FINS, LIMBS, AND TAILS - OUTGROWTHS AND AXIAL PATTERNING IN VERTEBRATE EVOLUTION

Current phylogenies show that paired fins and limbs are unique to jawe d vertebrates and their immediate ancestry. Such fins evolved first as a single pair extending from an anterior location, and later stabiliz ed as two pairs at pectoral and pelvic levels. Fin number, identity, a nd position are therefore key issues in vertebrate developmental evolu tion. Localization of the AP levels at wh ich developmental signals in itiate outgrowth from the body wall may be determined by Hox gene expr ession patterns along the lateral plate mesoderm. This regionalization appears to be regulated independently of that in the paraxial mesoder m and axial skeleton. When combined with current hypotheses of Hox gen e phylogenetic and functional diversity, these data suggest a new mode l of fin/limb developmental evolution. This coordinates body wall regi ons of outgrowth with primitive boundaries established in the gut, as well as the fundamental nonequivalence of pectoral and pelvic structur es. BioEssays 20:371-381, 1998. (C) 1998 John Wiley & Sons, Inc.