Evolution ofHoxa-11Expression in Amphibians: Is the Urodele Autopodium an Innovation?

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Abstract
In this paper we present preliminary results on the expression of Hoxa-11 in the frog Xenopus laevis and the newt Notophthalmus viridescens. In amniote limbs, Hoxa-11 is expressed exclusively in the prospective zeugopodium and is involved in the development of zeugopodial character identity. While the Hoxa-11 expression pattern in Xenopus is similar to those described in mice and chick, the newt Notophthalmus has a phase of autopodial Hoxa-11 expression starting with the development of digit III. Expression is particularly strong in the buds of the postaxial digits. This Hoxa-11 expression pattern is unique among tetrapods and adds to the list of developmental peculiarities of urodele limb development, which also include an anterior to posterior polarity of digit development. We propose a scenario to explain the evolutionary origin of urodele limb development. We assume that recent urodeles are derived from a lineage of amphibians which underwent partial digit reduction, leaving only two digits intact, most likely digits III and IV according to Morse's law. This implies that urodele digits I and II are homologous to amniote digits III and IV, and that the postaxial digits of urodeles are an evolutionary innovation (re-invention). Supporting evidence for this homology hypothesis is found in the developmental connections between the basale commune and the postaxial zeugopodial element.

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