Genetic basis and evolutionary context for structural color shift in the Buckeye butterfly
- Thayer, Rachel C
- Advisor(s): Patel, Nipam H;
- Nachman, Michael W
Abstract
In butterfly scales, nanostructures that scatter light create structural color, often with
visually delightful effects like iridescence. Structural colors used for signaling, thermal
regulation, or camouflage provide excellent case studies in the ecological
multifunctionality of color. Developmentally, intricate nanostructures must be precisely
sculpted, as a few nanometers’ difference in one dimension would change the hue. In
physics, the gyroid, an infinitely connected periodic surface once thought to be a purely
theoretical shape, was found reflecting green light in butterfly wings, and naturally
occurring photonic structures are a rich source of inspiration for optical engineering.
Despite interdisciplinary appeal, the biological processes that produce structural color
remain mysterious. Here, I take a telescopic approach to investigate the biology of
butterfly structural color, beginning with macroevolutionary trends, proceeding to
genus- and species-level variation, and concluding with genetic analysis. In chapter one,
I aggregate reflectance and morphological data for all 350 described butterfly structural
colors to interrogate the color gamut and phylogenetic distribution of structural color.
In the process, I comprehensively review what is known about how structurally colored
scales develop and evolve. In chapter two, I show that selective breeding shifted wing
color from brown to blue in buckeye butterflies (Junonia coenia) via a 74% increase in the
thickness of each scale lamina. By comparing ten related species in the genus Junonia, I
find that evolutionarily tuning lamina thickness has generated a wide range of
structural colors, from gold to magenta and green. A similar thickness increase explains
the appearance of blue scales in buckeyes with mutations in the optix wing patterning
gene. In chapter three, I use a large cross-breeding experiment between blue and brown
buckeyes and quantitative trait locus mapping to identify genetic loci that control the
evolved blue structural color. Hue is controlled by a different set of genes than the
arrangement of blue scales over the wing surface, and optix is the first specific gene
found to regulate the morphology of a photonic structure.