Biology Reference
In-Depth Information
To summarize, at fertilization, the oocyte methylome includes 1000
methylated single-copy CGIs, which represent only a minor fraction of
the total 23,000 CGIs. The sperm is hypermethylated as compared to the
oocyte, and intergenic repeats are the main targets of this methylation.
Oocyte-specific methylation of somatic promoters may regulate levels,
kinetics, or alternative splicing of genes during early embryonic develop-
ment. In contrast, sperm-specific methylation could prevent retrotransposon
expression following fertilization and at the time of zygotic genome activa-
tion, ensuring protection of the embryonic genome, as it does for the sperm
genome (
Bourc'his & Bestor, 2004
). However, as developed in
Section 3
,
the sperm-derived methylome is rapidly remodeled after fertilization, leav-
ing several families of retrotransposons methylation free, while the maternal
genome progressively loses DNA methylation through embryonic cell divi-
sions. The field will now need to dissect exactly which marks at which
sequences are only byproducts of gamete differentiation and which marks
can directly impact embryonic development.
3. IMMEDIATE INHERITANCE AND REINFORCEMENT OF
PARENTAL ASYMMETRY IN THE ONE-CELL ZYGOTE
As the immediate product of gametic fusion, the one-cell zygote rep-
resents the developmental stage at which parental differences are the most
apparent. Some of these are directly inherited from the gametes, but new
differences are also generated by maternally provided stores of proteins that
catalyze histone and DNA modifications, resulting in the amplification of
parental asymmetry. Although the two sets of parental chromosomes share
the same ooplasm environment, they maintain striking topological separa-
tion after fertilization, which remains even when they unite within the same
nucleus after the first mitosis and at least until the four-cell stage (
Mayer
et al., 2000
). This physical dissociation may be more than just a consequence
of the different developmental histories of the oocyte- and the sperm-
derived chromatin; it may also play a crucial role in the differential treatment
of the two parental pronuclei. Indeed, despite exposure to the same mater-
nally provided factors, the parental pronuclei undergo very different pro-
grams during the few hours following fertilization, as a prelude to zygotic
genome activation. The maternal pronucleus must be released from the
metaphase II arrest to complete meiosis and to reduce maternal genome con-
tent to one set of haploid chromosomes. Meanwhile, the paternal pronu-
cleus must be converted into an accessible, transcriptionally permissive
