Meet Kurt, a genetically important little guy with a bright breeding future
On August 6, the first cloned Przewalski’s horse was born in Texas. Kurt began with a cell nucleus from another of his kind frozen 40 years ago at the San Diego Zoo, and a surrogate run-of-the-mill domestic horse mother. The cloning is a project of San Diego Zoo Global, Revive & Restore, and Viagen Equine.
“This new Przewalski’s colt was born fully healthy and reproductively normal. He is head butting and kicking, when his space is challenged, and he is demanding milk from his surrogate mother,” said Shawn Walker, chief science officer at ViaGen.
I was excited at this news, because these last surviving wild horses had made quite a lasting impression on me when I was very young.
Wild, wild horses
The Catskill Game Farm was a magical place for a budding biologist in the 1960s. My urban experiences with wildlife were restricted to pigeons, squirrels, and the occasional tadpole scooped from the pond at the Brooklyn Botanical Gardens.
City kids like me could wander, at the Game Farm, in a huge fenced-in area among goats, sheep, pigs, deer, rabbits, and geese that roamed, waddled, and hopped about, and even a baby elephant. The sprawling menagerie, about a two-hour drive north, enchanted animal lovers from 1933 to 2006.
Owner Roland Lindemann took in “exotic” creatures too, representing 150 species in all. In 1958 the US Department of Agriculture deemed the place officially a zoo, but of the petting variety. Through the years, it was home to more than 2000 animals.
My favorite residents were a trio of placid Przewalski’s horses. They stood in their enclosure, child-height, behind signs that explained their fascinating history, and how the game farm was encouraging mating to increase the herd of the seriously endangered equines.
The San Diego Frozen Zoo website helped fill in my memories. The horses, Roland, Belina, and Bonnette, had arrived at the Catskill Game Farm in 1966. Bonnette begat Bolinda in 1969, and Belina begat Belaya, a year later.
Belina and Bonnette had apparently mated with their father. They’re lucky to have had babies, for such close inbreeding could pair shared recessive mutations, dooming offspring.
I suppose Roland couldn’t have gone on impregnating his daughters and granddaughters forever. So the five Catskill residents were relocated to the San Diego Zoo, to join more of their kind soon after the births.
Over the decades, traditional breeding gradually expanded the herd, which must have matched up blood relatives fairly often. Since the New York residents arrived, 149 more Przewalski’s horses have been born there, about a dozen living in San Diego now. The animals like to roll in the dust, eat, and groom each other.
A horse is a horse, of course, of course
The Przewalski’s (pronounced “sheh-VAHL-skeez”) horse, also known as the Asiatic or Mongolian wild horse, is named for army officer Nikolai Przewalski, who, in 1881, showed scientists at a museum in St Petersburg a hide and skull. But it probably wasn’t the first description, because the animals resemble horses depicted in cave paintings in France and Spain from 30,000 years ago. The animal was hardly a horse with no name.
The Przewalski’s horse and domestic horses are subspecies, and they join zebras and wild asses in the family Equidae. The animal is short and stocky, with a yellowish-tan (dun-coloured) coat. The head is large, the neck thick, with a dark stripe down the back and a plumed tail. The belly is pale and the backs of the legs striped like those of a zebra or Somali wild ass. There’s no forelock, but the chin and neck grow hairy in winter.
The wild horse is nearly seven feet long, stands four or so feet tall, and weighs 440 to 660 pounds. It lives 25 to 30 years. Przewalski’s horses eat grasses in the wild, and alfalfa, hay, and carrots at the San Diego Zoo.
Mares give birth to single foals, weighing in at about 66 pounds, following an 11-month gestation. Females are mature by three years, males two years older.
Although Przewalski’s horses have 66 chromosomes and domestic horses 64, they can have fertile offspring, which have 65 chromosomes. The hybrids look so much like the Przewalski’s horse that it takes a chromosome check to distinguish them. The event that set the Przewalski’s horses apart during equine evolution was likely a fusing of two small chromosomes into a large chromosome in domestic horses.
Comparing equine genomes can guide future breeding efforts and also provide glimpses of the deep past.
Researchers at the Centre for GeoGenetics at the Natural History Museum of Denmark sequenced the genome of a horse that lived 700,000 years ago, from cells in a fragment of a leg bone discovered jutting from the Yukon permafrost, and compared it to modern horse genomes.
“We estimate that the Przewalski’s and domestic horse populations diverged 38,000 to 72,000 years before the present, and find no evidence of recent admixture between the domestic horse breeds and the Przewalski’s horse. This supports the contention that Przewalski’s horses represent the last surviving wild horse population. We find similar levels of genetic variation among Przewalski’s and domestic populations, indicating that the former are genetically viable and worthy of conservation efforts,” they wrote in Nature.
Przewalski’s horses were plentiful on the vast grasslands that spanned east Asia to Spain and Portugal. But then about 15,000 years ago, as the glaciers receded and forests encroached on the steppe, the horses didn’t fare as well. Only a few were left in Mongolia, Poland, and southern Russia as the 18th century closed. Then farming drove the ancient horses into yet smaller habitats. A few were kept as pets.
Several references claim that all of today’s Przewalski’s horses descend from just 12 or 14 animals captured from the wild from 1910 to 1960, with four domestic horses entering the gene pool.
In 1977, the Foundation for the Preservation and Protection of the Przewalski Horse formed and facilitated the exchange of animals among zoos. By the 1980s, only a few remained in the wild in Mongolia, and the subspecies was soon declared extinct. But breeders increased the numbers, slowly, starting with the release of 16 horses in 1992.
Today, the global population is about 2000, half of them free-ranging, wandering with gazelles and red deer on reserves in Mongolia, Kazakhstan, and northern China. The animals are called “takhi” in Mongolia, which means “spirit.”
To be a clone
Mating pure Przewalski’s horses or trying to build a Przewalski-rich genome through mating hybrids selected for genetic diversity is slow going. But biotechnology speeds things up. Cloning creates a genetic replica of an individual, whereas mating combines genetic material from two individuals.
In fiction, scientists have cloned Nazis, politicians, dinosaurs, children, and organ donors. From 2013 to 2017 the TV show Orphan Black explored the confusing life of a woman with many clones. Real scientists have cloned sheep, mice, rats, cats, pigs, monkeys, dogs, deer, rabbits, and oxen.
Those with means can clone their pets. One company charges $35,000 for cats, $50,000 for dogs, and $85,000 for horses. The cat and horse genomes are about the same size, each larger than a dog genome, so cloning felines is either a bargain or the species is seriously undervalued by the powers-that-be.
Cloning pets is misguided, ignoring environmental influences on behavior and personality. But cloning to restore populations of endangered species is another matter.
The San Diego Frozen Zoo has been at the forefront of using reproductive technologies to assist restoring populations of species on the road to extinction. It houses more than 10,000 cell cultures, eggs, sperm, and embryos representing nearly 1000 types of organisms. Among the cells sit hundreds of potential future Przewalski’s horses on ice.
The cloning process, technically called “somatic cell nuclear transfer,” begins with cells from the desired species or subspecies – skin fibroblasts, in Kurt’s case.
After the cells thaw, their nuclei are separated and transferred to eggs, from domestic mares, that have had their nuclei removed. No sperm necessary: The transferred nuclei already have two copies of each chromosome, instead of one set from a sperm and one from an egg.
The not-technically-fertilized eggs divide a few times in glassware, forming and folding into tiny embryos, which are then transferred into surrogate regular-horse mothers. Then the introduced genomes guide development. The surrogate contributes only her mitochondria – bearing a few genes – and the cytoplasm from her donated deficient egg. Her genes are otherwise fully replaced.
Kurt’s biological dad was born in 1975 in the United Kingdom, and moved to the US in 1978, his skin cells stored in 1980, and he died in 1998. The clone is named for Kurt Benirschke, a founder of the Frozen Zoo.
The colt will join a breeding herd at the San Diego Zoo Safari Park when he’s older. He’s “expected to be one of the most genetically important individuals of his species. We are hopeful that he will bring back genetic variation important for the future of the Przewalski’s horse population,” said Bob Wiese, chief life sciences officer at San Diego Zoo Global.
Bring back the game farm
Perhaps the best news of all: The Catskill Game Farm came on the real estate market again in April! It has previously sold in 2012.
I hope that new owners will purchase the 200 acres and 100 buildings and restore the past, so, like the mythical Phoenix, the wonderful place lodged in so many people’s memories will be reborn. In these days of social distancing, stress, and fear, I can’t think of anything I’d rather do than walk among the gentle beasts.
Maybe new owners would even add a few Przewalski’s horses.