Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas, 78712.
Department of Integrative Biology, The University of Texas at Austin, Austin, Texas, 78712.
Evolution. 2022 Jan;76(1):42-57. doi: 10.1111/evo.14393. Epub 2021 Dec 6.
Carotenoids are pigments responsible for most bright yellow, red, and orange hues in birds. Their distribution has been investigated in avian plumage, but the evolution of their expression in skin and other integumentary structures has not been approached in detail. Here, we investigate the expression of carotenoid-consistent coloration across tissue types in all extant, nonpasserine species (n = 4022) and archelosaur outgroups in a phylogenetic framework. We collect dietary data for a subset of birds and investigate how dietary carotenoid intake may relate to carotenoid expression in various tissues. We find that carotenoid-consistent expression in skin or nonplumage keratin has a 50% probability of being present in the most recent common ancestor of Archosauria. Skin expression has a similar probability at the base of the avian crown clade, but plumage expression is unambiguously absent in that ancestor and shows hundreds of independent gains within nonpasserine neognaths, consistent with previous studies. Although our data do not support a strict sequence of tissue expression in nonpasserine birds, we find support that expression of carotenoid-consistent color in nonplumage integument structures might evolve in a correlated manner and feathers are rarely the only region of expression. Taxa with diets high in carotenoid content also show expression in more body regions and tissue types. Our results may inform targeted assays for carotenoids in tissues other than feathers, and expectations of these pigments in nonavian dinosaurs. In extinct groups, bare-skin regions and the rhamphotheca, especially in species with diets rich in plants, may express these pigments, which are not expected in feathers or feather homologues.
类胡萝卜素是使鸟类呈现大多数明亮的黄色、红色和橙色的色素。它们在鸟类羽毛中的分布已经得到了研究，但它们在皮肤和其他表皮结构中的表达进化尚未得到详细研究。在这里，我们在系统发育框架内调查了所有现存非雀形目物种（n=4022）和原始蜥脚类动物外群的组织类型中类胡萝卜素一致颜色的表达。我们收集了一部分鸟类的饮食数据，并研究了饮食中类胡萝卜素的摄入如何与各种组织中的类胡萝卜素表达相关。我们发现，在恐龙总目最近的共同祖先中，皮肤或非羽毛角蛋白中类胡萝卜素一致的表达有 50%的可能性存在。在鸟类冠群的基部，皮肤表达也有类似的可能性，但在该祖先中，羽毛表达明确不存在，并且在非雀形目新颌类中独立获得了数百次，与之前的研究一致。尽管我们的数据不支持非雀形目鸟类中组织表达的严格顺序，但我们发现支持非羽毛表皮结构中类胡萝卜素一致颜色的表达可能以相关的方式进化，并且羽毛很少是唯一的表达区域。饮食中类胡萝卜素含量高的类群也在更多的身体区域和组织类型中表现出表达。我们的结果可能为组织中非羽毛部位的类胡萝卜素的靶向检测提供信息，并为非鸟类恐龙中的这些色素提供预期。在已灭绝的群体中，裸露的皮肤区域和喙鞘，尤其是在饮食中富含植物的物种中，可能会表达这些色素，而这些色素在羽毛或羽毛同源物中是不存在的。
