Studies on fertilization in the teleost IV. Effects of aphidicolin and camptothecin on chromosome formation in fertilized medaka eggs.

To clarify the mechanisms of fish fertilization, the effects of inhibitors of DNA polymerase-alpha and DNA topoisomerases on nuclear behavior before and after fertilization were examined in eggs of the medaka, Oryzias latipes. Eggs underwent the fertilization process from sperm penetration to karyogamy of pronuclei, even when inseminated and incubated in the continuous presence of aphidicolin (DNA polymerase alpha inhibitor), camptothecin (DNA topoisomerase I inhibitor), etoposide, or beta-lapachone (DNA topoisomerase II inhibitor). However, continuous treatment with aphidicolin or camptothecin during fertilization inhibited the formation of sister chromosomes that were normally separated into blastomeres at the time of the subsequent cleavage. Sister chromosome formation appeared concomitantly with an increase in histone H1 kinase activity at the end of DNA synthesis, 30 min post insemination. However, non-activated eggs that were inseminated in saline containing anesthetic MS222 and aphidicolin had high levels of histone H1 kinase and MAP kinase activities, and transformation of the penetrated sperm nucleus to metaphase chromosomes occurred even in the presence of aphidicolin or camptothecin. The male chromosomes were normally separated into two anaphase chromosome masses upon egg activation. These results suggest that DNA polymerase alpha or DNA topoisomerase I, but not DNA topoisomerase II, may be required for the process by which the mitotic interphase nucleus transforms to separable metaphase chromosomes while the activity of MAP kinase is low, unlike the situation in meiotic division, during which MAP kinase activity is high and DNA replication is not required.