507over a half of mRNAs degraded after fertilization show behavior consistent with
polyadenylation-accelerated mRNA decay (Svoboda et al., unpublished). This
suggests a potential role for noncanonical poly(A) polymerases (reviewed in
Martin and Keller 2007 ) in maternal mRNA degradation. In fact, robust expres-
sion of some of the poly(A) polymerases occurs in mouse oocytes (Su et al. 2002 ).
It is off-note that degradation of miRNAs is also triggered by fertilization, while
the noncanonical poly(A) polymerase Wispy was implicated in maternal miRNA
adenylation and degradation in Drosophila (Lee et al. 2014 ).
10.2.2.5 mRNA Degradation Induced by ZGA
Mechanistically, mRNA degradation induced by ZGA fits the classical model of
mRNA degradation during developmental transitions where a trans-acting regula-
tory factor is produced through transcriptional activation in the later developmental
state (Alonso 2012 ). A well-studied example of a zygotic trans-acting factor induc-
ing degradation of maternal mRNAs after fertilization is microRNA (miRNA)-
mediated mRNA degradation in zebrafish embryos where miRNAs of the miR-430
family become strongly upregulated a few hours after fertilization and target mater-
nal mRNAs. The zygotic miRNA-mediated maternal mRNA degradation is rather
extensive—it was estimated that the miR-430 family may target up to 40 % of
maternal mRNAs (Giraldez et al. 2006 ).
A similar role was proposed for miR-427, a Xenopus orthologue of miR-430
(Lund et al. 2009 ). After fertilization, miR-427 is synthesized as multimeric pri-
mary transcripts, and the mature miR-427 accumulates to a high level (~10^9 mole-
cules) by the midblastula transition when the zygotic mRNA expression is initiated.
Presence of miR-427 cognate sequences in selected maternal mRNAs was shown to
be necessary and sufficient for their deadenylation. Conversely, inactivation of miR-
427 resulted in stabilization of those mRNAs (Lund et al. 2009 ).
The two examples above come from models in which development is very fast
and zygotic miRNAs appear when maternal mRNAs are still abundant (Fig. 10.3).
Fast development likely elicits higher pressure on removing maternal mRNAs (so
that their products cannot interfere with differentiation). In this sense, zygotic miR-
NAs offer a simple solution.
Mammals have orthologues of the miR-430 family, such as the miR-290 family
(reviewed in Svoboda and Flemr 2010 ). Transcription of the murine miR-290 family
initiates during ZGA at the 2-cell stage, but accumulation of mature miRNAs is first
observed from the 4-cell stage to the blastocyst (Tang et al. 2007 ; Zeng and Schultz
2005 ). Thus, the miR-290 family has a minimal if any role in maternal mRNA deg-
radation since the majority of maternal mRNAs is eliminated before miR-290 fam-
ily miRNAs reach functionally relevant levels. Furthermore, as mentioned above
(Sect. 10.2.1.1), genetic studies and functional analysis of the miRNA pathway in
the mouse OET model showed that miRNA activity is globally suppressed during
and nonessential for the entire OET (Suh et al. 2010 ; Ma et al. 2010 ). At this point,
there is no evidence that the global miRNA suppression and functional irrelevance
10 Clearance of Parental Products