THE ORIGIN AND EVOLUTION OF ANIMAL APPENDAGES

Animals have evolved diverse appendages adapted for locomotion, feedin g and other functions. The genetics underlying appendage formation are best understood in insects and vertebrates, The expression of the Dis tal-less (Dll) homeoprotein during arthropod limb outgrowth and of Dll orthologs (Dlx) in fish fin and tetrapod limb buds led us to examine whether expression of this regulatory gene may be a general feature of appendage formation in protostomes and deuterostomes. We find that DI I is expressed along the proximodistal axis of developing polychaete a nnelid parapodia, onychophoran lobopodia, ascidian ampullae, and even echinoderm tube feet. Dll/Dlx expression in such diverse appendages in these six coelomate phyla could be convergent, but this would have re quired the independent co-option of DIll/Dlx several times in evolutio n. It appears more likely that ectodermal Dll/Dlx expression along pro ximodistal axes originated once in a common ancestor and has been used subsequently to pattern body wall outgrowths in a variety of organism s. We suggest that this pre-Cambrian ancestor of most protostomes and the deuterostomes possessed elements of the genetic machinery for and may have even borne appendages.