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

Background: Vertebrate skin appendages are constructed of keratins produced by multigene families. Alpha (alpha) keratins are found in all vertebrates, while beta (beta) 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. Results: We found that the total number of alpha-keratins is lower in birds than mammals and non-avian reptiles, yet two alpha-keratin genes (KRT42 and KRT75) have expanded in birds. The beta-keratins, however, demonstrate a dynamic evolution associated with avian lifestyle. The avian specific feather beta-keratins comprise a large majority of the total number of beta-keratins, but independently derived lineages of aquatic and predatory birds have smaller proportions of feather beta-keratin genes and larger proportions of keratinocyte beta-keratin genes. Additionally, birds of prey have a larger proportion of claw beta-keratins. Analysis of alpha-and beta-keratin expression during development of chicken scales and feathers demonstrates that while alpha-keratins are expressed in these tissues, the number and magnitude of expressed beta-keratin genes far exceeds that of alpha-keratins. Conclusions: These results support the view that the number of alpha-and beta-keratin genes expressed, the proportion of the beta-keratin subfamily genes expressed and the diversification of the beta-keratin genes have been important for the evolution of the feather and the adaptation of birds into multiple ecological niches.