Regulation of the sperm-to-oocyte transition in Caenorhabditis briggsae hermaphrodites by the Cbr-met-2 and Cbr-fem-3 genes

Mol Reprod Dev. 2018 Jun;85(6):532-542. doi: 10.1002/mrd.22991. Epub 2018 May 23.


In Caenorhabditis briggsae hermaphrodites, spermatogenesis begins in the L4 larval stage and persists into early adulthood. Oogenesis begins after spermatogenesis; the sperm-to-oocyte transition is irreversible. The timing of this transition is believed to have evolved in response to selection to maximize the intrinsic growth rate. Sperm-to-oocyte transitions occurred early in Cbr-met-2 and Cbr-fem-3 mutants. These early transitions resulted in reduced brood sizes, but had little or no impact on the intrinsic growth rate. In Cbr-met-2; Cbr-fem-3 doubly mutant hermaphrodites, the transition to oogenesis occurred even earlier and brood size was further reduced, indicating that Cbr-met-2 and Cbr-fem-3 regulate the sperm-to-oocyte transition through separate pathways. Mutations in Cbr-met-2 also resulted in an increase in the frequency of males in mutant populations. These increased male frequencies were not caused by increased rates of X nondisjunction during oogenesis in mutant hermaphrodites. Rather, increases in the rates of outcrossing in mutant populations likely were an indirect effect of reduced brood sizes derived from self-fertilization. Based on these observations, it is possible that the timing of the sperm-to-oocyte transition in C. briggsae evolved in response to sexual selection on hermaphrodites to limit rates of outcrossing. Mutations in the orthologous Caenorhabditis elegans gene, Cel-met-2, did not impact the timing of the sperm-to-oocyte transition, consistent with the independent evolution of hermaphroditic reproduction in these species. Although brood sizes were reduced in Cel-met-2 mutant strains, increased male frequencies were not observed. Cbr- and Cel-met-2 mutations also differed in terms of germline mortality, observed in C. elegans, but not in C. briggsae.

Keywords: histone methyltransferase; oogenesis; reproduction; spermatogenesis.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Animals
  • Caenorhabditis / cytology
  • Caenorhabditis / genetics
  • Caenorhabditis / metabolism*
  • Helminth Proteins / genetics
  • Helminth Proteins / metabolism*
  • Hermaphroditic Organisms / cytology
  • Hermaphroditic Organisms / genetics
  • Hermaphroditic Organisms / metabolism*
  • Histone-Lysine N-Methyltransferase / genetics
  • Histone-Lysine N-Methyltransferase / metabolism
  • Male
  • Nuclear Proteins / genetics
  • Nuclear Proteins / metabolism
  • Oocytes / cytology
  • Oocytes / metabolism*
  • Oogenesis / physiology*
  • Spermatogenesis / physiology*
  • Spermatozoa / cytology
  • Spermatozoa / metabolism*


  • Helminth Proteins
  • Nuclear Proteins
  • Histone-Lysine N-Methyltransferase