August 6, 2005

Researchers Probe how an Ancient Microbe Thrives and Evolves without Sex

WOODS HOLE, MA - A January 2004 finding by biologists at the Josephine Bay Paul Center for Comparative Molecular Biology and Evolution added important evidence to the radical conclusion that a group of diminutive aquatic animals called bdelloid rotifers have evolved for tens of millions of years without sexual reproduction, in apparent violation of the rule that abandonment of sexual reproduction is a biological dead end. Now, MBL scientists are beginning to understand just what's different about these creatures' DNA that has enabled them to succeed where other asexual species have failed.

In a paper published in this week's Proceedings of the National Academy of Sciences (PNAS), MBL scientists Irina R. Arkhipova and Matthew Meselson provide evidence that suggests bdelloid rotifers--which probably gave up sex at least 50 million years ago but have still evolved into 370 species--handle DNA transposons more efficiently than other asexual species. Transposons are small snippets of "junk DNA" that sexual reproduction compensates for, but which often go unchecked and are believed to contribute to mutation (and eventually extinction) in species that reproduce asexually.

To learn more about the bdelloid rotifers' unique ability to evolve without sex, Arkhipova and Meselson studied portions of different bdelloid rotifer genomes and surveyed the diversity, structural organization, and patterns of evolution of DNA transposons.

The scientists found that DNA transposons in bdelloid rotifers are in a different, perhaps less damaging, location than those found in other creatures. Many bdelloid DNA transposons have the same surrounding sequences, which may indicate preferences for specific locations. Indeed, many of them appear to be located at the tip of the chromosome in an area called the telomere, different from the gene-rich portions of the genome, whereas most species tend to have DNA transposons dispersed throughout their genome.

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