Convergent Evolution of Cysteine-Rich Keratins in Hard Skin Appendages of Terrestrial Vertebrates

The study demonstrated that cysteine-rich keratins, crucial for the structural integrity of hard skin appendages like hair and claws in mammals, also evolved independently in reptiles and birds. These keratins, termed hard acidic (HAS) and hard basic (HBS) sauropsid-specific keratins, were found in various reptiles and birds, with HBS1 keratin specifically expressed in chicken feathers. Molecular phylogenetics indicated that these keratins evolved through convergent evolution, suggesting that similar biomechanical functions in different clades' hard skin appendages arose independently. The findings proposed an evolutionary model where HAS and HBS keratins became structural proteins in reptiles, with at least one HBS keratin being co-opted into feathers post-divergence of birds from reptiles.