Thanks to their domestication and favored pet status, dogs have enjoyed a genetic variability known to few other species.

Leigh Schindler

It may be time to revise that old maxim about humans and their canine companions. A man, it seems, is a dog’s best friend, and not vice versa.

A paper in the June 29th issue of Genome Research presents evidence suggesting that the domestication of dogs by humans has given rise to the immense diversity of the canine species by allowing otherwise harmful genetic mutations to survive.

“Dogs that would have otherwise died in the wild would have survived because humans would have allowed them to,” said Matt Webster, a geneticist at the University of Dublin and one of the study’s authors.

The stunning diversity of dogs—Canis lupus familiaris includes lumbering St. Bernards, sprightly Jack Russell terriers, and graceful greyhounds—has been a source of scientific interest since Darwin, who speculated that these creatures must have descended from several different species. (Scientists now know dogs have a single ancestral species, the gray wolf.)

“Within a single species you have this tremendous range of morphological variation, all this diversity—head shape, body shape, coat color, length—and a tremendous amount of variation in behavior,” said Leonid Kruglyak, a geneticist at Princeton University. “Where does all this come from? The parent species, which is the wolf, doesn’t show this diversity.”

Webster and his colleagues collected and sequenced DNA from the mitochondria of wolf and dog cells. Using this data, they looked for genetic mutations and calculated the rate at which mutations appeared.

Genetic mutations can be divided into two broad categories: nonsynonymous mutations actually change the protein that a stretch of DNA codes for,  while synonymous, or silent, mutations do not.

Webster and his colleagues found that the silent mutations occur at similar rates in dogs and wolves, but that nonsynonymous mutations accumulate twice as fast in dogs as they do in wolves. These random changes to proteins are usually harmful, and would have a weakly deleterious effect on dogs and their ability to survive, said Webster. 

“That suggests that during dog evolution there’s been a relaxation of selective constraint,” he said. “These additional changes that have happened during dog evolution have escaped the pressure of natural selection.”

Because humans made it easier for domesticated dogs to survive, random genetic mutations that reduced evolutionary fitness—and would have died out in wild dog populations—were able to persist. Furthermore, as humans bred dogs for more desirable traits, they may have exploited these random mutations, accentuating already present variation.

“A lot of the changes over dog evolution would have provided the raw material that humans have used to shape different breeds,” Webster said.

The result, then, is the phenomenal diversity in characteristics among different dogs and dog breeds today.

Elaine Ostrander, a geneticist at the National Human Genome Research Institute who worked on the institute’s dog genome project, praised Webster’s research and its use of mitochondrial DNA.

“For them to focus on mitochondrial DNA was an insightful decision,” Ostrander said. “It’s been neglected in canine genetics.”

Mitochondrial DNA, because it resides outside the cell nucleus, is passed down only from mother to offspring, and it accrues mutations particularly fast. While that might make mitochondrial DNA a natural place to study rates of genetic variation, it’s not yet clear whether Webster’s findings will apply to the nuclear genome.

“The mitochondrial genome is such a small percentage of the dog genome,” said Princeton’s Kruglyak. “The interpretations are somewhat speculative.”

Nevertheless, he conceded that the researchers’ findings and proposed explanation are reasonable, even if not definitive.

“It’s difficult to figure out what exactly happened over the last 10,000 years of dog domestication,” he said. “It’s not clear that any other species has been pushed by artificial human selection to the same extent. There’s definitely a very interesting set of questions to be answered.”

Originally published July 17, 2006


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