Terrestrial vertebrates have evolved hard skin appendages, such as scales, claws, feathers, and hair that play crucial roles in defense, predation, locomotion, and thermal insulation. The mechanical properties of these skin appendages are largely determined by cornified epithelial components. So-called “hair keratins,” cysteine-rich intermediate filament proteins that undergo covalent cross-linking via disulfide bonds, are the crucial structural proteins of hair and claws in mammals and hair keratin orthologs are also present in lizard claws, indicating an evolutionary origin in a hairless common ancestor of amniotes. Here, we show that reptiles and birds have also other cysteine-rich keratins which lack cysteine-rich orthologs in mammals. In addition to hard acidic (type I) sauropsid-specific (HAS) keratins, we identified hard basic (type II) sauropsid-specific (HBS) keratins which are conserved in lepidosaurs, turtles, crocodilians, and birds. Immunohistochemical analysis with a newly made antibody revealed expression of chicken HBS1 keratin in the cornifying epithelial cells of feathers. Molecular phylogenetics suggested that the high cysteine contents of HAS and HBS keratins evolved independently from the cysteine-rich sequences of hair keratin orthologs, thus representing products of convergent evolution. In conclusion, we propose an evolutionary model in which HAS and HBS keratins evolved as structural proteins in epithelial cornification of reptiles and at least one HBS keratin was co-opted as a component of feathers after the evolutionary divergence of birds from reptiles. Thus, cytoskeletal proteins of hair and feathers are products of convergent evolution and evolutionary co-option to similar biomechanical functions in clade-specific hard skin appendages.

中文翻译：

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半胱氨酸丰富的角蛋白在陆生脊椎动物坚硬的皮肤附件中的趋同演化。

陆生脊椎动物已经进化出坚硬的皮肤附属物，例如鳞片，爪子，羽毛和头发，它们在防御，掠食，运动和隔热中起着至关重要的作用。这些皮肤附件的机械性能在很大程度上取决于角质化的上皮成分。所谓的“毛发角蛋白”是富含半胱氨酸的中间丝蛋白，它们通过二硫键进行共价交联，是哺乳动物毛发和爪子的重要结构蛋白，而且在蜥蜴的爪子中也存在毛发角蛋白直系同源物，表明其进化起源在无毛的羊膜祖先中。在这里，我们显示出爬行动物和鸟类还具有其他富含半胱氨酸的角蛋白，而它们在哺乳动物中缺乏富含半胱氨酸的直系同源物。除了硬酸性（I型）蜥脚类（HAS）角蛋白外，我们确定了硬碱性（II型）蜥脚类动物特有的（HBS）角蛋白，它们在鳞翅目，乌龟，鳄鱼和鸟类中均得到保存。用新制备的抗体进行的免疫组织化学分析显示，鸡的HBS1角蛋白在羽毛的角质层上皮细胞中表达。分子系统学研究表明，HAS和HBS角蛋白的高半胱氨酸含量独立于头发角蛋白直系同源物的富含半胱氨酸的序列而演化，因此代表了趋同进化的产物。总之，我们提出了一种进化模型，其中HAS和HBS角蛋白在爬行动物的上皮角质化中作为结构蛋白进化，并且在鸟类从爬行动物进化分化之后，至少一种HBS角蛋白被选为羽毛的组成部分。因此，