The Drosophila MOS Ortholog Is Not Essential for Meiosis

Abstract

In metazoan oocytes, a metaphase arrest coordinates the completion of meiosis with fertilization. Vertebrate mos maintains the metaphase II arrest of mature oocytes and prevents DNA replication between the meiotic divisions. We identified a Drosophila homolog of mos and showed it to be the mos ortholog by two additional criteria. The dmos transcripts are present in Drosophila oocytes but not embryos, and injection of dmos into Xenopus embryos blocks mitosis and elevates active MAPK levels. In Drosophila, MAPK is activated in oocytes, consistent with a role in meiosis. We generated deletions of dmos and found that, as in vertebrates, dmos is responsible for the majority of MAPK activation. Unexpectedly, the oocytes that do mature complete meiosis normally and produce fertilized embryos that develop, although there is a reduction in female fertility and loss of some oocytes by apoptosis. Therefore, Drosophila contains a mos ortholog that activates a MAPK cascade during oogenesis and is nonessential for meiosis. This could be because there are redundant pathways regulating meiosis, because residual, low levels of active MAPK are sufficient, or because active MAPK is dispensable for meiosis in Drosophila. These results highlight the complexity of meiotic regulation that evolved to ensure accurate control over the reproductive process.

Drosophila Myt1 Is the Major Cdk1 Inhibitory Kinase for Wing Imaginal Disc Development

“…Further evidence of a role for Myt1 in G2/M regulation comes from studies of oocyte maturation in frogs, starfish, and nematodes (Palmer et al 1998;Okumura et al 2002;Peter et al 2002;Burrows et al 2006). Not all data indicate that Myt1 is required for G2 phase arrest, however, and there is no evidence that dMyt1 regulates oocyte maturation in Drosophila (Ivanovska et al 2004;Jin et al 2005). Nor is there evidence that dMyt1 activity is responsible for the timing of the G2/M meiotic transition that follows a prolonged 4-day-long G2 phase arrest, in Drosophila primary spermatocytes (D. Guha Majumdar, unpublished results).…”

“…Whether Myt1 has a role in Drosophila oocyte maturation remains unclear (Ivanovska et al 2004), however, Drosophila myt1 mutants exhibit pleiotropic cell-cycle defects during male and female gametogenesis, which suggest that dMyt1 has a role in developmentally regulated G2 phase arrest and in cell-cycle exit mechanisms that are normally coupled with terminal differentiation (Jin et al 2005).…”

Modulation of MAPK Activities During Egg Activation in Drosophila

“…20 Although Drosophila Mos is present in oocytes prior to ovulation, it is not necessary for cell cycle changes at egg activation, since Dmos null mutants complete meiosis normally despite decreased levels of ERK and MEK in mutant oocytes. 21 However, this finding does not eliminate the possibility of a role for modulation of MAPK activities during egg activation in Drosophila, given the numerous phenomena that occur during this time. Two Drosophila proteins that are established or potential MAPK targets, YA and GNU, become dephosphorylated during activation.…”

Transitioning from egg to embryo: Triggers and mechanisms of egg activation

“…In addition, maintenance of the meiotic arrest requires tension created on the kinetochores, suggesting participation of the spindle checkpoint that indirectly regulates Cdc2 (reviewed by Taylor et al, 2004). However, maintenance of the meiotic arrest differs in at least one respect from the situation in vertebrates: while Mos is present and active in mature oocytes of Drosophila, it is not required for the maintenance of the metaphase I arrest, because this arrest is normal in dmos null mutant oocytes (Ivanovska et al, 2004).…”