Injection of sperm extract mimics spatiotemporal dynamics of Ca2+ responses and progression of meiosis at fertilization of ascidian oocytes

Sperm extract (SE) of the ascidian, Ciona savignyi, injected into oocytes i nduced repetitive intracellular Ca2+ increases with kinetics consistent wit h those at fertilization and caused reinitiation and progression of meiosis as in fertilized oocytes with the formation of polar bodies. The Ca2+ resp onse comprised two sets of Ca2+ oscillations separated by 5 minutes and cor related with the first and second meiotic metaphase, The effects of SE were dose dependent and the critical dose corresponded roughly to a single sper matozoon. In the first Ca2+ transient observed by confocal microscopy, a Ca 2+ wave started from the SE injection site at the peripheral region of the oocyte and propagated across the ooplasm, The similar wave was produced by injection at the central region, starting from an arbitrary cortical area a fter 30 seconds, probably after SE had diffused to the cortex. The sensitiv ity to SE is thought to be preferentially higher in the cortex. The effecti ve component of SE was heat-unstable, and its molecular weight was estimate d as in the range between 10x10(4) and 3x10(4) using membrane filters. Thes e results suggest that, in ascidian fertilization, a cytosolic sperm protei n factor is introduced to the oocyte cortex and induces Ca2+ waves and ther eby meiotic resumption, leading to cell-cycle-correlated Ca2+ oscillations.