Agriculture Reference
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connection with the environment. As we began to understand the
molecular details of the epigenetic template, it has become clear that
several molecular processes are involved and operate in this loop, and
epigenetics based on such concepts cannot be easily de
ned by one set of
consistent criteria. Taken together, epigenetic processes have formidable
implications for our understanding of the diversity of life and evolution.
This became shockingly evident when it was discovered that plant,
animal, and even human genomes encoded roughly the same number of
classical protein-encoding genes (Meyerowitz 2002). Not only did the
sizes of genomes not correlate with the complexity of phenotypes that
they encoded, an observation referred to as the C-value paradox (Thomas
1971), but the biological complexity was also not correlated with the
number of genes in the genome.
IV. EPIGENETIC MEMORY
A. Alternation of Generations
Both plant and animal kingdoms are replete with species that exist as
very distinct and independent morphologies at different stages of the life
cycle. Some of the most dramatic transitions occur between free-living
forms of gametophyte and sporophyte generations in plants. The divi-
sion of the plant life cycle between sporophyte and gametophyte is
referred to as alternation of generations. In fact, alternation of genera-
tions appears to be somewhat neglected, but major phenomenon of great
signi
cance to being able to understand the molecular bases of epige-
netics clearly demonstrates that a single genome can result in two, often
extremely distinct, life-forms. The genome of the organism
how to form each life-form while it exists in the other form. Distinct
differences in gene expression between gametophytes and sporophytes
are evident (Walbot and Evans 2003; Yu et al. 2005). Differences in the
rate of progression of the developmental steps are generally referred to as
heterochrony, a term
remembers
first used by Ernst Haeckel (Gould 1977). Hetero-
chrony also emphasizes the importance of growth rate in development as
explained by D
Arcy Thompson in his topic On Growth and Form
(Thompson 1961). In fact, it is easily realized that controlling the rate
of cell division and expansion in different dimensions will allow the
development of all morphological shapes of tissues, organs, and entire
organisms. Barbara McClintock once remarked,
'
If I could control the
timing of gene action I could cause a fertilized snail egg to develop into
an elephant
(Wallace 1989).
 
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