495Fig. 10.5 RNA degradation during mammalian OET. Total RNA and poly(A) RNA changes
during OET in mice (left graph) and cows (right graph). Graphs are based on published RNA
analyses (Gilbert et al. 2009 ; Piko and Clegg 1982 ; Bachvarova and De Leon 1980 )
10.2 Maternal RNA Clearance
This section will review the fate of maternal RNA with a particular focus on degra-
dation of three specific types of RNAs: small RNAs, mRNAs, and ribosomal RNAs.
As mentioned above, vertebrate oocytes differ in the amount of deposited RNA that
will support the newly developing organism. Analyses of mammalian (mouse, cow,
rabbit, buffalo) and nonmammalian models (zebrafish, Xenopus, chicken) suggest
two distinct scenarios of maternal RNA clearance in vertebrate early development.
In mammals, total RNA is severely reduced (by 50 % or more) between oocyte’s
release from the ovary and the full zygotic genome activation in the cleaving embryo
(Fig. 10.5). This indicates that dependence on maternal RNAs in mammals does not
extend far into development. At the same time, it should be kept in mind that early
mammalian development is relatively slow; hence, maternal RNA degradation per
unit of time is perhaps not necessarily faster than in zebrafish or Xenopus whose
embryonic development runs at much faster pace (Fig. 10.3). Analysis of total RNA
content during the first 12 h of zebrafish development (unfertilized egg to 8-somite
stage) showed that RNA yield does not significantly differ among stages (de Jong
et al. 2010 ). Thus, it appears that fast-developing zebrafish embryos, which activate
their genome within a few hours after fertilization, efficiently replenish the maternal
RNA pool without a robust drop in RNA content. A similar situation is observed in
Xenopus where the total RNA content is relatively stable from fertilization to gas-
trulation (Brown and Littna 1964 ).
10 Clearance of Parental Products