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Background: The epidermal appendages of reptiles and birds are constructed of beta (β) keratins. The molecularphylogeny 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 ofthe 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.

Results: The subfamilies (claw, feather, feather-like, and scale) of β-keratin genes are clustered in the same 5' to 3' orderon 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 thatthe 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 monophyleticcluster of feather genes exist on macrochromosome 2 for each species. Expression sequence tag analysis for thechicken demonstrates that all feather β-keratin clades are expressed.

Conclusions: Similarity in the overall genomic organization of β-keratins in Galliformes and Passeriformes suggestssimilar organization in all Neognathae birds, and perhaps in the ancestral lineages leading to modern birds, such as theparavian Anchiornis huxleyi. Phylogenetic analyses demonstrate that evolution of archosaurian epidermal appendagesin 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.

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