Compatible limb patterning mechanisms in urodeles and anurans.

We have experimentally tested the similarity of limb pattern-forming mechanisms in urodeles and anurans. To determine whether the mechanisms of limb outgrowth are equivalent, we compared the results of two kinds of reciprocal limb bud grafts between Xenopus and axolotls: contralateral grafts to confront anterior and posterior positions of graft and host, and ipsilateral grafts to align equivalent circumferential positions. Axolotl limb buds grafted to Xenopus hosts are immunologically rejected at a relatively early stage. Prior to rejection, however, experimental (but not control) grafts form supernumerary digits. Xenopus limb buds grafted to axolotl hosts are not rejected within the time frame of the experiment and therefore can be used to test the ability of frog cells to elicit responses from axolotl tissue that are similar to those that are elicited by axolotl tissue itself. When Xenopus buds were grafted to axolotl limb stumps so as to align circumferential positions, the majority of limbs did not form any supernumerary digits. However, in experimental grafts, where anterior and posterior of host and graft were misaligned, supernumerary digits formed at positional discontinuities. These results suggest that Xenopus/axolotl cell interactions result in responses that are similar to axolotl/axolotl cell interactions. Furthermore, axolotl and Xenopus cells can cooperate to build recognizable skeletal elements, despite large differences in cell size and growth rate between the two species. We infer from these results that urodeles and anurans share the same limb pattern-forming mechanisms, including compatible positional signals that allow appropriate localized cellular interactions between the two species. Our results suggest an approach for understanding homology of the tetrapod limb based on experimental cellular interactions.