A Chip Off the Old Block: Equine Cloning
Embryos of the cloned mules from the University of Idaho
Photo: Courtesy University of Idaho
Imagine you have the perfect horse: He has excellent conformation, a beautiful disposition, great markings, and athletic ability. Now imagine you could have that exact same perfect horse again and again and again. You could even give them as gifts to loved ones on holidays and birthdays! But should you?
Dolly the Sheep was cloned 15 years ago, and the first horse was cloned a mere eight years ago in 2003. In "science years" this does not give us very much information about the overall impact of our actions--it takes time and research to comprehend the full effects of a major scientific feat.
In this article we'll explore how horses are cloned, why this process is becoming increasingly popular, how closely cloned offspring resemble the original animals, and potential perils and pitfalls.
What is Cloning?
Explaining the birds and the bees in horses and other mammals used to be easy. When a mare's egg or ovum (which has only one half of her entire "set" of DNA) is fertilized by a stallion's sperm (which only has one half of his DNA), a new cell is produced with a completely new complement of DNA (two copies of each chromosome) to create a genetically distinct animal. If the stallion donates an X chromosome, the foal will be a filly, and if the stallion donates a Y chromosome, the foal will be a colt.
With the advent of cloning technology, the story is no longer so simple. In reference to animals, cloning is the method of creating a copy that is genetically identical to the original. One of the most common techniques used to clone animals such as sheep, cows, and horses is somatic cell nuclear transfer (SCNT). This process of reproductive cloning involves taking an unfertilized egg from a mare, stripping it of its nucleus that contains the DNA, and inserting into the egg the DNA-filled nucleus from a cell of the horse to be cloned. Once the DNA is transferred into the egg, the egg is stimulated to begin dividing in a culture dish in a laboratory until it becomes an embryo developed enough to survive in a mare's uterus. The embryo is then transferred into a recipient mare using the typical embryo transfer process.
With cloning the DNA is not altered in any way. Thus, cloned horses are not considered "genetically engineered." Instead, cloning is often touted as an "advanced assisted reproduction technique"--like an über-advanced version of embryo transfer.
There are many reasons why owners choose to have their horses cloned. For example, an owner might have a particularly strong emotional attachment to a horse and wishes to have a clone produced to maintain that intimate bond. Other less emotional reasons for cloning horses also exist.
"Often, the goal of cloning is to create an animal for breeding, not performance," says Samantha A. Brooks, PhD, assistant professor of equine genetics at Cornell University, in New York. "In the case of aging mares, you can currently use embryo transfer and transfer the mare's egg to a younger mare to actually gestate. But if you clone the mare, then you have a whole other lifetime to produce foals from this mare."
This was the reasoning behind the cloning of Quarter Horse mare Royal Blue Boon, the world's all-time leading producer of cutting horses. She was 26 years old at the time (2006)--long past her performance and breeding career--so her owner had her cloned to preserve the mare's genetic material. The resulting clone, Royal Blue Boon Too, apparently is the spitting image of her donor.
Similarly, gelded horses can be cloned and their clones can stand at stud.
"If The Jockey Club allowed it, gelded horses like Funny Cide could be cloned, and those clones could then be stood at stud," says Brooks. "Based on our current understanding, having the clone as a sire would be essentially the same as having the original."
Scamper, for example, a champion barrel racing gelding registered with the American Quarter Horse Association (AQHA), was cloned in 2006. The resulting colt, Clayton, currently stands at stud and has produced apparently healthy offspring. While the AQHA does not register clones, breed registration is not required for a horse to compete in barrel racing and other rodeo events.
Cloning also can be used for species preservation. Some breeds that are near extinction can theoretically be cloned to maintain the breed. The Skyros Pony from Greece, the Przewalski (Mongolian wild horse), and the Kerry Bog Pony from Ireland are some breeds that are very low in numbers and could benefit from cloning.
Horse owners might also have a horse cloned to produce an athlete. One example of this can be inferred from our knowledge of cloned racing mules. Idaho Gem, born May 2003, is the world's first cloned equid and the first cloned mule. However, he was not cloned from the cells of an adult mule, but rather from the cells of a 45-day gestation mule fetus, cells that are considered by scientists as more likely to produce a healthy clone. Two additional cloned mule foals (from the same fetal cell line) were born in June and July 2003 and were named Utah Pioneer and Idaho Star, respectively.
All three mules were trained for racing; Idaho Gem has been particularly successful on the mule racing circuit, winning several races against naturally conceived mules.
However, because these mules were cloned from a fetal cell line, it is not possible to compare their performance with that of an existing animal. Additionally, these mules might not reflect the general health of animals cloned using the cells of an adult donor, a process which scientists say can be associated with more problems.
Potential benefits of cloning (or creating genetically identical embryos that can be cryogenically preserved, at least) also include producing a "bank" of cells, including stem cells, that can be used in the original horse in the event of injury or illness.
True to Form?
While cloning might seem like a surefire way to recreate a champion, this is far from the truth. Even identical human twins aren't really identical. Similarly, cloned foals are not exactly identical to their donors.
"Other factors, in addition to straight genetics, play a role in creating an animal," says Brooks. "Environment for sure, but also something called 'epigenetics,' which results in heritable changes of the structure or function of the DNA without altering the actual nucleotide sequence of the DNA."
Brooks explains that epigenetics can be altered by many things, including the maternal environment during pregnancy. This is because in cloning, the recipient mare is often not the donor mare that is being cloned; therefore, the development of the cloned foal is being impacted by a genetically distinct animal.
Thus, even with an exact identical genome, no two animals are exactly the same. The first cloned horse--Prometea, in 2003--even had a genetically distinct maternal environment because while she essentially foaled herself (she was gestated in the same female that donated the skin cell from which she was grown), her own maternal environment was not the same as that of the original mare's dam.
In terms of performance, Brooks advises, "Few performance genes have been described to date. That doesn't mean that genetics isn't important in determining a horse's performance level, but it is very complicated. There are lots of potential 'performance genes' and lots of interactions between these genes. With the completion of the equine genome we can now just begin to investigate the genetics of performance."
So far, however, clones appear to be very similar to their donors. Take world champion American Thoroughbred show jumper Gem Twist, for example. Breeder and trainer Frank Chapot of Chado Farms in N.J., with the France-based company Cryozootech, began surreptitiously attempting to clone the stunning gray gelding in 2006. In 2008 a dark bay colt was produced successfully and named Gemini. Though photos of the dark colt standing beside the gray donor might raise questions about their identicalness, it's worth noting that Gem Twist also was born a dark bay with similar markings.
"Gemini is now a 2-year-old and is about the same color as Gem Twist was at that age--a roan/gray," says Laura Chapot, Frank Chapot's daughter and Gem Twist's former rider. "It is really hard to tell at this age (if Gemini has the same athleticism as Gem Twist), but he is very well-built."
Scamper's clone, Clayton, on the other hand, does not appear physically similar to his donor, although previous interviews with his owner indicate the two horses have similar dispositions and other like characteristics.
There are no guarantees in life, and this includes cloning. In the case of Prometea, more than 800 embryos were created but only 22 were viable. Seventeen of these viable embryos were transferred into four recipient mares and only one live foal was produced.
Cloning success rates have improved since Prometea, but inefficiency still poses a bit of a challenge.
Katrin Hinrichs, DVM, PhD, a professor in the department of physiology and pharmacology at Texas A&M University, describes instances of high efficiency rates in equine cloning. "Cloning of one horse by Texas A&M in 2005 produced five viable foals from the transfer of only 13 embryos," she says.
However, public and scientific concerns surrounding cloned animals' health abound, which could cause potential cloning service customers to pause prior to purchase. Health issues reported in other cloned species, such as cattle and sheep, include respiratory distress at birth, increased birth weight, and major cardiovascular abnormalities that result in distension of the liver and major blood vessels, according to a study published in a 2002 edition of Nature. In a 2008 article on cloning equids, however, (see www.TheHorse.com/13086), Texas A&M researchers relayed that their cloned foals did not present similar major abnormalities.
Later, in 2010, Hinrichs and her research team published a study relaying their findings on the health of 14 foals they have cloned. This retrospective study relayed that cloned foals do require a little bit more veterinary attention than naturally conceived foals.
Six of the 14 cloned foals included in Hinrichs' study were considered normal. The remaining eight foals had various health issues including:
- Neonatal maladjustment syndrome (or dummy foal syndrome, a condition that results from inadequate oxygen supply);
- An enlarged umbilical remnant; and
- Angular deformities of the forelimbs.
"It seems (the occurrence of these types of health issues) are higher in the cloned foals than in normally conceived foals," Hinrichs concludes.
Two of the cloned foals in the study died within seven days of birth, but the remaining 12 foals survived, and their medical conditions were successfully treated using standard medical or surgical procedures.
Based on these results, Hinrichs says, "Some cloned foals require aggressive supportive care, and the birth of cloned foals should occur at a center equipped for the critical care of neonates."
In a previous interview, Cesare Galli, DVM, of the Laboratory of Reproductive Technology in Cremona, Italy, and one of the scientists involved with producing Prometea, said he hoped the birth of Prometea's own foal, Pegaso, (the first offspring of a cloned horse, produced via artificial insemination) would ease concerns about the health of cloned animals.
"Since she was born ... Prometea has turned out to be an absolutely normal animal in excellent health," Galli said. "Pegaso shows that cloned animals can grow normally and reproduce in a natural fashion."
Cloning is an exciting way to produce a genetic twin, but the lack of long-term information regarding the health of cloned animals is certainly worth consideration prior to embarking on a cloning expedition. Since cloning is still in its infancy, and most clones are still quite young, it is likely too early to make sweeping statements regarding the similarities and differences between clones and donors. Nonetheless, this will certainly be an interesting saga to watch unfold.
About the Author
Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.
POLL: Feeding Alfalfa