Studies on fertilization of the teleost. II. Nuclear behavior and changes in histone H1 kinase

In order to understand the dynamic responses of gamete nuclei upon fertiliz ation in the fish, Oryzias latipes, the relationship between changes in the activity of histone H1 kinase and nuclear behavior was examined during fer tilization. Kinase activity rapidly decreased concomitant with the initiati on of the propagative exocytosis of cortical alveoli following sperm attach ment to the egg plasma membrane post-insemination (PI). Activity again incr eased 30min PI. Similar changes in kinase activity, migration and syngamy o f pronuclei, and subsequent cleavage were observed with aphidicolin or acti nomycin D treatment, except that formation of abnormal metaphase chromosome s was retarded in aphidicolin-treated zygotes. Pretreatment of unfertilized eggs with cycloheximide or 6-dimethylaminopurine (6-DMAP) caused no nuclea r changes. The activity of histone H1 kinase in these eggs rapidly declined following sperm penetration and exocytosis, but did not undergo subsequent increase in the presence of these inhibitors. In these eggs with low histo ne H1 kinase activity, the fertilization process from sperm penetration to syngamy occurred normally, but the pronuclear membrane did not break down a nd the chromosomes did not condense. The present data suggest that in fish eggs, DNA replication as well as the synthesis and phosphorylation of prote ins, especially cyclin B, are required for normal formation of metaphase ch romosomes at the first cleavage, but not for fertilization events from sper m penetration through to nuclear migration resulting in syngamy.