Evolution and development of mammalian limb integumentary structures

The adaptive radiation of mammalian clades has involved marked changes in limb morphology that have affected not only the skeleton but also the integumentary structures. For example, didelphid marsupials show distinct differences in nail and claw morphology that are functionally related to the evolution of arboreal, terrestrial, and aquatic foraging behaviors. Vespertilionoid bats have evolved different volar pad structures such as adhesive discs, scales, and skin folds, whereas didelphid marsupials have apical pads covered either with scales, ridges, or small cones. Comparative analysis of pad and claw development reveals subtle differences in mesenchymal and ectodermal patterning underlying interspecific variation in morphology. Analysis of gene expression during pad and claw development reveals that signaling molecules such as Msx1 and Hoxc13 play important roles in the morphogenesis of these integumentary structures. These findings suggest that evolutionary change in the expression of these molecules, and in the response of mesenchymal and ectodermal cells to these signaling factors, may underlie interspecific differences in nail, claw, and volar pad morphology. Evidence from comparative morphology, development, and functional genomics therefore sheds new light on both the patterns and mechanisms of evolutionary change in mammalian limb integumentary structures.