Friday, October 22, 2010

Ravello meeting on Chance and Necessity, part III (last one)

I am at the Ravello meeting on Chance and Necessity in biology, on the 40th anniversary of Jacques Monod's seminal book, and will be posting a few entries while the meeting is going on this week.
The gathering is organized by Giorgio Bernardi, sponsored by International Union of Biological Sciences and Istituto Italiano di Studi Filosofici.
What follows are the raw and somewhat selective notes only, in order of presentation of the various speakers. Hopefully this will provide a feeling for what the meeting is about and generate some discussion. Throughout, parenthetical comments are my own, unless otherwise noted.
Denis Duboule, Constraints (necessity) and flexibility (chance) in the evolution of vertebrate morphologies.
Across vertebrates the structure of proximal bones is strongly constrained, while there is a lot of variation in distal structures, like the number and shape of digits. This pattern appears to be related to the pattern of deployment of a cluster of Hox genes during the development of vertebrate limbs. It is the differential regulation of distal Hox that generates the type of phenotypic variation that shows up in evolution. The reason the proximal pattern of the limb is much more constrained is because its regulation has been co-opted from the trunk, and the latter is obviously resistant to evolutionary change. (Nice and elegant explanation.) There are exceptions, like limbless lizards and snakes. But in those cases, obviously, you do also observe dramatic changes in the trunk. There is a similar reason why tetrapods cannot have symmetrical limbs: the developmental genes that cause the asymmetry are co-opted from the trunk, and changing the pattern would affect the trunk in inviable ways.
Walter Gehring, Chance and necessity in eye evolution.
Jacques Monod compared the eye to the camera to highlight both the similarities, as in the relation between form and function, and the difference between teleonomy - for the eye - and teleology - for the camera. Monod's insight is confirmed by modern research on how genetic control is deployed during the development of the eye: the observed patterns are clearly not the sort of thing that an engineer would put in place, but are instead the kind of hodgepodge that results from sequentially overlapping historical events. Eyes of vertebrates, insects, Cephalopoda, and other invertebrates have been thought as non homologous because they are morphologically different and because they develop differently. Molecular biology however shows a "deep homology" in the fact that all these eyes are affected by different version of the Pax6 gene. (But of course that raises the thorny question of the degree of congruence of homology at different levels: is the genetic one more fundamental than the developmental one? On what grounds?)
Takashi Gojobori, Chance and necessity in the evolution of connections between sensory and nervous systems.
Starts out with gene expression in Planaria brains, the most primitive of all structures that we recognize as brains. Turns out that half of known Planarian genes expressed in the head are shared by humans. Next, what about Hydra, which does not have a central neural system, just a diffuse nerve set? Again, half of the relevant genes are also expressed in human nerve cells. What about sea urchins, which have lost a central nervous system? Sure enough, gene expression patterns show that the arm of sea urchin larvae are degenerated from an ancestral more fully developed nervous system. Looking for connection between sensory and nervous systems back in the Hydra, because of the simplicity of their nervous system. Focus on gap junctions as precursors of fully formed sensory-nervous connections. (Once again, not much here about Monod, chance or necessity, but it’s near the end of the meeting...)

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