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

Abstract

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.

Keratin: Structure, mechanical properties, occurrence in biological organisms, and efforts at bioinspiration

“…Present knowledge suggests that differentiation from the germ cells into a particular cell type (here keratinocytes) involves the programmed sequential restriction and activation of different sets of genes; a detailed discussion of the gene-controlling mechanism of gene expression can be found in the literature [74][75][76].…”

Topographical mapping of α- and β-keratins on developing chicken skin integuments: Functional interaction and evolutionary perspectives

“…Apparently, compositional changes of keratins are associated with different avian lifestyles. A new subfamily of β-keratin genes, the feather-β-keratin genes, has evolved to form a fiber-like structure, in contrast to the scale-and claw-β-keratins that form interweaving filament bundles (9,11,26). Feather-β-keratins might have evolved from scale-β-keratins by losing the glycine and tyrosine-rich tail moieties, conferring more flexibility to feathers compared with the rigid scales and claws of birds.…”

“…Evolutionarily novel genes associated with feather development were largely unknown. The radiation and expansion of β-keratins are among the few obvious changes found on the lineage leading to birds (14,24,26,61). The emergence of novel, lineage-specific, morphological features is known to have occurred through gene family expansions (62).…”

Microscopic and immunohistochemical analyses of the claw of the nesting dinosaur, Citipati osmolskae

“…Beta-keratins (or 'corneous beta-proteins' [7][8][9][10]) are insoluble, rigid, fibrous, structural proteins distinct from alpha-keratins in composition and structure. They are expressed only in reptiles and birds (sauropsids) [11][12][13], suggesting that this gene family originated after the divergence of sauropsids from other vertebrates [5,[14][15][16]. The beta-keratins have in common a core of approximately 30 amino acids and produce filaments (microfibrils) 3 nm in diameter [17].…”

“…The beta-keratins have in common a core of approximately 30 amino acids and produce filaments (microfibrils) 3 nm in diameter [17]. The core incorporates amino acids such as proline and valine [11,[17][18][19] which confers hydrophobicity, therefore, increasing preservation potential.…”