Field of clones: How horse replicas came to dominate polo

In Argentina, equine cloning in polo is no longer a rarity. It’s now a mature industry — although ethical dilemmas surrounding it persist.

At the slightest touch of the reins, he felt a familiarity that shook him. It was 2016, and polo player Adolfo Cambiaso — considered the best in the world — was riding for the first time on a genetic clone of Cuartetera, his flagship mare. The same explosive start, the same agility in the curves, the same sustained stride in the long sprints. “It was the same,” he recalls. “Same movements, same head.... I couldn’t believe it.” It took only a few seconds for him to realize that his gamble — which many had dismissed as nonsense — had paid off.

Cambiaso, now 50, had seen before anyone else, back in 2006, the opportunity to preserve the genetics of his most exceptional horses through cloning and thus perpetuate his La Dolfina team, from the province of Buenos Aires, at the top of polo for generations.

That year, in the middle of the Palermo Open final — the ultimate temple of polo — his horse Aiken Cura suffered a devastating fracture and had to be put down. But before saying goodbye, Cambiaso made an unusual request to the veterinarians. “Just in case, before they put him to sleep, I said, ‘Let’s save some cells.’” It was nothing more than a hunch. He had heard the story of Dolly the sheep, the first mammal cloned from an adult cell, and the idea stuck in his mind.

That intuition two decades ago led to a radical change in the world of polo. La Dolfina, now with more than 150 cloned horses, has established unprecedented dominance, and Argentina has become the world center for horse cloning, far ahead of the United States and Europe. Over the years, laboratories have refined the procedure and improved the success rate — although it remains low.

Hence the high costs: Cloning a horse involves far more investment than that required to breed a good specimen the traditional way. And although equine cloning is no longer a rarity but a mature industry, the ethical dilemmas surrounding it — animal welfare, fair competition and the extent to which biology should be manipulated for sporting purposes — still persist.

Making genetic copies of mammals

In all mammals, including horses, the cloning process is similar. First, a somatic cell —a nonreproductive cell such as a skin cell — is taken from the animal to be cloned, and its nucleus, which contains the genetic information, is extracted. At the same time, scientists take an egg cell from the same species and remove its nuclear DNA, in a process called enucleation. The nucleus that was extracted from the first cell is then inserted into this “empty” egg cell.

Next, that egg with its new nucleus is stimulated chemically or by electrical impulses to begin dividing and form an embryo. The embryo is cultured in vitro for seven or eight days until it reaches the blastocyst stage, at which point it is implanted in a female who will carry the pregnancy to term.

The method is called somatic cell nuclear transfer. It was used to create Dolly the sheep in 1996, a milestone that proved it was possible to “reset” an animal’s DNA and bring it to an embryonic state capable of development, although with major challenges along the way.

Since Dolly, more than 25 species of mammals have been successfully cloned, including cattle, sheep, goats, pigs, horses, dogs, cats and wild species such as gray wolves and ferrets. The main limitations of cloning lie in the fact that the transferred nucleus does not always manage to reprogram itself completely, and that the mitochondria of the recipient egg and the genome of the transferred nucleus may have incompatibilities, explains Andrés Gambini, a veterinarian specializing in animal reproduction at the University of Queensland in Australia.

Mitochondria are small structures within cells that produce the energy necessary for them to function. They contain DNA of their own, and for this reason clones are not entirely genetically identical to each other. Although they share the same nuclear DNA from the original animal, the mitochondrial DNA will differ because it comes not from the original animal but from the female oocyte that was used in the cloning process. Though its DNA represents a tiny fraction of the total genome, it plays a critical role in the cell, and so those small variations can translate into differences of function and appearance, says Sebastián Demyda Peyrás, an equine geneticist at the University of Cordoba, Spain.

In addition, he says, “epigenetic patterns in cloning are altered much more frequently than in natural pregnancies. Both factors — mitochondrial replacement and epigenetics — influence the higher rate of miscarriages and the number of clones born with health problems, placental abnormalities or severe physical problems.” (Epigenetics refers to the way that genes may be turned on or off due to the addition or removal of small chemical groups, without affecting the DNA sequence itself.)

Despite its technical challenges, cloning has opened the door to many applications, such as species conservation, livestock breeding and even attempts to bring back extinct species. In the field of conservation, genetic material stored in biobanks can be used to reestablish a healthy breeding population, improving genetic diversity and increasing the number of animals that can reproduce, says Aleona Swegen, a reproductive veterinarian at the University of Newcastle, Australia, and coauthor of a 2024 overview of cloning in conservation in the Annual Review of Animal Biosciences. The main challenges, she says, are the need to find an adequate number of oocytes from closely related species and suitable surrogate mothers for gestation.

Cloning also continues to face challenges in domestic animals. A critical moment, different for each species, is when the embryo stops relying on RNA and proteins from the maternal egg and begins to use its own DNA, says Pablo Ross, chief scientific officer at STgenetics, a global leader in bovine reproductive biotechnology, and an animal geneticist at the University of California, Davis. In cattle, this step, known as embryonic genome activation, occurs when the embryo has between eight and 16 cells, while in horses it occurs when it has four to eight cells.

In horses, as in other species, public protocols for cloning already exist, but success depends on the expertise of the team and technical details that are not always included in manuals. A critical issue in horses is the source of the oocytes. One alternative is to obtain them from the ovaries of dead mares collected at slaughterhouses, although they can also be extracted from live females by transvaginal aspiration, a more invasive procedure but with better success rates.

With oocytes obtained using transvaginal aspiration, the proportion of embryos that reach the blastocyst stage is around 35 percent, compared to just 26 percent in oocytes obtained from slaughterhouses. And the difference widens in later stages: Among mares that remain pregnant after day 42, just over half of pregnancies derived from eggs obtained by transvaginal aspiration result in healthy foals, compared to just one in 10 when the eggs come from slaughterhouses.

In recent years, several advances have improved horse cloning, says Flávio Vieira Meirelles, a reproductive biotechnologist at the University of São Paulo, Brazil. These mainly involve methods for activation of the egg after inserting the nucleus, and cultivation conditions for the embryo. In addition, the efficiency with which the genes of the donated nucleus are reprogrammed — a process that is carried out by chemicals in the cytoplasm of the egg — has improved.

Greater success, too, is achieved when the donated nuclei come from adult stem cells — which are capable of renewing themselves and transforming into various tissues within an organ — compare