Mos in the Oocyte: How to Use MAPK Independently of Growth Factors and Transcription to Control Meiotic Divisions

Figure 1

Progression through meiosis and timing of fertilization: what does Mos do? Oocytes are arrested at prophase of the first meiotic division (prophase I). Under response to a physiological stimulus, MPF is activated and promotes breakdown of the nuclear envelope (GVBD for germinal vesicle breakdown) and formation of the metaphase I spindle. In insects, molluscs, and ascidians, oocytes arrest at metaphase I until fertilization. In the other cases, oocytes extrude the first polar body and enter the second meiotic division. In vertebrates, they arrest at metaphase II until fertilization. In echinoderms and cnidarians, they complete the second meiotic division by emitting the second polar boy, reform a nucleus (female pronucleus), and stop at the G1 phase of the first cell cycle until fertilization. In different species including the nematode Caenorhabditis elegans, fertilization occurs at prophase I and corresponds to the stimulus promoting meiotic maturation. Mos has been implicated: (i) in the initial step of MPF activation during reinitiation of meiotic division, (ii) during the metaphase I to metaphase II transition for the suppression of S-phase, for the microtubular spindle organization and for the reactivation of MPF to enter meiosis II, and (iii) in the arrest of oocyte maturation before fertilization.