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β-keratin or beta-keratin is a protein in the keratin family. β-keratin is rich in stacked β pleated sheets, in contrast to alpha-keratin, a fibrous protein rich in alpha helices. β-keratin is found in reptiles. It adds much more rigidity to reptilian skin than alpha-keratin does to mammalian skin. β-keratin is impregnated into the stratum corneum of the reptilian skin, providing waterproofing and the prevention of desiccation. In birds, scales, beaks, claws and feathers also contain β-keratin of the avian family. Phylogenetic studies of β-keratin sequences show that feather β-keratins evolved from scale β-keratins. The scale β-keratins form the basal group in avians. Duplication and divergence events then led to claw β-keratin genes, and further recombination resulted in new feather and feather-like avian β-keratin genes. Evidence for these duplication events comes from the correlation of feather β-keratin clade structure with their genomic loci. Changes in β-keratins may have also influenced the development of powered flight. A recent study using molecular dating methods to link the evolution of β-keratin genes to that of feathers reveals that the avian β-keratin family began diverging from the crocodile family about 216 million years ago.〔 But the feather β-keratin family did not begin diverging until 125 million years ago, a date consistent with the adaptive radiation of birds during the Cretaceous. β-keratins found in modern feathers have increased elasticity, a factor that may have contributed to their role in flight.〔 Thus, the feathered ancestors of birds including ''Anchiornis'' and ''Archaeopteryx'', whose flight capabilities have been questioned, would have had avian, but not feather β-keratins. The small alvarezsaurid dinosaur ''Shuvuuia deserti'' showed evidence of a featherlike skin covering. Analysis by Schweitzer ''et al.'' (1999) showed that these featherlike structures consisted of beta-keratin.〔.Schweitzer, Mary Higby, Watt, J.A., Avci, R., Knapp, L., Chiappe, L, Norell, Mark A., Marshall, M. (1999). "Beta-Keratin Specific Immunological reactivity in Feather-Like Structures of the Cretaceous Alvarezsaurid, ''Shuvuuia deserti'' Journal of Experimental Biology (Mol Dev Evol) 255:146-157〕 ==References== 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Beta-keratin」の詳細全文を読む スポンサード リンク
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