Ins and outs of meiosis in ascidians.

Ascidian oocytes are blocked in metaphase (M) of the first meiotic division. Fertilization triggers the completion of meiosis without any further arrest. In this review, we have analyzed the mechanisms that regulate the progression through meiosis in these oocytes. A primary signal from the fertilizing spermatozoon, probably soluble sperm factor(s), induces intracellular calcium release by activating the IP3 and CICR pathways and gates the fertilization current by triggering the generation of ADP ribose (ADPr). The calcium oscillations are not required for the inactivation of MPF observed at M-I release; however, ADPr may be indirectly involved in the activity of MPF associated kinase, Cdc2. MPF activity reaches a second peak at M-II followed by subsequent inactivation. Progression to M-II is dependent on the intracellular calcium oscillations. MAP kinase (MAPK) activity decreases at M-I exit and remains low during the completion of meiosis. Finally, although Cdc2, Cyclin B and MAPK-like proteins have been identified in ascidian oocytes, components of CSF still remain to be identified.