Agriculture Reference
In-Depth Information
Whitelaw 2008) and strongly suspected in many others (Boyko et al.
2010).
Any genetic change that
fixes transgenerationally epigenetic altera-
tions that increase
fitness would be under strong positive selection.
Waddington labeled this
fixation concept genetic assimilation
(Waddington 1953), and Schmalhauser (1949) called it stabilizing
selection. Even before Waddington and Schmalhauser, a related concept
called genetic accommodation was proposed by Baldwin (1896, 1902).
Genetic accommodation was eventually called the Baldwin effect (Simp-
son 1953; West-Eberhard 2003). The Baldwin effect was demonstrated
experimentally by Gause (1942) using adaptation of paramecia to salt.
This phenomenonwas con
rmed 40 years later using adaptation of plant
cells to salt (Bressan et al. 1990). Other examples of genetic assimilation
have continued to be reported in several species (Wootton 1976; McLeod
1984; Shapiro 1984, 1980; West-Eberhard 2003; Patton and Brylski 1987;
Kirschner 1992; Roth 1992; Cushman and Bohnert 1996) and some
assimilated phenotypes appear related to characters of ancestral species
(Carpenter 1989; Pryer and Hearn 2008). In view of this concept, it is
intriguing to consider that long-lived perennial plants such as trees may
possibly record hundreds of years of variable environmental exposure
in a cryptic epigenetic memory. Unfortunately, genetic assimilation/
accommodation continues to be poorly understood and generally dis-
missed as unimportant to evolution.
One of the earliest observed environment-directed changes in the
genome that is passed on to sexual progeny has been the occurrence
of genotrophs in
Linum usitatissimum
. Genotrophs are variant pheno-
types that are dependent on the nutritional environment of the parent
plant. This phenomenon was originally described by Durrant (1962,
1971) in the
flax varieties Stormont Cirrus and Lyrral Prince. Various
studies of genotrophs have revealed many environment-induced
changes in the
flax genome that may be responsible for this phenomenon
(Cullis 1979, 2005). Genotrophs have also been shown to have a close
linkage with speci
c RFLP patterns (Schneeberger and Cullis 1991).
Changes in the DNA sequence as well as in epigenetic patterns appear to
be involved in the generation of genotroph phenotypes (Cullis 1977,
2005).
Environment-induced changes in DNA sequence were long ago sug-
gested to be an important aspect of stress adaptation by McClintock
(1978). It has been shown that an altered environment may induce the
demethylation and thus activation of silenced transposons (McClintock
1978, 1984). Transposition events subsequently can lead to new inser-
tions that control gene expression (Fedoroff 2002; Martienssen 2008).
