Agriculture Reference
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framework of macroevolution by which the environment
first induces
“
variation (silent gene expression modules) and then selects
individuals that can initiate these modules (adapt) more effectively
(Taipale et al. 2010).
hidden
”
D. The Ping
Pong Trigger
-
It has been suggested that the alternation of phenotypes in the haploid/
diploid transitions could be causally related to ploidy status itself, and
ploidy-dependent gene expression has been demonstrated (Williamson
and Lehmann 1996; Kakutani 2002; Gu et al. 2003; Adams and Wendel
2005; Brunner et al. 2005; Yu et al. 2005; Huh et al. 2007). However, very
little difference in gene expression has sometimes also been observed
between cells of different ploidy states (Lee and Chen 2001; Birchler and
Veitia 2012). Most evidence point to the gamete fusion or fertilization
events as the trigger that normally initiates epigenetic events leads to
embryo development. It is also evident that ploidy changes, especially
via hybridization of dissimilar genomes, can result in major changes in
global allele-speci
c gene expression patterns since this often arrests
development (Chen 2007). It is, however, also clear that a more complex
process and other signals are involved in the control of epigenetic marks
that mediate development (Dilkes and Comai 2004), since it is some-
times possible for both haploid and diploid genomes to be competent to
allow the entire developmental program to be executed from a variety of
cell types including unfertilized egg cells (n), cells of the ovary and other
reproductive tissues (2n), microspores (n), and numerous types of
somatic cells (2n) (Mordhorst et al. 1998; Jacob and Moley 2005).
Development of haploid embryos is even under the control of speci
c
loci such as the Ig locus in maize or maternal diploid mice by the Igf2
cluster (Kono et al. 2004) and by the action of the Imprint Control
Element (ICE) that mediates expression of parentally imprinted genes
in mammals (Bell and Felsenfeld 2000; Kulinski et al. 2013). Meiosis and
fertilization are the usual events that are associated with very speci
c
patterns of gene expression (Yu et al. 2005) and changes in epigenetic
marks (Arnholdt-Schmitt 2004; Dilkes and Comai 2004; Chan et al. 2005)
related to development. However, many other unknown signals may be
involved.
The basis of heterosis (hybrid vigor) also appears to lie in the
transient heterozygous state of F
1
loci and resultant potential cooper-
ativity of the differential production of different allele products, such
as in hetero-protein complexes that cannot be produced by any com-
bination of homozygous loci (Vietia 2002; Dilkes and Comai 2004). It is
