Abstract
BackgroundThe epidermal appendages of reptiles and birds are constructed of beta (β) keratins. The molecular phylogeny of these keratins is important to understanding the evolutionary origin of these appendages, especially feathers. Knowing that the crocodilian β-keratin genes are closely related to those of birds, the published genomes of the chicken and zebra finch provide an opportunity not only to compare the genomic organization of their β-keratins, but to study their molecular evolution in archosaurians.ResultsThe subfamilies (claw, feather, feather-like, and scale) of β-keratin genes are clustered in the same 5' to 3' order on microchromosome 25 in chicken and zebra finch, although the number of claw and feather genes differs between the species. Molecular phylogenies show that the monophyletic scale genes are the basal group within birds and that the monophyletic avian claw genes form the basal group to all feather and feather-like genes. Both species have a number of feather clades on microchromosome 27 that form monophyletic groups. An additional monophyletic cluster of feather genes exist on macrochromosome 2 for each species. Expression sequence tag analysis for the chicken demonstrates that all feather β-keratin clades are expressed.ConclusionsSimilarity in the overall genomic organization of β-keratins in Galliformes and Passeriformes suggests similar organization in all Neognathae birds, and perhaps in the ancestral lineages leading to modern birds, such as the paravian Anchiornis huxleyi. Phylogenetic analyses demonstrate that evolution of archosaurian epidermal appendages in the lineage leading to birds was accompanied by duplication and divergence of an ancestral β-keratin gene cluster. As morphological diversification of epidermal appendages occurred and the β-keratin multigene family expanded, novel β-keratin genes were selected for novel functions within appendages such as feathers.
Highlights
The epidermal appendages of reptiles and birds are constructed of beta (β) keratins
We have identified a total of 111 complete β-keratin gene sequences in the G. gallus genome [7,8], which are distributed on three macrochromosomes (GGA1, 2 and 5), one intermediate chromosome (GGA6), two microchromosomes (GGA25, and 27), and "chromosome unknown" (GGA_Un) [37,38]
In the genome of T. guttata a total of 108 β-keratin genes are located on one macrochromosome (TGU2), two microchromosomes (TGU25 and 27), and "chromosome unknown" (TGU_Un)
Summary
The epidermal appendages of reptiles and birds are constructed of beta (β) keratins. The molecular phylogeny of these keratins is important to understanding the evolutionary origin of these appendages, especially feathers. Skin appendages such as claws, scales, beaks and feathers develop, and provide novel functions. These diverse epidermal structures are composed of beta (β) keratins, whose genes have been isolated from all major groups of reptiles including squamates, crocodilians, and chelo-. The scaleless (sc/sc) mutant chicken, which does not undergo scale and feather development, expresses β-keratins from all four subfamilies in its embryonic epidermis [14]. This embryonic epidermis is generated by the initial stem cell population of the embryonic ectoderm [15]
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