Mechanisms for blocking polyspermy in oocytes of the oyster Crassostrea gigas

We showed here the mechanisms to black polyspermy in the oocyte of the oyst er Crassostrea gigas. When the fertilized monospermic oocytes were insemina ted again with high sperm-oocyte ratio 15 min after the initial inseminatio n, sperm could not penetrate into the oocytes; the oocytes were still monos permic. Insemination in low-sodium artificial seawater induced polyspermy, suggesting the occurrence of a fast polyspermy block caused by depolarizati on of the plasma membrane at fertilization. Requirement of external sodium ion for blocking polyspermy continued for 15 min after fertilization. Relea se of proteases from oocytes at fertilization was not observed. Electron mi croscopic observations showed that re-inseminated sperm could undergo acros ome reaction and bind to the fertilized oocytes, but acrosomal process did not fuse with the oocyte plasma membrane. These results suggest that blocka ge of contact or fusion of the plasma membrane between sperm and oocyte act s as a slow polyspermy block in Crassostrea. (C) 1999 Wiley-Liss, Inc.