Posterior shift of Shh-Fgf signaling in axolotl limb regeneration drives sequential digit formation

Do conserved morphogen modules generate a given form only via a single spatiotemporal deployment, or can alternative deployments yield the same morphology? Axolotl limb regeneration provides a tractable test bed. SHH and FGF8, universally used in limb formation across vertebrates, also operate in axolotl, although the spatial domain of Fgf8 differs markedly from that in amniotes. A mutual Shh/Fgf feedback loop is likewise conserved. Here, we show that, in axolotl, the active domain of this loop shifts progressively posterior as digit formation proceeds. In step with this anterior-to-posterior displacement, digit-forming regions are sequentially induced posteriorly, explaining the axolotl's reversed order relative to the amniote posterior-to-anterior sequence. These findings indicate that conserved molecular toolkits can overcome differences in spatial deployment to produce equivalent final limb architectures, demonstrating that there is not a single route to a target morphology.