Investigations on the Activation of Strongylocentrotus Purpuratus Eggs by Homologous Sperm (or, Why Do the Male Gametes Lose it so Quickly?)
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Although egg activation and early fertilization events have been extensively observed in sea urchin Strongylocentrotus purpuratus, actual mechanisms involved in activation remain elusive. Diverse experiments were undertaken to clarify the trigger(s) involved. As one preliminary test, the naked egg was probed with various plant lectins in hopes of locating and identifying an hypothesized egg plasma membrane receptor for sperm. Theoretically, lectin binding might have directly activated the egg through a G-protein intermediate, or it could have blocked sperm access to the receptor. None of the lectins used either initiated activation parthenogenically or inhibited subsequent fertilization by sperm. These results do not imply that there are no sperm receptors on the egg cell membrane, but suggest that if they do exist, that the simple probes used in this study were not adequate to locate them. The next set of experiments focused on the other partner in fertilization; the sperm. Once acrosome-reacted, S. purpuratus sperm quickly lose viability, and by asking why, we hoped to find clues as to how activation occurs. Viability of acrosome-reacted sperm were assayed by percent of eggs fertilized and experiments were performed under several circumstances. To test one hypothesis that heavy calcium influx "poisons" sperm, reacted sperm were incubated in calcium-free seawater. Sperm viability over time was not enhanced and we conclude that calcium influx does not result in sperm degradation. We removed the vitelline layer from eggs and assayed fertilization to determine if the sperm lost an enzyme or lysin necessary for penetration of egg cell layers. Acrosome-reacted sperm were clearly able to fertilize these eggs for a longer period of time indicating that some substance necessary for vitelline layer penetration is lost after induction of the acrosome reaction. It was also observed that sperm binding to eggs was negatively affected with time after induction of the acrosome reaction, although sperm motility remained unaffected. These final results imply that degradation of the sperm protein bind in may be another factor contributing to the time-dependent loss of sperm viability.