MAPK INACTIVATION IS REQUIRED FOR THE G(2) TO M-PHASE TRANSITION OF THE FIRST MITOTIC CELL-CYCLE

Down-regulation of MAP kinase (MAPK) is a universal consequence of fer tilization in the animal kingdom, although its role is not known, Here we show that MAPK inactivation is essential for embryos, both vertebr ate and invertebrate, to enter first mitosis, Suppressing down-regulat ion of MAPK at fertilization, for example by constitutively activating the upstream MAPK cascade, specifically suppresses cyclin B-cdc2 kina se activation and its consequence, entry into first mitosis, It thus a ppears that MAPK functions in meiotic maturation by preventing unferti lized eggs from proceeding into parthenogenetic development. The most general effect of artificially maintaining MAPK activity after fertili zation is prevention of the G(2) to M-phase transition in the first mi totic cell cycle, even though inappropriate reactivation of MAPK after fertilization may lead to metaphase arrest in vertebrates, Advancing the time of MAPK inactivation in fertilized eggs does not, however, sp eed up their entry into first mitosis. Thus, sustained activity of MAP K during part of the first mitotic cell cycle is not responsible for l ate entry of fertilized eggs into first mitosis.