Regulation of Oocyte Maturation and Ovulation

Abstract

Before fertilization, the C. elegans oocyte undergoes maturation (exit from meiosis I prophase) and ovulation (expulsion from the gonad). We have defined the landmark events of oocyte development, maturation, and ovulation by time-lapse microscopy. The first indication of maturation is nuclear envelope breakdown, beginning 6 minutes before ovulation. Ovulation requires contraction of the myoepithelial sheath and dilation of the distal spermatheca for the oocyte to leave the gonad arm and enter the spermatheca where fertilization occurs. Sheath activity is cyclical, rising in rate and intensity with each ovulation. Laser ablation of either the proximal sheath or distal spermathecal cells results in sterility with mature oocytes trapped in the gonad (the Emo phenotype). Mutant analysis provides insight into the regulation of oocyte maturation and ovulation. First, in female mutants, oocytes arrest in diakinesis, failing to undergo meiotic maturation. Oocyte maturation resumes when sperm are introduced by mating. Sperm therefore appear to promote meiotic maturation. Second, sperm stimulate sheath contractile activity. In females and tumorous germline mutants, sheath contractile activity is low. Introduction of sperm induces contractions. In mutants where only sperm are present in the germ line, sheath contractions are observed at a constitutive high level without cyclic change or spermathecal dilation, suggesting that maturing oocytes are needed to trigger the cyclical pattern of activity at ovulation. Third, in the mutants lin-3(n1058), and let-23(sy10), the distal spermatheca fails to dilate on time or open fully; ovulation is unsuccessful resulting in the Emo sterile phenotype. Data will be presented indicating that LIN-3 is required continuously in the adult germ line for normal ovulation, while LET-23 appears to function in the spermathecal/sheath cells. Our results suggest that in the minutes following oocyte maturation, LIN-3 in the oocyte signals LET-23 in the spermathecal/sheath cells to induce ovulation. This work was supported by NSF Grant 9506220.