Lissamphibian Limbs and the Origins of Tetrapod Hox Domains

The expression and function of hox genes have played a key role in the debate on the evolution of limbs from fins. As an early branching tetrapod lineage, lissamphibians may provide information on the origin of the limb's hox domains and particularly how the plesiomorphic tetrapod pattern compares to the hox pattern present in fish fins. Here, we comparatively investigated the expression of hox genes in the developing limbs of axolotl and Xenopus laevis as well as in the fins of the direct developing cichlid Astatotilapia burtoni. In contrast to axolotl, which has only very low digital expression of hoxd11, Xenopus limbs recapitulate the reverse collinear hoxd expression pattern known from amniotes with clearly defined proximal and distal hoxd11 expression domains. For hoxa genes, we observe that in Xenopus limbs, as in axolotl, a clear distal domain of hoxa11 expression is present, although in the presence of a hoxa11 antisense transcript. Investigation of fins reveals the presence of hoxa11 antisense transcription in the developing fin rays in a domain similar to that of hoxa13 and overlapping with hoxa11 sense transcription. Our results indicate that full exclusion of hoxa11 from the autopod only became firmly established in amniotes. The distal antisense transcription of hoxa11, however, appears to predate the evolution of the limb, but likely originated without the concurrent implementation of the transcriptional suppression mechanism that causes mutually exclusive hoxa11 and hoxa13 domains in amniotes.