We love those tiny lapdogs, but how did they get so small, anyway?

Soon after humans began domesticating dogs 12,000 to 15,000 years ago, they started breeding them small. Now scientists have identified a piece of doggy DNA that reduces the activity of a growth gene, ensuring that small breeds stay small. It’s next to a gene known as the IGF1 gene. Medium and large dogs also have the IGF1 gene, but they do not have the same piece of DNA next to it, so their size is not restricted by that DNA.

Biologist Gordon Lark says, “Dogs have the biggest range of sizes of any mammal in existence. One of the big questions has always been, where does this range of sizes come from? By studying the Portuguese water dog, which has three-fold range of sizes?from 25 pounds to 75 pounds?we realized that IGF1 was a big player.” The IGF1 gene’s hormone helps humans and other mammals grow from birth to adolescence. But in small dogs, one or more mutations in the DNA next to the IGF1 gene suppress the gene?s activity, keeping small dogs from growing larger.

Lark and co-researcher Kevin Chase traveled to dog shows across the country to collect blood or cells. Their analyses confirmed that the location of genetic instructions for keeping dogs small was near the IGF1 gene. Lark says, “All dogs under 20 pounds have this?all of them. That’s extraordinary.” Oddly, so do Rottweilers, which are large. But even though they have the small-dog haplotype, other yet-unidentified genetic factors make them big, according to Chase.

Lark thinks that the genetic instructions to make dogs small probably arose because “a small wolf couldn’t survive in nature, but it could survive in company with humans.” This “unnatural selection” led to global proliferation of small dogs.

“Everybody treats their dogs like their babies, so it’s not surprising they would select for tiny dogs,” says Chase, who owns a pair of 4-pound, toy poodle-Maltese mixes. “Tiny dogs are not particularly functional…They’re just small and sweet.”

The two researchers discovered that the genetic code to make small dogs “was readily spread over a large geographic area by trade and human migration.” Or, as Lark puts it, “A local sailor came in with a small dog and everyone said, ‘Ooh, I want a dog like that! Can I use him to father my next dog?'” Meanwhile, other people bred dogs to be large for use as guard dogs, hunting dogs, and war dogs (like mastiffs) and even to pull carts.

Because small dog breeds are only distantly related to each other, the researchers believe the gene variation for small dogs probably first appeared early in the history of dogs. As a result of selective breeding of dogs by humans over many generations, dogs today exhibit the greatest diversity in body size of any mammal.

Art credit: freeimages.co.uk

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