500
2007 ; Houwing et al. 2008 ). piRNAs are the most abundant small RNA class in
zebrafish oocytes (Yao et al. 2014 ; Houwing et al. 2007 ). High piRNA abundance
was also observed in Xenopus oocytes (Armisen et al. 2009 ). In mammals, piRNAs
are present in fully grown oocytes; however, their abundance is much lower than
that observed in zebrafish or Xenopus models (Ohnishi et al. 2010 ; Garcia-Lopez
et al. 2014 ; Yang et al. 2012 ).
Regarding the clearance of maternal piRNAs, piRNA levels already gradually
decrease during Xenopus oocyte growth although they remain abundant at the time
of ovulation (Armisen et al. 2009 ). Their decay subsequently continues during early
development (Armisen et al. 2009 ; Harding et al. 2014 ). Fertilization clearly accel-
erates piRNA decay. Deep sequencing of developing zebrafish embryos revealed
that the population of maternal piRNAs was reduced by about a half within several
hours after fertilization and gradually disappeared within 2 days (Yao et al. 2014 ).
Postfertilization clearance of maternal piRNAs was also observed in mouse zygotes
(Ohnishi et al. 2010 ; Garcia-Lopez et al. 2014 ). The mechanism of maternal piRNA
clearance is unknown; maternal piRNAs may be targeted for degradation directly,
or their clearance may reflect turnover of maternal PIWI proteins.
10.2.1.4 Maternal Small RNAs and Their Clearance: General Summary
Taken together, the composition of the maternal small RNA population varies among the
vertebrate models. In general, miRNAs appear to be a relatively underrepresented small
RNA class in vertebrate oocytes. The same appears true for endo- siRNAs, with the nota-
ble exception of mouse oocytes. piRNAs are highly abundant in zebrafish and Xenopus
oocytes, while their levels are relatively low in mammalian oocytes. Deep sequencing of
porcine oocytes suggests that piRNAs and endo- siRNAs might be even less abundant
than miRNAs (Yang et al. 2012 ). Clearance of maternal small RNAs in vertebrates starts
during oocyte growth when piRNA levels begin to decline. piRNA clearance is subse-
quently accelerated upon fertilization. Meiotic maturation seems to initiate miRNA deg-
radation, which is also accelerated after fertilization. Thus, the general picture of
vertebrate small RNA turnover during OET implies that maternal small RNAs of all
types are eliminated after fertilization and become replaced by zygotic microRNAs,
which are the dominant small RNA class in the developing embryo.
10.2.2 Degradation of Maternal mRNAs
Regulation of mRNA stability and translation is controlled by cis- and trans-acting
factors. Eukaryotic mRNAs are protected by a 5′ cap structure and a 3′ poly(A) tail
that control translation and mRNA stability. mRNA degradation mechanisms
(reviewed in detail in Houseley and Tollervey 2009 ; Schoenberg and Maquat 2012 ;
Balagopal et al. 2012 ) typically involve deadenylation of the 3′ poly(A) tail and/or
decapping. In mammalian somatic cells, deadenylation coupled with decapping is
P. Svoboda et al.