Light environment influences mating behaviours during the early stages of divergence in tropical 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.

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.

The Evolutionary Importance of Intraspecific Variation in Sexual Communication Across Sensory Modalities

The evolution of sexual communication is critically important in the diversity of arthropods, which are declining at a fast pace worldwide. Their environments are rapidly changing, with increasing chemical, acoustic, and light pollution. To predict how arthropod species will respond to changing climates, habitats, and communities, we need to understand how sexual communication systems can evolve. In the past decades, intraspecific variation in sexual signals and responses across different modalities has been identified, but never in a comparative way. In this review, we identify and compare the level and extent of intraspecific variation in sexual signals and responses across three different modalities, chemical, acoustic, and visual, focusing mostly on insects. By comparing causes and possible consequences of intraspecific variation in sexual communication among these modalities, we identify shared and unique patterns, as well as knowledge needed to predict the evolution of sexual communication systems in arthropods in a changing world.

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.

Adult neurogenesis does not explain the extensive post-eclosion growth of Heliconius mushroom bodies

Among butterflies, Heliconius have a unique behavioural profile, being the sole genus to actively feed on pollen. Heliconius learn the location of pollen resources, and have enhanced visual memories and expanded mushroom bodies, an insect learning and memory centre, relative to related genera. These structures also show extensive post-eclosion growth and developmental sensitivity to environmental conditions. However, whether this reflects plasticity in neurite growth, or an extension of neurogenesis into the adult stage, is unknown. Adult neurogenesis has been described in some Lepidoptera, and could provide one route to the increased neuron number observed in Heliconius. Here, we compare volumetric changes in the mushroom bodies of freshly eclosed and aged Heliconius erato and Dryas iulia, and estimate the number of intrinsic mushroom body neurons using a new and validated automated method to count nuclei. Despite extensive volumetric variation associated with age, our data show that neuron number is remarkably constant in both species, suggesting a lack of adult neurogenesis in the mushroom bodies. We support this conclusion with assays of mitotic cells, which reveal very low levels of post-eclosion cell division. Our analyses provide an insight into the evolution of neural plasticity, and can serve as a basis for continued exploration of the potential mechanisms behind brain development and maturation.

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.

Does sexual conflict contribute to the evolution of novel warning patterns?

Why warning patterns are so diverse is an enduring evolutionary puzzle. Because predators associate particular patterns with unpleasant experiences, an individual's predation risk should decrease as the local density of its warning pattern increases, promoting pattern monomorphism. Distasteful Heliconius butterflies are known for their diversity of warning patterns. Here, we explore whether interlocus sexual conflict can contribute to their diversification. Male Heliconius use warning patterns as mating cues, but mated females may suffer costs if this leads to disturbance, favouring novel patterns. Using simulations, we show that under our model conditions drift alone is unlikely to cause pattern diversification, but that sexual conflict can assist such a process. We also find that genetic architecture influences the evolution of male preferences, which track changes in warning pattern due to sexual selection. When male attraction imposes costs on females, this affects the speed at which novel pattern alleles increase. In two experiments, females laid fewer eggs with males present. However, although males in one experiment showed less interest in females with manipulated patterns, we found no evidence that female colouration mitigates sex-specific costs. Overall, male attraction to conspecific warning patterns may impose an unrecognized cost on Heliconius females, but further work is required to determine this experimentally.

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'.

Prevalence and Adaptive Impact of Introgression

Alleles that introgress between species can influence the evolutionary and ecological fate of species exposed to novel environments. Hybrid offspring of different species are often unfit, and yet it has long been argued that introgression can be a potent force in evolution, especially in plants. Over the last two decades, genomic data have increasingly provided evidence that introgression is a critically important source of genetic variation and that this additional variation can be useful in adaptive evolution of both animals and plants. Here, we review factors that influence the probability that foreign genetic variants provide long-term benefits (so-called adaptive introgression) and discuss their potential benefits. We find that introgression plays an important role in adaptive evolution, particularly when a species is far from its fitness optimum, such as when they expand their range or are subject to changing environments.

Light environment influences mating behaviours during the early stages of divergence in tropical butterflies

Speciation is facilitated when traits under divergent selection also act as mating cues. Fluctuations in sensory conditions can alter signal perception independently of adaptation to the broader sensory environment, but how this fine-scale variation may constrain or promote behavioural isolation has received little attention. The warning patterns of Heliconius butterflies are under selection for aposematism and act as mating cues. Using computer vision, we extracted behavioural data from 1481 h of video footage, for 387 individuals. We show that the putative hybrid species H. heurippa and its close relative H. timareta linaresi differ in their response to divergent warning patterns, but that these differences are strengthened with increased local illuminance. Trials with live individuals reveal low-level assortative mating that is sufficiently explained by differences in visual attraction. Finally, results from hybrid butterflies are consistent with linkage between a major warning pattern gene and the corresponding behaviour, though the differences in behaviour we observe are unlikely to cause rapid reproductive isolation as predicted under a model of hybrid trait speciation. Overall, our results reveal that the contribution of ecological mating cues to reproductive isolation may depend on the immediate sensory conditions during which they are displayed to conspecifics.