Developmental basis of limb evolution.

Can developmental processes account for vertebrate limb homology, the overall similarity of definitive limb structure despite differences in different taxa which often relate to evolutionary adaptations? Relevant evidence is from molecular studies, from 'cut & paste' experimental embryology and from classical descriptive accounts of embryology and structure. There is striking evidence of a similar pattern of homologous regulatory gene expression (eg Shh, and Hox A & D genes) in tetrapod limb buds, and both similarity and differences when these are compared with expression patterns in a teleost fish paired fin bud. But these findings are as yet from too few tetrapod species (chick and mouse) to permit a 'molecular bauplan' for the limb to be proposed with any certainty. Further, the identification of similar networks of regulatory genes common to non-homologous developmental systems limits possibilities for finding a basis for classical structural homology in terms of expression of system-specific genes or gene networks. An integrated approach is needed, combining evidence from the fin-limb transition, and from study of the patterns and processes of amphibian and avian limb embryology, and this points towards a conserved developmental bauplan for the pentadactyl skeleton of the type earlier proposed by Alberch. Key features include the digital arch, restriction of digit number to a maximum of 5 and stereotyped connections between prechondrogenic condensations. But this is a dynamic and not rigidly fixed bauplan. It has no single set of skeletal elements (except proximally), since the position of joint formation in the prechondrogenic condensations is not stereotyped. Urodele amphibians in particular demonstrate heterochronic differences in the timing of events. Heterochrony may underlie some of the important changes in the pentadactyl pattern during evolution.