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August 03, 2010

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Wolf in dog’s clothing? Study points to twist in fur color

Feb. 6, 2009
Courtesy Stanford University
and World Science staff

Slip­ping amid trees or across snow, the wolf has glid­ed in­to leg­end on paws of white, gray or—in North Amer­i­ca—black. 

This last group owes an un­ex­pected debt to the cousins of the do­mes­tic dog, ge­neti­cists say: in an un­usu­al ev­o­lu­tion­ary twist, dogs that bred with wolves thou­sands of years ago passed a muta­t­ion for dark fur to their form­er an­ces­tors. As a re­sult, the Gray Wolf, or Ca­nis lu­pus, is not always gray.

Two wolf pups from Yel­low­stone Na­tion­al Park born to the same par­ents, a black fe­male and gray ma­le. (Im­age cour­te­sy of Dan­iel Stahler/NPS)

Slipping amid trees or across snow, the wolf has glided into legend on paws of white, gray or—in North America—black. This last group owes an unexpected debt to the cousins of the domestic dog, geneticists say. In an unusual evolutionary twist, dogs that bred with wolves thousands of years ago passed a mutation for dark fur to their former ancestors, the scientists say. As a result, the Gray Wolf, or Canis lupus, is no longer just gray. The effect was more than just cosmetic, they add: the resulting black wolves, inhabitants of North America, seem to have an evolutionary advantage over lighter-colored wolves in forests. It’s a rare instance of domestic animals contributing to the genetic variety of their wild counterparts in a way that affects both the recipients’ appearance and survival, the researchers claim. In this case, the domestic animals would have probably been the first Native Americans’ dogs. “We usually think of domestication as something that is carried out to benefit humans,” said geneticist Greg Barsh of Stanford University in California. “So we were really surprised to find that domestic animals can serve as a genetic reservoir that can benefit the natural populations from which they were derived. It’s also fascinating to think that a portion of the first Native American dogs, which are now extinct, may live on in wolves.” Canine geneticists generally agree that North American dogs today descend from European dogs. Barsh and Stanford graduate student Tovi Anderson joined many other scientists to conduct the study, published on Feb. 5 in the research journal Science. The group compared DNA collected from 41 black, white and gray wolves in the Canadian Arctic and 224 black and gray wolves in Yellowstone National Park in the Northwest U.S. with that of domestic dogs and gray and black coyotes. “We expected this to be a short research project to confirm that wolves and dogs shared the same genetic pathway that determines black coat color,” said Anderson. “The story got much more interesting when we expanded our research and began asking about the origin of the mutation in wolves.” Black wolves are much more common in Canadian Arctic forests than in the icy tundra—62% vs. 7% of the total population, respectively. Biologists have long suspected black fur is somehow advantageous to woodland wolves specifically, but weren’t sure why. Because black wolves gray with age, it seemed the root cause might be deeper than just coat color. Barsh’s laboratory, which studies genes affecting coat color and other biological pathways in mammals, found in 2007 that the gene for black fur in dogs, called beta-defensin, belongs to a family of genes previously believed to be involved in fighting infection. One version of the gene produces light or yellow-colored dogs and wolves; a mutant version produces black animals. “Wildlife biologists don’t really think that wolves rely much on camouflage,” said Barsh. “It’s possible there is something else going on here. For example, the protein responsible for the coat color difference has been implicated, in humans, in inflammation and infection, and therefore might give black animals an advantage that is distinct from its effect on pigmentation.” Although the “why” remains a mystery, the “how” is becoming clearer, according to the investigators. Anderson said she confirmed that the black-coat gene shows evidence of positive selection in forest wolves, suggesting evolutionary pressures have favored animals with the gene. She also found that the gene is dominant, meaning an animal with only one copy of the gene would still have a black coat. Anderson and collaborators used genetic tests to determine that the mutation was likely introduced into wolves by dogs in the last 10,000 to 15,000 years, about when the first Americans were migrating across the Bering land bridge into the continent. These people probably brought dogs, some of which carried the black-coat mutation estimated to have arisen about 50,000 years ago, according to the researchers. “It may have been easier for dogs to interact with wolves in North America than in Europe,” said Anderson. “There was probably a higher concentration of wolves, and the dogs, like the humans, were more migratory.” It’s uncertain whether there were any black wolves prior to the domestication of dogs, according to the scientists. “This is a mutation that had been cultivated by humans in the form of the domestic dog for thousands of years,” said Anderson. “Now we see that it not only entered the wild population, but also is benefiting them.”