Soluble sperm extract specifically recapitulates the initial phase of the Ca²⁺ response in the fertilized oocyte of P. occelata following a G-protein/PLCβ signaling pathway

Matured oocytes of the annelidan worm Pseudopotamilla occelata are fertilized at the first metaphase of the meiotic division. During the activation by fertilizing spermatozoa, the mature oocyte shows a two-step intracellular Ca²⁺ increase. Whereas the first Ca²⁺ increase is localized and appears to utilize the inositol 1,4,5-trisphosphate (IP3)-sensitive Ca²⁺ stores, the second Ca²⁺ increase is global and involves Ca²⁺ influx via voltage-gated Ca²⁺ channels on the entire surface of the oocyte. To study how sperm trigger the Ca²⁺ increases during fertilization, we prepared soluble sperm extract (SE) and examined its ability to induce Ca²⁺ increases in the oocyte. The SE could evoke a Ca²⁺ increase in the oocyte when it was added to the medium, but not when it was delivered by microinjection. However, the second-step Ca²⁺ increase leading to the resumption of meiosis did not follow in these eggs. Local application of SE induced a non-propagating Ca²⁺ increase and formed a cytoplasmic protrusion that was similar to that created by the fertilizing sperm at the first stage of the Ca²⁺ response, important for sperm incorporation into the oocyte. Our results suggest that the fertilizing spermatozoon may trigger the first-step Ca²⁺ increase before it fuses with the oocyte in a pathway that involves the G-protein-coupled receptor and phospholipase C. Thus, the first phase of the Ca²⁺ response in the fertilized egg of this species is independent of the second phase of the Ca²⁺ increase for egg activation.