Last Updated on March 31, 2022 by Allison Price
GENETIC BOOTTLENECK OVERCOMING
Przewalski’s horse, pronounced “shuh-VALskees”, faces the same challenge as many endangered species: recovering from an historic bottleneck. Przewalski’s horses today, which number around 2,000, are the descendants of 12 individuals who were saved from extinction in early 1900s. Mongolians refer to the horse as takhi (which means “spirit” or “worthy of worship”) .
This can cause inbreeding and fitness loss. Even if a species appears to be recovering in numbers, these negative effects can still occur. The reason is that the genetic diversity of endangered species continues its decline over the many generations required to repopulate it. This process is known as Genetic drift. Inbreeding depression can cause a seemingly healthy population to rapidly decline. The species may struggle to cope with environmental changes or disease–all because it has lost the genetic diversity that it requires to adapt. Larger species, like the Przewalski’s horse, are more susceptible to climate change, habitat loss, environmental disruption and climate change than smaller animals. Przewalkski’s horses may survive in a changing environment if their genetic diversity is increased now.
Genetic rescue can be used to reduce the negative effects of inbreeding. It involves introducing individuals from different populations. The Przewalski horse is the only one with unrelated populations. Forward-thinking conservationists managed to save living cells from more than a dozen Przewalski horses and freeze them at the SDZWA Frost Zoo(r). These cell lines have genetic diversity that was lost in recent times.
CLONING FOR CONSERVATION
A portion of the lost genetic diversity can be recovered now by cloning Przewalski’s historic horse from frozen cells. Reintroducing missing variants to the surviving populations can help increase genetic diversity through successful breeding. This is the hope of Kurt, the foal who was cloned using cells from the SDZWA Frozen Zo(r) in 1980. These cells were from a stallion who was born in 1975 in the UK and was then transferred to the USA in 1978. He lived until 1998. His Stud Book number 615 was (SB615). He was known by his zookeepers as “Kuporovic”. Find out more about the cloning procedure.
Because the SB615 cell line was selected for genetic rescue cloning, an analysis of the captive breeding pedigree revealed significant genetic variation. Kurt could become the most important North American captive breeding horse now that Kuporovic’s genetic variation is “live” in Kurt. He could also be the first to clone an animal that restores lost genetic variation.
GENETIC VARIATION IS IMPORTANT.
This is the second instance of SB615 changing the course of Przewalski’s horse conservation. In the early 1900s there were few Przewalski horses left in captivity. Many zoos crossed Przewalski’s horses from other breeders to save the species. The result was a conflict among zookeepers. Some believed that Hybrid horses were inferior to pure Przewalski’s horses. Therefore, lineages containing domestic horse ancestry were deliberately separated for decades. (Here, “pure” or “hybrid”, are terms used for animal husbandry. This is why Przewalski horses have different ancestry. They are descendants of only 12 founders. These lines were separated, creating two more bottlenecks in the species. In an attempt to remove domestic horse genetics from the hybrid line, North American zoos selectively bred males with wild horse ancestry in an effort purge them. It was not possible to breed domestic horse-gened stallions. This practice resulted in more severe inbreeding between Przewalski’s horse lines.
This became a concern in the 1980s and led to extensive pedigree analysis. The stallion SB615, a descendant from the hybrid horses but with unique ancestry, was actually the son of two wild founders, SB11, and SB12. SB615 actually had more unique variants of genes (or alleles) from wild founders than any other Przewalski horses.
SB615 was born to carry his unique genetics after he was informed of his valuable ancestry. In 2004, the North American captive breeding program changed. The genetic diversity of the subsequent generations has significantly improved since Zookeepers started breeding all Przewalski’s horses and mares, both “pure” as well as “hybrid”, and they began to breed them all. SB615 was the first “hybrid” stallions to be bred in North American Zoos. He has descendants that live in many zoos in the US. Although the program has greatly improved the genetic health and diversity of the recent generations, genetic drift continues.
THE PRZEWALSKI HORSE GENETIC REESCUE PROJECT’S EARLY DAYS
Kurt was named after Dr. Kurt Benirshke is a San Diego Zoo geneticist who had an ingenious idea in 1975. Dr. Benirshke founded the Frozen Zoo(r), which collects and freeze-preserves cell lines from endangered species. It also stores genetic diversity away safely before it is lost. The collection was an attempt to clone and develop reproductive technologies. Nearly fifty years later, Dr. Benirschke is able to bring his plans to life with the help of Revive & Restore at San Diego Zoo Wildlife Alliance and ViaGen Pets and Equine.
In February 2017, Revive & Restore founders Ryan Phelan and Stewart Brand met Blake Russell, president and CEO of ViaGen Pets and Equine, to discuss the possibility of cloning endangered animals. The company’s achievements in commercial cloning domestic species (including cattle, horses and pigs as well as sheep, goats, dogs, cats, and other animals) made it clear that this technology was not just theoretical or academic.
ViaGen Pets & Equine could use their vast knowledge of cloning domestic animals to save wild species? Which species could be the first to benefit? Revive & Restore reached to San Diego Wildlife Alliance in order to find out. Dr. Oliver Ryder was the one who identified many viable cell lines, decades-old, from endangered species. Revive & Restore and ViaGen Pets and Equine teamed up to select the Przewalski’s Horse as a most deserving candidate.
He will be joined by other foals at the San Diego Zoo Safari Park after he is weaned. We hope Kurt will mate with other foals at the San Diego Zoo Safari Park in five to ten year’s time, becoming the first Przewalski’s stallion cloned.
