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.
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.