Agriculture Reference
In-Depth Information
the reading of DNA sequence information through the epigenetic tem-
plate that is connected to the environment, such that new adaptive,
epigenetic states are achieved. It should also be considered that
Marsilia
,
Regnellidium
,and
Pilularia
are extant species that may display recapit-
ulation of phylogeny, or increasing differentiation, which may have
occurred evolutionarily in a nonlinear fashion. That is, they may not
have evolved directly from one to the other, but from extinct species that
had encoded in their genomes different ontogeny patterns (heterochro-
nies) that were possibly even silent, but eventually passed on to and
activated in Marsilia. Nevertheless, the example of the Marsilia hetero-
chrony demonstrates that a form of recapitulation or progressive differ-
entiation can occur as part of a postembryonic modular ontogenic
program further strengthening the Evo-Devo perspective (West-Eberhard
2003). Other clear examples of phylogenetic postembryonic recapitula-
tion have been reported (Carpenter 1989). Understanding the underlying
epigenetic mechanism of how these modules of developmental subpro-
grams were altered during evolution and remanifested in descendant
species will afford a dramatically better ability to make genetic modifi-
-
cations of more complex modular characteristics.
V. SEX, EPIGENETICS, AND THE GENOME
A. Origins of Complex Ontogeny
A central question at the heart of understanding epigenetics has been
what molecular processes began the metastable genetic memory process.
To address this question we need to examine why a recognition memory
systemwould be advantageous in natural selection. It has generally been
accepted that the major bene
t of the evolution of alternation of haploid
and diploid generations (sexual reproduction) is the generation of
variation by the genetic recombination process and its large contribution
to a selective advantage in changing environments. Certainly this is true,
evidenced by the fact that it is mediated by the highly complicated and
resource-invested molecular mechanisms of chromosome assortment
and recombination by DNA crossing over that greatly accelerates geno-
mic variation. In fact, organisms without a DNA recombination capabil-
ity are virtually nonexistent today. However, the initial advantages of a
diploid genome most likely stem from the buffering capacity of repli-
cated loci against deleterious mutations (Walbot and Evans 2003) and
the accelerated evolution afforded by creation of gene copies that can
mutate without negative selection pressure (Chen 2007). An initial
