Biology Reference
In-Depth Information
FIGURE 11.3
An abstract image of the
flow of genomic information for develop-
ment of the body plan. The regulatory
genome is represented by the coil at the
bottom of the diagram and the open triangles
denote its developmental readout. Progres-
sive spatial diversification of GRNs in
diverse regions of the developing organism
occurs on the grey platform as time proceeds.
In each regional domain a different regula-
tory state is installed (white region of vertical
cylinders with red arrows indicating the
progression of development of each body
part). These local regulatory states lead
eventually to the regional activation of
specific morphogenetic gene cassettes
(orange region of cylinders) and of differ-
entiation gene batteries (blue region of
cylinders). See text
for discussion and
ramifications.
conceived as a temporal diagram: it is intended as an infor-
mation flow diagram.
In each horizontally defined region of
Figure 11.3
there is
continuous flow of information from the genome vertically to
every level of process. But conceptual residence on one of the
horizontal planes provides no insight as to what comes
immediately before, or before that. The issue is what consti-
tutes explanation in development. If the explanation has to
start with the ex cathedra 'givens' that provide the mountings
for the upper-level planes, it can at best produce only isolated
islands of causality. An explanation of development must be
vertically related to the regulatory genome, the source code,
the root of the explanatory chain of causality, or it will instead
be rooted in a package of assumed phenomenology. An
explanation of the spatially differential functions at any level
must beginwith the origins of the differential regulatory states
in different domains. Therefore, what
Figure 11.3
tells us is
that explanation of development requires a system-wide and
deep perception of causal relations: it requires a virtual
framework of causality. This must originate in genomic
information, and it must permit a seamless vertical transit
from genomic information to any desired level of observation
and analysis. Of course therewill always bematters unknown,
but an ultimate goal is a framework explanation that may
retain a few islands of phenomenology floating within it.
Developmental biology has largely forever been the reverse:
a sea of phenomenology within which have floated a few
poorly connected islands of causality.
fertilized egg and terminating in the adult body plan. The
major features of the body plan are established in exactly
the same way in every individual of each species during
embryogenesis, i.e., the positions, identity, and internal
organization of organs, appendages and axial structures are
the same per species. The individual divergence of char-
acters within the species type that are due to genetic and
occasionally environmental differences affects only the last
phases of development, when finer-scale properties are
established. On the other hand, the same suite of funda-
mental developmental mechanisms is used throughout the
universe of animal forms to build the very diverse body
plans that define different animal clades. Animal clades are
hierarchically organized, indicating different levels of
shared developmental programs: the upper-level characters
that define phyla are shared by all members of that phylum
(e.g., the shell glands of molluscs; the dorsal nervous
systems of chordates; the jointed appendages of arthro-
pods); fish, amphibians, reptiles, mammals are classes
within the chordate subphylum vertebrates, and they all
utilize similar developmental mechanisms to construct
their axial skeletons, while within each of these classes the
various orders, such as carnivores or rodents, share more
detailed properties which, as is intuitively obvious, devolve
from the shared developmental processes specific to each
order, and so forth. In general, the genetic programs that
operate development are organized hierarchically, function
hierarchically, and display a hierarchical order in the
breadth of their distributions when diverse animal species
are compared
[20
22]
.
The fact that all members of each species build their
body plans in the same way, while members of different
species do so in different ways, can only be explained if the
control program for development is a heritable character-
istic and the genomes of different species harbor different
e
GENERAL FEATURES OF THE
DEVELOPMENTAL PROCESS
Development is the process by which the functional,
morphological animal organism is built, beginning with the
