The DNA that divides Chihuahuas and Great Danes

Scientists have identified a genetic marker that could explain why a Chihuahua is so much smaller than a Great Dane and why dogs have the widest range of body sizes among mammals.

Domestic dogs diverged from the wolf family more than 15,000 years ago. Because of selective breeding by humans since then, dogs exhibit a wide range of body types and behaviours. The origin of the genetic diversity between different breeds of dog is unknown.

Now researchers have studied the DNA sequences of more than 3,000 dogs from 143 breeds to pinpoint any gene variants that might be explain the size difference. In particular, they looked at Portuguese Water Dogs, which have the greatest variation in size of any individual breed.

They found that the smallest Portuguese Water Dogs had a particular piece of DNA sitting in their sequence, next to the gene that codes for the hormone insulin-like growth factor 1(IGF1). The IGF1 gene plays an important role in body size for dogs. The "small dog" variant suppresses the activity of the gene, inhibiting growth. The same sequence of DNA was found in other small breeds such as Chihuahuas, Toy Fox Terriers and the Pomeranian.

It was not there in larger breeds such as Irish wolfhounds, St Bernards and Great Danes, or in wild members of the dog family including wolves and jackals. The results have been published in Science.

"It is staggering to think that so many of the small dog breeds came about through selection on the same mutation in the same gene," said Carlos Bustamante, a professor of computational biology at Cornell University and one of the authors of the paper in Science. "These results suggest that while there are invariably differences among breeds... IGF1 has played an important role in the evolution of many small breeds by being a gene that consistently affects body size."

Researchers believe the mutation happened early in the domestication of wolves and the resulting smaller dogs were selectively bred by humans because they were easier to maintain in crowded villages and cities and easier to transport.

Reprinted Courtesy of Guardian News & Media Ltd