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Domains with coinciding H3K4me3 and H3K27me3 are also clearly
detectable in sperm (
Wu, Zhang, & Cairns, 2011
). These relatively large
chromatin “blocks” predominantly contain developmental genes, and even
entire clusters of such genes (e.g.,
hox
loci) (
Wu, Zhang, & Cairns, 2011
).
Because sperm cells are presumably similar or identical in their chromatin
states (this remains to be demonstrated), one can speculate that these
domains are bivalent. In addition, an interesting modality of sperm chroma-
tin is the combinatorial association of histone modifications, which, to our
knowledge, has not been reported in other cell types. A striking example is
H3K36me3, a mark commonly detected on the coding regions of expressed
genes, yet found on inactive developmental promoters in zebrafish sperm,
often overlapping with blocks of H3K27me3 (
Wu, Zhang, & Cairns,
2011
). Whether chromatin domains enriched in H3K27me3 and
H3K36me3 exist in embryos is uncertain, as only in rare occasions do we
observe overlapping domains enriched in these marks. To what extent this
and other cases of bi- or multivalency may also be the result of the partial
tetraploidy of zebrafish, with different alleles or duplicated regions in a dis-
tinct transcriptional state, has to our knowledge not be taken into consider-
ation and is worthy of close attention.
DE NOVO
DEPOSITION OF EMBRYONIC EPIGENETIC MARKS?
TWO MODELS
The presence of histone marks on the genome of pre-MBT embryos
suggests two alternatives as to the origin of these marks (
Lindeman et al.,
2011
). A first model puts forward a direct inheritance of modified histones
from gametes. To support this view, a significant proportion of genes in pre-
MBT embryos harbors the same histone modifications in sperm (
Andersen,
Reiner, et al., 2012; Wu, Zhang, & Cairns, 2011
). Furthermore, in mouse
(
Brykczynska et al., 2010; Puschendorf et al., 2008
) and
C. elegans
(
Arico,
Katz, van der Vlag, & Kelly, 2011
), male and female genomes are marked
by modified histones at the 1-cell stage, consistent with a transmission of
marks through fertilization. An implication of these findings is that develop-
mental instructions may already be predetermined by epigenetic states in
gametes, an idea supported by findings that high levels of H3K4me3 and
H3K14ac (a mark of transcriptional activity) in sperm correlate well with
transcriptional activation at or after ZGA (
Wu, Zhang, & Cairns, 2011
).
These marks may also reflect transcriptional activity in the germline of
7. TRANSGENERATIONAL INHERITANCE OR
