Biology Reference
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mammalian embryo (
Morgan, Santos, Green, Dean, &Reik, 2005
). Maternal
to zygotic transition begins within the first divisions of the newly formed
embryo, as early as the two-cell stage inmice and around the four- to eight-cell
stage in humans. Previously inherited silent states must be overcome rapidly to
set up chromatin states permissive for the activation of embryonic genes. Next,
the fully differentiated gametic state must be reset between fertilization and the
blastocyst stage to ensure the acquisition of pluripotency, the ability to generate
all cell lineages. This is followed by embryonic differentiation programs around
the time of implantationof the embryo in the uterus, concomitantlywith loss of
pluripotency and cell lineage diversification. These early events leave few
opportunities for inheritance of gametic information. Moreover, gametic
information that occurs in a parent-specific manner is naturally counter-
selected; allelic epigenetic specificities tend to be reduced to allow functional
equivalence of the two parental chromosomes and to limit haploinsufficiency,
where normal function is lost due to the availability of one active copy of a gene
instead of two.
However, functional non-equivalence of the two parental alleles of a gene
may also be useful, in certain developmental and physiological circumstances,
where it may be crucial to fine-tune expression levels. Early embryonic devel-
opment may be particularly prone to this type of single-dose regulation, as
minute differences in gene expression may be key for promoting differential
cell fates (
Guo et al., 2010; Miyanari & Torres-Padilla, 2012; Tang et al.,
2011
). Interactions through the placenta between the mother and the fetus
also seem to be especially sensitive to single-dose regulation, as dysregulation
of the monoallelic expression of imprinted genes is particularly deleterious in
biological pathways involved inmaternal-fetal exchanges (
Schulz et al., 2010
).
Here, we provide a survey of parental epigenetic asymmetries in mammals,
organized in a developmental time frame, from their establishment in gametes,
to their selective consolidation or erasure immediately after fertilization, to
their further restriction during embryonic development.
2. GENERATING ASYMMETRIC NUCLEAR-BASED
INFORMATION IN PARENTAL GAMETES
The cartography of the histone and DNA modifications that are dif-
ferentially carried by the mature oocyte and spermatozoa must first be
described to address the potential for long-lasting parental asymmetry in
the progeny. Gametogenesis provides a convenient period for parental
genomes to be distinctly marked, as they are physically separated in different
cell types and even different organisms. Profound differences exist between
