May 21, 2004
Collie or Pug? Study Finds the Genetic Code
Traditionally, appearance and a written pedigree have been used to define a dog's breed. But scientists had not been able to identify breed from DNA alone in more than a few cases until now.
"I was surprised that you could assign dogs to their breed with 99 percent accuracy," said Dr. Robert K. Wayne, an expert in canine evolution at the University of California, Los Angeles, who was not involved in the study. "That's pretty astounding."
More surprising to dog lovers might be some of the relationships among breeds that the research revealed. The German shepherd, for example, is closer genetically to mastiffs, boxers and other "guarding" dogs than to herding dogs. The fleet greyhound, Irish wolfhound, borzoi, or Russian wolfhound, and lumbering Saint Bernard count herding dogs among their closest kin. And the pharaoh hound and Ibizan hound, often called the oldest of breeds, are really recent constructions, as is the Norwegian elkhound.
The researchers emphasized that they had not yet found genes that account for differences in behavior and appearance among breeds - the mesmerizing stare, or "eye," of a border collie or the spotted coat of a Dalmatian but they now have a tool for studying genetic relationships among breeds that should help in that search.
"We can assign a dog to a breed, but we can't tell what behavior it will have," said Dr. Elaine Ostrander, a geneticist at the Fred Hutchinson Cancer Research Center and University of Washington. "There is huge variation in behavior between dogs within breeds."
Dr. Ostrander directed the study with a colleague, Dr. Leonid Kruglyak. The results were published in today's issue of Science magazine.
Dr. Ostrander said the research would help in the study of canine disease as well as human disease, because certain breeds of dogs are prone to some of the same genetic diseases as humans.
Dr. Wayne suggested that the research could someday be used to create a genetic gold standard for any given breed, or allow a veterinarian to identify a dog as purebred even without written papers.
The researchers sorted the 85 breeds into four major groups, based on genetic similarities. Three groups turned out to share physical characteristics, geographic origins or uses: guarding, herding and hunting. The fourth group consisted of ancient breeds that showed close genetic relationship to wolves. By contrast, the American Kennel Club sorts dogs primarily by use into sporting, working, herding, terrier, hound, toy and nonsporting groups.
Most of the 85 breeds fell into the hunting, herding or guarding groups and were created primarily in Europe or North America in the past 200 years to conform with the concept of purebred dogs, defined by appearance, behavior and closed gene pools, the researchers said. The oldest breeds tend to be most distinct, while the more recent creations, like retrievers, setters, pointers and hounds in the hunting group, are less well-defined genetically.
Because most breeds come from mixed ancestral stock, the differences among them result mainly from reproductive isolation, reliance on a limited number of "founders" and inbreeding to fix desired traits, Dr. Ostrander said.
The breeding practices have also left many purebred dogs susceptible to one or more of 350 genetic diseases.
The "ancient" group includes 14 geographically diverse breeds that are not usually grouped together, including the Asian chow chow, Shar-Pei, Shih Tzu, Pekingese, Tibetan terrier, Akita and Shiba Inu; the African basenji; the Middle Eastern Saluki; and the Siberian husky and Alaskan malamute.
The group's geographic and physical diversity surprised a number of geneticists not involved in the study and even the researchers themselves.
Dr. Kruglyak speculated that these breeds were directly descended from the first dogs and then spread out with their nomadic owners.
When and where dogs first separated from wolves is hotly disputed, with time estimates based on mitochondrial DNA evidence ranging from 15,000 to 135,000 years ago.
While generally praising the research, Dr. Wayne, who has proposed that dogs and wolves split 135,000 years ago, questioned the assignment of dogs to the ancient breed group, saying that any recent crossbreeding with wolves, as has happened with malamutes and Siberian huskies, could make a breed look primitive.
The researchers based their conclusions on DNA samples from 414 dogs representing 85 of the 152 breeds recognized by the American Kennel Club.
The research team drew genetic samples from 96 locations on the dog genome. Using variations in the DNA sequences from those locations, they were able to assign all but four of 414 dogs to their proper breed.
In one case they misidentified a beagle as a perro presa de Canario - a large, mastiff-like dog often used for guarding and fighting.
The study indirectly confirms and greatly expands on work published in 2003 by Dr. Mikko T. Koskinen, a Finnish geneticist, reporting success in using DNA to distinguish among five breeds.