TAKAHASHI, K. ; CHIU, C.-h.; AMEMIYA, C. ; RUDDLE, F.; LARSSON, H. ; WAGNER, G. P. ; Yale University, CT; Rutgers University, NJ; V. Mason Research Institute, WA; Yale University, CT; Yale University, CT; Yale University, CT: Sequence evolution of Hoxa-11 and Hoxa-13 suggests an early origin of the autopodial fieldearly origin of the autopodial field

The autopodium (i.e. hand and foot) develops from a semi-autonomous morphogenetic field of the limb bud in tetrapods, the autopodial field. During development the proximal boundary of this field is established by the expressions domains of Hoxa-11, proximal, and Hoxa-13, distal. The autopodium is the main morphological innovation that separates limbs from fins. It has been hypothesized that the origin of the autopodial field may have been caused by the evolution of the exclusive Hoxa-11/Hoxa-13 expression domain (Wagner and Chiu, 2001. J Exp Zool 291:226). This hypothesis predicts that Hoxa-13 has experienced directional selection during the fin limb transition. To test this prediction we sequenced Hoxa-11 and Hoxa-13 from basal actinopterygian and sarcopterygian taxa and analyzed the pattern of sequence evolution. The data confirms that the pattern of Hoxa-11 and Hoxa-13 differs in that Hoxa-13 has a higher rate of evolution in the sarcopterygian lineages than Hoxa-11, but rejects the prediction that this increase is limited to the fin-limb transition. In contrast, we find that the rate of evolution is increased at least as early as the stem of the rhipidistian clade, i.e. after the most recent common ancestor of coelacanths and tetrapods. This result suggests that Hoxa-13 was recruited into a new function early in the sarcopterygian lineage. A possible scenario to explain that finding is that the distal sarcopterygian fin is a developmentally autonomous field, unlike the actinopterygian fin. If this is correct, the autopodial field would be an apomorphic character of the rhipidistians rather than the tetrapods.