Alexander V. Spirov: Self-assemblage of gene nets in evolution via recruiting of new netters (continued)

2 Self-organisation, morphogenesis and evolution

Our growth of gene nets via recruiting of new netters is reminiscent, first of all, of the behavior of Stuart Kauffman's (1993) genomic regulatory network. Kitano (1994) started to use Kauffman's boolean networks to simulate a kind of metabolic reaction amongst networks of genomic regulatory elements. The GAs code for rules which construct boolean networks. Dellaert and Beer's (1994) paper models the genetic regulatory network by a Boolean network, extended to systems of multiple, communicating networks. In this way, the results of the GA application will be self-organizing systems whose attractor behaviour dictates predefined phenotypic traits. This is reminiscent also of the Clark et al. (1993) model of evolution of the network of regulatory genes.

The level of our formal description is far from the refined model of the Drosophila segmentation mechanism by the gene circuit method (Reinitz et al., 1995; Reinitz & Sharp, 1995). Unlike their approach, in our computations non-linear, Hill-type kinetics of gene activation are essential.

Simulations of genome growth during natural selection have shown that such a process would lead to modular organisation. The trend among the genes that the genome keeps is thus towards a modular genetic representation of the phenotype (Altenberg, 1995). The model of constructional selection assumes that modularity evolves by preferential duplication of genes with fewer pleiotropic effects (Altenberg, 1994). Altenberg proposes that the events surrounding the creation of new genes may play a special role in the evolution of the genotype to phenotype map because of their distinct property of adding new degrees of freedom to the genome. A recurrent theme in the biological literature is the concept of modularity, the fact that higher organisms are composed of semi-autonomous units (for references see Wagner & Altenberg, 1995). The concept of modularity was clearly expressed by John Bonner in his concept of gene nets (Bonner, 1988). According to Bonner's definition, a gene net is a network of gene actions and their products grouped into discrete units during the course of development.

We can point to numerous examples of gene elements that appear to have proliferated in the genome. As mentioned by Altenberg (1995), such a gene family as antennapedia-class vertebrate homeobox genes (Kappen et al., 1989; Schughart et al., 1989), shows periods of exponential evolutionary growth, possibly followed by logistic-like stasis as the genes saturated the available adaptive opportunities, departed from their original effects on organismal function, or became functionally burdened. One more conclusion from Altenberg's model is that during periods of exponential gene family growth, most new genes should come from genes that are themselves new. A clear example of such a situation has been discussed by Akam et al. (1988) for evolution of insect homeotic Antp-like genes.

CSTB Bulletin - Spring 96

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