The genomes of most animals are cluttered with mobile bits of DNA, called retrotransposons, whose origins are a mystery. Now a report in the September Nature Genetics suggests that retrotransposons can colonize and expand a genome surprisingly quickly. In just 3 million years, the study suggests, retrotransposons added enough copies of themselves to corn DNA to double the size of the plant's genome.
Over the last several years, researchers have come to realize that retrotransposons play an important role in evolution, expanding the sizes of genomes and, depending on where they insert, altering gene activity and function. "But before, we've never had a good feeling for how quickly and to what extent they can reorganize the genome," notes Dan Voytas, a molecular biologist at Iowa State University in Ames.
To begin to find this out, Jeffrey Bennetzen of Purdue University in West Lafayette, Indiana, and his colleagues analyzed the ends of 17 retrotransposons found in a region that they had sequenced a few years ago. Those ends, which can be 150 to 5000 bases long, are identical when the retrotransposon first sneaks into the genome. From that moment on, random mutations make them less and less alike. Bennetzan's team first determined that about 6.5 mutations occur annually per billion bases in this part of the corn genome. Based on the number of changes to the ends of the retrotransposons, they calculated that the retrotransposons entered the genome between 3 and 6 million years ago and over the ensuing millennia expanded that gene region from 80,000 bases to 240,000 bases, and the corn genome from 1.2 billion to 2.4 billion bases.
"We're surprised [the invasion] was so recent," says Bennetzen. "We would have guessed [the expansion] would have been more gradual, over tens of millions of years." Although they have no way of knowing what corn was like before this invasion, these retrotransposons paved the way for all sorts of new traits to evolve, he adds.