Multiple mechanisms of photoreceptor spectral tuning in Heliconius butterflies

Selection drives divergence of eye morphology in sympatric Heliconius butterflies

When populations experience different sensory conditions, natural selection may favor sensory system divergence, affecting peripheral structures and/or downstream neural pathways. We characterized the outer eye morphology of sympatric Heliconius butterflies from different forest types and their first-generation reciprocal hybrids to test for adaptive visual system divergence and hybrid disruption. In Panama, Heliconius cydno occurs in closed forests, whereas Heliconius melpomene resides at the forest edge. Among wild individuals, H. cydno has larger eyes than H. melpomene, and there are heritable, habitat-associated differences in the visual brain structures that exceed neutral divergence expectations. Notably, hybrids have intermediate neural phenotypes, suggesting disruption. To test for similar effects in the visual periphery, we reared both species and their hybrids in common garden conditions. We confirm that H. cydno has larger eyes and provide new evidence that this is driven by selection. Hybrid eye morphology is more H. melpomene-like despite body size being intermediate, contrasting with neural trait intermediacy. Overall, our results suggest that eye morphology differences between H. cydno and H. melpomene are adaptive and that hybrids may suffer fitness costs due to a mismatch between the peripheral visual structures and previously described neural traits that could affect visual performance.

Butterfly blues and greens caused by subtractive colour mixing of carotenoids and bile pigments

Butterflies often have conspicuously patterned wings, due to pigmentary and/or structurally wing scales that cover the wing membrane. The wing membrane of several butterfly species is also pigmentary coloured, notably by the bile pigments pterobilin, pharcobilin and sarpedobilin. The absorption spectra of the bilins have bands in the ultraviolet and red wavelength range, resulting in blue-cyan colours. Here, a survey of papilionoid and nymphalid butterflies reveals that several species with wings containing bile pigments combine them with carotenoids and other short-wavelength absorbing pigments, e.g., papiliochrome II, ommochromes and flavonoids, which creates green-coloured patterns. Various uncharacterized, long-wavelength absorbing wing pigments were encountered, particularly in heliconiines. The wings thus exhibit quite variable reflectance spectra, extending the enormous pigmentary and structural colouration richness of butterflies.

Adaptive introgression of a visual preference gene

Visual preferences are important drivers of mate choice and sexual selection, but little is known of how they evolve at the genetic level. In this study, we took advantage of the diversity of bright warning patterns displayed by Heliconius butterflies, which are also used during mate choice. Combining behavioral, population genomic, and expression analyses, we show that two Heliconius species have evolved the same preferences for red patterns by exchanging genetic material through hybridization. Neural expression of regucalcin1 correlates with visual preference across populations, and disruption of regucalcin1 with CRISPR-Cas9 impairs courtship toward conspecific females, providing a direct link between gene and behavior. Our results support a role for hybridization during behavioral evolution and show how visually guided behaviors contributing to adaptation and speciation are encoded within the genome.

Functional characterization of four opsins and two G alpha subtypes co-expressed in the molluscan rhabdomeric photoreceptor

BACKGROUND

Rhabdomeric photoreceptors of eyes in the terrestrial slug Limax are the typical invertebrate-type but unique in that three visual opsins (Gq-coupled rhodopsin, xenopsin, Opn5A) and one retinochrome, all belonging to different groups, are co-expressed. However, molecular properties including spectral sensitivity and G protein selectivity of any of them are not determined, which prevents us from understanding an advantage of multiplicity of opsin properties in a single rhabdomeric photoreceptor. To gain insight into the functional role of the co-expression of multiple opsin species in a photoreceptor, we investigated the molecular properties of the visual opsins in the present study.

RESULTS

First, we found that the fourth member of visual opsins, Opn5B, is also co-expressed in the rhabdomere of the photoreceptor together with previously identified three opsins. The photoreceptors were also demonstrated to express Gq and Go alpha subunits. We then determined the spectral sensitivity of the four visual opsins using biochemical and spectroscopic methods. Gq-coupled rhodopsin and xenopsin exhibit maximum sensitivity at ~ 456 and 475 nm, respectively, and Opn5A and Opn5B exhibit maximum sensitivity at ~ 500 and 470 nm, respectively, with significant UV sensitivity. Notably, in vitro experiments revealed that Go alpha was activated by all four visual opsins, in contrast to the specific activation of Gq alpha by Gq-coupled rhodopsin, suggesting that the eye photoreceptor of Limax uses complex G protein signaling pathways.

CONCLUSIONS

The eye photoreceptor in Limax expresses as many as four different visual opsin species belonging to three distinct classes. The combination of opsins with different spectral sensitivities and G protein selectivities may underlie physiological properties of the ocular photoreception, such as a shift in spectral sensitivity between dark- and light-adapted states. This may be allowed by adjustment of the relative contribution of the four opsins without neural networks, enabling a simple strategy for fine-tuning of vision.

Quantifying visual acuity in Heliconius butterflies

Heliconius butterflies are well-known for their colourful wing patterns, which advertise distastefulness to potential predators and are used during mate choice. However, the relative importance of different aspects of these signals will depend on the visual abilities of Heliconius and their predators. Previous studies have investigated colour sensitivity and neural anatomy, but visual acuity (the ability to perceive detail) has not been studied in these butterflies. Here, we provide the first estimate of visual acuity in Heliconius: from a behavioural optomotor assay, we found that mean visual acuity = 0.49 cycles-per-degree (cpd), with higher acuity in males than females. We also examined eye morphology and report more ommatidia in male eyes. Finally, we estimated how visual acuity affects Heliconius visual perception compared to a potential avian predator. Whereas the bird predator maintained high resolving power, Heliconius lost the ability to resolve detail at greater distances, though colours may remain salient. These results will inform future studies of Heliconius wing pattern evolution, as well as other aspects in these highly visual butterflies, which have emerged as an important system in studies of adaptation and speciation.

Opsin Gene Duplication in Lepidoptera: Retrotransposition, Sex Linkage, and Gene Expression

Color vision in insects is determined by signaling cascades, central to which are opsin proteins, resulting in sensitivity to light at different wavelengths. In certain insect groups, lineage-specific evolution of opsin genes, in terms of copy number, shifts in expression patterns, and functional amino acid substitutions, has resulted in changes in color vision with subsequent behavioral and niche adaptations. Lepidoptera are a fascinating model to address whether evolutionary change in opsin content and sequence evolution are associated with changes in vision phenotype. Until recently, the lack of high-quality genome data representing broad sampling across the lepidopteran phylogeny has greatly limited our ability to accurately address this question. Here, we annotate opsin genes in 219 lepidopteran genomes representing 33 families, reconstruct their evolutionary history, and analyze shifts in selective pressures and expression between genes and species. We discover 44 duplication events in opsin genes across ∼300 million years of lepidopteran evolution. While many duplication events are species or family specific, we find retention of an ancient long-wavelength-sensitive (LW) opsin duplication derived by retrotransposition within the speciose superfamily Noctuoidea (in the families Nolidae, Erebidae, and Noctuidae). This conserved LW retrogene shows life stage-specific expression suggesting visual sensitivities or other sensory functions specific to the early larval stage. This study provides a comprehensive order-wide view of opsin evolution across Lepidoptera, showcasing high rates of opsin duplications and changes in expression patterns.

Parallel Losses of Blue Opsin Correlate with Compensatory Neofunctionalization of UV-Opsin Gene Duplicates in Aphids and Planthoppers

Expanding on previous efforts to survey the visual opsin repertoires of the Hemiptera, this study confirms that homologs of the UV- and LW-opsin subfamilies are conserved in all Hemiptera, while the B-opsin subfamily is missing from the Heteroptera and subgroups of the Sternorrhyncha and Auchenorrhyncha, i.e., aphids (Aphidoidea) and planthoppers (Fulgoroidea), respectively. Unlike in the Heteroptera, which are characterized by multiple independent expansions of the LW-opsin subfamily, the lack of B-opsin correlates with the presence of tandem-duplicated UV-opsins in aphids and planthoppers. Available data on organismal wavelength sensitivities and retinal gene expression patterns lead to the conclusion that, in both groups, one UV-opsin paralog shifted from ancestral UV peak sensitivity to derived blue sensitivity, likely compensating for the lost B-opsin. Two parallel bona fide tuning site substitutions compare to 18 non-corresponding amino acid replacements in the blue-shifted UV-opsin paralogs of aphids and planthoppers. Most notably, while the aphid blue-shifted UV-opsin clade is characterized by a replacement substitution at one of the best-documented UV/blue tuning sites (Rhodopsin site 90), the planthopper blue-shifted UV-opsin paralogs retained the ancestral lysine at this position. Combined, the new findings identify aphid and planthopper UV-opsins as a new valuable data sample for studying adaptive opsin evolution.

Sex-linked gene traffic underlies the acquisition of sexually dimorphic UV color vision in Heliconius butterflies

The acquisition of novel sexually dimorphic traits poses an evolutionary puzzle: How do new traits arise and become sex-limited? Recently acquired color vision, sexually dimorphic in animals like primates and butterflies, presents a compelling model for understanding how traits become sex-biased. For example, some Heliconius butterflies uniquely possess UV (ultraviolet) color vision, which correlates with the expression of two differentially tuned UV-sensitive rhodopsins, UVRh1 and UVRh2. To discover how such traits become sexually dimorphic, we studied Heliconius charithonia, which exhibits female-specific UVRh1 expression. We demonstrate that females, but not males, discriminate different UV wavelengths. Through whole-genome shotgun sequencing and assembly of the H. charithonia genome, we discovered that UVRh1 is present on the W chromosome, making it obligately female-specific. By knocking out UVRh1, we show that UVRh1 protein expression is absent in mutant female eye tissue, as in wild-type male eyes. A PCR survey of UVRh1 sex-linkage across the genus shows that species with female-specific UVRh1 expression lack UVRh1 gDNA in males. Thus, acquisition of sex linkage is sufficient to achieve female-specific expression of UVRh1, though this does not preclude other mechanisms, like cis-regulatory evolution from also contributing. Moreover, both this event, and mutations leading to differential UV opsin sensitivity, occurred early in the history of Heliconius. These results suggest a path for acquiring sexual dimorphism distinct from existing mechanistic models. We propose a model where gene traffic to heterosomes (the W or the Y) genetically partitions a trait by sex before a phenotype shifts (spectral tuning of UV sensitivity).

Oviposition behavior is not affected by ultraviolet light in a butterfly with sexually-dimorphic expression of a UV-sensitive opsin

Animal vision is important for mediating multiple complex behaviors. In Heliconius butterflies, vision guides fundamental behaviors such as oviposition, foraging, and mate choice. Color vision in Heliconius involves ultraviolet (UV), blue and long-wavelength-sensitive photoreceptors (opsins). Additionally, Heliconius possess a duplicated UV opsin, and its expression varies widely within the genus. In Heliconius erato, opsin expression is sexually dimorphic; only females express both UV-sensitive opsins, enabling UV wavelength discrimination. However, the selective pressures responsible for sex-specific differences in opsin expression and visual perception remain unresolved. Female Heliconius invest heavily in finding suitable hostplants for oviposition, a behavior heavily dependent on visual cues. Here, we tested the hypothesis that UV vision is important for oviposition in H. erato and Heliconius himera females by manipulating the availability of UV in behavioral experiments under natural conditions. Our results indicate that UV does not influence the number of oviposition attempts or eggs laid, and the hostplant, Passiflora punctata, does not reflect UV wavelengths. Models of H. erato female vision suggest only minimal stimulation of the UV opsins. Overall, these findings suggest that UV wavelengths do not directly affect the ability of Heliconius females to find suitable oviposition sites. Alternatively, UV discrimination could be used in the context of foraging or mate choice, but this remains to be tested.

Parallel evolution of opsin visual pigments in hawkmoths by tuning of spectral sensitivities during transition from a nocturnal to a diurnal ecology

Light environments differ dramatically between day and night. The transition between diurnal and nocturnal visual ecology has happened repeatedly throughout evolution in many species. However, the molecular mechanism underlying the evolution of vision in recent diurnal-nocturnal transition is poorly understood. Here, we focus on hawkmoths (Lepidoptera: Sphingidae) to address this question by investigating five nocturnal and five diurnal species. We performed RNA-sequencing analysis and identified opsin genes corresponding to the ultraviolet (UV), short-wavelength (SW) and long-wavelength (LW)-absorbing visual pigments. We found no significant differences in the expression patterns of opsin genes between the nocturnal and diurnal species. We then constructed the phylogenetic trees of hawkmoth species and opsins. The diurnal lineages had emerged at least three times from the nocturnal ancestors. The evolutionary rates of amino acid substitutions in the three opsins differed between the nocturnal and diurnal species. We found an excess number of parallel amino acid substitutions in the opsins in three independent diurnal lineages. The numbers were significantly more than those inferred from neutral evolution, suggesting that positive selection acted on these parallel substitutions. Moreover, we predicted the visual pigment absorption spectra based on electrophysiologically determined spectral sensitivity in two nocturnal and two diurnal species belonging to different clades. In the diurnal species, the LW pigments shift 10 nm towards shorter wavelengths, and the SW pigments shift 10 nm in the opposite direction. Taken together, our results suggest that parallel evolution of opsins may have enhanced the colour discrimination properties of diurnal hawkmoths in ambient light.

Simple and complex, sexually dimorphic retinal mosaic of fritillary butterflies

Butterflies have variable sets of spectral photoreceptors that underlie colour vision. The photoreceptor organization may be optimized for the detection of body coloration. Fritillaries (Argynnini) are nymphalid butterflies exhibiting varying degrees of sexual dimorphism in wing coloration. In two sister species, the females have orange (Argynnis paphia) and dark wings (Argynnis sagana), respectively, while the males of both species have orange wings with large patches of pheromone-producing androconia. In spite of the differences in female coloration, the eyes of both species exhibit an identical sexual dimorphism. The female eyeshine is uniform yellow, while the males have a complex retinal mosaic with yellow and red-reflecting ommatidia. We found the basic set of ultraviolet-, blue- and green-peaking photoreceptors in both sexes. Males additionally have three more photoreceptor classes, peaking in green, yellow and red, respectively. The latter is the basal R9, indirectly measured through hyperpolarizations in the green-peaking R1-2. In many nymphalid tribes, including the closely related Heliconiini, the retinal mosaic is complex in both sexes. We hypothesize that the simple mosaic of female Argynnini is a secondary reduction, possibly driven by the use of olfaction for intraspecific recognition, whereas vision remains the primary sense for the task in the males. This article is part of the theme issue 'Understanding colour vision: molecular, physiological, neuronal and behavioural studies in arthropods'.

Insect opsins and evo-devo: what have we learned in 25 years?

The visual pigments known as opsins are the primary molecular basis for colour vision in animals. Insects are among the most diverse of animal groups and their visual systems reflect a variety of life histories. The study of insect opsins in the fruit fly Drosophila melanogaster has led to major advances in the fields of neuroscience, development and evolution. In the last 25 years, research in D. melanogaster has improved our understanding of opsin genotype-phenotype relationships while comparative work in other insects has expanded our understanding of the evolution of insect eyes via gene duplication, coexpression and homologue switching. Even so, until recently, technology and sampling have limited our understanding of the fundamental mechanisms that evolution uses to shape the diversity of insect eyes. With the advent of genome editing and in vitro expression assays, the study of insect opsins is poised to reveal new frontiers in evolutionary biology, visual neuroscience, and animal behaviour. This article is part of the theme issue 'Understanding colour vision: molecular, physiological, neuronal and behavioural studies in arthropods'.