On August 6, 2020, the first successfully cloned Przewalski’s horse was born at the Texas-based veterinary facility, Timber Creek Veterinary, along with a new hope for restoring some much-needed genetic diversity to the species. The successful birth of this foal is the culmination of the collaborative efforts between Revive & Restore, San Diego Zoo Global (SDZG), and ViaGen Equine, and lays the groundwork as an important model for future conservation efforts.
The new Przewalski’s foal (pronounced “shuh-VAL-skees”) has been affectionately dubbed Kurt, in honor of noted animal conservationist, geneticist and pathologist, Dr. Kurt Benirschke. Dr. Benirschke played an instrumental role in founding the Frozen Zoo®, a genetic library comprised of cryopreserved cell lines of endangered species. Established in the 1970s, this collection was built on a foundation of prescient hope, banking on the future development of reproductive and cloning technologies that did not yet exist.
Now thanks to his foresight, that gamble is paying off and the fruits of that labor are literally being brought to life almost 50 years later through Kurt the foal, who is as adorable as he is important to the future of his kind.
Przewalski’s horses, considered to be the last truly wild horse species, can be distinguished from their domestic relatives by their comparatively small, stocky frames. Their dun-colored bodies are carried by sturdy, short legs and their thick necks and large heads are topped off with a dark bristle-brush mane, similar to that of a zebra.
They are known to the people of Mongolia as takhi, which means “spirit” or “worthy of worship”, and for well into the 20th century wild populations roamed freely in their native habitat on the arid steppe of Mongolia and China. Due to a perfect storm of overgrazed pastures, deadly winters, and varying degrees of human interference, including poaching, capture, military presence, and political and cultural changes, the takhi species experienced a rapid decline, known as a genetic bottleneck. The last documented takhi sighting occurred around 1969, and as far as anyone could tell from there, it seemed that they ceased to exist in the wild.
Following World War II, the population of takhi living in zoo captivity underwent a second round of bottlenecking, and by the 1950s the breeding population had fallen to just 12 individuals. From there an international studbook, which would go on to become the Species Survival Plan, was established by a German zoologist in 1959, and conservation groups set to work organizing careful and intensive captive breeding programs to begin the recovery process. Virtually extinct in the wild, the species survived exclusively in zoos worldwide up until 1990, increasing their numbers to 961 individuals between 129 institutions in 33 countries.
It was at this point that conservationists felt that it was safe to begin reintroduction efforts. Working with in-country partners, they were able to successfully reintroduce some takhi back into the wild at three primary reintroduction sites in Mongolia, where the herds are legally protected and can continue to be actively monitored and managed.
Today, the Przewalski’s horse population has appeared to make a significant comeback. There are nearly 2,000 total living individuals, with an estimated 500 of those roaming freely back in the wild, and all of which are descendants of those original 12 individuals. While it is certainly a success that the needle on the species’ conservation status has been able to shift from “Extinct in the Wild” to “Endangered”, the species is not out of the woods yet in terms of their recovery.
Do You Catch My (Genetic) Drift?
Unfortunately, recovering a species from the brink of extinction isn’t as simple and straightforward as just breeding more of the animals. When a species experiences a genetic bottleneck, the population loses not only a drastic number of individuals, but also a good portion of its collective gene pool, due to the loss of the alleles, or gene variants, that were present in the original population. The genetic diversity of the remaining population drops to a low level, resulting in inbreeding and an overall decline in the species’ fitness, the measure of their ability to survive and reproduce in their current environment. It is under these conditions—declining, isolated, and lacking in genetic diversity—that a population is most strongly impacted by a process known as genetic drift.
Genetic drift is a mechanism of evolution that occurs in all populations, in which the allele frequencies of the population change from generation to generation. You can think of genetic drift as nature’s game of chance: the determination of which alleles are lost and which persist is unpredictable and determined purely by chance. Potential outcomes are completely random and could include the loss of some alleles, even those that may be beneficial, or the fixation (100% frequency) of other alleles, even those that may be detrimental. By contrast natural selection is more like nature’s game of skill: the possession of a particular allele, whether it is beneficial or harmful to an individual, plays a role in determining the persistence of that trait within a species.
Much like a dormant volcano, the negative impacts genetic drift can have on a species are not limited to a one-time chaotic event; the ripple effects can continue to be felt in the generations to come, even long after a species appears to be recovering. As a species recovers, whether that be through their own devices in the wild or through the help of a captive breeding program, the genetic diversity of the species continues to erode throughout the many generations that are vital to repopulation efforts, thanks to genetic drift. The loss of genetic variation can quickly crack the façade of what may seem like a healthy, recovering population on the surface, revealing inbreeding depression or an inability to cope with environmental changes or disease, since the option to adapt is no longer available.
Great Expectations for the Cloned Przelwalski’s Horse
One typical strategy to help mitigate further negative impacts of genetic drift on a population, is to introduce new individuals from an unrelated population to infuse some genetic diversity. For the Przelwalski’s horse species, however, there are no unrelated populations to pull from, as they are all descendents of the same 12 individuals. Fortunately, cell lines from over a dozen Przewalski’s horses had been preserved in the Frozen Zoo®, and contain genetic diversity that has been lost to the more recent generations. By cloning those genetically-diverse cell lines and periodically introducing them back into a population, you can continue to reset the current genetic diversity of that population using cloned Przewalski’s horses.
Genetic drift will continue to randomly erode that diversity, but for each reintroduction of lost variants from the cloned cell line, a different set of alleles will be lost at random. Thus, this cycle of repetitive cloning increases the chance of unique allele retention as the population increases, and furthermore helps ensure preservation of the original genetic diversity of the species.
This is where Kurt comes in.
Born via a domestic surrogate mother, Kurt was cloned from the cell line of a stallion that had been cryopreserved in the Frozen Zoo® since 1980. The stallion they chose, known as SB615 or Kuporovic, was selected following an analysis of the captive breeding pedigree, which revealed significantly more genetic variation than any living Przewalski’s horse.
The method they used to clone Kurt is known as interspecies cloning, meaning the oocyte, or egg cell, and donor somatic cell come from two different species. There are several reasons why this is the preferred method for cloning endangered species, all of which focus on minimizing depletion of existing reproductive resources of the endangered species, and capitalize on the predictability, reliability and flexibility of their domestic counterparts. Here’s an overview of how it works:
After he is weaned, Kurt will be joining a herd of other Przewalski’s horses at the San Diego Zoo Safari Park. As he grows, not only will he be carrying significantly more unique alleles from wild founders than any other living Przewalski’s horse, he will also be carrying the weight of the expectations that accompanies this major milestone in the conservation of his species. Luckily, he has four sturdy little legs that are well-equipped to help him shoulder that burden.
The hope is that as Kurt matures into the world’s first cloned takhi stallion over the next five to ten years that he will successfully mate, contributing to the genetic diversity of his species and, ultimately, to the future of conservation innovation.
The organization Revive & Restore supports efforts to use genetic tools to rescue endangered species by funding research through their Catalyst Science Fund to which Promega has pledged $3 million. This innovative fund from Revive & Restore awards grants to labs that are exploring the use of biotechnology to solve conservation challenges.
Did you like this post? Read more stories of work with endangered species funded by Revive and Restore:
- Black-Footed Ferrets: Back from the Brink
- Anti-Cancer Drugs are Pro-Coral
- Working in the Lab to save Animals in the Wild