Basic black. By interbreeding with black dogs thousands of years ago, North American wolves now come in dark colors.

Image courtesy of Monty Sloan; Wolf Park, Battle Ground, Indiana

Borrowed Gene Blackens Wolves

North American gray wolves that crisscross the frozen tundra after migrating caribou tend to be light colored, blending in with snow and ice. But dark wolves are common in forests, possibly because there they can slink through the woods unnoticed. Geneticists have pinpointed the gene variant that imparts this black fur and determined that it comes from domestic dogs that interbred with their wily cousins thousands of years ago.

Greg Barsh, a geneticist at Stanford University in Palo Alto, California, was chasing down the genetic underpinnings of black dogs when he first heard about unusual populations of black wolves from canine biologist Robert Wayne of the University of California, Los Angeles. They and their collaborators began to study black and gray wolves living in Yellowstone National Park, first looking for differences in genes known to influence color in birds, mice, cattle, and other animals. In these species, the gene agouti controls the relative amount of red and yellow pigments in the skin, and melanocortin 1 receptor (Mc1r) does the same for black and brown. Mutations that make Mc1r more efficient lead to darker fur. But "we didn't find any [relevant mutations] at all" in the Yellowstone wolves, Barsh recalls.

However, in 2007, Barsh and his colleagues determined that the loss of three letters of genetic code in another gene, called the K locus, was behind the black fur of Labrador retrievers, great Danes, German shepherds, and dozens of other breeds. And voilà, the same deletion was present in 102 of the 104 black wolves from Yellowstone the team tested and in all nine black wolves from the Canadian Arctic, Barsh and colleagues report online today in Science. None of the 120 gray Yellowstone wolves and 22 white Canadian Arctic wolves they studied had this deletion.

Not only do black-furred dogs and wolves have the same gene variant, but the DNA surrounding the K locus is also quite similar--and quite different from that of gray wolves--suggesting that the variant was introduced when wolves interbred with domestic dogs. Then, where being dark may have been an advantage, individuals carrying the dog gene proliferated. That variant occurs in 19% of the forest wolves but only in 2% of the tundra wolves, Barsh notes. The truncated K locus also showed up in black coyotes, implying that they also picked it up from dogs.

Black wolves are quite rare outside North America. So Barsh and his colleagues think that more than 10,000 years ago, black dogs migrating with people heading across the Bering Strait into North American interbred with wolves, introducing the K locus variant. "Typically, hybridization is thought to retard adaptation. However, every once in a while, [a variant] is introduced from one species into a second and is advantageous," says Hopi Hoekstra, an evolutionary biologist at Harvard University. "This is really an exceptional finding."

In this case, as in a recent case of gene borrowing between weeds (ScienceNOW, 13 November 2008), "humans were indirect agents in promoting these events," says Enrico Coen, a geneticist at the John Innes Centre in Norwich, U.K. Geneticist Sheila Schmutz of the University of Saskatchewan in Canada wonders what else the dogs might have contributed to wolves, say, to metabolism or immune system function. "It's possible that other traits may also have been introduced that are not necessarily visible traits."