Fins, limbs, and tails: outgrowths and axial patterning in vertebrate evolution

Current phylogenies show that paired fins and limbs are unique to jawed vertebrates and their immediate ancestry. Such fins evolved first as a single pair extending from an anterior location, and later stabilized as two pairs at pectoral and pelvic levels. Fin number, identity, and position are therefore key issues in vertebrate developmental evolution. Localization of the AP levels at which developmental signals initiate outgrowth from the body wall may be determined by Hox gene expression patterns along the lateral plate mesoderm. This regionalization appears to 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 regions of outgrowth with primitive boundaries established in the gut, as well as the fundamental nonequivalence of pectoral and pelvic structures. BioEssays 20:371–381, 1998.