Role of the cytoskeleton in sperm entry during fertilization in the freshwater bivalve Dreissena polymorpha

The present study examined the role of the cytoskeleton in sperm entry and migration through the egg cytoplasm during fertilization in the zebra musse l, Dreissena polymorpha (Bivalvia: Veneroida: Dreissenidae). Fertilization in this freshwater bivalve occurs outside the mantle cavity, permitting det ailed observations of fertilization. After its initial binding to the egg s urface, the sperm is incorporated in two stages: (1) a gradual incorporatio n of the sperm nucleus into the egg cortex, followed by (2) a more rapid in corporation of the sperm axoneme, and translocation of the sperm head throu gh the egg cytoplasm. Initial incorporation into the egg cortex was shown t o be microfilament dependent. Microfilaments were found in the sperm's pref ormed acrosomal filament, the microvilli on the egg surface, and in an acti n-filled insemination cone surrounding the incorporating sperm. Treatment o f eggs with cytochalasin B inhibited sperm entry in a dose- and time-depend ent manner. Microtubule polymerization was not necessary for initial sperm entry. Following incorporation of the sperm head, the flagellar axoneme entered th e egg cytoplasm and remained active for several minutes. Associated with th e incorporated axoneme was a flow of cytoplasmic particles originating near the proximal end of the flagella. Inhibition of microtubule polymerization prevented entry of the sperm axoneme, and the subsequent cytoplasmic curre nt was not observed. After sperm incorporation into the egg cortex, no appr eciable microfilaments were associated with the sperm nucleus. A diminutive sperm aster was associated with the sperm nucleus during its decondensatio n, but no obvious extension toward the female pronucleus was observed. The sperm aster was significantly smaller than the spindle associated with the female pronucleus, suggesting a reduced role for the sperm aster in amphimi xis.