Agriculture Reference
In-Depth Information
F. Epigenetic Control of Development and Evolution
The information encoded in the DNA sequence is neither self-reading
nor self-replicating. This reading and replication are dependent on the
cell machinery that must also be heritable. The machinery that we now
see has been built up over the long course of evolution and has infor-
mation content of its own that must function together with the DNA
sequence. Many intracellular structures cannot be made
de novo
and
have active and precise mechanisms that control their inheritance
(Warren and Wickner 1996; Estrada et al. 2003; Wiederkehr et al.
2003; Du et al. 2006). Some of the earliest observations of the inheri-
tances of subcellular structures were made with cortical variation in
ciliates, especially through the work of Tracey Sonneborn (Nanney 1958;
Beisson and Sonneborn 1965), and now include many self-replicating
entities, the most famous being prions (Cohen and Prusiner 1998), an
example of an inherited self-replicating protein structure (Wickner et al.
2004; Shorter and Lindquist 2005). Direct inheritance of the cellular
machinery or
has likely enhanced cellular function so
dramatically that natural selection has effectively worked against relying
on their less ef
“
toolboxes
”
cient
de novo
production solely from DNA-encoded
information.
Epigenetics represents the interactive control link between these two
worlds of transgenerational and developmental information, as Nanney
(1958) recognized long ago and is now increasingly accepted (Richards
2006). In fact, any of the processes that mediate the reading of the DNA
sequence information and its manifestation into phenotype can be
considered to be part of the epigenetic system based on the concept
of Waddington (1953), which was rooted in the earlier ideas of many
including Darwin (1868). Thus, from the earliest presentations of evo-
lution, transgenerational inheritance and development were considered
to be distinct but connected. The experimental approach of Morgan and
others who concentrated on transgenerational inheritance and largely
ignored the connection to development allowed rapid and ef
cient
advances in understanding transgenerational inheritance. Morgan
understood the connection to development, but success of his strictly
transgenerational approach overshowed any understanding of the
importance of integration and connections of these two worlds of
information (West-Eberhard 2003). Morgan was not interested in the
genetic basis of how a
fly forms six legs, but what is the genetic basis of a
fly having four legs instead of six. It is easy to see that these phenomena
are related and the legacy of Morgan continued to separate them as
experimental approaches. In addition, the
flawed interpretations of
