Wing scale ultrastructure underlying convergent and divergent iridescent colours in mimeticHeliconiusbutterflies

Abstract

Iridescence is an optical phenomenon whereby colour changes with the illumination and viewing angle. It can be produced by thin film interference or diffraction. Iridescent optical structures are fairly common in nature, but relatively little is known about their production or evolution. Here we describe the structures responsible for producing blue-green iridescent colour in Heliconius butterflies. Overall the wing scale structures of iridescent and non-iridescent Heliconius species are very similar, both having longitudinal ridges joined by cross-ribs. However, iridescent scales have ridges composed of layered lamellae, which act as multilayer reflectors. Differences in brightness between species can be explained by the extent of overlap of the lamellae and their curvature as well as the density of ridges on the scale. Heliconius are well known for their Müllerian mimicry. We find that iridescent structural colour is not closely matched between co-mimetic species. Differences appear less pronounced in models of Heliconius vision than models of avian vision, suggesting that they are not driven by selection to avoid heterospecific courtship by co-mimics. Ridge profiles appear to evolve relatively slowly, being similar between closely related taxa, while ridge density evolves faster and is similar between distantly related co-mimics.

Sub‐micrometer insights into the cytoskeletal dynamics and ultrastructural diversity of butterfly wing scales

“…Wing patterns are made of hundreds of thousands of scales that derive their color from pigments, ultrastructure, or a combination of the two . Scale structures interact with light through a variety of mechanisms that have been described from a biophysical perspective, including light‐trapping black, light polarization, high‐reflectance, transparency, and color‐selective iridescence . For instance, the coherent light‐scattering features of Morpho butterflies reflect specific wavelengths, producing their characteristic blue iridescence .…”

“…Qualitative and quantitative studies of scale ultrastructures have usually focused on a small number of scales using manual measurements taken from electron micrographs, but we are now starting to see new methods that can increase either resolution or sample size. Scanning probe microscopy (SPM) is unique in its ability to provide topographical data (eg, ridge height), but is limited to single scales . In contrast, small angle X‐ray scattering (SAXS) has been used across larger regions, and yielded precise measurements of gyroid crystals and ridge spacing; while this technique can be implemented on hundreds of wing samples, it requires access to a beamline, does not allow the extraction of multiple variables, and is more suited to homogenous wing surfaces rather than to complex, multicolor patterns.…”

“…6,7 Scale structures interact with light through a variety of mechanisms that have been described from a biophysical perspective, 8 including light-trapping black, [9][10][11] light polarization, 12 high-reflectance, 13 transparency, 14 and color-selective iridescence. [15][16][17][18][19] For instance, the coherent light-scattering features of Morpho butterflies reflect specific wavelengths, producing their characteristic blue iridescence. 20 In this butterfly, ridge stacking (the dense piling of multiple layers of lamellae) combined with short inter-ridge distances scatters long wavelengths of light while reflecting wavelengths in the blue range.…”

“…Scanning probe microscopy (SPM) is unique in its ability to provide topographical data (eg, ridge height), but is limited to single scales. 16,39,40 In contrast, small angle X-ray scattering (SAXS) has been used across larger regions, and yielded precise measurements of gyroid crystals and ridge spacing 16,41,42 ; while this technique can be implemented on hundreds of wing samples, it requires access to a beamline, does not allow the extraction of multiple variables, and is more suited to homogenous wing surfaces rather than to complex, multicolor patterns.…”

Structural color in Junonia butterflies evolves by tuning scale lamina thickness

“…The color itself is easier to quantify, but has limited 48 utility as a proxy for nanostructural dimensions, since structural colors and pigments often co-49 occur and covary. While recent studies [2][3][4] have made early headway toward describing 50 genetic regulation of structural colors, much work remains to decipher the evolutionary, 51 developmental, and genetic bases of structural coloration, and lab-tractable systems with 52 intraspecific variation in structural coloration are needed. We present a promising system, the 53 butterfly genus Junonia, with extensive variation in a simple structural color, and show how 54 structural simplicity is a tactical advantage when seeking to unravel mechanisms for the 55 biological production of nanostructures.…”