Wing scale ultrastructure underlying convergent and divergent iridescent colours in mimetic Heliconius butterflies

The actin cytoskeleton plays multiple roles in structural colour formation in butterfly wing scales

Vivid structural colours in butterflies are caused by photonic nanostructures scattering light. Structural colours evolved for numerous biological signalling functions and have important technological applications. Optically, such structures are well understood, however insight into their development in vivo remains scarce. We show that actin is intimately involved in structural colour formation in butterfly wing scales. Using comparisons between iridescent (structurally coloured) and non-iridescent scales in adult and developing H. sara, we show that iridescent scales have more densely packed actin bundles leading to an increased density of reflective ridges. Super-resolution microscopy across three distantly related butterfly species reveals that actin is repeatedly re-arranged during scale development and crucially when the optical nanostructures are forming. Furthermore, actin perturbation experiments at these later developmental stages resulted in near total loss of structural colour in H. sara. Overall, this shows that actin plays a vital and direct templating role during structural colour formation in butterfly scales, providing ridge patterning mechanisms that are likely universal across lepidoptera.

Divergent warning patterns influence male and female mating behaviours in a tropical butterfly

Traits under divergent ecological selection that also function during mating can be important in maintaining species boundaries. Few studies have considered mutual mate choice, where both males and females base mating decisions on the same trait. Wing colouration in Heliconius butterflies evolved as a warning signal but also functions as a mating cue. We investigated the contribution of visual preference to assortative mating in an aposematic butterfly Heliconius cydno in the context of reproductive isolation with its sympatric, visually distinct relative Heliconius melpomene. Heliconius cydno have conspicuous white bands on their forewings, whereas those of H. melpomene are red in colour. We predicted that both sexes of H. cydno contributed to assortative mating by exhibiting visual preference towards conspecific wing colouration. We analysed published and new data from preference experiments, in which males were presented with conspecific and H. melpomene females. We also recorded female responses and mating outcomes in choice experiments, involving conspecific males with either the original white or artificially painted red forewing bands. Both sexes of H. cydno responded more positively towards the conspecific colouration, and males strongly preferred females of its own colours. In contrast, male colouration did not predict mating outcomes in female choice experiments. As courtships are initiated by males in butterflies, our findings suggest that female visual preference might be of secondary importance in H. cydno. Our data also suggest that the contribution of visual preference to reproductive isolation might be unequal between H. cydno and its sympatric relative H. melpomene.

A meta-analysis of butterfly structural colors: their color range, distribution and biological production

Butterfly scales are among the richest natural sources of optical nanostructures, which produce structural color and iridescence. Several recurring nanostructure types have been described, such as ridge multilayers, gyroids and lower lamina thin films. While the optical mechanisms of these nanostructure classes are known, their phylogenetic distributions and functional ranges have not been described in detail. In this Review, we examine a century of research on the biological production of structural colors, including their evolution, development and genetic regulation. We have also created a database of more than 300 optical nanostructures in butterflies and conducted a meta-analysis of the color range, abundance and phylogenetic distribution of each nanostructure class. Butterfly structural colors are ubiquitous in short wavelengths but extremely rare in long wavelengths, especially red. In particular, blue wavelengths (around 450 nm) occur in more clades and are produced by more kinds of nanostructures than other hues. Nanostructure categories differ in prevalence, phylogenetic distribution, color range and brightness. For example, lamina thin films are the least bright; perforated lumen multilayers occur most often but are almost entirely restricted to the family Lycaenidae; and 3D photonic crystals, including gyroids, have the narrowest wavelength range (from about 450 to 550 nm). We discuss the implications of these patterns in terms of nanostructure evolution, physical constraint and relationships to pigmentary color. Finally, we highlight opportunities for future research, such as analyses of subadult and Hesperid structural colors and the identification of genes that directly build the nanostructures, with relevance for biomimetic engineering.

Measuring and Modelling Structural Colours of Euphaedra neophron (Lepidoptera: Nymphalidae) Finely Tuned by Wing Scale Lower Lamina in Various Subspecies

The nymphalid butterfly Euphaedra neophron (Hopffer, 1855) is the only structurally coloured species representing the genus along the Indian Ocean coast in East Africa and Southern Africa, with a distribution from southern Somalia to the Kwa-Zulu-Natal region of South Africa. The range of E. neophron is subdivided to several, geographically distinct populations, currently recognised as subspecies by taxonomists on the basis of violet, blue, and green-coloured morphs. We investigated the optical mechanism of all these morphs by various materials science techniques. We found that the structural colour is generated by the lower lamina of the cover scales and the different colours are tuned according to their thickness, which was also proved by modelling. The colour tuning of the different subspecies does not reflect any clinal pattern, be it geographical or altitudinal.

The genetic basis of structural colour variation in mimetic Heliconius butterflies

Structural colours, produced by the reflection of light from ultrastructures, have evolved multiple times in butterflies. Unlike pigmentary colours and patterns, little is known about the genetic basis of these colours. Reflective structures on wing-scale ridges are responsible for iridescent structural colour in many butterflies, including the Müllerian mimics Heliconius erato and Heliconius melpomene. Here, we quantify aspects of scale ultrastructure variation and colour in crosses between iridescent and non-iridescent subspecies of both of these species and perform quantitative trait locus (QTL) mapping. We show that iridescent structural colour has a complex genetic basis in both species, with offspring from crosses having a wide variation in blue colour (both hue and brightness) and scale structure measurements. We detect two different genomic regions in each species that explain modest amounts of this variation, with a sex-linked QTL in H. erato but not H. melpomene. We also find differences between species in the relationships between structure and colour, overall suggesting that these species have followed different evolutionary trajectories in their evolution of structural colour. We then identify genes within the QTL intervals that are differentially expressed between subspecies and/or wing regions, revealing likely candidates for genes controlling structural colour formation.

This article is part of the theme issue ‘Genetic basis of adaptation and speciation: from loci to causative mutations’.

Reflectance spectroscopy and machine learning as a tool for the categorization of twin species based on the example of the Diachrysia genus

In our work we used noninvasive point reflectance spectroscopy in the range from 400 to 2100 nm coupled with machine learning to study scales on the brown and golden iridescent areas on the dorsal side of the forewing of Diachrysia chrysitis and D. stenochrysis. We used our approach to distinguish between these species of moths. The basis for the study was a statistically significant collection of 95 specimens identified based on morphological feature and gathered during 23 years in Poland. The numerical part of an experiment included two independent discriminant analyses: stochastic and deterministic. The more sensitive stochastic approach achieved average compliance with the species identification made by entomologists at the level of 99-100%. It demonstrated high stability against the different configurations of training and validation sets, hence strong predictors of Diachrysia siblings distinctiveness. Both methods resulted in the same small set of relevant features, where minimal fully discriminating subsets of wavelengths were three for glass scales on the golden area and four for the brown. The differences between species in scales primarily concern their major components and ultrastructure. In melanin-absent glass scales, this is mainly chitin configuration, while in melanin-present brown scales, melanin reveals as an additional factor.

Ultrastructure of androconia and surrounding scales of nine species of Hesperiidae (Lepidoptera)

The ultrastructure of androconia and their surrounding scales of nine species in nine genera across four subfamilies of Hesperiidae is studied. This provides a basis for the classification and identification of some genera and species. The wing surface of the scent glands patches was cut with scissors, observed and photographed under an S-4800 scanning electron microscope (at 10.0 kV accelerated pressure). There were significant differences in the types of scent glands patches across subfamilies. The scent glands patches of Pyrginae and Dudaminae are mainly in the costal fold of the forewing, while those of Coeliadinae and Hesperiinae are mainly in the line or circular stigma on the wing surface. The length, breadth and aperture of the androconia were further measured and the data are analysed by variance and multiple comparisons. There are significant differences amongst the subfamilies, except for Dudaminae and Pyrginae. In Hesperiinae, Telicotacolon (Fabricius, 1775) and Ampittiavirgata (Leech, 1890) have no significant difference in the aperture of the androconia, but are significantly different from Thymelicusleoninus (Butler, 1878). There are significant differences in the aperture between Pyrgusalveus’s (Hübner, 1803) androconium and the second androconium of Loboclabifasciata (Bremer & Grey, 1853), but not with the first androconium of Loboclabifasciata. The morphology of androconia in the scent glands patches is very similar in Hesperiinae; all are rod-shaped and paddle-like. The scale types around the scent glands patches are different, but there are one or two similar types. To a certain extent, the aperture of the androconia reflects the genetic relationships between subfamilies and species. The differences in scale type and structure of scent glands patches can be used as a reference for the classification of subfamilies and genera in Hesperiidae.

Cortex cis-regulatory switches establish scale colour identity and pattern diversity in Heliconius

In Heliconius butterflies, wing colour pattern diversity and scale types are controlled by a few genes of large effect that regulate colour pattern switches between morphs and species across a large mimetic radiation. One of these genes, cortex, has been repeatedly associated with colour pattern evolution in butterflies. Here we carried out CRISPR knockouts in multiple Heliconius species and show that cortex is a major determinant of scale cell identity. Chromatin accessibility profiling and introgression scans identified cis-regulatory regions associated with discrete phenotypic switches. CRISPR perturbation of these regions in black hindwing genotypes recreated a yellow bar, revealing their spatially limited activity. In the H. melpomene/timareta lineage, the candidate CRE from yellow-barred phenotype morphs is interrupted by a transposable element, suggesting that cis-regulatory structural variation underlies these mimetic adaptations. Our work shows that cortex functionally controls scale colour fate and that its cis-regulatory regions control a phenotypic switch in a modular and pattern-specific fashion.

CARACTERIZACIÓN GENÉTICA DE LA POBLACIÓN DE Heliconius sara (Nymphalidae) EN LA ISLA GORGONA, COLOMBIA

La estructura genética de poblaciones de mariposas con distribución en islas y sus pares continentales ha sido poco documentada para el neotrópico. Este estudio presenta la caracterización de una población de Heliconius sara con distribución en la Isla Gorgona, ubicada en la región del Pacífico Oriental Colombiano. Para esto se examinaron secuencias parciales de un marcador mitocondrial incluyendo información obtenida del GenBank. Se comparó la diversidad y estructura genética con sus conespecíficos continentales y también con congéneres, con los que comparte un ancestro común cercano en el clado Sapho-Sara. Para el análisis de diversidad y estructura genética se realizó un análisis molecular de varianza. Este análisis muestra que la distancia entre la población de la isla y sus pares en el continente es consistente con la variación intraespecífica observada en otras especies del género Heliconius. Para la reconstrucción de la genealogía y datación reciente en el Pleistoceno superior del grupo monofilético de secuencias de H. sara, se realizó un análisis de inferencia bayesiana, así como una de máxima verosimilitud. Del análisis demográfico se seleccionó un modelo histórico de flujo asimétrico desde la isla hacia el continente que sugiere baja resistencia de la discontinuidad geográfica a la dispersión de esta mariposa diurna desde la isla. Este es el primer estudio en examinar un posible evento de aislamiento de una población insular de mariposas en Colombia.

Broader-Band and Flexible Antireflective Films with the Window-like Structures Inspired by the Backside of Butterfly Wing Scales

Antireflective performance is critical for most optical devices, such as the efficient solar energy utilization in photovoltaic cells of an aerospace craft and optical displays of scientific precise equipment. Therein, outstanding broad-band antireflection is one of the most crucial properties for antireflection films (ARFs). Unfortunately, it is still a challenging work to realize perfect "broader-band" antireflection because both the low refractive indices materials and time-consuming nanotexturing technologies are required in the fabricating process. Even in this case, a broader-band and flexible ARF with hierarchical structures is successfully developed, which is inspired by butterfly wing scales. First, the butterfly wings surface is treated with acid and stuck on a clean glass. Now, all the scales on the wings will form a strong adhesion with the glass substrate. Then, the wings are removed and the scales are left on the glass slide. Now the backside of scales is facing outward, the backside structures of the scales are coincidentally used as the template. Finally, the structure is replicated and the ARF with a controllable thickness is successfully fabricated by rotating PDMS on the biological template. In this work, the bionic ARFs realize the transmission of nearly 90% and more than 90% in the visible light and infrared region. It enhanced transmission to 13% under standard illumination compared with flat PDMS films of the same thickness. Furthermore, the ARF is flexible enough that it could bend nearly 180° to meet the special antireflection requirements in some extreme conditions. It is expected that this bioinspired AR film could revolutionize the technologies of broader-band antireflective materials and impact numerous applications from glass displays to optoelectronic devices.

Heliconius butterflies: a window into the evolution and development of diversity

Butterflies have become prominent models for studying the evolution and development of phenotypic variation. In Heliconius, extraordinary within species divergence and between species convergence in wing color patterns has driven decades of comparative genetic studies. However, connecting genetic patterns of diversification to the molecular mechanisms of adaptation has remained elusive. Recent studies are bridging this gap between genome and function and have driven substantial advances in deciphering the genetic architecture of diversification in Heliconius. While only a handful of large-effect genes were initially identified in the diversification of Heliconius color patterns, recent experiments have begun to unravel the underlying gene regulatory networks and how these have evolved. These results reveal an evolutionary story of many interacting loci and partly independent genetic architectures that underlie convergent evolution.

The evolution of structural colour in butterflies

Butterflies display some of the most striking examples of structural colour in nature. These colours originate from cuticular scales that cover the wing surface, which have evolved a diverse suite of optical nanostructures capable of manipulating light. In this review we explore recent advances in the evolution of structural colour in butterflies. We discuss new insights into the underlying genetics and development of the structural colours in various nanostructure types. Improvements in -omic and imaging technologies have been paramount to these new advances and have permitted an increased appreciation of their development and evolution.

Bioinspired Microstructured Materials for Optical and Thermal Regulation

Precise optical and thermal regulatory systems are found in nature, specifically in the microstructures on organisms' surfaces. In fact, the interaction between light and matter through these microstructures is of great significance to the evolution and survival of organisms. Furthermore, the optical regulation by these biological microstructures is engineered owing to natural selection. Herein, the role that microstructures play in enhancing optical performance or creating new optical properties in nature is summarized, with a focus on the regulation mechanisms of the solar and infrared spectra emanating from the microstructures and their role in the field of thermal radiation. The causes of the unique optical phenomena are discussed, focusing on prevailing characteristics such as high absorption, high transmission, adjustable reflection, adjustable absorption, and dynamic infrared radiative design. On this basis, the comprehensive control performance of light and heat integrated by this bioinspired microstructure is introduced in detail and a solution strategy for the development of low-energy, environmentally friendly, intelligent thermal control instruments is discussed. In order to develop such an instrument, a microstructural design foundation is provided.

Genetic switch in UV response of mimicry-related pale-yellow colors in Batesian mimic butterfly, Papilio polytes

In a Batesian mimic butterfly Papilio polytes, mimetic females resemble an unpalatable model, Pachliopta aristolochiae, but exhibit a different color pattern from nonmimetic females and males. In particular, the pale-yellow region on hind wings, which correspondingly sends important putative signals for mimicry and mate preference, is different in shape and chemical features between nonmimetic and mimetic morphs. Recently, we found that mimetic-type doublesex [dsx (H)] causes mimetic traits; however, the control of dimorphic pale-yellow colors remains unclear. Here, we revealed that dsx (H) switched the pale-yellow colors from UV-excited fluorescent type (nonmimetic) to UV-reflecting type (mimetic), by repressing the papiliochrome II synthesis genes and nanostructural changes in wing scales. Photoreceptor reactivities showed that some birds and butterflies could effectively recognize mimetic and nonmimetic pale-yellow colors, suggesting that a genetic switch in the UV response of pale-yellow colors may play essential roles in establishing the dimorphic female-limited Batesian mimicry.

The Paradox of Iridescent Signals

Signals reliably convey information to a receiver. To be reliable, differences between individuals in signal properties must be consistent and easily perceived and evaluated by receivers. Iridescent objects are often striking and vivid, but their appearance can change dramatically with viewing geometry and illumination. The changeable nature of iridescent surfaces creates a paradox: how can they be reliable signals? We contend that iridescent color patches can be reliable signals only if accompanied by specific adaptations to enhance reliability, such as structures and behaviors that limit perceived hue shift or enhance and control directionality. We highlight the challenges of studying iridescence and key considerations for the evaluation of its adaptive significance.

Structural color in Junonia butterflies evolves by tuning scale lamina thickness

In diverse organisms, nanostructures that coherently scatter light create structural color, but how such structures are built remains mysterious. We investigate the evolution and genetic regulation of butterfly scale laminae, which are simple photonic nanostructures. In a lineage of buckeye butterflies artificially selected for blue wing color, we found that thickened laminae caused a color shift from brown to blue. Deletion of the optix patterning gene also altered color via lamina thickening, revealing shared regulation of pigments and lamina thickness. Finally, we show how lamina thickness variation contributes to the color diversity that distinguishes sexes and species throughout the genus Junonia. Thus, quantitatively tuning one dimension of scale architecture facilitates both the microevolution and macroevolution of a broad spectrum of hues. Because the lamina is an intrinsic component of typical butterfly scales, our findings suggest that tuning lamina thickness is an available mechanism to create structural color across the Lepidoptera.

From iridescent blues to vibrant purples, many butterflies display dazzling ‘structural colors’ created not by pigments but by microscopic structures that interfere with light. For instance, the scales that coat their wings can contain thin films of chitin, the substance that normally makes the external skeleton of insects. In slim layers, however, chitin can also scatter light to produce color, the way that oil can create iridescence at the surface of water.

The thickness of the film, which is encoded by the genes of the butterfly, determines what color will be produced. Yet, little is known about how common thin films are in butterflies, exactly how genetic information codes for them, and how their thickness and the colors they produce can evolve.

To investigate, Thayer et al. used a technique called Helium Ion Microscopy and examined the wings of ten related species of butterflies, showing that thin film structures were present across this sample. However, the different species have evolved many different structural colors over the past millions of years by changing the thickness of the films.

Next, Thayer et al. showed that this evolution could be reproduced at a faster pace in the laboratory using common buckeye butterflies. These insects mostly have brown wings, but they can have specks of blue created by thin film structures. Individuals with more blue on their wings were mated and over the course of a year, the thickness of the film structures increased by 74%, leading to shiny blue butterflies. Deleting a gene called optix from the insects also led to blue wings. Optix was already known to control the patterns of pigments in butterflies, but it now appears that it controls structural colors as well.

From solar panels to new fabrics, microscopic structures that can scatter light are useful in a variety of industries. Understanding how these elements exist and evolve in organisms may help to better design them for human purposes.

Deep learning on butterfly phenotypes tests evolution's oldest mathematical model

Traditional anatomical analyses captured only a fraction of real phenomic information. Here, we apply deep learning to quantify total phenotypic similarity across 2468 butterfly photographs, covering 38 subspecies from the polymorphic mimicry complex of Heliconius erato and Heliconius melpomene. Euclidean phenotypic distances, calculated using a deep convolutional triplet network, demonstrate significant convergence between interspecies co-mimics. This quantitatively validates a key prediction of Müllerian mimicry theory, evolutionary biology's oldest mathematical model. Phenotypic neighbor-joining trees are significantly correlated with wing pattern gene phylogenies, demonstrating objective, phylogenetically informative phenome capture. Comparative analyses indicate frequency-dependent mutual convergence with coevolutionary exchange of wing pattern features. Therefore, phenotypic analysis supports reciprocal coevolution, predicted by classical mimicry theory but since disputed, and reveals mutual convergence as an intrinsic generator for the unexpected diversity of Müllerian mimicry. This demonstrates that deep learning can generate phenomic spatial embeddings, which enable quantitative tests of evolutionary hypotheses previously only testable subjectively.

Plasticity and divergence in ultraviolet reflecting structures on Dogface butterfly wings

The vast diversity of animal coloration is generated through a combination of pigment and structural colors. These colors can greatly influence the fitness and life history of an organism. Butterflies and their wing colors are an excellent model to study how these colors can impact the development and success of an organism. In this study, we explore species differences in structurally-based ultraviolet coloration in the Zerene butterfly. We show clear species differences in ultraviolet (UV) pattern and reflectance spectra. By varying larval diet, we show evidence for developmental plasticity in the structure and organization of UV reflecting scales in Zerene cesonia. We further show that feeding the larval host plant of Zerene eurydice to Z. cesonia does not result in greater similarity in scale structure or UV coloration to the sister species. These results not only demonstrate a connection between plasticity in a male ornamentation, UV wing pattern, and larval resource acquisition, but also identify candidate structural and organizational changes in wing scales responsible for the trait variation.

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

BACKGROUND

The color patterns that adorn lepidopteran wings are ideal for studying cell type diversity using a phenomics approach. Color patterns are made of chitinous scales that are each the product of a single precursor cell, offering a 2D system where phenotypic diversity can be studied cell by cell, both within and between species. Those scales reveal complex ultrastructures in the sub-micrometer range that are often connected to a photonic function, including iridescent blues and greens, highly reflective whites, or light-trapping blacks.

RESULTS

We found that during scale development, Fascin immunostainings reveal punctate distributions consistent with a role in the control of actin patterning. We quantified the cytoskeleton regularity as well as its relationship to chitin deposition sites, and confirmed a role in the patterning of the ultrastructures of the adults scales. Then, in an attempt to characterize the range and variation in lepidopteran scale ultrastructures, we devised a high-throughput method to quickly derive multiple morphological measurements from fluorescence images and scanning electron micrographs. We imaged a multicolor eyespot element from the butterfly Vanessa cardui (V. cardui), taking approximately 200 000 individual measurements from 1161 scales. Principal component analyses revealed that scale structural features cluster by color type, and detected the divergence of non-reflective scales characterized by tighter cross-rib distances and increased orderedness.

CONCLUSION

We developed descriptive methods that advance the potential of butterfly wing scales as a model system for studying how a single cell type can differentiate into a multifaceted spectrum of complex morphologies. Our data suggest that specific color scales undergo a tight regulation of their ultrastructures, and that this involves cytoskeletal dynamics during scale growth.

Quantitative characterization of iridescent colours in biological studies: a novel method using optical theory

Iridescent colours are colours that change with viewing or illumination geometry. While they are widespread in many living organisms, most evolutionary studies on iridescence do not take into account their full complexity. Few studies try to precisely characterize what makes iridescent colours special: their angular dependency. Yet, it is likely that this angular dependency has biological functions and is therefore submitted to evolutionary pressures. For this reason, evolutionary biologists need a repeatable method to measure iridescent colours as well as variables to precisely quantify the angular dependency. In this study, we use a theoretical approach to propose five variables that allow one to fully describe iridescent colours at every angle combination. Based on the results, we propose a new measurement protocol and statistical method to reliably characterize iridescence while minimizing the required number of time-consuming measurements. We use hummingbird iridescent feathers and butterfly iridescent wings as test cases to demonstrate the strengths of this new method. We show that our method is precise enough to be potentially used at intraspecific level while being also time-efficient enough to encompass large taxonomic scales.

Phenotypic variation in Heliconius erato crosses shows that iridescent structural colour is sex-linked and controlled by multiple genes

Bright, highly reflective iridescent colours can be seen across nature and are produced by the scattering of light from nanostructures. Heliconius butterflies have been widely studied for their diversity and mimicry of wing colour patterns. Despite iridescence evolving multiple times in this genus, little is known about the genetic basis of the colour and the development of the structures which produce it. Heliconius erato can be found across Central and South America, but only races found in western Ecuador and Colombia have developed blue iridescent colour. Here, we use crosses between iridescent and non-iridescent races of H. erato to study phenotypic variation in the resulting F₂ generation. Using measurements of blue colour from photographs, we find that iridescent structural colour is a quantitative trait controlled by multiple genes, with strong evidence for loci on the Z sex chromosome. Iridescence is not linked to the Mendelian colour pattern locus that also segregates in these crosses (controlled by the gene cortex). Small-angle X-ray scattering data show that spacing between longitudinal ridges on the scales, which affects the intensity of the blue reflectance, also varies quantitatively in F₂ crosses.

Living Light 2018: Conference Report

Living Light is a biennial conference focused on all aspects of light–matter interaction in biological organisms with a broad, interdisciplinary outlook. The 2018 edition was held at the Møller Centre in Cambridge, UK, from April 11th to April 14th, 2018. Living Light’s main goal is to bring together researchers from different backgrounds (e.g., biologists, physicists and engineers) in order to discuss the current state of the field and sparkle new collaborations and new interdisciplinary projects. With over 90 national and international attendees, the 2018 edition of the conference was strongly multidisciplinary: oral and poster presentations encompassed a wide range of topics ranging from the evolution and development of structural colors in living organisms and their genetic manipulation to the study of fossil photonic structures.