In a recently-published book, Eric Davidson, “the world's leading expert” in the field of evo-devo, offers his perspective “on the gene regulatory networks that control animal development”. The book is significant because Davidson “proposes a scenario to explain how these distinct types of network subcircuits have emerged in animal evolution.”
The complexity of the gene networks is well known. “These networks consist of huge sets of regulatory genes that control one another's expression as well as the expression of downstream effector genes via so-called cis-regulatory elements (to which the transcription factors bind).” The reviewer comments appreciatively on the way Davidson describes the genetic control of development in living animals: “In essence, through a stepwise process, development subdivides the embryo into territories, subterritories, and "progenitor fields" that make up a given body part of the adult animal. This stepwise subdivision is accomplished by gene regulatory network subroutines called subcircuits that consist of small sets of genes and cis-regulatory elements.” Something of this complexity can be appreciated by reference to the figure that accompanies the review.
“The final chapter, on the evolution of gene regulatory networks, is the most speculative and most stimulating. Here, Davidson outlines a possible evolutionary origin of kernels …”. The reviewer refers to three stages of evolution. “In a third stage … , the animal body parts and, concomitantly, the territorial subdivision of the developing embryo became more and more elaborate until finally the underlying subcircuits were locked down into kernels that could no longer be changed without deleterious consequences. According to Davidson, this "triumph of the bilaterian versions of animal body plans" was in place sometime before the Cambrian and has persisted, constraining metazoan evolution ever since with tremendous success.”
The concept being developed here can be related to irreducible complexity (although Davidson clearly thinks these IC systems can be constructed by evolutionary processes). The “kernel” is a term drawn from information science, referring to the core of an operating system. When these kernels were in place, they could no longer be modified by evolutionary processes, because any change was deleterious and was not preserved.
We thus have the developmental architectures of animals in place sometime before the Cambrian. This is a scenario that is discussed in that much denigrated paper by Steve Meyer (“The Origin of Biological Information and the Higher Taxonomic Categories” Proceedings of the Biological Society of Washington 117(2004):213-239.). Meyer writes: “Can neo-Darwinism explain the discontinuous increase in CSI that appears in the Cambrian explosion--either in the form of new genetic information or in the form of hierarchically organized systems of parts?” Davidson’s book undoubtedly puts more substance into the argument, spelling out some essential elements of the complexity that has to be in place for the Cambrian Explosion.
The reviewer continues: “The proposed link between the evolution of kernels and the evolution of bilaterian body plans is exciting, but it awaits validation through more comparative analyses of gene networks. As Davidson himself concedes, "for the identification of kernels … an overwhelming feature of the evidence thus far is its thinness."” This seems to be a realistic assessment of the state of knowledge. There is also the significant absence of ancestors of the Cambrian animals, despite continuing discoveries about the Ediacaran fauna. But it seems to me that the convergence between the thinking of Davidson and Meyer is such that it gives strength to the argument of Meyer’s 2004 paper. I wonder if others see the situation like this?
A Kernel Bears Fruit
Science 314, 17 November 2006: 1085-1086.
A review of: "The Regulatory Genome. Gene Regulatory Networks in Development and Evolution" by Eric H. Davidson, Academic Press (Elsevier), Burlington, MA, 2006.
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