The vast scope of the human genome project may seem overwhelming, but compared to some other creatures, we really don't have all that much DNA. Even the lowly, single-celled amoeba has 100 times more than people do--and ferns have even more. The wide range of genome sizes has puzzled researchers for decades. Now a study suggests that part of the answer may be that some organisms are more efficient at weeding useless DNA out of their genome.
More DNA doesn't necessarily mean more genes. In some cases, organisms have just collected lengths of repetitive, useless DNA. This so-called "junk DNA" accumulates from occasional mistakes during cell division, when DNA is shuffled and copied into nonsensical sequences. If the garbled DNA doesn't hurt the organism, it just lingers in the genetic code. Although some is shed as more mutations occur, junk DNA tends to accumulate.
Several years ago, evolutionary biologist Dmitri Petrov of Harvard University discovered that the fruit fly Drosophila, which has a relatively small genome, discards junk DNA at a surprisingly high rate. Perhaps, Petrov surmised, creatures with smaller genomes toss out junk DNA faster than those burdened with large genomes. To find out, he and his colleagues compared two organisms with genomes of very different sizes: the fruit fly and the Hawaiian cricket, which has a genome 11 times larger. They examined stretches of junk DNA between known genetic landmarks and found that deletions in fruit fly DNA were on average four times longer than similar deletions in cricket DNA, the researchers report in the 11 February Science.
Petrov says there might be other factors that contribute to hefty genomes, but the rate of DNA loss seems to be important. "This doesn't solve the puzzle, but it's a good step," he says. Pierre Capy, a geneticist at CNRS in Gif-sur-Yvette, France, says he is impressed by the team's work, but cautions that to prove the theory, the work needs to be extended outside of the insect world.