CHIU, C.-H. P.*; WAGNER, G. P.*: Molecular Evolution Across the Fin-Limb Transition

Evolutionary innovations in morphological characters are realized through the evolutionary modification of genes controlling the development of the characters. We follow this line of thought and examine the evolution of genes involved in the development of fins and limbs to gain insights into the molecular underpinnings of the origin of tetrapod limbs. Specifically we study the coding and non-coding regions of Hoxa-11 and Hoxa-13. The coding region of Hoxa-11 does not show a burst of substitutions in most of its coding region during the fin-limb transition. There is, however, an association between the presence of a cluster of Alanine residues in the Hoxa-11 protein and the derived expression pattern of Hoxa-11 typical for eu-tetrapods. In the non coding region we find several conserved sequences. One protein binding sequence is found in the intron and is conserved between zebra-fish, coelacanth and tetrapods. Another one is found in the 5′ non-coding region and shows a clade specific pattern of sequence conservation and protein binding. This result suggests that it may represent a tetrapod specific enhancer element. The coding region of Hoxa-13 also shows a tendency to acquire Alanine repeats, but in contrast to Hoxa-11 this event is not associated with the fin-limb transition but with the origin of the mammals. We conclude that the comparison of sequences among fish and tetrapods reveals several candidate differences that may be involved in major developmental transformations during the evolution of vertebrates.