Evolution of Hoxa-11 expression in amphibians: Is the urodele autopodium an innovation?

In this paper we present preliminary results on the expression of Hoxa-11 i n the frog Xenopus laevis and the newt Notophthalmus viridescens. hi amniot e limbs, Horn-II is expressed exclusively in the prospective zeugopodium an d is involved in the development of zeugopodial character identity. While t he Horn-II expression pattern in Xenopus is similar to those described in m ice and chick, the newt Notophthalmus has a phase of autopodial Horn-II exp ression starting with the development of digit III. Expression is particula rly strong in the buds of the postaxial digits. This Horn-II expression pat tern is unique among tetrapods and adds to the list of developmental peculi arities of urodele Limb development, which also include an anterior to post erior polarity of digit development. We propose a scenario to explain the e volutionary origin of urodele limb development. We assume that recent urode les are derived from a lineage of amphibians which underwent partial digit reduction, leaving only two digits intact, most likely digits LU and TV acc ording to Morse's law, This implies that urodele digits I and II are homolo gous to amniote digits III and IV, and that the postaxial digits of urodele s are an evolutionary innovation (re-invention), Supporting evidence for th is homology hypothesis is found in the developmental connections between th e basale commune and the postaxial zeugopodial element.