Thursday, November 12, 2015

Reconstructing the Last Wild Horses

Portrait of Przewalski's Horse taken at Smithsonian National Zoological Park in Washington, D.C., USA.  Joe Ravi

How the Last Wild Horses Can Be Saved by Cheap Gene-Sequencing   written by Becky Ferreira  September 24, 2015

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.

Captive Przewalski’s horses. Image: BS Thurner Hof

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. 

Mongolian horse Emgl1, early 1900s. Image: Copyright Museum of domesticated animals "Julius Kühn" at the University of Halle-Wittenberg  Compare with the Tarpan, below:

Only known photo of an alleged live tarpan, which may have been a hybrid or feral animal, 1884.  Tarpan at the Moscow zoo. This male tarpan was then 18 years old, dark grey with white spot on front left tibia. Mane was very long; the tail was cut by keepers. The tarpan was caught in 1866 on Zagradovsk steppe and lived until 1880 in Novovorontsovka, until it reached Moscow zoo on May 29th 1884. He was castrated from 3 years of age, and possibly not a pure specimen. His height at withers was 133 cm.
 “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.+

Przewalski’s horse colt, in captivity in Germany. Image: Michael Gäbler

“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.
So what about inbreeding and a genetic bottleneck in the Przewalski's horse?  Well, the news is not all bad, but it still looks a little worrisome to me.  If the breeding programs continue to outcross to horses like the reconstructed Tarpan, and other similar horses, then the picture looks better.  But then there will be those who complain that the offspring of such matings will not be "real" Przewalski's.

That kind of thinking has got the purebred domestic dog into real trouble, thanks to the AKC, the KC and other breed snobs who have little understanding of what makes for a good, sound, healthy animal. 

from: Wikipedia

Every Przewalski horse presently living is descended from 9 of the 13 horses captured in 1945. Two of these were hybrids, one sired from a wild horse stallion and domestic mare and the other from a wild stallion and a tarpan mare. These 13 horses were descended in turn from approximately 15 captured around 1900. A cooperative venture between the Zoological Society of London and Mongolian scientists has resulted in successful reintroduction of these horses from zoos into their natural habitat in Mongolia; and as of 2011 there is an estimated free-ranging population of over 300 in the wild. From a population of 13 horses held in captivity in 1945, the total number by the early 1990s was over 1,500. 

In 1977, the Foundation for the Preservation and Protection of the Przewalski horse was founded in Rotterdam, the Netherlands, by Jan and Inge Bouman. The Foundation started a program of exchange between captive populations in zoos throughout the world to reduce inbreeding, and later began a breeding program of its own. As a result of such efforts, the extant herd has retained a far greater genetic diversity than its genetic bottleneck made likely. 

Since 1986, Chinese researchers have bred Prezewalski's horses in captivity, with the program seeing over twenty years of success.

While dozens of zoos worldwide have Przewalski's horses in small numbers, there are also specialized reserves dedicated primarily to the species. The world's largest captive breeding program for Przewalski's horses is at the Askania Nova preserve in Ukraine. Several dozen Przewalski's horses were also released in the area evacuated after the Chernobyl accident, which now serves as a deserted de facto nature reserve. In Chernobyl, the population reproduced at a high rate, reaching up to 200 individuals until poachers decreased their number to just 60 in recent years. As of 2011, it was estimated that only 30–40 individuals remained. An intensely researched population of free-ranging animals was also introduced to the Hortobágy National Park puszta in Hungary; data on social structure, behavior and diseases gathered from these animals is used to improve the Mongolian conservation effort. 

Several American zoos also collaborated in breeding Equus ferus przewalskii from 1979 to 1982.Recent advances in equine reproductive science in the United States also have potential to further preserve and expand the gene pool. In October 2007, scientists at the Smithsonian Institution's National Zoo successfully reversed a vasectomy on a Przewalski's horse — the first operation of its kind on this species and possibly the first ever on any endangered species. While normally a vasectomy may be performed on an endangered animal under limited circumstances, particularly if an individual has already produced many offspring and its genes are overrepresented in the population, scientists realized the animal in question was one of the most genetically valuable Przewalski's horses in the North American breeding program. The first birth by artificial insemination occurred on July 27, 2013 at the Smithsonian Conservation Biology Institute

Le Villaret, located in the Cevennes National Park in southern France and run by the Association Takh, is a breeding site for Przewalski's horses that was created to allow the free expression of natural Przewalski's horse behaviors. Eleven zoo-born horses were brought to Le Villaret in 1993. Horses born there are adapted to life in the wild: they are free to choose their own mates and must forage on their own. Such a unique breeding site was necessary to produce the individuals that were reintroduced to Mongolia in 2004 and 2005. In 2012 there were 39 individuals at Le Villaret. 

The Przewalski's Horse Reintroduction Project of China was initiated in 1985 when 11 wild horses were imported from overseas. After more than two decades of effort, the Xinjiang Wild Horse Breeding Centre has bred a large number of the horses, of which 55 were released into the Kalamely Mountain area. The animals quickly adapted to their new environment. In 1988, six foals were born and survived, and by 2001 there were over 100 horses at the centre. As of 2013, the center hosted 127 horses divided into 13 breeding herds and three bachelor herds.

Reintroductions organized by western European countries started in 1990s. These were later stopped, mostly for financial reasons. Prague Zoo started a new cycle of transporting horses to the wild, which, with the support of public and many strategic partners, continues today.

The stallion Vaska, the first Przewalski Horse captured from the wild by Europeans to reach the West in 1899. The photo was taken in Askania Nova in 1904. / Source: G. I. Riberger – this image is in the public domain.
Przewalski’s hybrid horse Theodor at the Zoological Department of the Agricultural Institute of the University of Halle, Germany, early 1900s. (Copyright Museum of domesticated animals “Julius Kühn” at the University of Halle-Wittenberg).
MONGOLIA - CIRCA 1986: stamp printed by Mongolia, shows Przewalski horse, circa 1986
In the second half of the 20th century the wild population collapsed dramatically and the last wild Przewalski's stallion was sighted in 1969.

Cro-Magnon cave painting of primitive horse from Lascaux, southern France (20,000 years ago) Alex Greely/Alpha/FPG