Ancient horse's DNA fills in picture of equine evolution: a 700,000-year-old fossil proves astoundingly well preserved.
Clocking in at about 700,000 years old, the horse DNA is much older than the previous record holder, the genome of an 80,000-year-old Denisovan, an extinct evolutionary cousin of Neandertals and modern people (see Back Story, Page 6).
The extreme age of the horse's genetic material has raised hopes that scientists can isolate and analyze even older DNA, perhaps a million years old or more. The ancient DNA also provides scientists with some of the first clues about the genetic changes that accompanied horse domestication.
An international team deciphered the genome of the horse from the Middle Pleistocene epoch, along with those of a 43,000-year-old horse, a modern donkey and five contemporary domestic horse breeds. Using those data, the researchers pushed back the emergence of the ancestor of horses, zebras, asses and donkeys to about 4 million to 4.5 million years ago. That makes the ancestor twice as old as previously thought, the team reports in the July 4 Nature.
Prior to the new work, researchers had retrieved snippets of DNA that were more than 100,000 years old from cave bear fossils and ice cores. But those snippets were very short, providing nothing like the information contained in the billions of chemical units, or nucleotides, that make up a genome, an organism's complete set of genetic instructions. "It was literally nothing more than a few nucleotides," says study coauthor Ludovic Orlando, an evolutionary biologist at the University of Copenhagen.
So Orlando and his colleagues were astounded by how well preserved the biological molecules were in the ancient horse foot bone. The fossil was found in permafrost at the Thistle Creek site in the Canadian Yukon.
Experts on ancient DNA say that natural deep freezes such as permafrost are the place to look for really old specimens. "They took advantage of the best possible conditions," says Carles Lalueza-Fox of the Institute of Evolutionary Biology in Barcelona. But, he says, the question remains: "How far back can you go in nonpermafrost environments?"
The study may help settle a debate over whether a Mongolian equine called the Przewalski's (pronounced sheh-VAL-skees) horse is really wild and has no domesticated ancestry. Named for the Russian colonel who led an expedition in 1881 that found them, Przewalski's horses were extinct in the wild for decades until a captive breeding program reintroduced them to Mongolia in the mid-1990s.
Some experts consider these sturdy steppe animals a separate species (Equus ferus przewalskii) and the last wild horse. Others insist the wild horses are a subspecies, a feral offshoot of domestic horses (Equus ferus caballus) like the American mustang, Chincoteague pony or Australian brumby. The debate has been difficult to resolve because, until the new study, scientists had no examples of wild ancestral horses to compare with Przewalski's and domestic horses.
By lining up the DNA from the ancient and modern horses, the researchers concluded that Przewalski's horse is a separate, truly wild species that split sometime between 38,000 and 72,000 years ago from the lineage that led to domestic horses. Since that time the two groups have not interbred, the researchers found. "It is 100 percent wild," says Eske Willerslev of the University of Copenhagen. "There's not domestic genetics present in that horse."
Despite stemming from only 13 or 14 animals in zoo breeding programs, Przewalski's horses have retained more genetic diversity than the domestic horse breeds the team examined. That diversity is good news for conservation efforts. "It might mean we could have a very good chance at saving that horse population," Orlando says.
Meanwhile, the researchers are gleaning information about horse domestication by identifying genes that differ among domestic horses, Przewalski's horses and the fossils. So far, genes involved in production of blood and sperm, muscle organization and coat color show signs of being important for domestication.
Caption: Przewalski's horses (one shown in Mongolia) are the last truly wild species of horse, a new study of ancient DNA suggests.
Caption: Researchers extracted DNA from these pieces of a 700,O00-year-old horse bone and compiled the world's oldest genome.
Please note: Illustration(s) are not available due to copyright restrictions.
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|Title Annotation:||STORY ONE|
|Author:||Saey, Tina Hesman|
|Date:||Jul 27, 2013|
|Previous Article:||Chromosome variations.|
|Next Article:||From horses to humans.|