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

The archosauria consist of two living groups, crocodilians, and birds. Here we compare the structure, expression, and phylogeny of the beta ()-keratins in two crocodilian genomes and two avian genomes to gain a better understanding of the evolutionary origin of the feather -keratins. Unlike squamates such as the green anole with 40 -keratins in its genome, the chicken and zebra finch genomes have over 100 -keratin genes in their genomes, while the American alligator has 20 -keratin genes, and the saltwater crocodile has 21 -keratin genes. The crocodilian -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 -sheets, which form the 2-3nm filament. Overall the expression of alligator -keratin genes in the integument increases during development. Phylogenetic analysis demonstrates that a crocodilian -keratin clade forms a monophyletic group with the avian scale and feather -keratins, suggesting that avian scale and feather -keratins along with a subset of crocodilian -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 -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 -keratins. The expansion of the avian specific feather -keratin genes accompanied the diversification of birds and the evolution of feathers. J. Exp. Zool. (Mol. Dev. Evol.) 320B: 393-405, 2013. (c) 2013 Wiley Periodicals, Inc.