How the Last Wild Horses Can Be Saved by Cheap Gene-Sequencing

Humans have long romanticized wild horses as raw manifestations of natural beauty, but our admiration hasn't always carried over into protecting them from endangerment.

This is especially true of the fascinating Mongolian subspecies known as Przewalski's horse, the last truly wild horse in the world. All other "wild" horses, such as mustangs or Chincoteague ponies, are actually feral descendants of domesticated lineages, whereas Przewalski's horse has never been domesticated by humans. Przewalski's horses have, however, been affected by humans in other ways, such as hunting and habitat encroachment, which thinned their herds until the species finally became extinct in the wild in the 1960s.

For a few decades, the surviving population was kept in captive containment, but starting in the 1990s, Przewalski's horses slowly began to be reintroduced to the Mongolian steppes again. Currently, the species has a world population of about 2,000 individuals, roughly one quarter of which has been rewilded. Throughout the process, conservationists have tried to release the horses most resembling the pre-captivity wild population in order to up the odds that they will repopulate their own territories.

In a new paper published today in Current Biology, a team of scientists used gene sequencing to demonstrate how this population bottleneck has affected the surviving population of Przewalski's horses. This kind of information is incredibly useful for the ongoing efforts to reintroduce the horses into the wild, and according to lead author and evolutionary biologist Ludovic Orlando, it has broader implications for managing other recovering species as well.

"One important finding is also that even though Przewalski's horses went through an extreme demographic collapse, the population seems to be recovering, and is still genetically diverse," Orlando told me. "There is, thus, hope for endangered populations, fighting similar demographic issues."

Orlando's team sequenced the genomes of 11 modern Przewalski's horses, along with five museum samples dating back to over 100 years ago. The researchers then cross-referenced that data with the genomes of 28 horses from domesticated lineages to determine how much the true Przewalski's horse stock had been diluted by crossbreeding.

The result was a much clearer picture of the gene flow between wild and tame horses over the last five-odd millennia of domestication and beyond. "In our work, we clearly illustrate the power of genomics and high-throughput DNA sequencing approaches," Orlando said.

"This type of study would have been impossible even a couple of years ago, because it would have involved international consortia of hundreds of researchers and would have been extremely costly," he continued. "High-throughput DNA sequencing made it affordable, even for non-model organisms, for single labs."

The team confirmed that Przewalski's horses diverged from modern domesticated horses about 45,000 years ago, but that occasional interbreeding continued to leave its mark in the Przewalski's horses' genomes. In particular, it seems that a lot of crossbreeding occurred in the early 1900s, and that has real implications for the survival of the remaining Przewalski's horses.

"Ideally you want to preserve the captive stock to be as close to the original wild stock as possible," Orlando explained. "Our work has identified lineages within the current population of Przewalski's horses that remained almost unaffected by the domestic influx that entered the population in its early captive history."

"Those horses can, thus, be proposed to best reflect the ancestral gene pool of Przewalski's horses, a component that one could aim at maximizing in future conservation programmes," he said.

That's not to imply that the success of reintroduction efforts is entirely dependant on ancestry, however. For example, a particularly harsh 2010 winter in Mongolia crippled the fledgling population of rewilded Przewalski's horses, killing about 60 percent of the already meagre population of 138 individuals.

"[T]he success of conservation programmes is not just a matter of the genetic viability of the population, but is a lot related to environmental conditions as well as conservation strategies [and] geographic locations," Orlando said.+

"Either way, one should aim at a long-term survival of preserved populations," he continued. "It, thus, remains essential to involve locals in conservation policies, as this is the only way to make sure that the whole range or environment is maintained, to best reflect the environment that the population has been naturally adapted to."

That's why Orlando and his colleagues plan to continue fleshing out the genetic history and survival techniques of these iconic horses, as well as using genomic techniques to shed light on equine evolution. "Screening a lot more of ancient horses through time—wild, and also domestic—we aim at reconstructing the history of 5,500 years of domestication for horses," he told me.

Just as wild horses remain a powerful symbol of underexplored frontiers, domestic horses have played a starring role in the development of human civilization. The more we understand the evolutionary history of these diverse and magnificent creatures, the more we can responsibly uphold our enduring partnership with them, be it on the wild Mongolian steppes or in bustling urban stables.