Genetic, microstructural, and macrostructural mechanisms of color production in birds

Abstract

Colorful birds are classic and charismatic systems for investigating fundamental questions in evolutionary biology. A detailed understanding of the physical basis of coloration is crucial for identifying the origins of variation in color, and ultimately for interpreting the evolutionary significance of coloration – yet many aspects of color production in feathers remain unexplored. While attention has historically focused on color at the level of pigments or nanostructures within feathers, such simple categorizations of color production may be incomplete or misleading. My dissertation examines color production at several interacting scales, from the molecular mechanisms that underlie nanostructures and pigments to the evolution of hidden plumage layers and modified microscale feather branches. In my first chapter, I summarize recent progress and emerging areas of research in avian coloration genetics, with an emphasis on frontiers and unanswered questions. In my second and third chapters, I investigate how two relatively unexplored and underappreciated aspects of plumage – macroscale layers of visible and concealed plumage, and microscale feather branches – alter the appearance of colorful plumage. I first show that total plumage reflectance depends on optical interactions among visible and concealed layers of plumage, and that many birds have evolved hidden white and black achromatic layers of feathers which dramatically enhance plumage brightness and saturation. Next, I explore the evolution and development of modified feather microstructures in colorful feathers. I show that microstructural modifications are an important component of color production, and I identify two distinct developmental pathways which produce modified microstructures in structurally colored and carotenoid-pigmented feathers. While my focus is on the remarkably colorful tanager genus Tangara, I show that both hidden achromatic feather layers and microstructural modifications are widespread and convergent traits in colorful birds. Overall, my dissertation illustrates how the color diversity of birds has evolved through the recruitment, cooption, and modification of existing genes, developmental pathways, and feather structures. Feathers are remarkably complex and beautiful tissues that resist simple categorizations of “structurally colored” or “pigmented,” and appreciating the true complexity of color production across different scales yields new insights into the evolution and function of coloration.