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 through successive generations, meiosis must be followed by mitosis after the recovery of diploidy by fertilization. However, the coordination from meiotic to mitotic cycle is still unclear. Mos, the c-mos protooncogene product, is a key regulator of meiosis in vertebrates. In contrast to the previous observation that Mos functions only in vertebrate oocytes that arrest at meiotic 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, meiosis I is followed directly by repeated embryonic mitotic cycles, and its reinstatement restores meiosis II and subsequent cell cycle arrest. These observations imply that after meiosis I, oocytes have a competence to progress 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 cycle. We propose that a role of Mos that is conserved in invertebrate and vertebrate oocytes is not to support metaphase II arrest but to prevent the meiotic/mitotic conversion after meiosis I until fertilization, directing meiosis II to ensure the reduction of ploidy.

Detection of c-mos related products in the dogfish (Scyliorhinus canicula) testis.

The objective of the present paper was to do a comparative study to assess somatic versus germ cell localization of c-mos products in the testis. In mouse and amphibian oocytes, c-mos activity is necessary for meiotic maturation. Lack of c-mos expression has been reported in somatic cells of male and female gonads while transcripts have been found in germ cells of testis and ovary. Using a v-mos probe, we report here the detection of a c-mos related transcript (1.7 kb) in the dogfish Scyliorhinus canicula testis. Western blot analysis detects two proteins of 106 and 32 kDa. A specific immunostaining was exclusively localized in the interstitial tissue while the germinal compartment was completely negative. In conclusion, our results indicate for the first time the presence of c-mos products in an elasmobranch species and, moreover, their presence in somatic testicular cells rather than germ cells. Therefore, this finding in an ancient vertebrate indicates that c-mos activity does not have a direct universal role in the regulation of spermatogenesis.