Biology Reference
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is often a direct or indirect consequence of mechanisms involved in trans-
poson silencing. This idea is further supported by genome-wide compari-
sons of DNA methylation patterns between sperm cells and the
endosperm, which indicate that maternally and paternally imprinted genes
are preferentially associated with DMRs. This model applies correctly to
most of the maternally imprinted genes but is not universal (
Wohrmann
et al., 2012
). Interestingly, maternally imprinted genes are also associated
with strong accumulation of 24-nt siRNAs that target neighboring TE
sequences in sperm cells, which likely contributes to silence the paternal
allele prior to fertilization via RdDM. Most TEs targeted by 24-nt siRNAs
in the sperm cells also correspond to targets of DME in the vegetative
nucleus. This raises the possibility that loss of silencing in the vegetative
nucleus might generate small RNAs that are translocated to the sperm cells,
where they act to reestablish silencing via RdDM (
Calarco et al., 2012;
Slotkin et al., 2009
).
In addition to DNA methylation, imprinting also relies on the activity of
PRC2 and the deposition of H3K27me3 (Reviewed in
K¨hler &
Grossniklaus, 2005; K¨hler et al., 2012
). This is the case at the paternally
imprinted
PHERES
locus, where imprinting requires both the demethyla-
tion of a repeated sequence and PRC2 targeting to the promoter region of
the maternal
PHERES
allele. The current model suggests that DNA meth-
ylation protects against PRC2 targeting, whereas hypomethylation of
repeats and TEs favors PRC2-mediated repression, which in turn promotes
non-CG methylation in the endosperm, possibly via RdDM. PRC2 is fur-
ther involved in the regulation of maternal imprinting by repressing paternal
alleles at several maternally imprinted genes. Whether specific patterns of
DNA methylation or siRNA targeting are similarly involved in the recruit-
ment of PRC2 proteins to their targets is unknown.
4.3. Epigenetic reprogramming in germ cells
The establishment of specific chromatin states in the different gametophytic
cell types might also play an important role in genome reprogramming
(
Calarco et al., 2012; Ibarra et al., 2012; Jullien et al., 2012; Slotkin et al.,
2009
), that is, the resetting of the differentiated state of the gametes, the selec-
tive erasure of chromatin marks accumulated during vegetative growth, and
the establishment of a fully totipotent state in the newly formed organism.
In animals, it is generally accepted that DNA methylation patterns are
reprogrammed through waves of active and passive demethylation and
