Multiple triggers of oocyte maturation in nemertean worms: The roles of calcium and serotonin

To analyze the process of oocyte maturation in nemertean worms, oocytes wit h a large nucleus (=germinal vesicle, or GV) were removed from gravid ovari es of Cerebratulus lacteus and Micrura alaskensis. Following transfer to na tural seawater (NSW), fully grown oocytes spontaneously matured as indicate d by their completion of germinal vesicle breakdown (GVBD), whereas GVBD wa s reversibly blocked if the oocytes were initially placed in calcium-free s eawater (CaFSW). Similarly, calcium ionophore treatments triggered GVBD in calcium-containing artificial seawater (ASW) but not in CaFSW, suggesting t hat external calcium influx may facilitate maturation. However, compared to the overall levels of maturation elicited by ASW, significantly higher per centages of GVBD were achieved with NSW or with ASW that had been condition ed with marine sediment. Moreover, calcium channel blockers decreased GVBD rates in ASW but not in NSW, which is consistent with the view that substan ces other than external calcium ions can trigger maturation. Accordingly, o ocytes underwent equally high levels of GVBD when treated with serotonin (= 5-hydroxytryptamine, or 5-HT) in ASW or CaFSW. The 5-HT-induced maturation was blocked by inhibitors of 5-HT receptors but continued to occur in the p resence of calcium channel blockers or the calcium chelator BAPTA. In addit ion, oocytes microinjected with fluorescent calcium indicators underwent GV BD in response to 5-HT without displaying marked calcium transients during confocal imaging runs. Collectively, such findings suggest that nemertean o ocytes can mature via multiple pathways that may include external calcium i nflux or a 5-HT-induced signaling cascade that lacks prominent calcium fluc tuations. J. Exp. Zool. 287:243-261, 2000. (C) 2000 Wiley-Liss, Inc.