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mixtures than H3K4me3-marked regions, arguing for a greater likelihood
that sperm H3K27me3 marks play a role in paternal epigenetic
transmission. In support of this, a correlation was found between high enrich-
ment of H3K27me3 marks in spermatozoa and repression of associated genes
in early embryogenesis (
Brykczynska et al., 2010; van der Heijden et al.,
2006
).
Themajor sex-specific constraint exerted on the oocyte is to produce stores
of RNAs and proteins to support embryonic development immediately after
fertilization. Accordingly, a state of acute transcriptional activity characterizes
the oocyte during the growing phase of development. Our current knowledge
about histone variants andmodifications in oocytes comes mainly from immu-
nofluorescence studies and genetic deletions of histone-modifying enzymes
(
Abe, Tsai, Jin, Pfeifer, & Szabo, 2011; Ciccone et al., 2009; Gu, Wang, &
Sun, 2010; Liu, Kim, & Aoki, 2004
). Growing oocytes gain increasing levels
of histone acetylation, in agreement with a transcriptional burst, which are then
reversed into hypoacetylated states as meiosis is resumed (
Kim, Liu, Tazaki,
Nagata, &Aoki, 2003
). Methylationmarks associatedwith active transcription
such as H3K4me2/3 and H3K79me2/3, and those associated with repression
such as H3K9me2/3 and H3K27me2/3, are also globally detected in mouse
oocyte nuclei (
Mayer, Smith, Fundele, & Haaf, 2000; Ooga et al., 2008;
Sarmento et al., 2004
). Due to the difficulty in obtaining large numbers of
oocytes for ChIP assays, studies addressing the modification status of specific
loci have not been amenable so far. However, recent advances in techniques
applicable to small quantities of material should soon allow the genome-wide
analysis of the histone modification landscape of the mammalian oocyte
(
Dahl & Collas, 2008; Goren et al., 2010
). Currently, it is not known which
specific loci carry differential histonemarks in oocyte and spermand could thus
impart asymmetric information to the embryo. Importantly, not only are
oocyte nucleosomes passed to the zygote, both in the form of incorporated
and free histones, but also the enzymes necessary to posttranslationally modify
histones in the early embryo. Further studies in early embryos are required to
determine howstably information contained inoocyte and spermnucleosomes
is transmitted during preimplantation development, and most importantly,
whether it can impact the embryonic program.
2.2. Sex-specific DNA methylation patterns
The acquisition of DNA methylation patterns in the gametes has been more
extensively studied than histone modifications, partly because of the larger
