MAGNESIUM REQUIREMENT IN FERTILIZATION PROCESS OF SEA URCHINS

The Mg2+ requirement in fertilization was investigated in sea urchins. It was found that when sea urchin eggs were inseminated in sea water free of Mg2+ , little fertilization took place. Even when spermatozoa pre-treated with dissolved egg-jelly to induce the acrosome reaction, which needs Ca2+ , were used, the fertilization rate remained quite low in the absence of Mg2+ . In Strongylo-centrotus intermedius, the lowest concentration of Mg2+ required for 50% fertilization was 0.05 mM in the presence of 10 mM Ca2+ , whereas that of calcium was 3 mM in the presence of 49 mM Mg2+ . These critical concentrations increased when the concentration of the other ion decreased. Removal of Mg2+ or Ca2+ or both from the suspending medium had little adverse effect on sperm motility. The elevation of the fertilization membrane was also induced by butyric acid independent of the presence or absence of Mg2+ and/or Ca2+ . These results indicate that Mg2+ are required at least in some process(es) between acrosome reaction and fertilization membrane elevation, such as sperm penetration or membrane fusion.