ACCORDING TO scientists, dogs exhibit the greatest range in body size of any mammal—from Yorkshire Terriers that fit inside a purse to gigantic Great Danes, however, the genetic origin of this diversity has baffled scientists—until now.
Researchers have identified a variation in a single gene that plays a key role in making small dogs small. "The best way to describe the role of the gene is, it's like the 'reduce' button on a Xerox machine," said lead researcher Elaine Ostrander of the National Human Genome Research Institute's Cancer Genetics Branch in Bethesda, Maryland.
More than 20 scientists from eight institutions in the United States and the U.K. participated in the study. The discovery could ultimately lead to improved medical care for both dogs and humans, by helping scientists track down genetic causes for complex diseases such as cancer, diabetes, and hormone disorders.
To conduct the study, the researchers traveled to dog shows and other events, collecting blood samples from 3,241 canines ranging in size from Chihuahuas to Mastiffs. After analysing the DNA, researchers discovered the genetic instructions for restricting growth were located near the gene named IGF-1, for insulin-like growth factor 1.
In humans and other mammals, the hormone regulated by IGF-1 helps animals grow from birth to adolescence. But in small dogs, the researchers found, one or more mutations located next to the IGF-1 gene did just the opposite and suppressed growth.
"The surprise was, all small dogs have this particular gene in common," said study co-author Gordon Lark, a biologist at the University of Utah in Salt Lake City. "We didn't expect that."
Researchers believe the newly discovered genetic instructions are at least 12,000 years old, which is around the same time that dogs evolved from wolves. Since the variant is only found in small breeds, Lark speculates that pint-size pups descended from undersize wolves. These smaller wolves might have been likely candidates for domestication by ancient humans, he said.
"You can imagine that a small wolf would be much more adoptable [by humans]," he said. "Or that a small wolf would say, The only way I'm going to survive is by adopting a human." The genetic variant for smallness, he said, was looked for in modern wolves but not found.