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Maternal transcript degradation and onset of ZGA seem to be functionally
connected in that zygotic transcription provides proteins and microRNAs
(miRNAs) with a feedback effect enhancing the efficiency of maternal
RNA clearance (reviewed in
Walser & Lipshitz, 2011
).
During pre-MBT development, the highly condensed maternal and
paternal genomes are remodeled in preparation for embryonic transcription.
This remodeling involves removal, exchange, and deposition of DNA- and
chromatin-associated proteins on the zygotic genome. It also entails bio-
chemical modifications of DNA, such as DNA demethylation and methyl-
ation (
Andersen, Reiner, Aanes, Alestr¨m, & Collas, 2012; Mhanni &
McGowan, 2004
). In addition, recent evidence shows that chromatin rem-
odeling prior to and during the MBT involves the establishment of specific
“marks” on the genome, in the form of posttranslationally modifications of
histones including methylation and acetylation of specific lysine (K) residues
(
Aday, Zhu, Lakshmanan, Wang, & Lawson, 2011; Lindeman et al., 2011;
Vastenhouw et al., 2010
). In this review, we present recent evidence of
chromatin changes taking place during zebrafish pre-MBT development,
prior to ZGA onset, and as the embryo developments past the MBT. These
observations suggest a view of prepatterning of developmental transcription
by epigenetic marking of developmentally regulated genes.
2. TURNING ON GENE EXPRESSION AT THE MZT
Transcripts loaded in the oocyte during oogenesis (maternal tran-
scripts) have been shown to direct early embryonic development before
activation of the embryonic genome. These maternal transcripts have started
to be characterized in zebrafish (
Abrams & Mullins, 2009; Lindeman &
Pelegri, 2010
); however, only recently has high-throughput RNA sequenc-
ing (RNA-seq) analysis provided a first attempt at estimating the relative
abundance of these mRNAs during development through the MBT period
(
Aanes et al., 2011; Pauli et al., 2012; Vesterlund, Jiao, Unneberg, Hovatta,
& Kere, 2011
). In addition to highlighting novel transcripts and splice
variants, this study identifies five main cohorts of differentially expressed
genes during this period (
Fig. 3.1
).
The first cohort encompasses maternal mRNAs, detected in unfertilized
and fertilized eggs, and involved in cell cycle regulation, DNA replication,
DNA repair, and protein trafficking (
Fig. 3.1
, panel i). These maternal
transcripts display different kinetics of degradation in the pre-MBT period.
Nonetheless, a subset of maternal transcripts subsists and becomes upregulated
