Agriculture Reference
In-Depth Information
Here we have a remaining point of confusion about genetics and
epigenetics, which complicates our efforts to standardize our terminol-
ogy. We now readily accept that epigenetic events can be directed by the
environment and pass through many serial mitoses (Youngson and
Whitelaw 2008). However, such cases as with
flax and other examples
with transposons (McClintock 1984) strongly imply that phenotype
changes based on sequence alterations (e.g., by activation of transposi-
tion) can also sometimes be directed by the environment (Grandbastien
1998). In fact, the
first discovered mutants in
Drosophila
included
phenotypes that were not clearly Mendelian (Cavalli and Paro 1998).
The WHITE gene, discovered by Muller (1930), was demonstrated to
result from a position effect variegation (PEV) phenomenon where there
has been the rearrangement of a segment of DNA. This results in the
spreading of a heterochromatic, silenced region around theWHITE locus
(Zhimulev et al. 1986). The phenotype is patchy as a result of the
ontogenic timing of the rearrangement. If this rearrangement occurs
early in embryogenesis, the portion of the eye that becomes white
will be larger than if it occurs later (Lu et al. 1996). This kind of
phenomenon was also clearly seen in the early reports of Barbara
McClintock on early and late transposition events in maize pericarp
that can also be directed by the environment (McClintock 1965, 1984).
We are tempted then and do call such phenomena epigenetic as well, but
they do involve DNA sequence changes. Observations of environment-
directed phenotype changes continued to be observed for many decades,
and became a sort of black sheep of conventional genetics and evolution
(Jablonka and Raz 2009).
D. Growth Is a Common Phenotypic Link Between Plant and Animal
Epigenetics
Growth, replication of cells by division, and their expansion lies at the
heart of both increasing organism size and populations or both. It also
controls multicellular development to form complex structures.
Importantly, whatever form an organism takes, its growth rate and
final size or population are the most important features that adapt it to
environmental change. That natural selection occurs when the envi-
ronment cannot support the growth and reproduction rate of an
organism is central to Darwin
is theory. Therefore, control of growth
must always remain delicately attuned to the environment. Growth
control cannot be genetically too rigid and this may explain the
extraordinary plasticity of the growth of a wide range of taxa. Remark-
ably, but understandably in keeping with this need for plasticity,
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