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Sperm nuclear expansion, meiotic maturation of the maternal chromatin, and events involving the association of the male and female pronuclei leading to the two-cell stage were observed in Mulinia zygotes using the fluorochromes DAPI and Hoechst. The effects of ultraviolet irradiation on the fertilizing sperm were also examined. Incorporated sperm nuclei underwent changes in diameter that were temporally correlated with meiotic processes of the maternal chromatin. Following its entry, the sperm nucleus underwent a rapid, initial enlargement, which was correlated with germinal vesicle breakdown. Sperm nuclear expansion ceased during the period in which the egg was engaged in polar body formation and was re-initiated with formation and enlargement of the female pronucleus. The rates of enlargement of the male and female pronuclei were 0.59 and 0.65 μm/min, respectively. Following their migration into apposition with one another, the male and female pronuclei synchronously underwent events characteristic of prophase as separate structures; i.e., chromosome condensation, and nuclear envelope breakdown. The two groups of chromosomes that formed became organized on the metaphase plate in preparation of the first cleavage division; hence, there was no fusion of pronuclei. Ultraviolet irradiation of fertilizing sperm had no apparent affect on sperm nuclear transformations leading to the development of a male pronucleus or on female pronuclear development. However, events subsequent to the apposition of the pronuclei were affected and included asynchrony of prophase and the nondisjunction of chromosomes at anaphase. These observations are discussed in relationship to events regulating transformations of the sperm nucleus and experiments to generate gynogenetic bivalve embryos.

Expansion of the Sperm Nucleus and Association of the Maternal and Paternal Genomes in Fertilized Mulinia lateralis Eggs