Worm Species Lost 7,000 Genes After Evolving to Fertilize Itself; Genes for Sperm Competition Proteins Among Those Lost

Reproduction in most animal species requires breeding between two individuals. But some worms have evolved the ability to go it alone. In these species, a single individual can breed with itself to produce offspring. A new University of Maryland (UMD)-led study found that gaining this ability, known as "selfing," may have caused a worm species to lose a quarter of its genome, including genes that give male sperm a competitive edge during mating. "Our results suggest that genes that are essential for tens of millions of years can suddenly become useless or liabilities, even, when the sex system changes," said Eric Haag, PhD, a Professor of Biology at UMD and lead investigator of the study, which was published in the January 5, 2018 issue of Science. The article is titled “Rapid Genome Shrinkage in a Self-Fertile Nematode Reveals Sperm Competition Proteins.” 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. Dr. Haag's group, which focuses on the evolution of sex, has long studied C. briggsae because of their unusual reproductive behavior. To study how selfing shaped the evolution of C. briggsae, Erich Schwarz, PhD, an Assistant Research Professor of Molecular Biology and Genetics at Cornell University and co-corresponding 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.
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