Agriculture Reference
In-Depth Information
pathways in epigenetic changes may be a genome-wide phenomenon,
because (i) the chromatin protein structure exists at all loci, and (ii) the
double-stranded nature of DNA allows the possibility of transcribing in
both directions that could result in double-stranded regulatory RNAs
complementary to any locus. The strong indications that resetting epi-
marks at the early embryo stage of the meiotic (sexual) cycle after a global
clearing of marks also leaves the intriguing and centrally important
question of what serves as the template and its copying and thus the
possible mechanisms by which the environment can in
uence the
fidelity for both initially setting and resetting marks. Controlling
the epigenetic templating process also would obviously set andmaintain
the pluripotent subprograms during the cell divisions required for post-
zygotic progressive embryo development and ultimately phenotype.
Whatever the template reading and resetting process be, the epigenetic
error balance should permit enough transgenic epigenetic variation to
allow rapid and adaptive response to changing environmental patterns
of ecological niches compared with low-frequency DNA sequence muta-
tions. These more frequent changes could create greater phenotype
plasticity in a higher proportion of the population. For example, if
transition to
flowering induced by epigenetic changes that result from
vernalization produces an advantage for a
flowering delay until expe-
riencing a cold period, this epigenetic process should be maintained by
requiring high-
delity resetting at each generation. If the environment
changes and no cold period is experienced, it would be an advantage to
allow a frequent epigenetic change (epimutation) that does not require
the
flowering program to be reset by a cold period to become assimilated
(Dennis and Peacock 2007). Although epimarks and gene expression
control are well established in the vernalization process, how the
environment directs epichanges in vernalization remains unclear
(Swiezewski et al. 2007). Recent studies in
A. thaliana
have shown
that vernalization involves a polycomb-based epigenetic memory
system (Song et al. 2013).
Although maternal phenotype effects have been observed for more
than a century (Correns 1990), there has been a signi
cant upsurge in the
interest in inheritable maternal traits (Galloway 2005; Mathieu et al.
2007; Donohue 2009; Johannes et al. 2009; Whittle et al. 2009; Lang-
Mladek et al. 2010; Pillot et al. 2010; Paszkowski and Grossniklaus 2011;
Verhoeven and Van Gurp 2013). These are traits that are acquired by the
maternal parent and passed on to subsequent progeny. These traits are
highly in
uenced by the environment, but they were not considered
important to evolution because (i) they have very low and/or metastable
inheritance; (ii) the fact that parent and progeny environments are often