THE GENETIC MECHANISMS UNDERLYING PIGMENTATION AND THEIR EVOLUTIONARY IMPORTANCE IN BIRDS

Abstract

Integumentary pigmentation is a phenotype of fundamental importance to animals, with major impacts on survival and fitness. Thus, understanding the mechanisms underlying pigmentation can help illuminate general principles about how adaptive variation is generated and maintained in populations. Here, I present a dissertation that is aimed at understanding the developmental, regulatory, and genetic mechanisms that underlie variation in avian plumage color, and their evolutionary importance. In my first chapter, I addressed how the modular organization of plumage traits may impact their evolution. The production of color in developing feathers is a modular process, with several mechanisms combining to produce the complete feather phenotype. A modular trait organization is predicted to increase phenotypic evolvability by reducing negative pleiotropic interactions with functionally unrelated traits. Through phylogenetic comparative analysis, I show that separate mechanisms producing feather colors show independent, uncorrelated patterns of evolutionary change, consistent with their modular organization. My results show that developmental modularity can have detectable impacts on trait evolution. For my second chapter, I identified gene expression variation associated with melanin pigmentation in the Zebra Finch. I found differential expression of several functionally important genes that synthesize melanin. In addition, I found changes in expression in the signaling pathways that govern transcription of melanogenesis genes. These signaling pathways differ from those previously reported to drive major pigmentation differences, indicating that the regulation of melanogenesis is flexible in how it generates similar phenotypic outcomes. For my third chapter, I identified the genetic basis for loss of sexual dimorphism in a domestic color morph of Zebra Finch. With whole-genome sequencing, I found a major divergence peak between dimorphic and monomorphic finches containing the gene Norrie Disease Protein (NDP). NDP is a signaling molecule that regulates transcription of several melanogenesis genes, and is underexpressed when dimorphism is lost. Sexual dimorphism can be lost repeatedly and rapidly in many groups. My work shows that relatively simple genetic changes in the regulation of important signaling molecules can influence sexual dimorphism in a patch-specific manner, facilitating this rapid evolution