Commun Biol 2026 Jun 20; doi: 10.1038/s42003-026-10519-y.

Hoxc13 and homologs of mammalian hair keratins are required for the cornification of nuptial pads in Xenopus frogs.

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The evolution of skin appendages has involved innovation at the levels of keratin intermediate filament proteins, which build stable cornified structures, and regulatory factors, which determine the anatomical location of skin appendages. The keratin components of nails and claws have evolved at the digit tips under the control of the transcription factor Hoxc13 in a common ancestor of tetrapods. The additional expression of hoxc13 at other skin sites allowed the evolution of hair in mammals. Here, we show that the cornification of epithelial spines within nuptial pads of Xenopus frogs involves the expression of the type II hair keratin homolog, krt59. The expression of hoxc13 is enriched in the skin of the forelimbs, and CRISPR-Cas9-mediated inactivation of hoxc13 abolished both the expression of krt59 and epithelial cornification in nuptial pads of X. tropicalis. We conclude that cornified nuptial spines have evolved in parallel to mammalian hair by adaptation of the ancestral cornification program of skin appendages.

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