508
for OET would apply to all mammals in general. However, the minimal contribution
of zygotic miRNAs to maternal mRNA degradation could be a general rule in mam-
mals as massive maternal mRNA degradation precedes ZGA in bovine embryos as
well (Gilbert et al. 2009 ) (Fig. 10.5).
Taken together, maternal mRNA degradation occurs in phases initiated by termi-
nation of transcription, resumption of meiosis, fertilization, and ZGA. The length
and intensity of each phase may differ among vertebrates. What also differs is the
stage of development until which maternal mRNAs contribute to gene expression.
In mammals, maternal mRNAs are essentially gone between 8-cell and blastocyst
stages, and maternal mRNAs do not control germ cell development. In contrast,
maternal mRNAs are still present during organogenesis in zebrafish and Xenopus,
and they control germline development in the embryo.
10.2.3 Elimination of Maternal Ribosomal RNA and Maternal
Ribosomes
Ribosomal RNA (rRNA) forms the RNA component of the ribosome. Three rRNA
molecules are present in the larger ribosomal subunit (5S, 5.8S, and 28S) and one in
the smaller subunit (18S). 5S rRNAs are encoded by clusters of tandem repeats
transcribed by RNA polymerase III (reviewed in Ciganda and Williams 2011 ). 18S,
5.8S, and 28S rRNAs are transcribed by RNA polymerase I as a single transcript
(47S rRNA), which is subsequently endonucleolytically processed to give a rise to
18S, 5.8S, and 28S rRNAs (reviewed in Henras et al. 2008 ). Genes encoding 47S
rRNAs are tandemly arrayed in specific genomic locations and exhibit unique con-
trol of gene expression (reviewed in Grummt and Langst 2013 ).
rRNA forms the bulk of maternal RNA (63–95 % of total RNA) (Piko and
Clegg 1982 ; Davidson 1976 ; Phillips 1982 ). In Xenopus laevis embryos, rRNA
synthesis is detected during gastrulation (Brown and Littna 1964 ), but maternal
ribosomes can support development much further, as demonstrated in nu mutants,
which are defective in ribosomal synthesis. Homozygous nu mutants, which carry
only ribosomes provided by their heterozygous mothers, can develop until the
swimming tadpole stage (Brown and Gurdon 1964 ). It was estimated that Xenopus
oocyte contains 3.6 μg rRNA, which corresponds to 10^12 ribosomes (Davidson
1976 ). Interestingly, the total RNA content is relatively stable from fertilization to
gastrulation (Brown and Littna 1964 ), which suggests a minimal rRNA turnover
in the absence of its transcription and, subsequently, balanced embryonic rRNA
transcription and turnover in Xenopus embryos.
A different situation exists in mammals. Although the mammalian oocyte is rela-
tively small and rRNA expression starts early in development (Abe et al. 2015 ;
Knowland and Graham 1972 ; Zatsepina et al. 2003 ), biochemical data suggest that
majority of mouse ribosomes are not in polysomes during ovulation and fertiliza-
tion, presumably being stored for later use (Bachvarova and De Leon 1977 ;
P. Svoboda et al.