Gibson the Great Dane - the world's tallest dog according to the Guinness Book of World Records - stands 1.07 m from floor to shoulder. Gibson plays with his friend, Zoie, a 19 cm chihuahua.
Credit: Deanne Fitzmaurice
SYDNEY: Much of the amazing variation in size of different breeds of domestic dogs stems from tiny differences in a single gene, say researchers.
Tracking down this mechanism may one day help pave the way for genetic testing of canines, helping dog breeders and owners determine the eventual size of their dogs before they are even born. Similar tests might even reveal traits like temperament, coat colour, and predisposition to genetic diseases.
Dogs have the greatest variation of any mammal in existence, according to the authors of the study, which appears today in the U.S. journal Science. But "one of the biggest questions has always been: where does this range of sizes come from?" said co-author K. Gordon Lark of the University of Utah in Salt Lake City.
Dogs (Canis familiaris) evolved from wolves, and were domesticated by humans between 12,000 and 15,000 years ago. Since then, the dog has been bred to vary in a wide range of traits, small size notable among them.
Curious about the genetic source of this variation, the team compared DNA samples from hundreds of Portuguese water dogs - a breed with a large natural variation in size. The comparisons showed them that changes in one small region of the genome were strongly linked to the variation.
But to pinpoint the exact genetic change that makes some dogs big and some dogs small, the team - made up of 21 scientists led by geneticists from the National Human Genome Research Institute in Bethesda, Maryland - compared that stretch of DNA in samples they took from 526 dogs representing 23 small and 20 "giant" breeds.
Changes, or mutations, in DNA can alter the function of the proteins it encodes, or even prevent them from being produced at all.
When the team compared the dog DNA samples, they found that all the small breeds shared several mutations in a small part of the DNA of one gene which regulates the production of insulin-like growth factor (IGF1) - a protein key to promoting growth.
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Portuguese water dog, Pipi
Image: Ian Grey
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"In the big dogs, the [level] of insulin like growth factor is larger than in the small dogs," said Alan Wilton, an expert in dog genetics at Sydney’s University of New South Wales, who was not involved in the study.
Exactly which of the small series of mutations restricts production of IGF1 remains unclear. According to the researchers, though, the result is that dogs with the mutations stay small. "All dogs under nine kilograms have this – all of them," said Lark. "That’s extraordinary."
In a strange twist, Rottweilers, a very large breed of dog, also share this series of mutations. According to the researchers, this indicates that IGF1 is not the only gene that influences size. "It’s certainly not the only gene that’s involved in size, but it’s obviously a major gene that’s responsible for a lot of the difference in size between big breeds and small breeds," commented Wilton.
the headline is wrong--mutation is not identified in this study
the authors have not found the mutation or causal genetic alteration that leads to altered IGF1 function. I really don't understand how they got the manuscript accepted at this stage.