by Marcia King
To many people, cloning is a pretty creepy process: Taking cells from a donor animal, placing them into eggs that had their own genetic material removed, mechanically introducing the cell or fusing the two with a jolt of electricity, and voila! Life begins. The living embryo is then implanted in a mare for carrying to term.
But in the not very distant past, many considered organ transplantation and in vitro fertilization techniques (i.e., test tube babies) to be abominations, as well.
In fact, cloning is another – albeit imperfect – breeding tool in development for a world that already encompasses artificial insemination, in vitro fertilization, embryo transfer, and surrogate mares.
Cloning Uncloaked
“At this point, cloning is still in its early stages,” says Dirk K. Vanderwall, DVM, PhD (Animal Physiology), and Diplomate of the American College of Theriogenologists. Vanderwall was a member of the team that produced three cloned mules from fetal cells in 2003 – the first successful equine clones.
“The success rate in equines is poor, as it is in other species; less than three per cent of cloned embryos result in the birth of live offspring. The process is expensive, due to its inefficiency. Also, in non-horse species (primarily cattle and sheep) cloned offspring have a high incidence of neonatal health problems.
“However, clones that survive the neonatal period do not show any evidence of long-term health problems.”
Fortunately, none of the equine clones (mules and horses) so far have developed neonatal complications. As far as long-term health status, equine clones are not expected to encounter problems related to their cloning. “We’re not too worried about that,” says Katrin Hinrichs, DVM, PhD (comparative medical sciences), professor at the College of Veterinary Medicine, Texas A & M University. “It does not appear to be true that clones age prematurely or die of premature old age.”
Those aging misconceptions resulted from inaccurate reporting in the media concerning the death of “Dolly,” a sheep that was the world’s first clone. Dr. Hinrichs, the lead scientist in the project that produced the first cloned horse in North America (in 2005, using skin cells from an adult horse), says that researchers learned Dolly had shorter than normal tips on her chromosomes; young lambs normally have long tips and old sheep have short tips.
“Since then, studies in cattle found that mammary gland cells, which is what they used for Dolly, produce calves with short tips on their chromosomes. When skin cells were used for cloning instead, chromosomes were normal length. At any rate, Dolly didn’t die of old age; she died from a viral-induced lung tumor that spread and killed sheep throughout the barn where she was housed.”
Still, given the expense and minimal success rates, why bother? What value does cloning offer?
Cloning Contributes
Equine cloning promises several benefits, but contrary to another widely held misconception, reproducing an exact replica of a stellar performer or beloved pet is not one of them.
The primary value of equine cloning is in creating a breeding animal that could, in turn, produce competitive offspring. “Cloning will become just one more tool the veterinarian can offer a client who has a horse they want a foal out of, but which is unable to reproduce,” predicts Dr. Hinrichs.
“For example, an old stallion that doesn’t have good sperm, an older mare, or a gelding are good prospects for cloning,” she says.
“If you’re really determined that this genotype is something you want to carry on, we could do cloning, produce a foal with the same genes, and breed that foal.” Because the clone carries the exact same genes as its donor, the clone will be able to produce the same offspring that the donor horse could or would have if it remained fertile.
Another benefit: cloning expands the ability to preserve genetic material from endangered or exotic species such as Przewalski horses, Dr. Vanderwall says.
Further, horse clones could facilitate research into career-ending and life-ending genetic diseases. “As a scientist, I am excited about the possibility that we may be able to conduct studies on cloned horses,” says Ernest Bailey, PhD (genetics), professor in the Department of Veterinary Science, Gluck Equine Research Center, University of Kentucky, and the coordinator of the USDA-NRSP8 Horse Genome Project.
“Our current studies suffer from variation that results from genetic differences among the test animals. This causes us to require large numbers of animals before we can achieve statistical significance for some of our studies.
“With less genetic variation among clones, we could quickly determine whether or not a given treatment had an effect and use fewer horses. Using fewer horses is a more responsible approach to research and would also reduce the costs of research by reducing the number of animals we used.”
A Copy … But Not Exact
As for producing a clone from the cells of a living or dead superstar performer and expecting that clone to deliver the same kind of performance, that’s not likely to happen anytime soon, for clones are not exact replicas of their donors.
For starts, although clones carry the same genes and genetic information as their donors, this genetic information is not necessarily expressed identically from one animal to the other.
Reports Dr. Vanderwall, “The genes can be turned on or turned off differently from one individual animal to another.” Whichever genes are expressed (turned on) will affect, among other things, growth rate, muscle strength, health status, color, etc.
“Two of our three clones (the mules Idaho Gem and Idaho Star) are extremely similar in most every characteristic – coat colour, behaviour, etc. – whereas their identical brother Utah Pioneer has a darker coat colour and has always been less gregarious with people.”
Another factor in performance of clones is that, for various reasons, cloned horses may be born a bit undersized.
The horse clone produced by Texas A&M was about 20 to 30 lbs underweight. The three genetically identical cloned mules were of varying body length, but all slightly shorter than the normal range. Notes Hinrichs, “We know from natural twinning in horses that a twin that’s born very small is not going to grow to its potential.”
Despite these differences, studies conducted in other species demonstrated that when the clone grows and makes eggs or sperm, everything is set back to normal. “Even if they, themselves, have some differences, their offspring are normal,” Hinrichs states.
Today & Tomorrow
Currently, scientists at the two American cloning laboratories, as well as two other teams in Italy and England, are investigating the various components of cloning, working to increase the number of successful births. Texas A & M researchers are studying horse oocytes (immature eggs) and methods for stimulating oocytes to develop in embryos.
For the future, Dr. Vanderwall anticipates that due to its high costs, cloning will be used on a limited basis.
“Even though the cost will come down with the increasing efficiency of the cloning process, I foresee cloning being a niche application for certain circumstances,” he says. “I think the cloning of geldings will continue as the primary use, being able to pass on the genes that would be otherwise lost to the breeding population.”
Bottom line, cloning is not like some horror movie come to life nor weird science. It’s advanced science. Cloning is not a means of creating life where no life existed before.
Cloning is a reproduction tool that, like other reproductive measures now being used, implants some sort of living material into an egg that, if successful, results in a viable embryo. Does it truly matter whether that material comes from skin cells or sperm?
