Characterizing the Molecular Basis of Supergene Mimicry in Butterflies Project Summary Sex-limited polymorphism is widespread in animals, including a variety of human traits and diseases, yet we lack a general functional understanding of sex-limited polymorphism in any organism. “Supergene” mimicry in the swallowtail butterfly Papilio polytes stands out as a particularly striking example of sex-limited polymorphism and one that is amenable to functional characterization. While much theoretical work has explored the evolutionary dynamics of supergene mimicry, its molecular and developmental basis is virtually unexplored. Over the last five years we have determined that a single gene, doublesex, controls the mimicry switch in P. polytes and we have studied the origin and evolution of mimicry, behavioral aspects of mimicry, natural selection in nature, and we have developed tools and methods for CRISPR and multiple functional genomics assays. We propose to investigate the functional basis of supergene mimicry in P. polytes by integrating genomics, functional genetics, molecular and developmental biology, providing the single most comprehensive investigation of its kind. In so doing, this work will greatly expand the known role of the sexual determination pathway, and generate general insights into molecular and cellular causes of sexual differentiation, dimorphism, and sex-limited polymorphism.