Repetitive Intracellular Ca2+ Increases at Fertilization and the Role of Ca2+ in Meiosis Reinitiation from the First Metaphase in Oocytes of Marine Bivalves

Abstract

Spawned oocytes of marine bivalves Limaria hakodatensis, Mytilus edulis, Crassostrea gigas, and Hiatella flaccida are arrested at the first metaphase (metaphase-I) until fertilization. We have measured changes in intracellular Ca2+ ([Ca2+]i) at fertilization in the single oocytes of these bivalves using the fluorescent Ca2+ indicator fura-2. Shortly after insemination, these oocytes displayed a transient [Ca2+]i increase which was usually followed by a period during which [Ca2+]i was kept higher than the resting level (elevated [Ca2+]i period). During this period, [Ca2+]i showed oscillatory increases superimposed on an elevated [Ca2+]i level in Limaria, Crassostrea, and Hiatella, whereas a sustained elevation without pulses occurred in Mytilus. After [Ca2+]i returned to the resting level, repetitive transient [Ca2+]i increases appeared in Limaria, Mytilus, and Hiatella. The [Ca2+]i increases still occurred following external Ca2+ removal shortly after fertilization in all four bivalve species. In contrast, external Ca2+ removal immediately abolished a [Ca2+]i increase induced by excess-K+ seawater in Mytilus. Using another fluorescent Ca2+ indicator, calcium green, we found that during the first transient in Mytilus, [Ca2+]i increased uniformly over the whole oocyte. These results strongly suggest that the fashion of [Ca2+]i increases at fertilization in bivalve oocytes fertilized at metaphase-I differs not only from that in deuterostomes but also from that in protostomes oocytes of which are fertilized at the first prophase.