What do Chihuahuas, terriers and Pekingese dogs have in common, apart from their diminutive size? The answer is a mutation in a single gene.
This mutation, which lies in one of the body鈥檚 master growth-regulator genes, is so widespread that it must have been present early in the history of dogs鈥� domestication.
Elaine Ostrander at the National Human Genome Research Institute in Bethesda, Maryland, US, and colleagues surveyed genetic variants in a small region of chromosome 15 in Portuguese water dogs, a breed with an unusually broad size range. They found that smaller water dogs tended to carry one particular variant of the gene for Insulin-like growth factor 1 (IGF1), an important growth regulator. Dogs carrying the variant, or allele, for small size also had less IGF1 in their bloodstream.
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When the researchers then turned to other breeds, they found the same variant in every small breed they looked at, but hardly ever in very large breeds such as St. Bernards, Irish wolfhounds, and great Danes.
鈥淚GF1 functions a lot like the 鈥渞educe鈥� button on a Xerox machine, where things can get increased or reduced proportionately,鈥� says Ostrander.
Constellation of factors
However, many other genes must also affect size. The small allele is also common in a few large dog breeds such as mastiffs and especially rottweilers. 鈥淭he fact that you can see it in the mastiff or the rottweiler means it鈥檚 not the whole story,鈥� says Ostrander. 鈥淚t can be tolerated in large dogs as long as there is a constellation of other factors that overwhelm it.鈥�
Since dogs as large as great Danes and as small as terriers occur in the archaeological record more than 10,000 years ago, the small-dog variant of IGF1 must be at least that old, the researchers note.
However, the variant is not known in wolves, dogs鈥� closest wild relatives. 鈥淓ither man selected a small wolf, which doesn鈥檛 exist now, or they found a small variant of dog that everybody liked,鈥� says Gordon Lark, a geneticist at the University of Utah in Salt Lake City, US, who was involved in the study.
Apart from the light it sheds on the evolutionary history of dogs, Ostrander鈥檚 study also represents a big first step in understanding how a suite of genes interact to produce a complex trait such as body size. 鈥淲e鈥檙e really very clueless about how complex traits work,鈥� says John Fondon III at the University of Texas Southwestern Medical Center in Dallas, US, who was not involved in the study.
The multitude of dog breeds, with their vast range of body sizes and shapes, offer an ideal test bed for addressing this problem, he says.
Journal reference: Science (vol 316, p 112)
