Biology Reference
In-Depth Information
a fundamental rule of hierarchy. In a true hierarchy, command flows only one way
d
Captains
give orders to Sergeants, but Sergeants do not give orders to Captains. In the morphogenetic
mechanisms described in this topic, the chain of command can loop back so that, to pursue
the military analogy, a lowly Private at the end of a long chain of command ends up giving an
order to the Brigadier General back at the top. Thus a 'high level' genetic module that invokes
the creation of migratory machinery in a cell, including the expression of target receptors, can
be switched off by signals initiated by the receptors once they have bound their targets
(Chapter 12).
It is therefore most sensible to view morphogenetic processes as being multilayered,
without necessarily considering those layers to follow the rules of a hierarchy.
Why are Morphogenetic Mechanisms Organized the Way They are?
'Why' questions are always dangerous to ask in science because they run the risk of
descent into teleology and metaphysics. Nevertheless, asked with appropriate caution,
a question about whether one would expect something to be the way it is, from basic princi-
ples, can help to highlight any unexpected features it has, and help to suggest future lines of
enquiry. In biology, 'why' questions usually translate as 'what is there about this system that
would confer a selective advantage over rival systems if they all competed during natural
selection?' Such questions demand that one imagines some alternative system for accom-
plishing the same task, with which the attributes of real biological systems can be compared.
At the lowest level, real morphogenetic mechanisms rest on the self-assembly of macromol-
ecular complexes, a process that is directed primarily by the information present in the
subunits themselves, but with some other regulatory controls. It is difficult to imagine any
mechanism that does not rest at the bottom on self-assembly, but it is clear that self-assembly
is not itself sufficient to produce complex organisms, for the reasons discussed in Chapter 3.
An alternative to multilayered, shared-integron mechanisms for morphogenesis that are
based on a foundation of self-assembly therefore requires that alternatives be found to either
the multilayering or the sharing of integrons, because there are no realistic alternatives to the
foundation being self-assembly.
The presence of many layers gives morphogenetic systems great potential for feedback
and control. Low-level, local feedback can organize and optimize structures at the very
fine (
10 s) temporal scale. Higher level, more diffuse
feedback systems can integrate the activities of local systems and regulate their number,
while feedback at even larger spatial scales can coordinate the activities of 10
e
100 nm scale
structures with the embryo as a whole. Having only low-level, short-range feedback would
allow an organism to make accurate structures, but these would be hopelessly uncoordinated
once the organism reached a size much more than that of a single cell. Having only long-
range feedback would allow large-scale coordination, but it would not allow the structures
being coordinated to be made with enough accuracy to be of any actual use. Multiple layers
of control therefore confer direct selective advantages on morphogenetic systems. They
would therefore be expected to be favoured during evolution by natural selection.
Sharing of modules/integrons is also likely to be favoured, but for slightly convoluted
reasons. Organisms that succeed in evolving complex developmental programmes and
body plans must be founded on mechanisms that are amenable to rapid evolution: an
<
100 nm) spatial scale and the short (
<
