Breeds apart: purebred dogs defined by DNA differences.

As fans of the famous Westminster Dog Show will attest, dogs come in all shapes and sizes. The most thorough DNA analysis yet of purebred dogs suggests that canine breeds, typically defined by physical features and family history, can also be discerned genetically with great accuracy.

"At a DNA level, breeds are a very real concept. Every poodle is more closely related to a poodle than it is to a dog of any other breed," says Elaine Ostrander of the Fred Hutchinson Cancer Research Center in Seattle. In the May 21 Science, she and her colleagues describe their analysis of DNA from 85 dog breeds.

The new study is the latest spin-off from efforts to use man's best friend to investigate human health. Over the past few centuries, dog owners have created hundreds of breeds with strikingly different temperaments and physical characteristics. This inbreeding, however, has generated many breed-specific problems, such as deafness and osteoporosis, that also afflict people.

By comparing closely related breeds that differ in their prevalence of diseases, researchers are now tracking down genes responsible for many illnesses. Ostrander and her colleagues analyzed the DNA from 414 dogs representing many of the most popular breeds recognized by the American Kennel Club, one of the groups funding the research. The researchers examined about 100 places on chromosomes where a short DNA sequence occurs repetitively. The exact number of repeats can vary from dog to dog.

The team's hypothesis was that various combinations of these sequences, called microsatellites, could provide breed-defining genetic fingerprints. Indeed, using the microsatellite data, a computer program blindly assigned 410 out of 414 dogs to the correct breed.

"It's a very nice study that advances the state of canine genetics," says Patrick Venta of Michigan State University in East Lansing.

With the microsatellite data, the researchers also drew a family tree of the breeds. In one cluster of closely related breeds, presumably the oldest ones, all the dogs trace their ancestry back to Asia and Africa. These dogs' DNA shows the greatest similarity to wolf DNA, Ostrander and her colleagues report. This diverse collection of old breeds includes the Siberian husky, the Alaskan malamute, the sharpei, and the Akita.

The computer analysis also revealed three other clusters of dogs. They appear to have originated from European efforts to create breeds specialized for hunting, which include bloodhounds and golden retrievers; guarding, such as mastiffs, bulldogs, boxers, rottweilers, and German shepherds; and herding, such as collies and Belgian sheepdogs. Somewhat surprisingly, says Venta, genetics puts greyhounds and Saint Bernards in the herding cluster.

Owners of Ibizan and pharaoh hounds may not embrace the new study, he adds. These breeds resemble the royal dogs depicted in ancient Egyptian art, and many owners consider their dogs direct descendants of those canines. However, the microsatellite data indicate that the two varieties are probably European re-creations of the original breed, notes Ostrander. Ibizan and pharaoh hounds tamed up in the hunting cluster. "There are some people who will be disappointed," says Lisa Puskas, who breeds the dogs and has written several books on them.