Dynamic evolution of the alpha (α) and beta (β) keratins has accompanied integument diversification and the adaptation of birds into novel lifestyles

Abstract

BackgroundVertebrate skin appendages are constructed of keratins produced by multigene families. Alpha (α) keratins are found in all vertebrates, while beta (β) keratins are found exclusively in reptiles and birds. We have studied the molecular evolution of these gene families in the genomes of 48 phylogenetically diverse birds and their expression in the scales and feathers of the chicken.ResultsWe found that the total number of α-keratins is lower in birds than mammals and non-avian reptiles, yet two α-keratin genes (KRT42 and KRT75) have expanded in birds. The β-keratins, however, demonstrate a dynamic evolution associated with avian lifestyle. The avian specific feather β-keratins comprise a large majority of the total number of β-keratins, but independently derived lineages of aquatic and predatory birds have smaller proportions of feather β-keratin genes and larger proportions of keratinocyte β-keratin genes. Additionally, birds of prey have a larger proportion of claw β-keratins. Analysis of α- and β-keratin expression during development of chicken scales and feathers demonstrates that while α-keratins are expressed in these tissues, the number and magnitude of expressed β-keratin genes far exceeds that of α-keratins.ConclusionsThese results support the view that the number of α- and β-keratin genes expressed, the proportion of the β-keratin subfamily genes expressed and the diversification of the β-keratin genes have been important for the evolution of the feather and the adaptation of birds into multiple ecological niches.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-014-0249-1) contains supplementary material, which is available to authorized users.

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

“…Furthermore, comparative genomic studies showed that the number of α-and β-keratin genes and β-keratin gene diversity were important for feather evolution and the adaptation of birds to diverse ecological niches (13,14,24,25). Apparently, compositional changes of keratins are associated with different avian lifestyles.…”

“…A recent study found that regulatory innovations of feather development genes predate the origin of feathers, suggesting that the avian dinosaur ancestor already had the nonkeratin protein-coding toolkit for making feathers (12). While fewer new genes have been found in bird genomes (13) and the α-keratin gene family has shrunk in birds relative to mammals and reptiles (14,15), the expansion of β-keratin genes is one of the most unique genomic features of birds (13,14).…”

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].…”

Avian genomics: fledging into the wild!

“…A study of keratin genes, which are important for the evolution of feathers early or just before the avian radiation, is an example of how to use whole genome information to gain knowledge about flight evolution in birds in general (Greenwold et al 2014). Comparative gene family evolution across birds, and also mammals, has been shown to become highly informative by two exemplary studies: the adaptive gene complex of avian haemoglobin isoform expression (Opazo et al 2014) and a general vertebratewide analysis of hedgehog genes, which are key switches during embryonal development (Pereira et al 2014).…”