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New Data Fuel Dogfight Over the Origins of Man's Best Friend
29 November 2011 5:21 pm
Dogs are everywhere, but where did Fido, Rover, and Lassie get their start? Most researchers agree that dogs descend from wolves, but they fiercely debate when and where humans and canines first became best friends. In the latest volley, two new studies place dog domestication in southern China, south of the Yangtze River, some 16,000 years ago. That location contradicts results published last year suggesting that dogs come from the Middle East, but the new work is far from the final word, according to many dog-origins experts.
"I'm not convinced," says Greger Larson, an evolutionary biologist at Durham University in the United Kingdom.
Because archaeologists have trouble telling wolf remains from dog remains, the fossil record has not been very useful for reconstructing canine history. So in the past decade, geneticists have turned to DNA for clues. In 2002, geneticist Peter Savolainen of the KTH Royal Institute of Technology in Stockholm, Ya-ping Zhang of the Kunming Institute of Zoology in China, and colleagues analyzed mitochondrial DNA, which is passed down through females, from 38 wolves and more than 500 dogs around the world. They found the full range of genetic diversity—a marker of a species' origin—in southern East Asia and only subsets of that diversity elsewhere. They conclude that dogs were domesticated in East Asia, and just once.
But a 2010 study looking at single base differences throughout dogs' nuclear DNA called the East Asian origin into question, pointing instead to the Middle East as the home of the first Fidos. Savolainen faulted that analysis, however, because it didn't include data from southern East Asia.
Now Savolainen, Zhang, and their colleagues have confirmed their earlier results by looking at a different part of the dog genome: the Y chromosome, which only males inherit. They sequenced 14,000 bases of the Y chromosomes of 151 dogs, 10 wolves, and two coyotes from around the world. They found more than 50 differences in the DNA sequences and using them reconstructed the dog's history. The team concludes that dogs originated from a large number of domesticated wolves in southern East Asia, south of the Yangtze River. The canine DNA samples from that region had a higher diversity in their Y chromosomes than samples from the Middle East or Europe, the researchers reported online 23 November in Heredity. Other regions may have contributed to the early dog gene pool through wolf-dog hybridizations, but rarely, they note.
In a second paper published online 18 October in Ecology and Evolution, Savolainen and colleagues cast further doubt on the idea that dogs got their start in the Middle East. For this work, they again analyzed mitochondrial DNA, this time comparing samples from 345 dogs from Southwest Asia with mitochondrial DNA from 1556 dogs from across the Old World. For the most part, genetic diversity in the DNA in the Middle Eastern samples was a subset of the overall genetic diversity found in all the samples, indicating that this region was not where dogs were first domesticated, Savolainen says.
"The evidence for both [mitochondrial] DNA and Y chromosome is quite compelling in pointing to a single Asian center of domestication," says Renato Mariani Costantini, a pathologist who has done genetic studies of dogs at the "Gabriele d'Annunzio" University of Chieti-Pescara in Italy. Hannes Lohi, a molecular biologist at the University of Helsinki also finds Savolainen's data convincing, suggesting that although dogs likely came very early to the Middle East, they probably got their start in Southeast Asia.
But others disagree. Dogs' place of origin does not have to be where genetic diversity is currently the greatest, Larson cautions. Dogs may have been more genetically diverse in other places at one time, but the diversity may have subsequently disappeared in those regions, he says. Pontus Skoglund, a population geneticist at Uppsala University in Sweden, is also cautious: "The Y chromosome and mitochondrial DNA represent just two gene regions; it would be much better to compare many more gene regions as was done by the other group in 2010." Furthermore, although the dog-wolf issue makes its analysis problematic, the fossil record doesn't support an East Asia origin, others note.
Robert Wayne, the University of California, Los Angeles, evolutionary biologist who led the studies that point to a Middle East origin for dogs, faults the new study as well, saying there are not enough differences in the Y chromosomes to really reconstruct a clear evolutionary history of canines. To try to do just that, his team is sequencing the genomes of wolves from the possible places where dogs came from and are analyzing DNA from ancient dogs, including what some say is a 31,000-year-old dog fossil. "Our genomewide approach, I feel, is better; [it] suggests a more complex origin with contributions to the dog genome from the Middle East, Europe, and Asia that reflect possible multiple origins and a history of [interbreeding] between early dogs and wolves," Wayne says. "Study of a single gene or region is unlikely to reveal this complexity."