KEY POINTS:
In a pioneering investigation of canine DNA scientists have discovered the genetic trick that has allowed small dogs to shrink in size from their wolf-like ancestors.
A major study of more than 3200 dogs from 143 different breeds has found the part of the canine genome that is responsible for producing diminutive stature in breeds such as bichon frise, chihuahua and pekinese.
The part of the canine genome responsible for small size is not a gene but a "regulatory sequence" that controls a separate gene responsible for a key growth factor.
Scientists found that all small dogs they examined had the same kind of regulatory sequence, which they believe can explain the huge variations in dog size - the largest of any mammal.
"The identification and characterisation of a key genetic variant that accounts for differences in dog size is particularly exciting because the underlying gene is present in all dogs and other diverse species, including humans," said Eric Green, of the US National Human Genome Research Institute.
The findings would also lead to a better understanding of many human conditions that are affected by the same insulin-like growth factor gene, said Elaine Ostrander, head of cancer genetics at the genome research institute.
"Nearly all of what we learn from studying body structure, behaviour and disease susceptibility in dogs helps us to understand some aspect of human health and biology," said Dr Ostrander.
"By learning how genes control body size in dogs, we are apt to learn something about how skeletal body size is genetically programmed in humans.
"We also will increase our data set of genes likely to play a role in diseases such as cancer, in which regulation of cell growth has been lost," she said.
Dogs evolved from wolves between 12,000 and 15,000 years ago when they were domesticated by humans for hunting and guarding. At some point soon after, the genetic variant for small size evolved and spread rapidly. Although the scientists have not yet been able to pinpoint the precise changes to the DNA that account for the size of small dogs, they believe it resides within a variation in the segment of DNA that lies next to the gene for an insulin-like growth factor.
"All dogs under 20lb [9kg] have this - all of them," said Gordon Lark of the University of Utah, who was part of the large team whose results are published in the journal Science. "That's extraordinary.
"Since this is found in all small dogs, it either got into dogs when they were first domesticated, or it was a small wolf that dogs descended from. The small dog [genetic variant] is not found in wolves today."
The genetic variant spread so rapidly in dogs because people evidently favoured smallness. Small dogs were more easily managed and they were useful in capturing small game and vermin. In later centuries, breeds of toy dogs were bred specifically for the purposes of keeping a cute pet.
"Everybody treats their dogs like their babies, so it's not surprising they would select tiny dogs," said Kevin Chase, a biologist at the University of Utah.
"Tiny dogs are not particularly functional. They don't hunt with you. They don't protect your house. They don't pull carts.
"They're just small and sweet as a pet."
- INDEPENDENT
