Winning Bet: Horse and mule clones cross the finish line

Given Funny Cide’s thrilling attempt at the Triple Crown and Seabiscuit’s reign at the movies, this is arguably the year of the horse. Cloning researchers would agree. Following hard on the hooves of news that a U.S. research team had cloned a mule, an Italian group this week reports the first cloning of a true horse. The female foal, dubbed Prometea, is actually a clone of the mare that gave birth to it.

ALL IN THE FAMILY. Prometea, the first cloned horse, and her genetically identical mother. G. Lazzari/Nature

Artificial reproduction in the equine family is notoriously difficult–there have been just two horse foals created using standard in vitro fertilization techniques–so Prometea’s cloning by Italian scientists has drawn praise even from competitors. “They’ve made a huge advance,” says Gordon Woods of the University of Idaho in Moscow.

About 2 months ago, in a Science report online, Woods and his colleagues described how they created a mule, named Idaho Gem, through cloning. Mules, typically the result of breeding a male donkey with a female horse, are usually sterile. Woods’ team harvested mature eggs from horse mares and replaced each egg’s DNA using a nucleus from a fetal mule cell. Immediately after chemically triggering the eggs to start dividing, the scientists surgically implanted the resulting embryos into the oviducts of horse mares.

Of more than 300 embryos transferred, just 21 lasted as long as 2 weeks. Only three of them went to term; on May 4, Idaho Gem was the first to be born.

Like several other groups, Woods’ team has also been racing to clone a horse. In the Aug. 7 Nature, Cesare Galli of the Consortium for Zootechnical Improvement in Cremona, Italy, and his colleagues claim victory by reporting Prometea’s birth on May 28.

In contrast to the mule work, the Italian team obtained immature horse eggs from ovaries obtained at a slaughterhouse. The eggs matured in laboratory dishes before investigators replaced each egg’s DNA using a nucleus from skin cells of a male Arabian thoroughbred or of a female Haflinger, an Austrian breed.

After triggering more than 800 eggs to begin dividing, Galli’s group permitted the resulting embryos to grow to a stage at which the researchers could non-surgically implant the 22 surviving embryos into the uteruses of surrogate mares. Four detectable pregnancies resulted, but only one went full term. By chance, the mare that provided the DNA for Prometea also gestated her.

Genetically speaking, this makes Prometea a twin of her own birth mother.

Since Prometea was created from DNA of an adult horse, her birth sets the stage for the cloning of prized show and racehorses. Moreover, the Italian’s group strategy is much more practical than the costly and complex approach used to create Idaho Gem, notes Katrin Hinrichs of Texas A&M University in College Station.

Beyond the commercial potential of cloning horses, Prometea’s birth from a genetically identical mother seems to challenge the hypothesis that spontaneous abortion occurs when a mother’s immune system doesn’t recognize a fetus as genetically different, suggests Galli.

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