Przewalski's horses, the last truly wild horses, were discovered living on the Asian steppes back in the 1870s. But by the 1960s, they had become extinct in the wild, surviving only in captivity. That remaining captive population was descended from about a dozen wild-caught horses and a handful of domesticated ones. With 2,109 individuals today, Przewalski's horses are still endangered, and about a fourth of them have been reintroduced into Chinese and Mongolian reserves.
Now, researchers have sequenced the complete genomes of 11 living Przewalski's horses (Equus ferus ssp. przewalskii) that represent all of the founding lineages. They also sequenced five museum specimens dating back between 1878 and 1929. While they found selection signatures and gene variants specific to Przewalski's horses, over a hundred years of captivity has left a mark: lower genetic diversity and increased inbreeding. The findings were published in Current Biology this week.
"The novelty of our approach is to have not only surveyed the present-day genomic diversity of Przewalski's horses, but also to monitor their past genomic diversity," University of Copenhagen's Ludovic Orlando says in a statement. "That way we could assess the genetic impact of more than 100 years of captivity in what used to be a critically endangered animal."
The large international team compared those genomes to that of 28 domesticated horses (from five breeds) and also of one hybrid. The biggest differences between the domesticated and wild horses were with genes involved in processes ranging from metabolism and muscle contraction to reproduction and cardiac disorders.
Furthermore, the researchers found that the ancestors of domesticated and Przewalski’s horses mixed after they had diverged 45,000 years ago. They even continued to mix after humans began domesticating horses some 5,500 years ago. "We also show that very early in captivity – in the early 1900s – domestic horses contributed significantly to some lineages of the Przewalski's horse pedigree," Orlando adds. "It implies that not all of the surviving Przewalski's lineages represent the gene pool of wild horses equally." In some cases, about a quarter of a Przewalski's horse genome consisted of gene variants inherited from domesticated horses.
Still, the findings are welcomed news for other endangered animals. “Even though Przewalski's horses went through an extreme demographic collapse, the population seems to recover, and is still genetically diverse," Orlando says.
Mongolian horse Emgl1 at the Zoological Department of the Agricultural Institute of the University of Halle, Germany, in the early 1900s. Credit: Museum of domesticated animals "Julius Kühn" at the University of Halle-Wittenberg
Image in the text: Mongolian horses in Mongolia. Ludovic Orlando