Reinitiation of meiosis in polychaete (annelida) oocytes

Abstract

This thesis presents ultrastructural and biochemical information on meiotic reinitiation during oocyte maturation in the polychaetes, Arenicola marina, A. defodiens and Nereis virens. The ultrastructural changes during meiotic maturation was characterised in the oocytes of Arenicola marina and Nereis virens using transmission electron microscopy in addition to germinal vesicle breakdown, release of the prophase I block was signified by major cortical changes in both species. The ultrastructure of fertilization in A. marina was independent of whether the oocytes were matured in vivo and spawned or matured in vitro by CMF. Oocyte maturation in Arenicola marina is controlled by a hormonal cascade that is initiated by the prostomial maturation hormone, PMH, and followed by the coelomic maturation factor, CMF (Watson and Bentley, 1997). Results presented here demonstrated that PMH has a molecular mass greater than 10 kDa, yet how this molecule triggers CMF activity remains unknown. M-phase promoting factor (MPF) consists of two subunits, cdkl and cyclin B, and is responsible for the control of mitosis and meiosis. The cytoplasmic "second messenger" that transduces the hormone signal to the activation of MPF in the oocyte cytoplasm was investigated in the two Arenicola species and is discussed. MPF regulation was investigated in Arenicola marina and Nereis virens oocytes. MPF activation was driven by the dephosphorylation of cdkl and phosphorylation of cyclin B. The results indicate that as with all other higher eukaryotes, the precursor of MPF in A. marina oocytes was maintained inactive by the phosphorylation of threonine 14 and tyrosine 15 (or equivalent residues) on the cdkl subunit. In contrast to other organisms, however, only a fraction of the cdkl present was complexed to cyclin B and utilised during meiotic reinitiation. All the cdkl in N. virens oocytes was joined with cyclin B but results suggest that the inactive complex contained tyrosine-only phosphorylated cdk1.