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non-CG methylation (
Duran-Figueroa & Vielle-Calzada, 2010; Havecker
et al., 2010; Singh et al., 2011
). This suggests that non-CG methylation in
plants plays a role in defining germ cell identity in the ovule (
Garcia-
Aguilar et al., 2010
).
The second transition, from spores to gametophytes, is also marked by
dramatic chromatin changes (
Fig. 6.2
). In both male and female reproduc-
tive organs, the gametes are enclosed in a multicellular gametophyte
together with “accessory companion cells” that participate in reproduction
but are not transmitted to the next generation. Analyses of tagged versions of
the Histone Three Related (HTR) gene family indicate that female gamete
specification is accompanied by the eviction of most somatic H3 isoforms
(
Ingouff et al., 2010
). At maturity, a limited repertoire of HTR proteins
is retained: the H3.3 variant HTR5 is the only one detected in the egg,
and only one H3.1 protein (HTR3) and two H3.3 variants (HTR8 and
HTR14) are found in the central cell (
Fig. 6.2
). Thus, the two female gam-
etes show distinct histone-based signatures. Similarly, they are dimorphic for
two histone marks associated with silent chromatin, H3K9me2 and
H3K27me3 (
Pillot et al., 2010
). In the sperm cells, only HTR5 and
HTR10 (a sperm-cell specific H3.3 variant) are found, whereas HTR8
and HTR14 are detected in the vegetative nuclei (
Ingouff et al., 2010
).
Genome-wide analyses indicate that the formation of the sperm and vege-
tative cells also involves dynamic changes and distinct DNA methylation
patterns. Interestingly, plant sperm cells show high levels of both CG and
non-CG methylation (
Calarco et al., 2012; Ibarra et al., 2012
), suggesting
that contrary to mammals, DNA methylation is mostly retained during
reproduction. However, whereas CG methylation remains relatively con-
stant during sperm development, CHH methylation is transiently reduced
at TE loci in the spores and reestablished later. By contrast, the vegetative
nuclei showed loss of CG methylation but maintenance of high levels of
CHH methylation. Thus, similar to the female gametophyte, the male
gametophyte produces epigenetically divergent cell types. Patterns of
DNA methylation in female gametophytic cell types are less well defined.
Active and passive DNA demethylation takes place in the central cell,
through the action of DNA glycosylases (
Choi et al., 2002; Gehring,
Bubb, & Henikoff, 2009; Hsieh et al., 2009
) and the repression of mainte-
nance DNMTs in young ovules (
Jullien, Susaki, Yelagandula, Higashiyama,
& Berger, 2012; Jullien et al., 2008
). The central cell is therefore expected to
have lower levels of DNA methylation.
