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Evolutionarily conserved developmental pathways
Animals have evolved diverse appendages adapted for locomotion, feeding and other functions. The genetics underlying appendage formation is best understood in insects and vertebrates. The expression of the Distal-less (Dll) homeoprotein during arthropod limb 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 of appendage formation in protostomes and deuterostomes. Dll is expressed along the proximodistal axis of developing polychaete annelid parapodia (Annelida are segmented worms), onychophoran lobopodia (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 in such diverse appendages in these coelomate phyla could be convergent, but this would have required the independent co-option of Dll/Dlx several times in evolution. It appears more likely that ectodermal Dll/Dlx expression along limb proximodistal axes originated once in a common ancestor and has been used subsequently to pattern body wall outgrowths in a variety of organisms. It is suggested that this pre-Cambrian ancestor of most protostomes and the deuterostomes possessed elements of the genetic 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).
REFERENCE
Panganiban, G., et al. (1997). The origin and evolution of animal appendages. Proc. Natl. Acad. Sci. 94 (10): 5162-5166. Medline abstract: 97289737
date revised: 30 June 97
Developmental Pathways conserved in Evolution
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