Biology Reference
In-Depth Information
5. TRANSGENERATIONAL INHERITANCE OF EPIGENETIC
VARIATION
Epigenetics historically refers to the stable propagation of chromatin
states during DNA replication. However, alternative epigenetic states at
genetically identical alleles (epialleles) can be stably transmitted through mei-
osis in rare instances. No more than a dozen natural epialleles with clear phe-
notypic impact have been reported in plants (
Weigel & Colot, 2012
). This
suggests that transgenerational epigenetic inheritance (TEI) may in fact be a
rather uncommon source of phenotypic diversity in natural populations. On
the other hand, natural strains of
Arabidopsis
differ for numerous epialleles
(
Vaughn et al., 2007
). In addition, spontaneous (Reviewed in
Becker
et al., 2011; Schmitz et al., 2011
) or experimentally induced epialleles
(
Johannes et al., 2009; Reinders et al., 2009; Teixeira et al., 2009
) are easily
identified in laboratory conditions and can be meiotically propagated across
many generations. This suggests that epialleles are widespread but rarely
identified, that they are rapidly eliminated upon occurrence, or, alterna-
tively, that the conditions that favor the emergence of epialleles are severely
constrained during plant development in natural populations.
Changes inDNAmethylation patterns have been associated to all molecularly
characterized epialleles (Reviewed in
Becker &Weigel, 2012; Weigel & Colot,
2012
).Moreover, differential methylation inor aroundTEs or repeat sequences is
often associatedwith the occurrence of epialleles. Somewhat conflictingwith this
observation, long-term analyses of single cytosine methylation stability in
Ara-
bidopsis
lines indicate that, while spontaneous single cytosinemethylation variants
arise at a relatively high frequency (several orders of magnitude higher than
nucleotidic mutations), DNA methylation changes in TEs and repeats (typically
large segments of contiguous methylation) are rare, with a spontaneous epi-
mutation rate in the range of nucleotidic mutation rate (
Becker et al., 2011;
Schmitz et al., 2011
). As expected, siRNAs are associatedwithmost differentially
methylated TEs, but not with single cytosinemethylation polymorphisms. Thus,
siRNA-dependent maintenance of DNA methylation may account for the sta-
bility of repeat-associated epialleles. This, however, generates an interesting par-
adox: while repeats are protected againstepigeneticalterations through RNAi,
they apparently generate most evolutionary relevant instances of TEI.
One possibility to reconcile these observations is that repeat-associated
siRNA themselves might contribute to TEI, as suggested by the phenomenon
of paramutation. First described inmaize, paramutation refers to the epigenetic
