Rapid genome shrinkage in a self-fertile nematode reveals sperm competition proteins

Examining the consequences of selfing The Caenorhabditis genus of nematodes includes a mix of closely related outcrossing and self-fertilizing (selfing) species. Genome size differs widely among these different species. Yin et al. generated a genome assembly for the outcrossing nematode C. nigoni and compared it with that of its close relative, the selfing C. briggsae. C. briggsae has experienced a substantial decrease in genome size since the two species' recent divergence. The underlying causes of this size difference appear to involve a decrease in protein-coding genes and changes in other types of sequences that have homology with RNAs expressed primarily in C. nigoni males. One of the implicated gene families, the mss family, compromises sperm competitiveness. Thus, in nematodes, selfing appears to result in a decrease in genome size owing to selection to reduce male reproductive function. Science , this issue p. 55