THURSDAY, May 29, 2003 (HealthDayNews) -- Scientists have now disproved two stubbornly held notions: Mules can't be bred and horses can't be cloned.
Idaho Gem, the world's first cloned mule and first cloned equine, was born on May 4 at 3 a.m. Details of the historic event can be found in the May 29 online issue of Science.
"It's exciting, but it hasn't solved all the problems," says Robert Foote, professor emeritus of animal physiology at Cornell University.
Cloning is a highly individualized process, specific to each species. A breakthrough for one species, therefore, does not necessarily mean a breakthrough for others. "If I were going to try to clone another species I don't think that I would have any great leads, except to remind one to look at the early stages of the embryo and to modify the media in different ways to see if that was helpful," Foote adds.
Mules are normally created by breeding a male donkey, which has 62 chromosomes, with a female horse, which has 64 chromosomes. The resulting offspring have 63 chromosomes and cannot reproduce on their own.
Horses and other members of the equine family have long presented a reproductive challenge to scientists. Multiple laboratories around the world have been largely unsuccessful with in vitro fertilization (IVF), the exception being two foals produced in France in the late 1980s. IVF is routinely done in humans and in cattle.
Study author Gordon L. Woods hypothesized that horses had lower cell activity than humans, meaning their cells divided more slowly. Bolstering this theory is the fact that horses have a much lower overall mortality rate from cancer (8 percent) than humans (24 percent). Moreover, while older, light-colored horses develop melanomas, those tumors rarely metastasize. In humans, by contrast, melanomas are among the most virulent of all cancers, spreading like wildfire all over the body.
Woods and his colleagues then started looking for a chemical explanation for this apparent lower cell activity. When they contrasted the red blood cells of horses with those of humans, they found less calcium, a universal regulator of cell activity. "We observed that the intracellular concentration was 2.3 times lower in horse cells," reports Woods, who is professor of animal and veterinary science at the University of Idaho in Moscow. "That fit with the model."
To test the theory further, Woods and his colleagues crossbred a donkey, who was the father of a racing mule named Taz, with Taz's mother, a mare. This produced a pregnancy and, at 45 days, the researchers removed the fetus from the mother. They then took cells from the fetus and placed them in an environment with an increased calcium concentration. The authors eventually implanted 305 oocytes (eggs) in surrogate mares. Idaho Gem is cloned from the cells of that fetus, though the fetus itself didn't develop into anything. "The fetus was created for establishing a cell line to create a clone from," Woods explains.
While clearly a breakthrough, much still needs to be done, Foote says. One issue is to see if the clone grows up normally. Also, the clones came from 45-day-old fetuses, which is fairly early. "It's possible that some of the cells in the fetus didn't have to be completely reprogrammed," Foote says. Historically, the older the cells, the harder it is to clone them.
Dolly the sheep, the first cloned mammal, developed arthritis at an early age and developed a lung disorder that prompted doctors to euthanize her earlier this year. She was 6.
It's also not yet possible to know whether the increased calcium is responsible for the cloning success. "Maybe they were able to because they varied the medium slightly and made it more appropriate for equine compared to other species," Foote says. "The media certainly are important."
One lingering question is why a mule and not a horse?
The answer has to do with bucks -- the green kind. Don Jacklin, one of the project's financial backers, is a mule-racing enthusiast and president of the American Mule Racing Association. "Don has a huge interest in mules, and he wanted the first cloned equine to be a mule," Woods says simply.
Woods is optimistic that horse cloning will be easier. "We think that cloning a mule is more difficult than it would be for a horse," Woods says. "Instead of being asked to jump two feet, we were asked to jump three feet. Having done so, we're optimistic that the same procedures will be successful in cloning horses."
First, Idaho Gem has two identical mule brothers about to arrive: One on June 9 or thereabouts and one on or around Aug. 2.
"Those pregnancies are proceeding just picture perfect," Woods says.
And Idaho Gem is developing into a picture-perfect racer. "Within 12 minutes [of being born] he stood up and we were drying him off with towels and the little rascal bucked with his hind end like he was going to kick us," says Woods, who arrived shortly after the birth. "When he was 33 hours old, we took him out of the stall, out on the grass and you should've seen this guy dart. He darted around like a little rabbit. We were out for about an hour and at the end it took six of us acting like a human fence to catch him."
A big day for science, a big day for mule racing.
More information
For more on mule racing, visit the American Mule Racing Association. The Oak Ridge National Laboratory has a cloning fact sheet.
