Meeting Abstract

S3-1.4  Saturday, Jan. 5  The deep homology of the tetrapod limb: Combining fossil and genetic datasets. DAVIS, Marcus C; Kennesaw State University mdavi144@kennesaw.edu

The evolution of tetrapod limbs from fish fins was a significant functional shift. But how significant was this shift in terms of morphology and gene regulation? The fossil record provides insight into the morphological changes. However, to understand the underlying mechanisms we must peer into the gene regulatory networks of living vertebrates. Until recently, data from gene expression and functional studies in tetrapods and teleosts supported the notion that the distal region of the tetrapod limb, the autopod (wrist, ankle, and digits), was an evolutionary novelty. In contrast, the fossil data suggests that the autopod was already present in fish fins prior to the origin of tetrapods, and was subsequently modified for new adaptive roles in terrestrial locomotion, feeding, and support. Data from phylogenetically more basal extant taxa has helped to reconcile these datasets. Hox genes encode transcription factors that provide positional identity along animal axes, including the axes of the fins/limbs. Our analysis of Hox expression in a basal actinopterygian, the North American paddlefish, Polyodon spathula reveals patterns of expression long considered to be developmental hallmarks of the autopod and shown in tetrapods to be controlled by a ‘digit enhancer’ regulatory region. But we also observe differences: For example, in Polyodon, early and late phases of HoxD expression overlap proximodistally, whereas in tetrapods these phases are spatially segregated. These data demonstrate that aspects of Hox expression once considered unique to autopod sare, in fact, ancestral to tetrapod limbs. However, our data also show that tetrapod limbs exhibit a unique regulatory context – different in key ways from the fins of fish. Together, these results suggest that novelty in the tetrapod limb has arisen by changes in regulation of an ancient and conserved pattern of gene expression.