Fins, fossils, and phylogenies: The evo‐devo of vertebrate appendage patterning

The natural diversity of vertebrate appendages is rich starting material for studying the mechanisms of pattern formation and morphological variation. Our understanding of the molecular mechanisms of appendage patterning, however, is largely drawn from model systems‐mouse, chick, and zebrafish‐ with highly derived appendage morphologies; these mechanisms are thus unlikely to represent the primitive vertebrate condition. To assess the ‘primitive vs. derived’ features of appendage patterning mechanisms, we examined the expression and regulation of key appendage patterning genes in phylogenetically basal vertebrates (skates, sharks, and paddlefish) whose paired fins have retained primitive skeletal patterns. Components of the Shh signaling cascade and 5′ Hox gene clusters, for instance, are expressed in basal vertebrate fins, but key regulatory differences are seen with regards to both tetrapod limbs and teleost fins. Combined with data from model systems and the fossil record, developmental analyses of basal vertebrate taxa will fundamentally inform our understanding of vertebrate appendage evolution.