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reproduction and subsequent development.
Fertilization in fl owering plants is
accomplished by double fertilization,
resulting in an embryo and nutritive endo-
sperm tissues. Studies on DNA methyl-
ation during this process in Arabidopsis
have revealed several major themes
regarding heredity of cytosine methylation
and its transgenerational variation. First,
maintenance of cytosine methylation via
the MET, CMT and DRM methyl-
transferases is reduced in both the male
and female germlines and endosperm but
increases dramatically in the embryo,
suggesting a process of general de-
methylation followed by a system that
remethylates the progeny genome (Jullien
et al. , 2012) and probably also contributes
to hybrid vigour (Groszmann et al. , 2011).
DME specifi cally demethylates endosperm
genes throughout the genome, especially
those associated with transposon repeats,
resulting in maternally derived, or
imprinted, patterns of endosperm gene
expression facilitating the functional
attributes of this tissue in embryo support
(Gehring et al. , 2006, 2009). Endosperm
genome demethylation occurs in concert
with increased siRNA production from the
demethylated DNA regions, indicating an
RNA-mediated process (Hsieh et al. , 2011;
Mosher et al. , 2011). siRNAs have been
shown to be mobile between cell and tissue
types (Dunoyer et al. , 2010; Molnar et al. ,
2010), so that endosperm and even pollen
(Borges et al. , 2011) may contribute
information directing the methylation
pattern of the embryo genome. This pro-
cess of epigenome transfer, maintenance
and imprinting represents one of the few
examples of well-characterized epigenome
dynamics in eukaryotes, and is also likely
to provide opportunities for selective
modifi cation of the epigenome between
generations.
Studies on transgenerational methyl-
ation patterns in Arabidopsis , where single
seed descent lineages have been studied for
both DNA sequence and methylome
polymorphisms, indicate that patterns of
DNA methylation from generation to
generation can be quite stable (Becker et
al. , 2011; Schmitz et al. , 2011). In these
elegant reports, while stably inherited
methylation polymorphisms were observed
and infrequent, they were more common
than DNA sequence polymorphisms,
suggesting that the epigenome provides
additional plasticity for heritable change.
The ability of siRNAs to mediate heritable
cis - and trans -effects on siRNA production
in progeny of tomato lines harbouring wild
species introgressions provides further
evidence for a plastic epigenome and its
possible contributions to heterosis and
transgressive segregation (Shivaprasad et
al. , 2012). The fact that all of these
processes occurs within a biochemical
context of sequence-specifi c siRNAs and
methylases/demethylases also suggests that
such changes have increased opportunity
for reversion or modifi cations that may
have selective advantages, as in the
examples of stress epigenome changes
described below. In short, the accumulation
of ovule, endosperm and pollen siRNAs
contribute to the process of genome and
epigenome inheritance from parents to
their embryos and in a manner facilitating
both short- and long-term genome
modifi cations with selective advantage.
Finally, possibly related to the under-
lying processes that mediate germline and
embryo epigenome dynamics, cytosine
methylation status appears to infl uence
meiotic recombination rates in ways more
complex than originally thought, as deter-
mined through the study of a population
segregating for the met1 hypomethylation
mutation (Mirouze et al. , 2012). This
analysis suggested that, rather than an
overall repressive effect on recombination,
DNA methylation actually may reduce the
recombination-repressive effect of hetero-
chromatin and repress recombination in
euchromatin. Localized variation also was
observed within chromatin states, indicat-
ing additional complexity.
17.3.2 Fruit development and ripening
Tomato is a long-studied model system for
analysis of fl eshy fruit development and
 
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