This article is from Hakai Magazine, an online publication about science and society in coastal ecosystems. Read more stories like this at hakaimagazine.com.
One woman hopes technology can resurrect the now extinct Abaco Island horse breed, writes Stacey McKenna.
A horse named Nunki died in a Bahamian forest in July 2015, marking the end of the Abaco Island horse. But thanks to 21st-century technology, the extinction may be temporary. Two years on, Milanne Rehor, head of the Wild Horses of Abaco Preservation Society, has received approval from the Bahamian government for an ambitious plan: using Nunki’s DNA and selective breeding to clone the breed back into being.
How horses came to the Abacos, an archipelago in the Bahamas, remains a mystery, but the herd’s roots can be traced to the steeds brought by Spanish explorers to the Americas 500 years ago.
In the 1960s, the population was around 200 until a child died while trying to ride one of the horses. The locals responded by killing all but three. When Rehor arrived in 1992 — drawn to the islands by a love for the sea and a fascination with the equines — the population had rebounded to an evolutionarily shaky 30 individuals. Although numbers were up, genetic diversity had plummeted, leaving them vulnerable.
In 1999, Hurricane Floyd drove the horses from the pines to a citrus farm, where a pesticide-riddled diet caused toxins to accumulate in their bodies. No Abaco Island horses were born after 1999. When Nunki died, the breed disappeared. She had became ill after antibiotic treatment for a cut.
“When you get down to the point where there are no more individuals alive of a species, cloning is your only chance,” says Katrin Hinrichs, a veterinary physiologist and pharmacologist at Texas A&M University.
Cloning extinct animals can work, though it comes with limitations. Reviving entire species such as the woolly mammoth is still a ways off, and the only successfully resurrected animal — a subspecies of ibex — died within minutes.
But Rehor and the Abaco Island horse have a shot at changing this. Equine cloning has high-profile precedents, such as replicating Gem Twist, a gelding and one of showjumping’s greatest athletes. His clones — Gemini and Murka’s Gem —now pass his champion genetics on to new generations.
Furthermore, upon Nunki’s death in 2015, Rehor had the foresight to have a veterinarian collect tissue and send the sample to a Texas cloning laboratory, ViaGen. Technicians cultured the cells, then stored them cryogenically.
Nearly every cell in a living animal contains a complete set of that animal’s chromosomes — all the information needed to re-create the animal in full. In normal fertilization, the oocyte reduces its own chromosomes by half and the sperm provides the second half. When the sperm delivers its share, it triggers the egg to begin the process of becoming an embryo.
In cloning, you start with a mature, healthy egg from a donor mare. But instead of fertilizing that egg, you remove all of its chromosomes and replace them with the nucleus from the cell of the animal to be cloned — in this case, Nunki.
“Now you have an egg with two sets of chromosomes, just as if it had been fertilized. You tell it it’s been fertilized, and then it develops into an embryo,” says Hinrichs.
The embryo is cultured for about seven days until it can be deemed viable, and is then transferred into a recipient mare. On average, only about 10 percent of clone embryos develop normally. For every three that are successfully transferred, two will result in pregnancy and one in a live foal. The procedure is costly, but ViaGen has offered Rehor $US2 million in pro bono work. Cloning is hardly guaranteed, but it is possible.
Hinrichs warns, however, that cloning will not save the Abaco Island horse in the long run. It may bring Nunki back, but in order to revive the breed, the cloned mares must reproduce.
“So that’s the puzzle. If you have to keep breeding back to non-Abaco horses then, in the end, you’d end up with a non-Abaco horse,” she says.
Even if it is too late to revive a herd of pure Abaco Island horses, there’s still value in preserving Nunki’s genes.
Because clones keep genes alive, and cloned horses can be introduced to herds to breed naturally, “cloning offers an amazing opportunity to enhance the gene pool of gene-limited species,” says Hinrichs. Saving Nunki’s genes could one day add outside genetics to a herd facing a bottleneck.
The precise benefits Abaco genes could confer aren’t known, says Gus Cothran, a veterinarian at Texas A&M University. However, when Cothran compared the Abaco Island horse’s DNA against a database of horse breeds, he found them to be historically significant and genetically unusual. Living on an island, the herd had preserved and amplified its Old World genes in a way that few feral breeds have.
Thus, Cothran says, saving the genes is worth the effort. “It’s an insurance policy. You’re trying to keep something that you may need in the future, but you don’t yet know that you need.”