c-Mos forces the mitotic cell cycle to undergo meiosis II to produce haploid gametes

The meiotic cycle reduces ploidy through two consecutive M phases, meiosis I and meiosis II, without an intervening S phase. To maintain ploidy throug h successive generations, meiosis must be followed by mitosis after the rec overy of diploidy by fertilization. However, the coordination from meiotic: to mitotic cycle is still unclear. Mos, the c-mos protooncogene product, i s a key regulator of meiosis in vertebrates. In contrast to the previous ob servation that Mos functions only in vertebrate oocytes that arrest at meio tic metaphase II, here we isolate the first invertebrate mos from starfish and show that Mos functions also in starfish oocytes that arrest after the completion of meiosis II but not at metaphase II. In the absence of Mos, me iosis I is followed directly by repeated embryonic mitotic cycles, and its reinstatement restores meiosis II and subsequent cell cycle arrest. These o bservations imply that after meiosis I, oocytes have a competence to progre ss through the embryonic mitotic cycle, but that Mos diverts the cell cycle to execute meiosis II and remains to restrain the return to the mitotic cy cle. We propose that a role of Mos that is conserved in invertebrate and Ve rtebrate oocytes is not to support metaphase II arrest but to prevent the m eiotic/mitotic conversion after meiosis I until fertilization, directing me iosis II to ensure the reduction of ploidy.