The Przewalski’s Horse (TAKHI) Project

OVERCOMING A GENETIC BOTTLENECK

The Przewalski’s horse (pronounced “shuh-VAL-skees”) population faces the same struggle that many endangered species face—recovering from a severe historic bottleneck. Today’s Przewalski’s horses, of which there are now approximately 2,000, are descendants of just 12 individuals saved from extinction in the early 1900s. The Mongolians call the horse by another name takhi, which means “spirit,” or “worthy of worship.” Learn more about the species.

Genetic bottlenecks like this can lead to inbreeding and loss of fitness. These negative impacts may emerge long after a species seems to have recovered in numbers. This is because the genetic diversity of endangered species continues to decline over the many generations it takes to repopulate the species, due to a process known as genetic drift. A seemingly healthy recovered population can quickly decline due to inbreeding depression. Or the species can struggle to cope with disease or environmental changes—all because the population has lost the genetic variation it needs to adapt. Additionally, larger species like the Przewalski’s horse are more vulnerable to environmental disruption, habitat loss, and climate change than are smaller animal species. Increasing their genetic variation now may help the Przewalkski’s horse species survive in a changing world.

Genetic rescue is one strategy to reduce the negative impact of inbreeding by introducing individuals from unrelated populations. But for the Przewalski’s horse, there are no unrelated populations to draw from. However, forward-thinking conservationists saved living cells from over a dozen Przewalski’s horses and cryopreserved them at the SDZG Frozen Zoo. These cell lines contain genetic diversity that has been lost to recent generations.

CLONING FOR CONSERVATION

Now a portion of this lost genetic diversity may be recovered by cloning historic Przewalski’s horse from frozen cells. Successful breeding can increase genetic diversity by reintroducing lost variants to the surviving population. This is the hope for the new foal, Kurt, who was cloned from cells that had been cryopreserved at the SDZG Frozen Zoo in 1980. These were cells from a stallion that was born in 1975 in the UK, was transferred to the US in 1978, and lived until 1998. He was recorded as Stud Book number 615 (SB615) and known as “Kuporovic” by his zookeepers. Learn more about this cloning process.

The SB615 cell line was chosen for genetic rescue cloning because an analysis of the captive breeding pedigree revealed that the genome offers significantly more genetic variation than any living Przewalski’s horse. Now that the genetic variation from Kuporovic “lives” again in Kurt, Kurt may become the most important horse in the North American captive breeding population. He may also become the first cloned animal to restore lost genetic variation to its species.

THE IMPORTANCE OF GENETIC VARIATION AND SB615

This is the second time that SB615 has changed the course of Przewalski’s horse conservation. In the early 1900s, there were very few Przewalski’s horses left in captivity. To save the species, several zoos interbred their Przewalski’s horses with domestic horses. This created a clash amongst zookeepers. Some felt that “hybrid” horses were inferior to “pure” Przewalski’s horses, and so lineages containing domestic horse ancestry were intentionally segregated for decades. (Here “pure” and “hybrid” are meant as animal husbandry terms.) Due to this, different Przewalski’s horses today have ancestry from only a few of the 12 founders. Separating these lines created two additional bottlenecks for the species. Later, in an effort to purge domestic horse genetics from the “hybrid” line, zoos in North America selectively bred only those males that had “pure” wild horse ancestry. Stallions with domestic horse genes were not bred. This practice led to increasingly more severe inbreeding within both lines of Przewalski’s horse. 

By the 1980s, this became a major concern and prompted an extensive pedigree analysis. It was discovered that the stallion SB615, although a descendant of the hybrid horses, actually possessed unique ancestry from two wild founders, SB11 and SB12. In fact, SB615 carried significantly more unique alleles (or variants of genes) from those wild founders than other living Przewalski’s horses.

Due to the discovery of his valuable ancestry, SB615 was bred to pass on his unique genetics. The captive breeding program in North America changed course in 2004. Zookeepers began breeding all viable Przewalski’s horses, mares and stallions, both “pure” and “hybrid” and the genetic diversity of subsequent generations substantially improved. SB615 was one of the first stallions of the “hybrid” line to be bred in North American zoos, and today he has descendants living in several zoos across the US. While the program greatly improved the genetic health of recent generations, genetic drift continues to erode genetic diversity. 

The saving of cryopreserved cell lines that was begun in 1975 by Dr. Kurt Benirschke is now led by Oliver Ryder, Ph.D., Director of Genetics for San Diego Zoo Global. Here, he examines part of the Frozen Zoo collection. (Courtesy SDZG)