Retinal perception and ecological significance of color vision in insects

Ambient light spectrum affects larval Mexican jumping bean moth (Cydia saltitans) behavior despite light obstruction from host seed

Spectral differences in ambient light can affect animal behavior and convey crucial information about an individual's environment. The ability to perceive and respond to differences in ambient light varies widely by taxa and is shaped by a species' ecology. Mexican jumping bean moths, Cydia saltitans, spend their entire larval period encased in fallen host seeds and contend with potentially lethal environmental temperatures when host seeds are in direct sunlight. We investigate if and how C. saltitans larvae in host seeds respond to lighting conditions associated with these thermal risks. In a temperature-controlled experiment, we identified that larvae demonstrated distinct behavioral ("jumping") responses corresponding to four lighting treatments (white, red, green, and purple), despite extremely minimal light penetration through host seed walls. Red light induced the greatest larval activity (measured by probability of movement and by displacement from origin), suggesting that larvae have mechanisms to perceive low levels of red light and/or to detect subtle increases in heat produced by red/near infrared-biased light spectra, possibly providing them with an early-warning mechanism against thermal stress. Our findings highlight the interplay of environmental lighting, behavior, and potential thermosensory adaptations in a species with a visually constrained environment.

Regulation of opsin and circadian clock genes on mate-finding behavior of the day-flying red moth, Phauda flammans (Walker)

First, significantly higher mate-finding success was found under light condition than under constant darkness condition in Phauda flammans, a typical diurnal moth. We speculate that mate-finding behavior in P. flammans may be influenced by the light-sensitive opsin genes Long wavelength opsin (PfLW), Ultraviolet opsin (PfUV) and Blue opsin (PfBL), which are potentially regulated by both light-cues and endogenous circadian rhythms. Second, the circadian clock genes Period (PfPer), Timeless (PfTim), Cryptochrome1 (PfCry1), Cryptochrome2 (PfCRY2), Cryptochrome3 (PfCry-like), Clock (PfClk), Cycle (PfCyc), Vrille (PfVri), and Slimb (PfSli) were identified in P. flammans. Third, circadian rhythms in the relative expression levels of opsin and circadian clock genes were demonstrated via quantitative real-time PCR analysis, with peak expression coinciding with the mate-finding peak. Notably, the relative expression of PfLW in males P. flammans was significantly higher than that in females P. flammans at the mate-finding peaks Zeitgeber time (ZT) 8 and ZT 10 under light, while the expression of the opsin gene PfBL showed a similar pattern at ZT 10 under light. Additionally, the expression of the clock gene PfCry-like was significantly higher in males than in females at ZT 8 and ZT 10 under light, while PfPer, PfTim, PfClk and PfCyc exhibited similar male-biased expression patterns at ZT 10 under light. Conversely, PfCry1 and PfVri expression was significantly higher in females than in male at ZT 8 under light. In conclusion, sex differences were detected in the expression of opsin and circadian clock genes, which indicated that light-mediated regulation of these genes may contribute to the daytime mate-finding behavior of P. flammans.

Investigating the Influence of Varied Light-Emitting Diode (LED) Wavelengths on Phototactic Behavior and Opsin Genes in Vespinae

The phototactic behavior of insects is commonly used to manage pest populations in practical production. However, this elusive behavior is not yet fully understood. Investigating whether the opsin genes play a crucial role in phototaxis is an intriguing topic. Vespinae (Hymenoptera: Vespidae) are a common group of social wasps that are closely associated with human activities. Efficiently controlling wasp populations while maintaining ecological balance is a pressing global challenge that still has to be resolved. This research aims to explore the phototactic behavior and key opsin genes associated with Vespinae. We found significant differences in the photophilic rates of Vespula germanica and Vespa analis under 14 different light conditions, indicating that their phototactic behavior is rhythmic. The results also showed that the two species exhibited varying photophilic rates under different wavelengths of light, suggesting that light wavelength significantly affects their phototactic behavior. Additionally, the opsin genes of the most aggressive hornet, Vespa basalis, have been sequenced. There are only two opsin genes, one for UV light and the other for blue light, and Vespa basalis lacks long-wavelength visual proteins. However, they exhibit peak phototaxis for long-wavelength light and instead have the lowest phototaxis for UV light. This suggests that the visual protein genes have a complex regulatory mechanism for phototactic behavior in Vespinae. Additionally, visual protein sequences have a high degree of homology among Hymenoptera. Despite the hypotheses put forward by some scholars regarding phototaxis, a clear and complete explanation of insect phototaxis is still lacking to date. Our findings provide a strong theoretical basis for further investigation of visual expression patterns and phototactic mechanisms in Vespinae.

Opsin mutants alter host plant selection by color vision in the nocturnal invasive pest Tuta absoluta

In insects, vision is crucial in finding host plants, but its role in nocturnal insects is largely unknown. Vision involves responses to specific spectra of photon wavelengths and opsins plays an important role in this process. Long-wavelength sensitive opsin (LW opsin) and blue-sensitive opsin (BL opsin) are main visual opsin proteins and play important in behavior regulation.We used CRISPR/Cas9 technology to mutate the long-wavelength-sensitive and blue wavelength-sensitive genes and explored the role of vision in the nocturnal invasive pest Tuta absoluta. Light wave experiments revealed that LW2(-/-) and BL(-/-) mutants showed abnormal wavelength tropism. Both LW2 and BL mutations affected the preference of T. absoluta for the green environment. Mutations in LW2 and BL are necessary to inhibit visual attraction. The elimination of LW2 and BL affected the preference of leaf moths for green plants, and mutations in both induced a preference in moths for white plants. Behavioral changes resulting from LW2(-/-) and BL(-/-) mutants were not affected by sense of smell, further supporting the regulatory role of vision in insect behavior. To the best of our knowledge, this is the first study to reveal that vision, not smell, plays an important role in the host-seeking behavior of nocturnal insects at night, of which LW2 and BL opsins are key regulatory factors. These study findings will drive the development of the "vision-ecology" theory.

Parasite manipulation of host phenotypes inferred from transcriptional analyses in a trematode-amphipod system

Manipulation of host phenotypes by parasites is hypothesized to be an adaptive strategy enhancing parasite transmission across hosts and generations. Characterizing the molecular mechanisms of manipulation is important to advance our understanding of host-parasite coevolution. The trematode (Levinseniella byrdi) is known to alter the colour and behaviour of its amphipod host (Orchestia grillus) presumably increasing predation of amphipods which enhances trematode transmission through its life cycle. We sampled 24 infected and 24 uninfected amphipods from a salt marsh in Massachusetts to perform differential gene expression analysis. In addition, we constructed novel genomic tools for O. grillus including a de novo genome and transcriptome. We discovered that trematode infection results in upregulation of amphipod transcripts associated with pigmentation and detection of external stimuli, and downregulation of multiple amphipod transcripts implicated in invertebrate immune responses, such as vacuolar ATPase genes. We hypothesize that suppression of immune genes and the altered expression of genes associated with coloration and behaviour may allow the trematode to persist in the amphipod and engage in further biochemical manipulation that promotes transmission. The genomic tools and transcriptomic analyses reported provide new opportunities to discover how parasites alter diverse pathways underlying host phenotypic changes in natural populations.

Mechanisms of spectral orientation in a diurnal dung beetle

Ball rolling dung beetles use a wide range of cues to steer themselves along a fixed bearing, including the spectral gradient of scattered skylight that spans the sky. Here, we define the spectral sensitivity of the diurnal dung beetle Kheper lamarcki and use the information to explore the orientation performance under a range of spectral light combinations. We find that, when presented with spectrally diverse stimuli, the beetles primarily orient to the apparent brightness differences as perceived by their green photoreceptors. Under certain wavelength combinations, they also rely on spectral information to guide their movements, but the brightness and spectral directional information is never fully disentangled. Overall, our results suggest the use of a dichromatic, primitive colour vision system for the extraction of directional information from the celestial spectral gradient to support straight-line orientation. This article is part of the theme issue 'Understanding colour vision: molecular, physiological, neuronal and behavioural studies in arthropods'.

High diversity of arthropod colour vision: from genes to ecology

Colour vision allows animals to use the information contained in the spectrum of light to control important behavioural decisions such as selection of habitats, food or mates. Among arthropods, the largest animal phylum, we find completely colour-blind species as well as species with up to 40 different opsin genes or more than 10 spectral types of photoreceptors, we find a large diversity of optical methods shaping spectral sensitivity, we find eyes with different colour vision systems looking into the dorsal and ventral hemisphere, and species in which males and females see the world in different colours. The behavioural use of colour vision shows an equally astonishing diversity. Only the neural mechanisms underlying this sensory ability seems surprisingly conserved-not only within the phylum, but even between arthropods and the other well-studied phylum, chordates. The papers in this special issue allow a glimpse into the colourful world of arthropod colour vision, and besides giving an overview this introduction highlights how much more research is needed to fill in the many missing pieces of this large puzzle. This article is part of the theme issue 'Understanding colour vision: molecular, physiological, neuronal and behavioural studies in arthropods'.

Improved Captures of the Invasive Brown Marmorated Stink Bug, Halyomorpha halys, Using a Novel Multimodal Trap

Simple Summary

Pest monitoring using traps is a key component of integrated pest management. For several insects, trapping is achieved using visual or olfactory stimuli. Although the combination of both is supposed to provide higher efficacy, this has often been overlooked in trap design. Through laboratory bioassays and field experiments we evaluated the use of UV-A and visible light in combination with olfactory stimuli to improve trapping of the invasive brown marmorated stink bug, Halyomorpha halys (Hemiptera: Pentatomidae). Our results may be useful for the improvement of monitoring strategies for early pest detection. Additionally, the higher efficacy of the multimodal traps would allow their use in attract-and-kill or push–pull strategies within integrated pest management.

Abstract

Capture strategies for the brown marmorated stink bug, Halyomorpha halys (Hemiptera: Pentatomidae), are challenging. Here we developed and evaluated a multimodal trap which combines visual and olfactory stimuli. Visual stimuli consisted of LEDs emitting UV-A and visible light. Olfactory stimuli were comprised of the synthetic aggregation pheromone and odours from trapped H. halys individuals. Stink bug attraction at different wavelengths was evaluated in laboratory two-choice bioassays, and different prototypes of the trap were tested in 2021 in natural, agricultural, and urban settings. Traps with a combination of UV-A and blue or green visible wavelengths provided higher H. halys attraction (up to ~8-fold) compared to traditional sticky or small pyramidal traps. The concurrent presence of synthetic pheromone and LED had a synergistic effect on H. halys positive phototaxis. Further development and implementation of the multimodal trap is discussed for prospective use in attract-and-kill or push–pull strategies.

Shaping an optical dome: The size and shape of the insect compound eye

The insect compound eye is the most abundant eye architecture on earth. It comes in a wide variety of shapes and sizes, which are exquisitely adapted to specific ecosystems. Here, we explore the organisational principles and pathways, from molecular to tissular, that underpin the building of this organ and highlight why it is an excellent model system to investigate the relationship between genes and tissue form. The compound eye offers wide fields of view, high sensitivity in motion detection and infinite depth of field. It is made of an array of visual units called ommatidia, which are precisely tiled in 3D to shape the retinal tissue as a dome-like structure. The eye starts off as a 2D epithelium, and it acquires its 3D organisation as ommatidia get into shape. Each ommatidium is made of a complement of retinal cells, including light-detecting photoreceptors and lens-secreting cells. The lens cells generate the typical hexagonal facet lens that lies atop the photoreceptors so that the eye surface consists of a quasi-crystalline array of these hexagonal facet-lenses. This array is curved to various degree, depending on the size and shape of the eye, and on the region of the retina. This curvature sets the resolution and visual field of the eye and is determined by i) the number and size of the facet lens - large ommatidial lenses can be used to generate flat, higher resolution areas, while smaller facets allow for stronger curvature of the eye, and ii) precise control of the inter facet-lens angle, which determines the optical axis of the each ommatidium. In this review we discuss how combinatorial variation in eye primordium shape, ommatidial number, facet lens size and inter facet-lens angle underpins the wide variety of insect eye shapes, and we explore what is known about the mechanisms that might control these parameters.

Chromosomal-Level Genome Assembly of a True Bug, Aspongopus chinensis Dallas, 1851 (Hemiptera: Dinidoridae)

The true bug, Aspongopus chinensis Dallas, 1851 (Hemiptera: Dinidoridae), is a fascinating insect with prolonged diapause and medicinal properties but also a notorious pest. However, because of the lack of genomic resources, an in-depth understanding of its biological characteristics is lacking. Here, we report the first genome assembly of A. chinensis anchored to 10 pseudochromosomes, which was achieved by combining PacBio long reads and Hi-C sequencing data. This chromosome-level genome assembly was 1.55 Gb in size with a scaffold N50 of 156 Mb. The benchmarking universal single-copy ortholog (BUSCO) analysis of the assembly captured 96.6% of the BUSCO genes. A total of 686,888,052 bp of repeat sequences, 18,511 protein-coding genes, and 1,749 noncoding RNAs were annotated. By comparing the A. chinensis genome with that of 8 homologous insects and 2 model organisms, 213 rapidly evolving gene families were identified, including 83 expanded and 130 contracted gene families. The functional enrichment of Gene Ontology and KEGG pathways showed that the significantly expanded gene families were primarily involved in metabolism, immunity, detoxification, and DNA/RNA replication associated with stress responses. The data reported here shed light on the ecological adaptation of A. chinensis and further expanded our understanding of true bug evolution in general.

Evidence for UV-green dichromacy in the basal hymenopteran Sirex noctilio (Siricidae)

A precondition for colour vision is the presence of at least two spectral types of photoreceptors in the eye. The order Hymenoptera is traditionally divided into the Apocrita (ants, bees, wasps) and the Symphyta (sawflies, woodwasps, horntails). Most apocritan species possess three different photoreceptor types. In contrast, physiological studies in the Symphyta have reported one to four photoreceptor types. To better understand the evolution of photoreceptor diversity in the Hymenoptera, we studied the Symphyta Sirex noctilio, which belongs to the superfamily Siricoidea, a closely related group of the Apocrita suborder. Our aim was to (i) identify the photoreceptor types of the compound eye by electroretinography (ERG), (ii) characterise the visual opsin genes of S. noctilio by genomic comparisons and phylogenetic analyses and (iii) analyse opsin mRNA expression. ERG measurements revealed two photoreceptor types in the compound eye, maximally sensitive to 527 and 364 nm. In addition, we identified three opsins in the genome, homologous to the hymenopteran green or long-wavelength sensitive (LW) LW1, LW2 and ultra-violet sensitive (UV) opsin genes. The LW1 and UV opsins were found to be expressed in the compound eyes, and LW2 and UV opsins in the ocelli. The lack of a blue or short-wavelength sensitive (SW) homologous opsin gene and a corresponding receptor suggests that S. noctilio is a UV-green dichromate.

CRISPR/Cas9-mediated knockout of LW-opsin reduces the efficiency of phototaxis in the diamondback moth Plutella xylostella

BACKGROUND

Opsins are crucial for animal vision. The identity and function of opsins in Plutella xylostella remain unknown. The aim of the research is to confirm which opsin gene(s) contribute to phototaxis of P. xylostella.

RESULTS

LW-opsin, BL-opsin and UV-opsin, were identified in the P. xylostella genome. LW-opsin was more highly expressed than the other two opsin genes, and all three genes were specifically expressed in the head. Three P. xylostella strains, LW-13 with a 13-bp deletion in LW-opsin, BL + 2 with a 2-bp insertion in BL-opsin, and UV-29 with a 5-bp insertion and a 34-bp deletion in UV-opsin, were established from the strain G88 using the CRISPR/Cas9 system. Among the three opsin-knockout strains, only male and female LW-13 exhibited weaker phototaxis to lights of different wavelengths and white light than G88 at 2.5 lx due to defective locomotion, and LW-13 was defective to sense white, green and infrared lights. The locomotion of LW-13 was reduced compared with G88 at 2.5, 10, 20, 60, 80, 100, and 200 lx under the green light, but the locomotion of LW-13 female was recovered at 80, 100 and 200 lx. The defective phototaxis to the green light of male LW-13 was not affected by light intensity, while the defective phototaxis to the green light of female LW-13 was recovered at 10, 20, 60, 80, 100, and 200 lx.

CONCLUSION

LW-opsin is involved in light sensing and locomotion of P. xylostella, providing a potential target gene for controlling the pest. © 2021 Society of Chemical Industry.

Haze of glue determines preference of western flower thrips (Frankliniella occidentalis) for yellow or blue traps

In a wind tunnel we compared the colour preference for western flower thrips to four types of colour plates (clear, white, blue and yellow) applied with two types of glue (diffuse Stikem versus clear D41). Further the results for blue and yellow preference were validated in two greenhouses. In the wind tunnel, we found a clear preference of yellow over blue when a clear glue (D41) was used. However, with a more diffuse (whitish) glue (Stikem) the preference for yellow over blue disappeared, whereby the attraction to yellow decreased (58%) while the attraction to blue increased (65%). In the greenhouses, we found similar effects as in the wind tunnel with a decrease in attraction to yellow (35%) and increase in attraction to blue (32%) for Stikem compared to D41. Light measurements showed an increase of 18% of blue, 21% of violet light, 8% of yellow and 9% of green light reflected on the yellow Stikem trap versus the yellow D41 trap. On blue plates there was only 4% increase of blue light, 8% decrease of yellow light reflected when Stikem glue was used compared to D41 glue. It is not yet clear if the change of light reflection ratio blue/yellow caused by the glue type plays a role in the change of attraction. The reflective properties of glue are so far an unknown factor in colour choice and may explain partially the different results on colour preference. A small review on thrips colour preference is discussed to determine possible other factors of influence on colour choice.

Wireless steerable vision for live insects and insect-scale robots

Vision serves as an essential sensory input for insects but consumes substantial energy resources. The cost to support sensitive photoreceptors has led many insects to develop high visual acuity in only small retinal regions and evolve to move their visual systems independent of their bodies through head motion. By understanding the trade-offs made by insect vision systems in nature, we can design better vision systems for insect-scale robotics in a way that balances energy, computation, and mass. Here, we report a fully wireless, power-autonomous, mechanically steerable vision system that imitates head motion in a form factor small enough to mount on the back of a live beetle or a similarly sized terrestrial robot. Our electronics and actuator weigh 248 milligrams and can steer the camera over 60° based on commands from a smartphone. The camera streams "first person" 160 pixels-by-120 pixels monochrome video at 1 to 5 frames per second (fps) to a Bluetooth radio from up to 120 meters away. We mounted this vision system on two species of freely walking live beetles, demonstrating that triggering image capture using an onboard accelerometer achieves operational times of up to 6 hours with a 10-milliamp hour battery. We also built a small, terrestrial robot (1.6 centimeters by 2 centimeters) that can move at up to 3.5 centimeters per second, support vision, and operate for 63 to 260 minutes. Our results demonstrate that steerable vision can enable object tracking and wide-angle views for 26 to 84 times lower energy than moving the whole robot.

Butterfly dichromatism primarily evolved via Darwin's, not Wallace's, model

Sexual dimorphism is typically thought to result from sexual selection for elaborated male traits, as proposed by Darwin. However, natural selection could reduce expression of elaborated traits in females, as proposed by Wallace. Darwin and Wallace debated the origins of dichromatism in birds and butterflies, and although evidence in birds is roughly equal, if not in favor of Wallace's model, butterflies lack a similar scale of study. Here, we present a large‐scale comparative phylogenetic analysis of the evolution of butterfly coloration, using all European non‐hesperiid butterfly species (n = 369). We modeled evolutionary changes in coloration for each species and sex along their phylogeny, thereby estimating the rate and direction of evolution in three‐dimensional color space using a novel implementation of phylogenetic ridge regression. We show that male coloration evolved faster than female coloration, especially in strongly dichromatic clades, with male contribution to changes in dichromatism roughly twice that of females. These patterns are consistent with a classic Darwinian model of dichromatism via sexual selection on male coloration, suggesting this model was the dominant driver of dichromatism in European butterflies.

Color vision in insects: insights from Drosophila

Color vision is an important sensory capability that enhances the detection of contrast in retinal images. Monochromatic animals exclusively detect temporal and spatial changes in luminance, whereas two or more types of photoreceptors and neuronal circuitries for the comparison of their responses enable animals to differentiate spectral information independent of intensity. Much of what we know about the cellular and physiological mechanisms underlying color vision comes from research on vertebrates including primates. In insects, many important discoveries have been made, but direct insights into the physiology and circuit implementation of color vision are still limited. Recent advances in Drosophila systems neuroscience suggest that a complete insect color vision circuitry, from photoreceptors to behavior, including all elements and computations, can be revealed in future. Here, we review fundamental concepts in color vision alongside our current understanding of the neuronal basis of color vision in Drosophila, including side views to selected other insects.

Hypothesis on monochromatic vision in scorpionflies questioned by new transcriptomic data

In the scorpionfly Panorpa, a recent study suggested monochromatic vision due to evidence of only a single opsin found in transcriptome data. To reconsider this hypothesis, the present study investigates opsin expression using transcriptome data of 21 species including representatives of all major lineages of scorpionflies (Mecoptera) and of three families of their closest relatives, the fleas (Siphonaptera). In most mecopteran species investigated, transcripts encode two opsins with predicted peak absorbances in the green, two in the blue, and one in the ultraviolet spectral region. Only in groups with reduced or absent ocelli, like Caurinus and Apteropanorpa, less than four visual opsin messenger RNAs have been identified. In addition, we found a Rh7-like opsin in transcriptome data derived from larvae of the mecopteran Nannochorista, and in two flea species. Peropsin expression was observed in two mecopterans. In light of these new data, we question the hypothesis on monochromatic vision in the genus Panorpa. In a broader phylogenetic perspective, it is suggested that the common ancestor of the monophyletic taxon Antliophora (Diptera, Mecoptera and Siphonaptera) possessed the full set of visual opsins, a Rh7-like opsin, and in addition a pteropsin as well as a peropsin. In the course of evolution individual opsins were likely lost in several lineages of this clade.

Representation of the stomatopod's retinal midband in the optic lobes: Putative neural substrates for integrating chromatic, achromatic and polarization information

Stomatopods have an elaborate visual system served by a retina that is unique to this class of pancrustaceans. Its upper and lower eye hemispheres encode luminance and linear polarization while an equatorial band of photoreceptors termed the midband detects color, circularly polarized light and linear polarization in the ultraviolet. In common with many malacostracan crustaceans, stomatopods have stalked eyes, but they can move these independently within three degrees of rotational freedom. Both eyes separately use saccadic and scanning movements but they can also move in a coordinated fashion to track selected targets or maintain a forward eyestalk posture during swimming. Visual information is initially processed in the first two optic neuropils, the lamina and the medulla, where the eye's midband is represented by enlarged regions within each neuropil that contain populations of neurons, the axons of which are segregated from the neuropil regions subtending the hemispheres. Neuronal channels representing the midband extend from the medulla to the lobula where populations of putative inhibitory glutamic acid decarboxylase-positive neurons and tyrosine hydroxylase-positive neurons intrinsic to the lobula have specific associations with the midband. Here we investigate the organization of the midband representation in the medulla and the lobula in the context of their overall architecture. We discuss the implications of observed arrangements, in which midband inputs to the lobula send out collaterals that extend across the retinotopic mosaic pertaining to the hemispheres. This organization suggests an integrative design that diverges from the eumalacostracan ground pattern and, for the stomatopod, enables color and polarization information to be integrated with luminance information that presumably encodes shape and motion.