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3.1. Reshuffling of histone variants and modifications
Very early on, within the first 3 hours after entry into the oocyte, the con-
densed sperm chromatin is submitted to extensive, active remodeling, while
the maternal chromatin is relatively stable. Paternal protamines are stripped
and replaced with oocyte-supplied histones, using H3.3 histone variants,
which can be incorporated in a DNA replication-independent manner,
through the control of the histone assembly chaperone HIRA ( Loppin
et al., 2005; Torres-Padilla, Bannister, Hurd, Kouzarides, & Zernicka-
Goetz, 2006 ). However, some H3.1 and H3.2 variants, which can be assem-
bled only through DNA replication, are still detected on paternal chromatin
by immunofluorescence at this stage. This observation suggests a possible
participation of the limited amount of sperm-derived histones to zygotic
chromatin, at least transiently before the first S-phase ( Fig. 9.2 ; van der
Heijden et al., 2008 ). The maternal pronucleus is, on the other hand, exclu-
sively loaded with canonical H3 histones, H3.1 and H3.2, in direct contin-
uation with the oocyte chromatin state ( van der Heijden et al., 2005 ).
The parental pronuclei also display different histone modifications before
the onset of the first S-phase (for review, see Burton & Torres-Padilla,
2010 ). Compared to maternal chromatin, the newly formed paternal chro-
matin is hyperacetylated, a state that could participate in the global
decondensation of the paternal pronucleus ( Adenot, Mercier, Renard, &
Thompson, 1997; Santos, Peters, Otte, Reik, & Dean, 2005 ). Acetylated
histones H4 may be directly incorporated in this modified form at the time
of protamine to histone exchange ( Sobel, Cook, Perry, Annunziato, & Allis,
1995 ). In terms of histone methylation, however, the paternal pronucleus is
globally depleted, while the maternal pronucleus is enriched, as was the
oocyte, for H3K4me2/3, H3K9me2/3, and H3K27me1/2/3 marks ( van
der Heijden et al., 2005 ). By the final stage of the zygote (PN5), after rep-
lication and incorporation of replication-dependent histones, the asymmetry
between the two parental genomes in terms of histone variants and the
majority of histone acetylation and methylation marks mostly disappears.
H3K9me2/3 marks represent an exception, as these modifications con-
tinue to specifically decorate the maternal genome, but not the paternal one
after the first S-phase ( Liu et al., 2004 ). The combination of histone hyper-
acetylation and lack of repressive H3K9 methylation may participate in the
more precocious transcriptional activation of the paternal pronucleus, which
can be observed starting from the S-phase of the one-cell zygote and
onward, while maternal pronucleus activity is not detectable until the
two-cell stage ( Aoki, Worrad, & Schultz, 1997 ). As developed later in this
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