title: Mos limits the number of meiotic divisions in urochordate eggs.
abstract: Mos kinase is a universal mediator of oocyte meiotic maturation and is produced during oogenesis and destroyed after fertilization. The hallmark of maternal meiosis is that two successive M phases (meiosis I and II) drive two rounds of asymmetric cell division (ACD). However, how the egg limits the number of meioses to just two, thereby preventing gross aneuploidy, is poorly characterized. Here, in urochordate eggs, we show that loss of Mos/MAPK activity is necessary to prevent entry into meiosis III. Remarkably, maintaining the Mos/MAPK pathway active after fertilization at near physiological levels induces additional rounds of meiotic M phase (meiosis III, IV and V). During these additional rounds of meiosis, the spindle is positioned asymmetrically resulting in further rounds of ACD. In addition, inhibiting meiotic exit with Mos prevents pronuclear formation, cyclin A accumulation and maintains sperm-triggered Ca(2+) oscillations, all of which are hallmarks of the meiotic cell cycle in ascidians. It will be interesting to determine whether Mos availability in mammals can also control the number of meioses as it does in the urochordates. Our results demonstrate the power of urochordate eggs as a model to dissect the egg-to-embryo transition.
authors.affiliation: Developmental Biology Unit UMR 7009, University Marie and Pierre Curie, Univ. Paris 06, Paris, France. remi.dumollard@obs-vlfr.fr
authors.forename: Rémi
authors.initials: R
authors.lastname: Dumollard
authors.forename: Mark
authors.initials: M
authors.lastname: Levasseur
authors.forename: Céline
authors.initials: C
authors.lastname: Hebras
authors.forename: Philippe
authors.initials: P
authors.lastname: Huitorel
authors.forename: Michael
authors.initials: M
authors.lastname: Carroll
authors.forename: Jean-Philippe
authors.initials: JP
authors.lastname: Chambon
authors.forename: Alex
authors.initials: A
authors.lastname: McDougall
mesh_terms: D000818:Animals; D002455:Cell Division; D002938:Ciona intestinalis; D004625:Embryo, Nonmammalian; D020935:MAP Kinase Signaling System; D008540:Meiosis; D010063:Ovum / Q000166:cytology*; D017231:Proto-Oncogene Proteins c-mos / Q000502:physiology*; D014561:Urochordata / Q000166:cytology* / Q000196:embryology; D015053:Zygote
chemical_list: D017231:Proto-Oncogene Proteins c-mos