504
than eightfold during 48 h) (Puschendorf et al. 2006 ). Analysis of the 3′UTR
composition of maternal transcripts showed that transcripts with higher turnover tend
to contain U-rich motifs in their 3′UTRs (Svoboda, unpublished observation). However,
the molecular mechanism associated with U-rich motifs remains unknown. There are
many mechanisms that could cause selective transcript destabilization during Phase 0
such as RNA interference (RNAi, reviewed in Svoboda 2014 ), Staufen-mediated
mRNA decay (reviewed in Park and Maquat 2013 ), or AU-rich element-mediated
mRNA destabilization (summarized in Barreau et al. 2005 ). Interestingly, unstable
transcripts were also derived from retrotransposons suggesting that RNAi, which is
highly active in mouse oocytes (Flemr et al. 2013 ; Tam et al. 2008 ; Watanabe et al.
2008 ), might be one of the molecular mechanisms operating during Phase 0.
The functional significance (if any) of Phase 0 of mRNA degradation is unclear.
The abovementioned countdown aspect of Phase 0 is not associated with the main-
tenance of developmental competence. The evidence comes from artificial exten-
sion of Phase 0. Since high cAMP levels prevent resumption of meiosis (Eppig et al.
1983 ; Schorderet-Slatkine et al. 1982 ), inhibitors of phosphodiesterase III allow to
reversibly prevent resumption of meiosis in oocytes. Mammalian oocytes cultured
in the presence of such inhibitors for 18–24 h apparently retain developmental com-
petence (Naruse et al. 2012 ; Somfai et al. 2003 ). However, Phase 0 could be associ-
ated with establishment of developmental competence at the end of the growth
phase, when the oocyte undergoes nuclear and cytoplasmic changes manifested as
termination of transcription and chromatin remodeling of the non-surrounded
nucleolus (NSN into the surrounded nucleolus (SN) chromatin configuration. NSN
nuclei show rather diffuse DNA staining with several bright foci (chromocenters)
containing constitutive heterochromatin, while SN nuclei are characterized by a
bright ring surrounding the nucleolus. Nuclear transplantation experiments suggest
that both nuclear and cytoplasmic factors (but not chromatin remodeling itself)
determine the meiotic and developmental competence of fully grown SN oocytes in
antral follicles (Inoue et al. 2008 ). Cytoplasmic factors contributing to the acquisi-
tion of developmental competence during the NSN/SN transition are poorly under-
stood. Deep sequencing of NSN and SN oocyte transcriptomes revealed differences
in several metabolic pathways, and more than 200 transcripts reduced more than
twofold during the NSN/SN transition (Ma et al. 2013b). At the same time, there is
a minimal overlap between downregulated transcripts identified in two independent
studies (Puschendorf et al. 2006 ; Ma et al. 2013b), which precludes conclusions
about transcripts degraded during Phase 0. In any case, the idea that RNA decay
during Phase 0 might be contributing to NSN/SN transition and acquisition of
developmental competence is worthy of further investigation.
10.2.2.3 mRNA Degradation Induced During Meiotic Maturation
The first major wave of maternal mRNA degradation appears during meiotic matu-
ration. In mammals, 25–50 % reduction in poly(A) RNA has been observed during
meiotic maturation (Lequarre et al. 2004 ; Piko and Clegg 1982 ; Bachvarova et al.
P. Svoboda et al.