The Przewalski's horse is the world's last truly wild horse, found in small numbers on the steppes of Mongolia and China. Their mere existence today is astonishing—by the 1960s they had been declared extinct in the wild. A dozen remained in private zoos in several countries, and scientists undertook an ambitious breeding program. Today, there are around 2,000 of the animals, about one quarter of which live in the wild.
Researchers have now sequenced the genomes of more than a dozen of the creatures, and compared the results to genomes from domesticated horses. The scientists show that Przewalski's and domesticated horses are genetically distinct but diverged only 45,000 years ago, a relatively short stretch in evolutionary time, says Ludovic Orlando, a researcher at the University of Copenhagen. (While there are other "wild horses" in North America and elsewhere, these are actually the feral descendants of domesticated animals, Orlando explains.)
The analysis by Orlando and colleagues, detailed in a study published September 24 in the journal Current Biology, shows that Przewalski's and domesticated horses have significant differences in genes that govern metabolism, cardiac disorders, muscle contraction, reproduction and behavior. It gives scientists clues about how domestication can change an animal's genetics, and helps them better understand how modern horses came to be, he adds. It also shows that species can come back from tiny populations.
"Apparently even 12 horses have a lot of genetic variation and the broader lesson is that we should not give up on a species...we should not abandon them to extinction as long as there is a breeding pair," says Ernest Bailey, a researcher at the University of Kentucky who wasn't involved in the study.
Przewalski's horses do look different from domesticated horses; they have a stockier and shorter body, to go along with brown coats and stiff manes. They are also much more aggressive, explaining why they have never been domesticated, Orlando says.
The study shows that by the time horses were domesticated, likely around 5,500 years ago in what is now Kazakhstan, the two subspecies had already branched apart, although this was already known.
The analysis also shows that there has been some cross-breeding between the two animals, but that, surprisingly, a significant portion of remaining Przewalski's horses remain free of domesticated genes, Orlando says; some had suspected there'd have been more crossing.
"The gene flow from one to the other has been very limited, much more limited than popularly thought," Bailey says.
Finding Przewalski's horses free of domestic DNA will help identify individuals and groups to target for future captive breeding efforts, he says, a process that likely needs to be continued because although the animal has rebounded, its total population is still small enough to be vulnerable to disease and the like.
