FROM FINS TO LIMBS - TOWARDS A MOLECULAR APPROACH TO THE EVOLUTION OFVERTEBRATE PAIRED APPENDAGES

Over the past few years, several genes involved in the development of the tetrapod limbs have been isolated and characterized. Thus, it beco mes possible to study some of the developmental mechanisms responsible for the morphogenesis of these complex structures through a novel app roach. Alternatively, these genes can be used as tools to examine, at the molecular level, some aspects of the fascinating question of the f in to limb transition. Such an approach provides additional facts whic h illustrate both the similarities an differences in the morphologies of these homologous structures. In particular, the study of zebrafish (Danio rerio) Hox genes suggests a scenario for the fin-limb transitio n in which the distal parts of the autopods (the digits) are neomorphi c structures produced by unequal proliferation of the posterior part o f an ancestral appendix. Such an asymmetric morphogenesis could he the reason why tetra-pods have their major limb axes bent anteriorly, tow ards their distal ends, while the homologous fin axis extends straight from proximal to distal.