PRUM, R. O. ; University of Kansas, Lawrence, KS: Mechanisms of Innovation in Evolution of Feathers

Feathers are complex integumentary appendages with extraordinary diversity in structure and function. Recent research into the hierarchical and modular nature of feather development and morphology provides new perspectives on the mechanisms of innovation in feather evolution. Feather development and morphology are characterized by numerous morphological modules that vary in their levels of independence, covariation, and interaction. Feather complexity has evolved through a series of developmental novelties. The initial novelty was the origin of the tubular feather germ and follicle. The tubular germ was followed by novel compartmentalization of the germ into barb ridges, helical growth of barb ridges, the origin of barbule plates, and other stages. Molecular developmental studies have demonstrated a fundamental role for a molecular signaling module composed of Sonic Hedgehog and Bone morphogenetic protein 2 in the development and evolution of innovations in feather morphology. This Shh/Bmp2 module was primitively incorporated in the developmental mechanisms of archosaurian scale placodes, but was co-opted repeatedly for the control of development and differentiation during the evolution of various feather innovations. Molecular developmental research provides insight into the roles of developmental and morphological hierarchy and modularity in macroevolutionary innovations of feathers.