Epigenetics of Reproduction in Animals - Building the Most Complex Structure on Earth - page 66

rump), Shea, like many other biologists, believes that the genotype has both informa-
tion with indicative content, which tells the organism where the abrasion will take place
postnatally, and information with imperative content for producing skin thickenings.
Analogous to the old genecentric concept of “masters and servants,” he believes in the
existence of genetic representations or tokens and consumers specialized in “reading”
these genomic representations or the genetic information and producing respective
phenotypic outputs ( Shea, 2012 ). But again, in examples of transgenerational develop-
mental plasticity, there are no “genomic representations or tokens” to read, and both
“indicative” and “imperative” information come from epigenetic sources (see Chapter
4, section Transgenerational developmental plasticity - Insights into the nature of evo-
lutionary morphological change). For instance, locusts of species Schistocerca gregaria
(Forskål), in response to crowding, develop several new morphological, behavioral,
physiological, and life history characters that are transmitted to the next generation,
although no changes in genes are involved in this phase transition.
From an informational point of view, in cases of developmental plasticity, genes
themselves serve as channels of upstream epigenetic information, as proposed by
Griffiths and Gray (1994) .
Is There Any Genetic Program in the Genome?
Conventionally, a genetic program implies the information contained in the genome,
presumed to determine patterns of gene expression leading to cell differentiation and
sequential events, and ultimately leading to the development of all phenotypic traits.
The program specifies where and when the new organ will develop, which cells will
participate in the development of the organ, and how different types of cells will be
arranged in the intricate patterns of tissues in the developing organ.
The route from genes to morphological characters is convoluted and, in many
respects, still escapes us. This is the reason why we have no hypothesis or model
yet of the sequential steps of the development of an organ. There is no evidence that
any part of the genome (a gene or a number of genes) codes or determines steps for
developing a brain, a heart, or even a strand of hair. It is true that genes or gene net-
works are involved or responsible for developing such characters, but their involve-
ment is not linear: a number of different types of cells have to be differentiated and
arranged in specific spatial patterns in order to form the organ. But both cell differ-
entiation ( Christophersen and Helin, 2010 ) and dedifferentiation (return of differenti-
ated cells to pluripotency) ( Takahashi and Yamanaka, 2006 ), as well as activation of
gene regulatory networks ( Cabej, 2012, pp. 23-24, 39-80 ), are epigenetically rather
than genetically determined.
Since the normal development of the phenotype requires precise implementation
of the program in time and space, the genome must also receive updated information
on the implementation of a preceding step of the developmental program, release the
relevant signal to implement the next step, and so on. Moreover, the genome has to
restrict the effects of specific circulating inducers on the developing organ and block
their effects on the rest of cells of the animal body. Relevant supporting evidence is
still absent.