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

Abstract

To reveal impacts of sexual mode on genome content, we compared chromosome-scale assemblies of the outcrossing nematode <i>Caenorhabditis nigoni</i> to its self-fertile sibling species, <i>C. briggsae</i><i>C. nigoni</i>'s genome resembles that of outcrossing relatives but encodes 31% more protein-coding genes than <i>C. briggsae</i><i>C. nigoni</i> genes lacking <i>C. briggsae</i> orthologs were disproportionately small and male-biased in expression. These include the <i>male secreted short</i> (<i>mss</i>) gene family, which encodes sperm surface glycoproteins conserved only in outcrossing species. Sperm from <i>mss</i>-null males of outcrossing <i>C. remanei</i> failed to compete with wild-type sperm, despite normal fertility in noncompetitive mating. Restoring <i>mss</i> to <i>C. briggsae</i> males was sufficient to enhance sperm competitiveness. Thus, sex has a pervasive influence on genome content that can be used to identify sperm competition factors.