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the previous generation, as proposed in C. elegans ( Arico et al., 2011 ).
An exciting idea emerging from these observations is that inheritance of
modified histones through fertilization may constitute a mechanism of
“bookmarking” gene expression patterns between generations.
A second model involves the removal of modified histones or modifica-
tions themselves fromspermchromatin and restoration of thesemodifications
after fertilization, on the same target genes as those premarked in sperm. We
and others have observed that genes enriched inmodifiedhistones in zebrafish
sperm are not all marked in pre-MBT embryos ( Lindeman et al., 2011; Wu,
Zhang, & Cairns, 2011 ), suggesting removal of histone marks from parental
gametes. This is also the case for DNA methylation, as genes methylated in
sperm are not necessarily methylated in the embryo ( Andersen, Reiner,
et al., 2012; Wu, Zhang, & Cairns, 2011 ). Conversely, a significant number
of genesmarkedbymodifiedhistones in pre-MBTembryos are notmarked in
sperm ( Lindeman et al., 2011; Wu, Zhang, &Cairns, 2011 ), arguing in favor
of postfertilization de novo embryonic epigenetic marking.
8. CONCLUSIONS AND PERSPECTIVES
A prepatterning of developmental gene expression by post-
translationally modified histones is consistent with an instructive function
of histone modifications on gene expression patterns. Maintenance of tran-
scriptional repression before ZGA by H3K27me3 (or H3K9me3) may
ensure temporal and spatial modulation of embryonic gene activation to pat-
tern development. From recent studies, a working model of epigenetic con-
trol of developmental gene expression in zebrafish emerges ( Fig. 3.5 ). The
model puts forward a prepatterning of developmental transcription during
pre-MBT development, followed by a priming of transcription initiation
or repression at the MBT, when the embryonic genome is turned on. This
priming leads to distinct chromatin states on genes pending on their tran-
scriptional fate.
The jury remains open on which of the transgenerational inheritance or
removal/redeposition mechanisms prevails to epigenetically prepare the
embryonic genome for transcriptional activation. The models proposed
are not mutually exclusive, as while some genes may remain epigenetically
marked in sperm and embryos, others clearly lose or gain modifications.
Even in a context of removal and reestablishment of histone marks, a chal-
lenging question is how this remarking is directed to previously marked sites.
This may involve instructions “encoded” in the sperm chromatin or DNA,
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