Spatiotemporal characteristics and mechanisms of intracellular Ca2+ increases at fertilization in eggs of jellyfish (Phylum Cnidaria, Class Hydrozoa)

We have clarified, for the first time, the spatiotemporal patterns of intracellular Ca2+ increases at fertilization and the Ca2+-mobilizing mechanisms in eggs of hydrozoan jellyfish, which belong to the evolutionarily old diploblastic phylum, Cnidaria. An initial Ca2+ increase just after fertilization took the form of a Ca2+ wave starting from one cortical region of the egg and propagating to its antipode in all of four hydrozoan species tested: Cytaeis uchidae, Cladonema pacificum, Clytia sp., and Gonionema vertens. The initiation site of the Ca2+ wave was restricted to the animal pole, which is known to be the only area of sperm-egg fusion in hydrozoan eggs, and the wave propagating velocity was estimated to be 4.2-5.9 μm/s. After a Ca2+ peak had been attained by the initial Ca2+ wave, the elevated Ca2+ gradually declined and returned nearly to the resting value at 7-10 min following fertilization. Injection of inositol 1,4,5-trisphosphate (IP3), an agonist of IP3 receptors (IP3R), was highly effective in inducing a Ca2+ increase in unfertilized eggs; IP3 at a final intracellular concentration of 12-60 nM produced a fully propagating Ca2+ wave equivalent to that observed at fertilization. In contrast, a higher concentration of cyclic ADP-ribose (cADPR), an agonist of ryanodine receptors (RyR), only generated a localized Ca2+ increase that did not propagate in the egg. In addition, caffeine, another stimulator of RyR, was completely without effect. Sperm-induced Ca2+ increases in Gonionema eggs were severely affected by preinjection of heparin, an inhibitor of Ca2+ release from IP3R. These results strongly suggest that there is a well-developed IP3R-, but not RyR-mediated Ca2+ release mechanism in hydrozoan eggs and that the former system primarily functions at fertilization. Our present data also demonstrate that the spatial characteristics and mechanisms of Ca2+ increases at fertilization in hydrozoan eggs resemble those reported in higher triploblastic animals.