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.