A DOG鈥橲 nose can tell us a lot about evolution. Whether a breed has the scrunched-up nose of a boxer or the long snout of a greyhound depends on whether it inherits certain small stretches of repeating DNA sequences. The surprise finding that such easily achieved genetic changes can produce obvious new traits hints that this may be one way in which rapid evolution takes place.
Within the coding regions of genes, a particular three-letter sequence of DNA bases can be repeated many times. These repeats direct the production of long strings of the same amino acid within a protein. But when DNA is copied prior to cell division, the copying enzymes sometimes incorporate more or fewer of these repeats than they should. Such errors can alter the function of proteins in which they occur, and they can happen up to 100,000 times as often as single-base mutations, which are thought to be the usual drivers of evolution.
To see whether variations in repeats have been important in evolution, John Fondon and Harold Garner of the University of Texas Southwestern Medical Center in Dallas sequenced a total of 37 repeated regions from 17 developmental genes of 92 breeds of dogs. The length of the repeats varied across almost all genes and dog breeds.
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One such gene called Runx-2 plays a role in determining head shape. Long-nosed breeds such as greyhounds had repeats within the gene that code for an unusually high number of copies of the amino acid glutamine, followed by unusually few copies of the amino acid alanine. Pug-nosed breeds such as boxers had the reverse (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.0408118101). The changes alter the activity of the gene鈥檚 protein product, which controls the differentiation of bone-producing cells. 鈥淲e think this is the factor primarily responsible for the length of a dog鈥檚 nose,鈥� Garner says.
鈥淲e are not claiming that we understand all of rapid evolution, but we think this plays a major role that is largely underappreciated,鈥� he adds. Most obviously, it could explain how dogs evolved their many shapes and sizes within just a few thousand years.
