© 1999 by The Society for Integrative and Comparative Biology
Vertebrate Axial and Appendicular Patterning: The Early Development of Paired Appendages1
*Department of Biology, University College London Gower Street, London WC1E 6BT UK
School of Animal and Microbial Sciences, University of Reading Whiteknights, Reading, RG6 6AJ UK
Correspondence: 2E-mail: m.coates{at}ucl.ac.uk
Determination of paired fin or limb number, identity and position are key issues in vertebrate development and evolution. Phylogenies including fossil data show that paired appendages are unique to jawed vertebrates and their immediate ancestry; that such fins evolved first as a single pair in an anterior location; that appendicular endoskeletons are primitively AP asymmetric; and that pectoral and pelvic fins primitively differ. It is conjectured that Hox gene expression patterns along the lateral plate mesoderm establish boundaries that contribute to localisation of AP levels at which signals initiate outgrowth from the body wall. Such regionalisation may be regulated independently of that in the paraxial mesoderm and axial skeleton. When combined with current hypotheses of Hox gene phylogenetic and functional diversity, these data suggest a new model of fin/limb developmental evolution. This coordinates body wall outgrowth regions with primitive boundaries established in the gut, and the fundamental non-equivalence of pectoral and pelvic structures.