ROLE OF CALCIUM DURING RELEASE OF MOLLUSK OOCYTES FROM THEIR BLOCKS IN MEIOTIC PROPHASE AND METAPHASE

Bivalve oocytes constitute an excellent model to study the mechanisms which control cell division, since they arrest at different stages of the meiotic process and respond quite synchronously to various externa l signals regulating their evolution. Different ent situations were ob served, according to the species, which required a more careful analys is. For example, the same serotonin (5-HT) signal was found to drive p rophase-arrested oocytes of Spisula to the female pronucleus stage, wh ereas Ruditapes oocytes stopped in metaphase I. In this last species, 5-HT treatment thus represents a necessary prerequisite for further de velopment since sperm cannot not penetrate prophase-arrested oocytes, in contrast to the situation found in Spisula. The reasons for these d ifferences cannot be found at the level of the receptors and second me ssengers. In both species, we found that Ca2+ was involved as a second messenger and required to release oocytes from their blocks in propha se and metaphase I. This is an unexpected and puzzling situation since MPF is activated by the prophase Ca2+ surge and inactivated during me taphase-anaphase transition. The mechanisms underlying these paradoxic al situations are discussed, taking into account results from previous observations and experiments. Briefly, our data suggest: i) that the absence of K+ sensitive voltage-gated Ca2+ channels may be responsible for the unfertilizability of prophase-arrested arrested Ruditapes ooc ytes; and ii) that functional variations in the network of activated k inases present in prophase and metaphase oocytes may be responsible fo r the divergent responses exhibited by Spisula and Ruditapes oocytes. In particular, these oocytes differ in their sensitivity to Ca2+/calmo dulin antagonists and in tyrosine phosphorylation of their activated p 34cdc2 kinase.