Linking the molecular evolution of avian beta (β) keratins to the evolution of feathers

Abstract

Feathers of today's birds are constructed of beta (β)-keratins, structural proteins of the epidermis that are found solely in reptiles and birds. Discoveries of "feathered dinosaurs" continue to stimulate interest in the evolutionary origin of feathers, but few studies have attempted to link the molecular evolution of their major structural proteins (β-keratins) to the appearance of feathers in the fossil record. Using molecular dating methods, we show that before the appearance of Anchiornis (∼155 Million years ago (Ma)) the basal β-keratins of birds began diverging from their archosaurian ancestor ∼216 Ma. However, the subfamily of feather β-keratins, as found in living birds, did not begin diverging until ∼143 Ma. Thus, the pennaceous feathers on Anchiornis, while being constructed of avian β-keratins, most likely did not contain the feather β-keratins found in the feathers of modern birds. Our results demonstrate that the evolutionary origin of feathers does not coincide with the molecular evolution of the feather β-keratins found in modern birds. More likely, during the Late Jurassic, the epidermal structures that appeared on organisms in the lineage leading to birds, including early forms of feathers, were constructed of avian β-keratins other than those found in the feathers of modern birds. Recent biophysical studies of the β-keratins in feathers support the view that the appearance of the subfamily of feather β-keratins altered the biophysical nature of the feather establishing its role in powered flight.

Ground tit genome reveals avian adaptation to living at high altitudes in the Tibetan plateau

“…Avian feather b-keratin genes encode proteins vital for the formation of feathers, and recent biophysical studies have demonstrated the role of b-keratin genes in forming the elasticity of the feather 32 . Cameron et al 33 found that birds capable of flight generally have a higher b-keratin composition in their …”

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

“…However, molecular dating studies show that the basal β-keratins of birds began diverging from their archosaurian ancestor earlier than 200 Mya (million years ago). But, the subfamily of feather-β-keratins, as found in living birds, did not begin to diverge until approximately 143 Mya (27). These findings combine to suggest it is likely that avian dinosaur ancestors might have evolved some primitive types of β-keratin to make their ancient feathers.…”

Evolution of dinosaur epidermal structures

“…The latter possibility is suggested by: possession of feathers and protofeathers in theropods [2][3][4][5]; presence of filamentous coverings in at least some pterosaurs [18]; and development of elaborate midline scale frills in hadrosaurs and sauropods (electronic supplementary material, S1). As archosaur scales, claws and feathers are composed of b-keratins [20,21], it is possible that the elaboration of all complex ornithodiran epidermal structures was underpinned by the same developmental and regulatory mechanisms ( [22]: which, for unknown reasons, were not expressed in the majority of non-coelurosaurian dinosaurs). Molecular phylogenies of b-keratin families indicate that those found in feathers are the latest diverging among archosaurs and may not have appeared prior to the evolution of crown birds, whereas scale and 'feather-like' b-keratins diverged earlier [21].…”

“…Additional homology tests are required, such as identification of b-keratin antibody reactivity, detailed comparisons between collagenous and epidermal structures, and identification of microstructural differences between elongate scales and protofeathers [5,22,26]. However, as reptilian scale and feather b-keratins are compositionally similar [20,21], it may be difficult to distinguish elongate scales from genuine protofeathers on the basis of biogeochemistry and gross morphology. Current data indicate that feathers and their filamentous homologues are probably theropod synapomorphies but fail to support the hypothesis that protofeathers are plesiomorphic for Dinosauria.…”