Vertebrate axial and appendicular patterning: The early development of paired appendages

Determination of paired fin or limb number, identity and position are key i ssues 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 an terior location; that appendicular endoskeletons are primitively AP asymmet ric,, and that pectoral and pelvic fins primitively differ. It is conjectur ed that Hox gene expression patterns along the lateral plate mesoderm estab lish boundaries that contribute to localisation of AP levels at which signa ls initiate outgrowth from the body wall. Such regionalisation may be regul ated independently of that in the paraxial mesoderm and axial skeleton. Whe n combined with current hypotheses of Hox gene phylogenetic and functional diversity, these data suggest a new model of fin/limb developmental evoluti on. This coordinates body wall outgrowth regions with primitive boundaries established in the gut, and the fundamental non-equivalence of pectoral and pelvic structures.