Iridescence is widespread in the living world, occurring in organisms as diverse as bacteria, plants, and animals. Yet, compared to pigment-based forms of coloration, we know surprisingly little about the developmental and molecular bases of the structural colors that give rise to iridescence. Birds display a rich diversity of iridescent structural colors that are produced in feathers by the arrangement of melanin-containing organelles called melanosomes into nanoscale configurations, but how these often unusually shaped melanosomes form, or how they are arranged into highly organized nanostructures, remains largely unknown. Here, we use functional genomics to explore the developmental basis of iridescent plumage using superb starlings (Lamprotornis superbus), which produce both iridescent blue and non-iridescent red feathers. Through morphological and chemical analyses, we confirm that hollow, flattened melanosomes in iridescent feathers are eumelanin-based, whereas melanosomes in non-iridescent feathers are solid and amorphous, suggesting that high pheomelanin content underlies red coloration. Intriguingly, the nanoscale arrangement of melanosomes within the barbules was surprisingly similar between feather types. After creating a new genome assembly, we use transcriptomics to show that non-iridescent feather development is associated with genes related to pigmentation, metabolism, and mitochondrial function, suggesting non-iridescent feathers are more energetically expensive to produce than iridescent feathers. However, iridescent feather development is associated with genes related to structural and cellular organization, suggesting that, while nanostructures themselves may passively assemble, barbules and melanosomes may require active organization to give them their shape. Together, our analyses suggest that iridescent feathers form through a combination of passive self-assembly and active processes.

中文翻译：

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羽毛基因表达阐明了非洲椋鸟羽毛彩虹色的发育基础

虹彩在生物世界中很普遍，发生在细菌、植物和动物等多种生物体中。然而，与基于颜料的着色形式相比，我们对产生彩虹色的结构颜色的发育和分子基础知之甚少。鸟类表现出丰富多样的虹彩结构颜色，这些颜色是通过将称为黑素体的含黑色素细胞器排列成纳米级结构而在羽毛中产生的，但这些通常形状异常的黑素体是如何形成的，或者它们是如何排列成高度有序的纳米结构的，在很大程度上仍然是未知的. 在这里，我们使用功能基因组学来探索使用超级椋鸟 (Lamprotornis superbus) 的虹彩羽毛的发育基础，这些椋鸟会产生虹彩蓝色和非虹彩红色羽毛。通过形态学和化学分析，我们证实虹彩羽毛中空心、扁平的黑素体是基于真黑素的，而非虹彩羽毛中的黑素体是实心和无定形的，这表明高褐黑素含量是红色的基础。有趣的是，小羽中黑素体的纳米级排列在羽毛类型之间惊人地相似。在创建一个新的基因组组装后，我们使用转录组学来证明非虹彩羽毛的发育与色素沉着、新陈代谢和线粒体功能相关的基因相关，这表明非虹彩羽毛的生产成本比虹彩羽毛高。然而，彩虹色羽毛的发育与结构和细胞组织相关的基因有关，这表明，虽然纳米结构本身可能被动组装，但小枝和黑素体可能需要主动组织才能赋予它们形状。总之，我们的分析表明，虹彩羽毛是通过被动自组装和主动过程的结合形成的。