MOLECULAR PATHWAYS TO PARALLEL EVOLUTION .1. GENE NEXUSES AND THEIR MORPHOLOGICAL CORRELATES

Aspects of the regulatory interactions among genes are probably as old as most genes are themselves. Correspondingly, similar predisposition s to changes in such interactions must have existed for long evolution ary periods. Features of the structure and the evolution of the system of gene regulation furnish the background necessary for a molecular u nderstanding of parallel evolution. Patently ''unrelated'' organs, suc h as the fat body of a fly and the liver of a mammal, can exhibit frac tional homology, a fraction expected to become subject to quantitation . This also seems to hold for different organs in the same organism, s uch as wings and legs of a fly. In informational macromolecules, on th e other hand, homology is indeed all or none. In the quite different c ase of organs, analogy is expected usually to represent attenuated hom ology. Many instances of putative convergence are likely to turn out t o be predominantly parallel evolution, presumably including the case o f the vertebrate and cephalopod eyes. Homology in morphological featur es reflects a similarity in networks of active genes. Similar nexuses of active genes can be established in cells of different embryological origins. Thus, parallel development can be considered a counterpart t o parallel evolution. Specific macromolecular interactions leading to the regulation of the c-fos gene are given as an example of a ''contro ller node'' defined as a regulatory unit. Quantitative changes in gene control are distinguished from relational changes, and frequent paral lelism in quantitative changes is noted in Drosohpila enzymes. Evoluti onary reversions in quantitative gene expression are also expected. Th e evolution of relational patterns is attributed to several distinct m echanisms, notably the shuffling of protein domains. The growth of suc h patterns may in part be brought about by a particular process of com pensation for ''controller gene diseases,'' a process that would spont aneously tend to lead to increased regulatory and organismal complexit y. Despite the inferred increase in gene interaction complexity, whose course over evolutionary time is unknown, the number of homology grou ps for the functional and structural protein units designated as domai ns has probably remained rather constant, even as, in some of its bran ches, evolution moved toward ''higher'' organisms. In connection with this process, the question is raised of parallel evolution within the purview of activating and repressing master switches and in regard to the number of levels into which the hierarchies of genic master switch es will eventually be resolved.