Abstract

Salamanders are the only tetrapods capable of regenerating limbs as adults, however the evolutionary origin of this remarkable ability remains unclear. The only other sarcopterygian capable of regenerating its fin endoskeleton are the lungfishes. Regeneration of dermal fin rays is common among teleost fish, yet fin endoskeleton regeneration has only been reported in living representatives of the non-teleost clade Cladistia, family Polypteridae. Therefore, the explanation for the phylogenetic distribution of vertebrate appendage regeneration as a trait remains elusive. Here, we combine experimental studies in non-teleost actinopterygians and comparative RNA -seq analyses in an effort to resolve the evolutionary origin of limb and fin regeneration. First, we demonstrate that among actinopterygians, fin endoskeleton regeneration is not restricted to living representatives of early diverging clade Cladistia, but is also present in species of the other two non-teleost clades: the paddlefish, Polyodon spathula (Chondrostei) and the spotted gar, Lepisosteus oculatus (Holostei). Next, we generated transcriptome assemblies of regenerating and non-regenerating appendages for Polypterus, lungfish, axolotl, zebrafish and mouse digit tips. Our comparative RNA -seq analysis provides compelling evidence for a shared appendage regeneration program between axolotl limbs and Polypterus and lungfish fins. Altogether, our findings provide strong support for an evolutionary scenario in which an appendage endoskeleton regeneration program first arose in osteichthyes and was subsequently lost in amniotes and teleosts.