Evolution in the fast lane. For want of repeating DNA, the Great Pyrenees gains an extra toe.

Why a Pug is Not a Collie

Liz is a staff writer for Science.

Most dogs have five digits, but Great Pyrenees have six. The reason, according to a new study: a gene involved in foot development in this breed is missing 51 bases. That wayward DNA consists of a pattern of three bases repeated over and over again. The gain and loss of these so-called tandem repeats may help explain why dogs have evolved so quickly, researchers now say.

There are more than 100 dog breeds, most of them dating back little more than 1000 years. They all belong to the same species, despite contrasting looks and behavior. Typically researchers expect such differences to arise from mutations. But modern dogs have changed much faster than the pace at which mutations accumulate.

Physicist Harold Garner and evolutionary biologist John Fondon III from the University of Texas Southwestern Medical Center in Dallas decided to look into tandem repeats as an alternative. Tandem repeat alterations pop up more frequently than mutations because they arise from a sequencing stutter: Enzymes copying repetitive regions of DNA sometimes lose track of where they are, occasionally leaving a few repeats out of the new copy--or adding one or two extra.

The researchers sequenced the repetitive regions in 17 equivalent developmental genes from 92 dog breeds. They also measured the skulls with a laser scanner, and correlated those differences with variations in repeat number.

Several patterns emerged. Toe number was affected by a gene called Alx-4. Snout lengths correlated quite well with the number of repeats in another gene, Runx-2: Breeds with collie-like noses had more of a particular tandem repeat, while those with pug-like faces had more of a different tandem. And when the researchers compared bull terrier DNA, they found that terriers have one more repeat unit than they did in the 1950s, which could explain why the nose used to be droopier, the researchers note. Repeat numbers didn't vary as much in wolves, however, they report online this week in the Proceedings of the National Academy of Sciences.

Breeding can skew "evolution" because breeders select for just a few traits without regard to what their canines need to survive in harsher environments. Thus, in wolves, fewer changes wrought by tandem repeats are likely to hold up. Even so, the findings are still quite appealing, says Robert Wayne from the University of California, Los Angeles. "Tandem repeats, generally regarded as junk DNA, offer a novel mechanism for evolutionary change."

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Posted in Evolution