Molecular evolution and expression of archosaurian β‐keratins: Diversification and expansion of archosaurian β‐keratins and the origin of feather β‐keratins

The archosauria consist of two living groups, crocodilians, and birds. Here we compare the structure, expression, and phylogeny of the beta (β)‐keratins in two crocodilian genomes and two avian genomes to gain a better understanding of the evolutionary origin of the feather β‐keratins. Unlike squamates such as the green anole with 40 β‐keratins in its genome, the chicken and zebra finch genomes have over 100 β‐keratin genes in their genomes, while the American alligator has 20 β‐keratin genes, and the saltwater crocodile has 21 β‐keratin genes. The crocodilian β‐keratins are similar to those of birds and these structural proteins have a central filament domain and N‐ and C‐termini, which contribute to the matrix material between the twisted β‐sheets, which form the 2–3 nm filament. Overall the expression of alligator β‐keratin genes in the integument increases during development. Phylogenetic analysis demonstrates that a crocodilian β‐keratin clade forms a monophyletic group with the avian scale and feather β‐keratins, suggesting that avian scale and feather β‐keratins along with a subset of crocodilian β‐keratins evolved from a common ancestral gene/s. Overall, our analyses support the view that the epidermal appendages of basal archosaurs used a diverse array of β‐keratins, which evolved into crocodilian and avian specific clades. In birds, the scale and feather subfamilies appear to have evolved independently in the avian lineage from a subset of archosaurian claw β‐keratins. The expansion of the avian specific feather β‐keratin genes accompanied the diversification of birds and the evolution of feathers. J. Exp. Zool. (Mol. Dev. Evol.) 320B: 393–405, 2013 . © 2013 Wiley Periodicals, Inc.