AN ULTRASTRUCTURAL ANALYSIS OF EARLY FERTILIZATION EVENTS IN THE GIANT SCALLOP, PLACOPECTEN-MAGELLANICUS (MOLLUSCA, PELECYPODA)

We examined the ultrastuctural events occurring during early fertiliza tion in the giant scallop, Placopecten magellanicus. The unfertilized oocytes had not completed their meiotic maturation and were blocked at the metaphase I stage. At fertilization, there was a local loss of mi crovilli, and a fertilization cone of 6 mu m in length by 4 mu m in di ameter was produced. As the spermatozoon penetrated the oocyte, its he ad rotated by 90 to 180 degrees. The condensed electron-opaque state o f the sperm chromatin transformed into a dispersed form of male pronuc leus; this event was coupled with the beginning of the formation of th e pronuclear envelope and the development of the sperm aster. The male pronucleus was completely formed 60 min postfertilization, before com pletion of meiosis, and spots of electron-dense chromatin still remain ed. During sperm nuclear reorganization, meiotic maturation was resume d. At anaphase I, 30 min after fertilization, the dyad chromosomes mov ed into a cytoplasmic protrusion that became the first polar body. The chromatin located within this protrusion was condensed in a compact e lectron-dense mass. A gap of 0.2 mu m separated the first polar body f rom the zygote. At anaphase II, 60 min after fertilization, a second p rotrusion located just below the first polar body developed into the s econd polar body. Unlike the first polar body, the second polar body h ad a flat shape, remained closely associated with the oocyte surface, and the chromatin located within it dispersed. Nearly 90 min after fer tilization, the pronuclei completed their migration toward each other and their pronuclear envelopes became into close apposition without in terdigitating. At this stage, there was a simultaneous condensation of the chromosomes and a breakdown of the pronuclear envelopes of both p ronuclei. Intact sperm mitochondria still persisted 90 min after ferti lization. The chromosomes attached to microtubules and became aligned on the metaphase plate of the first mitotic spindle. Hence, for this s pecies, no zygotic nucleus is formed. This type of pronuclear associat ion is similar to the Ascaris-type of fertilization.