ANALYSIS OF FERTILIZATION AND POLYSPERMY IN SEROTONIN-SPAWNED EGGS OFTHE ZEBRA MUSSEL, DREISSENA-POLYMORPHA

Eggs isolated from animals spawned with 10(-3) M serotonin were insemi nated with sperm concentrations ranging from 10(3)-10(6) sperm/ml. Mul tiple sperm attached to the surface of the egg and sperm incorporation occurred within 3 min postinsemination (PI). Sperm mitochondria, cent rioles, and flagellum were also incorporated. Incorporation was essent ially complete by 6 min PI. In the egg cortex, the sperm head rotated 180 degrees, and a rapid translocation of the sperm through the cytopl asm towards the egg interior began by 5-6 min PI. In heavily polysperm ic inseminations, translocations of the sperm were either minimal or n onexistent. In monospermic eggs, nuclear decondensation occurred after translocation was complete, beginning by 9-10 min PI. A male pronucle us began to develop in the cytoplasm by 21 min PI and enlarged to 20 m u m before fusing with the female pronucleus. Oscillation of the egg c ytoplasm and mitotic spindle apparatus was observed immediately prior to cleavage. Cleavage occurred at 60 min PI. Sperm incorporation and p ronuclear formation were confirmed with fluorescent and confocal micro scopy using the DNA-specific dyes Hoescht 33342 and 7-aminoactinomycin D. In sperm concentrations >10(4) sperm/ml, 26-76% of the eggs exhibi ted polyspermy. The high polyspermy suggests that rapid, effective pol yspermy were not present or were ineffective in a significant proporti on of serotonin-spawned eggs. (C) 1996 Wiley-Liss, Inc.