Agriculture Reference
In-Depth Information
Richards (2006) has clearly and concisely described three important
categories of these connecting loops that result in obligatory, facilitated,
or pure epigenetic phenotype variation (Vaughn et al. 2007; Zhai et al.
2008). In the
first, the epistatus, such as methylation of a genome DNA
sequence, can obligate another change such as movement, or not, of a
transposon that alters the sequences and gene expression at or near the
transposition sites (Richards 2006). Facilitated epigenetic changes are
potentiated (made more likely to occur) by a particular genome sequence
change, for example, classical mutation of a methyltransferase, creating
a gene expression alteration or gene sequence disruption. These genetic
mutations make more likely epimark errors or changes at loci often
unlinked to the mutation. Being unlinked to the genetic change and
having a higher instability allow both independent segregation and a
non-Mendelian pattern of heredity caused by the epimark alterations.
Pure epigenetic phenomena are stochastic errors in epimarks just like
DNA sequence errors, but can also form a loop if, like facilitated changes,
they occur at loci that are important to read DNA sequences. Although
there has been revealed some clear evidence that epigenetic changes are
not always stochastic, but are also environment-directed (Youngson and
Whitelaw 2008; Paszkowski and Grossniklaus 2011), the extent and
importance to adaptation of directed processes are still not well de
ned.
Recognition of the interconnected worlds of genetics and epigenetics has
already allowed the isolation of gene sequence mutations that affect
many epigenetic (no changes in DNA sequences) phenomena, mainly by
the identi
cation of mutations that affect silencing or derepression of
marker loci (Zhu 2009). Important components of major control path-
ways that effect the repression or derepression of marker genes have also
been found, but especially those loci that control the speci
city of gene
repression or derepression are still largely unknown.
A useful paradyme is emerging where adaptation to different environ-
ments occurs through a process of development. Acclimation to the
environment thus should be viewed in the same complexity and evolu-
tionary signi
cance of other aspects of development, for example,
flowering, tuber formation, and so on. Thus, environmental acclimation
by adaptation is achieved by following an appropriate developmental
pathway or channel as Wadsworth described. The work of Lerner and
Amzollag (2005) has, in the recent past, most closely adhered to this
paradyme. A close examination of their results and the paradyme by
which they were directed may be an excellent platform to apply modern
epigenetic technologies.
In other words, we should view stress acclimation (adaptation) as
another developmental program that is initiated and maintained for
