Selfing in worms causes genome loss

Print edition : February 02, 2018

Two C. briggsae worms (the smaller worm is male, the larger worm is a hermaphrodite). A study has found that the worm lost a quarter of its DNA when it evolved to self-fertilize. Photo: Da Yin/University of Maryland via The New York Times

Reproduction in most animal species requires breeding between two individuals. But some worms have evolved the ability to go it alone.

Now, a study led by a biologist at the University of Maryland (UMD) has found that gaining this ability, known as “selfing”, may have cost a worm species a loss of a quarter of its original genome.

“Our results suggest that genes that are essential for tens of millions of years can suddenly become useless or liabilities, when the sex system changes,” said Eric Haag, a biologist at the UMD and lead author of the study, published in the journal Science on January 5.

A million years ago, a species of tiny worms called Caenorhabditis briggsae evolved the ability to breed via selfing. As a result, most C.briggsae are hermaphrodites with both male and female sex organs. Haag’s group, which focuses on the evolution of sex, has long studied C.briggsae because of their unusual reproductive behaviour.

To study how selfing shaped the evolution of C.briggsae, Erich Schwarz, a molecular biologist and geneticist at Cornell University and co-author of the study, sequenced the genome of Caenorhabditis nigoni, the closest relative of C. briggsae. C.nigoni always reproduce by mating with other individuals, or outcrossing. By comparing the genomes of the two species, the researchers found that the selfing C.briggsae worms had 7,000 fewer genes than C.nigoni. Over time, C.briggsae lost approximately a quarter of its genome.

Because the two worms differ primarily in their method of reproduction, the researchers hypothesised that the shift from outcrossing to selfing had led to the gene loss. To confirm this, they compared gene activity in C.nigoni males and females and found that almost three-quarters of the genes that C.briggsae lost were more active in C.nigoni males than females.

Seeking possible sex-related functions for the lost genes, the researchers focused on a family of “male secreted short” (mss) genes that C.nigoni had but C. briggsae did not have. In fact, no known selfing Caenorhabditis species have mss genes. And mss genes are only active in the male worms of outcrossing species, the scientists had shown in a previous study.

T.V. Jayan

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