Animals have evolved diverse appendages adapted for locomotion, feeding and other functions. The genetics underlying appendage formation are 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 us to examine whether expression of this regulatory gene may be a general feature of appendage formation in protostomes and deuterostomes. We find that Dll is expressed along the proximodistal axis of developing polychaete annelid parapodia, onychophoran lobopodia, ascidian ampullae, and even echinoderm tube feet. Dll͞Dlx expression in such diverse appendages in these six coelomate phyla could be convergent, but this would have required the independent co-option of Dll͞Dlx several times in evolution. It appears more likely that ectodermal Dll͞Dlx expression along proximodistal axes originated once in a common ancestor and has been used subsequently to pattern body wall outgrowths in a variety of organisms. 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.
Phenotypic analysis of both zebrafish and mouse has shown that fibroblast growth factor 8 (FGF8) is required for many developmental decisions. To further our understanding of the FGF8 signaling process, we sought to identify new transcriptional targets of the pathway. Here, we propose that two zebrafish ETS genes, pea3 and erm, are general targets of FGF8 signaling, based upon the following observations: both genes are expressed around all early FGF8 signaling sources, both genes are downregulated in fgf8 mutant embryos in all tissues known to require fgf8 function, a pharmacological inhibitor of the FGF pathway completely abolishes expression of both genes, and ectopic expression of fgf8 is sufficient to induce both genes. The finding that pea3 and erm are common transcriptional targets of FGF8 signaling suggests that they are general mediators of FGF8 signaling during development. In addition, we observed that pea3 is often expressed close to an FGF8 source, and erm is expressed in a broader domain. To test whether this differential expression is established by FGF8, we have induced FGF8 ectopically and show that it is sufficient to recapitulate the endogenous nested expression pattern of pea3 and erm.
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