about 6,000 to 10,000 years ago—that divergence was actually much more ancient. “In fact,” Makova says, “they probably shared a common ancestor as far back as 160,000 years ago, long before horse domestication. This is a major shift in our understanding.”
To bolster their conclusions, the team also sequenced a small portion of the Przewalski’s horse’s nuclear DNA—as well as a similar sample from a third species, the Somali wild ass. Adding this information, Makova explains, allowed the researchers to "calibrate the molecular clock of horse evolution," thus narrowing the window of time for sub-species divergence.
In addition to the earlier split, their data suggest that present-day Przewalski’s horses have a much more diverse gene pool than was previously hypothesized, Makova says. For this, she credits at least in part the work of conservationists. “They not only began new breeding efforts and built wildlife reserves in California and Ukraine, but they also made sure to avoid inbreeding among close relatives.”
Makova and her team hope their findings will help guide future conservation efforts. “The idea is to gradually reintroduce Przewalski’s horse into the wild,” she says. “Now that we have a more thorough understanding of the different maternal lineages, we can diversify the animal’s gene pool even more. This will be a way to ensure that members of wild species suffer as few recessive diseases as possible and have the best opportunity to flourish once they are introduced into the appropriate habitat.”
In addition, the researchers hope to further studies of horse evolution by sequencing the genomes of additional breeds of domestic horses, and, eventually, by sequencing the complete genome of Przewalski’s horse. “More genetic data means a more precise evolutionary clock,” Makova explains. “The more we know, the more we can adjust the time frame for when Przewalski’s horse and other horses diverged from their common ancestor.”