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 with a large nucleus (=germinal vesicle, or GV) were removed from gravid ovaries of Cerebratulus lacteus and Micrura alaskensis. Following transfer to natural seawater (NSW), fully grown oocytes spontaneously matured as indicated by their completion of germinal vesicle breakdown (GVBD), whereas GVBD was reversibly blocked if the oocytes were initially placed in calcium-free seawater (CaFSW). Similarly, calcium ionophore treatments triggered GVBD in calcium-containing artificial seawater (ASW) but not in CaFSW, suggesting that external calcium influx may facilitate maturation. However, compared to the overall levels of maturation elicited by ASW, significantly higher percentages of GVBD were achieved with NSW or with ASW that had been conditioned with marine sediment. Moreover, calcium channel blockers decreased GVBD rates in ASW but not in NSW, which is consistent with the view that substances other than external calcium ions can trigger maturation. Accordingly, oocytes 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 presence of calcium channel blockers or the calcium chelator BAPTA. In addition, oocytes microinjected with fluorescent calcium indicators underwent GVBD in response to 5-HT without displaying marked calcium transients during confocal imaging runs. Collectively, such findings suggest that nemertean oocytes can mature via multiple pathways that may include external calcium influx or a 5-HT-induced signaling cascade that lacks prominent calcium fluctuations. J. Exp. Zool. 287:243-261, 2000. Copyright 2000 Wiley-Liss, Inc.