The evolution and development of structural colour in Heliconius butterflies

In nature, nanostructures can interact with light to produce a striking array of colours called structural colours. Such colours have gained considerable attention in evolutionary biology and, more recently, as potential routes for the design of advanced optical materials. Vivid examples of structural colour are found in butterflies, resulting from the modification of their wing scales to contain optically precise nanostructures. In Heliconius, such optical nanostructures take the form of multilayer reflectors on the cuticle scale ridges which reflect light through constructive interference. While structural colours in Heliconius have been characterised optically, their underlying development remains unknown. Throughout this thesis, I explore the development of structural colour in Heliconius, both in terms of the cellular processes guiding the precise formation of the multilayer reflectors as well as the underlying genetic control. Using high-resolution microscopy and perturbation experiments, I demonstrate a crucial role of the actin cytoskeleton in the development of optical nanostructures in Heliconius sara. In addition, I reveal a previously undescribed network of actin in developing butterfly scale cells which forms at the time of optical nanostructure formation. I then use differential expression analysis between structurally coloured and non-structurally coloured subspecies of Heliconius erato and Heliconius melpomene to reveal a number of candidate genes that may control optical nanostructure formation. Furthermore, I reveal a lack of convergence in the genes controlling structural colour development in Heliconius. Finally, I demonstrate the development of structural colour in Heliconius is subject to environmental influences, specifically dietary stress. This thesis lays the foundation for investigations into the study of structural colour development in diverse butterfly species.