Convergent Evolution of Cysteine-Rich Keratins in Hard Skin Appendages of Terrestrial Vertebrates
Convergent Evolution of Cysteine-Rich Keratins in Hard Skin Appendages of Terrestrial Vertebrates
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DOI：
10.1093/molbev/msz279
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发表时间：
2020-04-01
影响因子：
10.7
通讯作者：
Eckhart, Leopold
中科院分区：
文献类型：
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作者：
Ehrlich, Florian;Lachner, Julia;Eckhart, Leopold
陆生脊椎动物进化出坚硬的皮肤附属物，如鳞片、爪子、羽毛和毛发，这些附属物在防御、捕食、运动和隔热方面发挥着关键作用。这些皮肤附属物的机械性能在很大程度上由角化的上皮成分决定。所谓的“毛发角蛋白”，富含半胱氨酸的中间丝蛋白，通过二硫键进行共价交联，是哺乳动物毛发和爪子的关键结构蛋白，蜥蜴爪中也存在毛发角蛋白同源基因，这表明进化起源于羊膜动物无毛的共同祖先。这里，我们展示了爬行动物和鸟类也有其他富含半胱氨酸的角蛋白，这些角蛋白在哺乳动物中缺乏富含半胱氨酸的同源蛋白。除了硬酸性(I型)蜥蜴特异性(HAS)角蛋白外，我们还鉴定了硬碱性(II型)蜥蜴特异性(HBS)角蛋白，它们在鳞龙、海龟、鳄鱼和鸟类中都是保守的。用新制备的抗体进行免疫组织化学分析，发现鸡HBS1角蛋白在羽毛角化上皮细胞中表达。分子系统发育学表明，HAS和HBS角蛋白的高半胱氨酸含量是从发角蛋白同源基因富含半胱氨酸的序列独立进化而来的，因此代表了趋同进化的产物。综上所述，我们提出了一个进化模型，在该模型中，Has和HBS角蛋白作为爬行动物上皮角化的结构蛋白进化，在鸟类和爬行动物进化分化后，至少有一种HBS角蛋白被增选为羽毛的成分。因此，毛发和羽毛的细胞骨架蛋白是趋同进化和进化共同选择的产物，具有类似的生物力学功能。
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 newlymade 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.