Thermal Plasticity in a Cross‐Sexual Transfer Trait: Geographic Variation and Phenotypic Integration of Blue Wing Colour in Female Butterflies

ABSTRACT Within‐sex phenotypic variation can arise through co‐option of sexual differentiation mechanisms. Recently, several such cross‐sexual‐transfer traits have been identified, but we lack a mechanistic understanding of their geographic variation, environmental influences, and phenotypic integration with other traits. Male Polyommatus icarus butterflies are blue, whereas female wing coloration varies from brown to blue. Here, we show that female wing colour varies in a geographic mosaic, with the spring generation being bluer than the summer generation. Laboratory experiments revealed that females developed both bluer wings and increased phenotypic integration between the amount of blue and total wing area at low temperature, qualitatively matching differences between spring and summer generations. Scanning electron microscopy revealed that female and male colour develop through similar mechanisms, supporting the cross‐sexual‐transfer hypothesis. Our study establishes a promising study system on cross‐sexual‐transfer and a solid foundation for research on the fitness consequences and evolutionary history of this trait.