Erik Sperling

Making heads or tails of lowly worm

YALE (US)—A group of researchers is using microRNA genes to untangle the history of the large—and still largely misunderstood—group of segmented worms known as annelids, which evolved millions of years ago and can be found in every corner of the world.

“These genes are excellent evolutionary markers,” says lead author Erik Sperling, a graduate student in Yale University’s geology and geophysics department. “Once a microRNA gene is fixed in a species, it is very rarely lost. As such, organisms with similar microRNAs are closely related to one another.”

Annelids include earthworms, leeches, and bristle worms.

MicroRNAs are small, non-coding genes that have long been known to play an important role in developmental biology but which have never before been used to study the evolutionary relationships between organisms.

Building on previous work done at Dartmouth College, which demonstrated the potential of using microRNAs to decipher evolutionary history, the team applied a form of high-throughput sequencing technology at the Yale Center for Genomics and Proteomics that uses a novel strategy to reveal the microRNA complement of an organism.

They discovered that annelids represent a unique evolutionary branch separate from other organisms like mollusks and peanut worms. The research also showed that the ancestral annelid more closely resembled a kind of bristle worm that lived on the seafloor, as opposed to the classic belief that it was a kind of burrowing worm that lived in the ocean mud.

The team’s microRNA sequencing results also agree with the order in which the different annelids and their relatives appear in the fossil record—something that previous hypotheses about their relationships had failed to do, says coauthor Derek Briggs, Yale’s Frederick William Beinecke Professor of Geology and Geophysics.

“This study is an elegant example of how new methods can reconcile results from molecular sequencing of living animals with information from the fossil record,” Briggs explains.

Researchers from Dartmouth College, North Carolina State University, and the University of Lyon contributed to the paper, which was published online Sept. 9 in the journal Proceedings of the Royal Society B.

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