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Evolutionarily conserved developmental pathways
Animals have evolved diverse appendages adapted for locomotion, feeding and otherfunctions. The genetics underlying appendage formation is best understood in insectsand vertebrates. The expression of the Distal-less (Dll) homeoprotein during arthropodlimb outgrowth and of Dll orthologs (Dlx) in fish fin and tetrapod limb buds led to a proposal that this regulatory gene is a general feature ofappendage formation in protostomes and deuterostomes. Dll is expressedalong the proximodistal axis of developing polychaete annelid parapodia (Annelida are segmented worms), onychophoranlobopodia (onychophorans share affinity with both annelids and arthropods), ascidian ampullae (ascidians are Urochordates), and even echinoderm tube feet (echinoderms are deuterostome invertebrates). Dll/Dlx expression insuch diverse appendages in these coelomate phyla could be convergent, but thiswould have required the independent co-option of Dll/Dlx several times in evolution. Itappears more likely that ectodermal Dll/Dlx expression along limb proximodistal axesoriginated once in a common ancestor and has been used subsequently to pattern bodywall outgrowths in a variety of organisms. It is suggested that this pre-Cambrianancestor of most protostomes and the deuterostomes possessed elements of thegenetic machinery for appendages, and may have even borne appendages. Since Dll/Dlx genes are also expressed in the annelid and onychophoran CNS, Dll/Dlx function may well have arisen in the CNS before becoming involved in body wall outgrowths (Panganiban, 1997).
Panganiban, G., et al. (1997). The origin and evolution of animal appendages. Proc. Natl. Acad. Sci. 94 (10): 5162-5166. PubMed Citation: 9144208
date revised: 30 June 97
Developmental Pathways conserved in Evolution
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