VALENTINE, J.W.: The Evolutionn of Genomic and Morphological Complexity in Metazoans: The Topology of Gene Activities

A useful measure of the complexity of an object or process is the size of its minimum description. For metazoan bodies, a thorough description would have to begin with subatomic particles and encompass all the constructional levels of the somatic hierarchy. The immensity of such descriptions makes them impractical, but indices that are somewhat sensitive to complexity, as cell-type number, may be substituted for many purposes. The minimum description of the development of a metazoan body through the activities of its genome is also immense, and a practical index is required if genomic complexities are to be usefully evaluated. Neither the size of the genome nor the number of genes correlate at all well with morphological complexity, suggesting that they cannot be used to describe or index genomic complexity. However, genes are often expressed many times and in many different settings, in response to different combinatorial signals to enhancer modules in the promoter complex. It is hypothesized that if each expression event is considered as a separate genetic entity, the total number of expression events would serve as an index of the developmental complexity of an organism. Plotted through time, gene expression events have a tree-like structure, although a given gene’s transcription may occur in a number of places along a number of branches. From the standpoint of an entity such as a cell, expression events form networks. The mean number of gene expression events per cell type is probably rather similar across metazoans; it is the addition of cell types and their positioning that are chiefly involved in the evolution of metazoan complexity, requiring new branches and branch extensions on the gene expression tree.