Polyspermy block of Spisula eggs is prevented by cytochalasin B

Area of Science:

• Reproductive biology
• Cellular and molecular biology
• Marine invertebrate embryology

Background:

• Polyspermy, the fertilization of an egg by multiple sperm, is typically prevented by a block to further sperm entry.
• The surf clam (Spisula solidissima) is a model organism for studying fertilization mechanisms.

Purpose of the Study:

• To investigate the timing and location of the block to polyspermy in Spisula solidissima eggs.
• To determine the role of the egg's outer coat and plasma membrane in preventing polyspermy.

Main Methods:

• Fertilization assays were performed on Spisula solidissima eggs.
• The vitelline layer was experimentally removed from eggs.
• The effect of cytochalasin B on sperm receptivity was assessed.

Main Results:

• Spisula solidissima eggs exhibited a rapid decrease in sperm receptivity, becoming resistant to fertilization within 15 seconds.
• Removal of the vitelline layer did not alter the timing or effectiveness of the polyspermy block.
• The observed changes in sperm receptivity were sensitive to low concentrations of cytochalasin B, indicating a plasma membrane event.

Conclusions:

• The primary block to polyspermy in Spisula solidissima is established at the plasma membrane, not by the vitelline layer.
• The fertilization process involves rapid alterations in the egg plasma membrane surface.
• Cytochalasin B sensitivity suggests cytoskeletal involvement in the post-fertilization block to polyspermy.