Continuous pharyngeal endoderm links external and internal gills
Amphibians develop both external and internal gills during ontogeny, offering an opportunity to investigate the developmental relationship between these positionally distinct respiratory organs. Although internal gills of vertebrates are widely accepted to arise from pharyngeal endoderm, external gills have long been regarded as purely ectodermal outgrowths, obscuring their relationship to other vertebrate gills. Here, we combine histological analysis with direct lineage tracing in the Mexican axolotl (Ambystoma mexicanum) and the African clawed frog (Xenopus laevis) to resolve the embryonic origin of amphibian gills. We show that the external gill develops as a continuous epithelial extension of the pharyngeal endoderm, which forms its basal epithelium and reaches the distal gill tip. In the frog, this extension remains continuous with the epithelium giving rise to the internal gills. Rather than representing separate epithelial structures, external and internal gills therefore arise from a shared epithelial domain of the pharyngeal endoderm. These findings resolve a longstanding question concerning the embryonic origin of amphibian gills and provide a developmental viewpoint for understanding how spatially diverse vertebrate gills can evolve through repeated modification of a conserved endodermal tissue.