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In-Depth Information
maintainphenotypic aspects of the tolerant cells (Bressan et al. 1985; 1985;Grafi
et al. 2011). In fact, there exists evidence that the stress adaptation process
is closely associated with the environment-directed metastable stages of
development, referred to as canalization (channeled or directed) accord-
ing to Waddington
'
s terminology (Amzallag 2000, 2005).
VII. EVOLUTION: THE ENGINE OF IDEAS
A. Epigenetic Changes, Germ Cells, and the Environment
The separation of epigenetic changes from transgenerational inheritance
was originally assumed by interpreters of Weisman
s theory to be
primarily physical, based on germ versus somatic cell lineages. How-
ever, it is now clear that development and heredity are never completely
separated, and this view has now become increasingly recognized
(Jablonka and Raz 2009). Jablonka and Lamb (Jablonka and Lamb
2007) also have reminded us that for most of evolution, germ and
differentiated cells resided inseparably together in the unicellular world.
In complex extant species, germ cells are still products of ontogeny, and
do not form again directly from immediate (without any lineage) meiosis
of fused gametes (zygotes) (Morgan et al. 2005). The germ cell lineage
is not maintained in an uninterrupted
'
epigenetic state. The
epigenetic changes that occur, even during very short germ cell to germ
cell lineages, as in mammals, are considerable and control differentia-
tion of the zygote into germ cells (gametogenesis) that begins in mam-
mals at the embryo epiblast stage (Lawson et al. 1999; Saitou et al. 2003;
Cantone and Fisher 2013) and continues even after the next generation
of mature gametes are formed (Hajkova et al. 2002; Petronis 2010). The
context of the MST (Jablonka and Lamb 2008) has led many to propose
that despite the common physical lineage of pregerm and somatic
embryonic cells that form other somatic cells, the two worlds of heredity,
transgenerational germ cells and developmental somatic cell lines
(mostly not transgenerational), are in many ways connected but distin-
guished mainly by the molecular bases of their gene expression includ-
ing suppression of somatic programs and activation of pluripotent
programs (Nanney 1958; Surani and Reik 2007). The molecular connec-
tions between these worlds now form the frontier of transgenerational
epigenetic inheritance (Richards 2006). It is dif
“
Germ
”
cult to overstate that
to understand such connections we need to know much more about
how epigenetics controls development, where germ and somatic lines
become separated (Autran et al. 2011).
