WAGNER, G P; STADLER, P ; METSCHER, B; TAKAHASHI, K; AMEMIYA, C; RUDDLE, F; Yale University; University of Leipzig; Southern Indiana University; Yale University; Virigina Mason Research Center; Yale University; ; : The Origin of Tetrapod Limbs: Developmental Evolution meets Molecular Evolution

The transition from aquatic to terrestrial life is one of the most dramatic evolutionary transitions in the history of vertebrates. Of the many phenotypic changes associated with this transition the changes in the paired appendages are the most intensely studied. In this paper we discuss how the definition of an evolutionary novelty defines the research agenda for studying the origin of tetrapod limbs. The first step is a clear understanding of the mode of character evolution. This step sets the agenda for the investigation of the developmental basis of this innovation. In the case of the tetrapod limb there is a broad agreement that the critical innovation is the origin of the autopod, i.e. the hand and the foot. The next question is what are the developmental innovations are that underlie the origin of the morphological novelty. It is proposed that there are possibly four distinct events that require explanation: 1. The loss of dermal fin rays, 2. The origin of the autopodial morphogenetic field, 3. The origin of the mesopodium (ankle and wrist) and 4. The origin of digits. It is not yet clear to what extend these events happened independently of each other or whether some of them are mechanistically linked. Experimental developmental data on derived species like mouse do not provide a clear picture. It is argued that hypotheses about the evolutionary recruitment of developmental genes, in particular transcription factor genes, can be tested by studying the evolution of gene expression as well as the evolution of coding as well as non-coding sequences. We will present preliminary evidence suggesting that this is in deed the case.