Vision in click beetles (Coleoptera: Elateridae): pigments and spectral correspondence between visual sensitivity and species bioluminescence emission

Glowing wonders: exploring the diversity and ecological significance of bioluminescent organisms in Brazil

Bioluminescence, the emission of light by living organisms, is a captivating and widespread phenomenon with diverse ecological functions. This comprehensive review explores the biodiversity, mechanisms, ecological roles, and conservation challenges of bioluminescent organisms in Brazil, a country known for its vast and diverse ecosystems. From the enchanting glow of fireflies and glow-in-the-dark mushrooms to the mesmerizing displays of marine dinoflagellates and cnidarians, Brazil showcases a remarkable array of bioluminescent species. Understanding the biochemical mechanisms and enzymes involved in bioluminescence enhances our knowledge of their evolutionary adaptations and ecological functions. However, habitat loss, climate change, and photopollution pose significant threats to these bioluminescent organisms. Conservation measures, interdisciplinary collaborations, and responsible lighting practices are crucial for their survival. Future research should focus on identifying endemic species, studying environmental factors influencing bioluminescence, and developing effective conservation strategies. Through interdisciplinary collaborations, advanced technologies, and increased funding, Brazil can unravel the mysteries of its bioluminescent biodiversity, drive scientific advancements, and ensure the long-term preservation of these captivating organisms.

Opsin diversity and evolution in the Elateroidea superfamily: Insights from transcriptome data

Vision plays a vital biological role in organisms, which depends on the visual pigment molecules (opsin plus chromophore). The expansion or reduction of spectral channels in the organisms is determined by distinct opsin classes and copy numbers resulting from duplication or loss. Within Coleoptera, the superfamily Elateroidea exhibits a great diversity of morphological and physiological characteristics, such as bioluminescence, making this group an important model for opsin studies. While molecular and physiological studies have been conducted in Lampyridae and Elateridae, other families remain unexplored. Here, we reused transcriptome datasets from Elateroidea species, including members of Elateridae, Lampyridae, Phengodidae, Rhagophthalmidae, Cantharidae, and Lycidae, to detect the diversity of putative opsin genes in this superfamily. In addition, we tested the signature of sites under positive selection in both ultraviolet (UV)- and long-wavelength (LW)-opsin classes. Although the visual system in Elateroidea is considered simple, we observed events of duplication in LW- and UV-opsin, as well as the absence of UV-opsin in distinct families, such as larval Phengodidae individuals. We detected different copies of LW-opsins that were highly expressed in the eyes of distinct tribes of fireflies, indicating the possible selection of each copy during the evolution of the sexual mating to avoid spectrum overlapping. In Elateridae, we found that the bioluminescent species had a distinct LW-opsin copy compared with the non-bioluminescent species, suggesting events of duplication and loss. The signature of positive selection showed only one residue associated with the chromophore binding site in the Elateroidea, which may produce a bathochromic shift in the wavelength absorption spectra in this family. Overall, this study brings important content and fills gaps regarding opsin evolution in Elateroidea.

Insect visual sensitivity to long wavelengths enhances colour contrast of insects against vegetation

The sensitivity of animal photoreceptors to different wavelengths of light strongly influence the perceived visual contrast of objects in the environment. Outside of the human visual wavelength range, ultraviolet sensitivity in many species provides important and behaviourally relevant visual contrast between objects. However, at the opposite end of the spectrum, the potential advantage of red sensitivity remains unclear. We investigated the potential benefit of long wavelength sensitivity by modelling the visual contrast of a wide range of jewel beetle colours against flowers and leaves of their host plants to hypothetical insect visual systems. We find that the presence of a long wavelength sensitive photoreceptor increases estimated colour contrast, particularly of beetles against leaves. Moreover, under our model parameters, a trichromatic visual system with ultraviolet (λ_max = 355 nm), short (λ_max = 445 nm) and long (λ_max = 600 nm) wavelength photoreceptors performed as well as a tetrachromatic visual system, which had an additional medium wavelength photoreceptor (λ_max = 530 nm). When we varied λ_max for the long wavelength sensitive receptor in a tetrachromatic system, contrast values between beetles, flowers and leaves were all enhanced with increasing λ_max from 580 nm to at least 640 nm. These results suggest a potential advantage of red sensitivity in visual discrimination of insect colours against vegetation and highlight the potential adaptive value of long wavelength sensitivity in insects.

Geographic variation in the matching between call characteristics and tympanic sensitivity in the Weeping lizard

Effective communication requires a match among signal characteristics, environmental conditions, and receptor tuning and decoding. The degree of matching, however, can vary, among others due to different selective pressures affecting the communication components. For evolutionary novelties, strong selective pressures are likely to act upon the signal and receptor to promote a tight match among them. We test this prediction by exploring the coupling between the acoustic signals and auditory sensitivity in Liolaemus chiliensis, the Weeping lizard, the only one of more than 285 Liolaemus species that vocalizes. Individuals emit distress calls that convey information of predation risk to conspecifics, which may respond with antipredator behaviors upon hearing calls. Specifically, we explored the match between spectral characteristics of the distress calls and the tympanic sensitivities of two populations separated by more than 700 km, for which previous data suggested variation in their distress calls. We found that populations differed in signal and receptor characteristics and that this signal variation was explained by population differences in body size. No precise match occurred between the communication components studied, and populations differed in the degree of such correspondence. We suggest that this difference in matching between populations relates to evolutionary processes affecting the Weeping lizard distress calls.

RNA-Seq analysis of the blue light-emitting Orfelia fultoni (Diptera: Keroplatidae) suggest photoecological adaptations at the molecular level

Bioluminescence in Diptera is found in the Keroplatidae family, within Arachnocampininae and Keroplatinae subfamilies, with reported occurrences in Oceania, Eurasia, and Americas. Larvae of Orfelia fultoni, which inhabit stream banks in the Appalachian Mountains, emit the bluest bioluminescence among insects, using it for prey attraction, similarly to Arachnocampa spp. Although bioluminescence has a similar prey attraction function, the systems of Arachonocampininae and Keroplatinae subfamilies are morphologically/biochemically distinct, indicating different evolutionary origins. To identify the possible coding genes associated with physiological control, ecological adaptations, and origin/evolution of bioluminescence in the Keroplatinae subfamily, we performed the RNA-Seq analysis of O. fultoni larvae during day and night and compared it with the transcriptomes of Arachnocampa luminosa, and reanalyzed the previously published proteomic data of O. fultoni against the RNA-Seq dataset. The abundance of chaperones/heat-shock and hexamerin gene products at night and in luciferase enriched fractions supports their possible association and participation in bioluminescence. The low diversity of copies/families of opsins indicate a simpler visual system in O. fultoni. Noteworthy, gene products associated with silk protein biosynthesis in Orfelia were more similar to Lepidoptera than to the Arachnocampa, indicating that, similarly to the bioluminescent systems, at some point, the biochemical apparatus for web construction may have evolved independently in Orfelia and Arachnocampa.

Opsin Evolution in Flower-Visiting Beetles

Flowers have evolved signals that exploit the sensory systems of insect visitors. In the case of visual cues, color signals are thought to have been shaped in large part by the spectral sensitivity of key pollinators, such as hymenopterans. Beetles were some of the first plant pollinators, pre-dating the angiosperm radiation but with the exception of a few well-studied species, the evolution of flower-visiting beetle visual systems is poorly understood. Thus, the ability of beetles to detect and distinguish flower color signals and perhaps their potential role in shaping flower coloration is not well understood. Traditional models of pollinator visual systems often assume a putative tri- or tetrachromatic flower-visitor, as is found in bees, flies and butterflies. Beetles are unique among modern pollinators as ancestrally they did not possess the machinery for trichromatic vision, lacking the blue-sensitive photoreceptor class. Research on the evolution of visual genes responsible for wavelength sensitivity (opsins) has revealed that beetles with putative tri- and tetrachromatic visual systems have evolved independently, along multiple lineages. We explore the evolution of beetle visual genes using newly generated and publicly available RNA-seq data from 25 species with flower associations, including previously unexplored key flower-visitor groups and 20 non-flower visiting relatives. Our findings serve as a resource to inform and guide future studies on beetle-flower interactions, where insight from both signal and receiver is needed to better understand these poorly explored systems.

Sinopyrophorinae, a new subfamily of Elateridae (Coleoptera, Elateroidea) with the first record of a luminous click beetle in Asia and evidence for multiple origins of bioluminescence in Elateridae

The new subfamily Sinopyrophorinae within Elateridae is proposed to accommodate a bioluminescent species, Sinopyrophorusschimmeli Bi & Li, gen. et sp. nov., recently discovered in Yunnan, China. This lineage is morphologically distinguished from other click-beetle subfamilies by the strongly protruding frontoclypeal region, which is longitudinally carinate medially, the pretarsal claws without basal setae, the hind wing venation with a well-defined wedge cell, the abdomen with seven (male) or six (female) ventrites, the large luminous organ on the abdominal sternite II, and the male genitalia with median lobe much shorter than parameres, and parameres arcuate, with the inner margin near its apical third dentate. Molecular phylogeny based on the combined 14 mitochondrial and two nuclear genes supports the placement of this taxon far from other luminescent click-beetle groups, which provides additional evidence for the multiple origin of bioluminescence in Elateridae. Illustrations of habitus and main diagnostic features of S.schimmeli Bi & Li, gen. et sp. nov. are provided, as well as the brief description of its luminescent behavior.

Phylogenomic analyses and divergence time estimation of Elateroidea (Coleoptera) based on RNA-Seq data

Bioluminescence, the emission of visible light in a living organism, is an intriguing phenomenon observed in different species and environments. In terrestrial organisms, the bioluminescence is observed mainly in beetles of the Elateroidea superfamily (Coleoptera). Several phylogenetic studies have been used different strategies to propose a scenario for the origin and evolution of the bioluminescence within this group, however some of them showed incongruences, mainly about the relationship of the bioluminescent families. In order to increase the number of molecular markers available for Elateroidea species and to propose a more accurate phylogeny, with high supported topology, we employed the Next-Generation Sequencing (NGS) methodology to perform the RNA-Seq analysis of luminescent (Elateridae, Phengodidae, Rhagophthalmidae, and Lampyridae) and non-luminescent (Cantharidae) species of Neotropical beetles. We used the RNA-Seq data to construct a calibrated phylogeny of Elateroidea superfamily using a large number of nuclear molecular markers. The results indicate Lampyridae and Phengodidae/Rhagophthalmidae as sister-groups, suggesting that the bioluminescence evolved later in Elateridae than other families (Lampyridae, Phengodidae, and Rhagophthalmidae), and indicating the Upper Cretaceous as the period for the main diversification of Elateroidea bioluminescent species.

Ginger and the beetle: Evidence of primitive pollination system in a Himalayan endemic alpine ginger (Roscoea alpina, Zingiberaceae)

The Himalayan endemic alpine genus Roscoea, like other members of ginger family, exhibits the combination of floral traits that would fit pollination by long distant foragers such as bees, birds or flies. We studied the pollination biology of Roscoea alpina, observed potential floral visitors and determined their foraging behaviour, visitation frequency and pollination efficiency, to seek evidence in support of the pollination syndrome hypothesis. We also measured the floral spectra of R. alpina flowers to evaluate if signals fit with the currently known framework for observed floral visitors. We found that R. alpina have autonomous selfing and pollinator-mediated crossing, but lack apomixis. We observed that a beetle (Mylabris sp.), and a moth (Macroglossum nycteris) visit the flowers of R. alpina for pollen and nectar feeding respectively. Our field observations, the stigmatic pollen count and fruit set data indicated that the visit by the beetle was legitimate, while that of the moth was illegitimate. Emasculated flowers visited by beetles set as many fruits and seeds/fruit as auto-selfed and naturally pollinated flowers, while emasculated flowers excluded from beetle visits did not set fruit and seed; indicating that a single visit of a beetle to the flowers of R. alpina can facilitate pollination. We found that flower spectral signal of R. alpina does not fit typical spectra previously reported for beetle or bee-visited flowers. Our results suggest that, to ensure reproductive success in alpine habitat, R. alpina has evolved autonomous selfing as a predominant mode of reproduction, while beetle pollination would promote genetic diversity of this plant species. The visitation of beetles to the flowers of R. alpina, despite floral signal mismatch with the classically associated beetle vision, suggests that a different visual processing may operate in this plant-pollinator interaction at high altitudes.

Excited-State Dynamics of Oxyluciferin in Firefly Luciferase

The color variations of light emitted by some natural and mutant luciferases are normally attributed to collective factors referred to as microenvironment effects; however, the exact nature of these interactions between the emitting molecule (oxyluciferin) and the active site remains elusive. Although model studies of noncomplexed oxyluciferin and its variants have greatly advanced the understanding of its photochemistry, extrapolation of the conclusions to the real system requires assumptions about the polarity and proticity of the active site. To decipher the intricate excited-state dynamics, global and target analysis is performed here for the first time on the steady-state and time-resolved spectra of firefly oxyluciferin complexed with luciferase from the Japanese firefly (Luciola cruciata). The experimental steady-state and time-resolved luminescence spectra of the oxyluciferin/luciferase complex in solution are compared with the broadband time-resolved firefly bioluminescence recorded in vivo. The results demonstrate that de-excitation of the luminophore results in a complex cascade of photoinduced proton transfer processes and can be interpreted by the pH dependence of the emitted light. It is confirmed that proton transfer is the central event in the spectrochemistry of this system for which any assignment of the pH-dependent emission to a single chemical species would be an oversimplification.

A new orange emitting luciferase from the Southern-Amazon Pyrophorus angustus (Coleoptera: Elateridae) click-beetle: structure and bioluminescence color relationship, evolutional and ecological considerations

Bioluminescent click-beetles display a wide variation of bioluminescence colors ranging from green to orange, including an unusual intra-specific color variation in the Jamaican Pyrophorus plagiophthalamus. Recently, we collected individuals of the Pyrophorus angustus species from the Southern Amazon forest, in Brazil, which displays an orange light emitting abdominal lantern. This species was also previously described from Central America, but displaying a bioluminescence spectrum from 536 nm (dorsal) to 578 nm (ventral). The biogeographic variation of the bioluminescence color in this species could be an adaptation to environmental reflectance and inter/intraspecific sexual competition. Here, we cloned, sequenced, characterized and performed site-direct mutagenesis of this new orange emitting luciferase. The in vitro luciferase spectrum displayed a peak at 594 nm, KM values for ATP and d-luciferin of 160 μM and 17 μM, respectively, and an optimum pH of approximately 8.5. Comparative multialignment and site-directed mutagenesis using different color emitting click-beetle luciferases from P. angustus, Fulgeochlizus bruchi and Pyrearinus termitilluminans luciferases cloned by our group showed an integral role of residue 247 in bioluminescence color modulation.

Four photoreceptor classes in the open rhabdom eye of the red palm weevil, Rynchophorus ferrugineus Olivier

The red palm weevil (RPW) is a severe palm pest with high dispersal capability. Its visual sense allows it to navigate long distances and to discriminate among differently colored traps. We investigated the RPW compound eyes with anatomical and electrophysiological methods. The ommatidia are composed of eight photoreceptor cells in an open rhabdom arrangement with six peripheral and two central photoreceptors. The photoreceptor signals are relatively slow and noisy. The majority of recorded photoreceptors have broad spectral sensitivity with a peak in the green, at 536 nm. Three minor classes of photoreceptors have narrower spectral sensitivities with maxima in the UV (366 nm), green (520 nm) and yellow (564 nm). Sensitivity below 350 nm is very low due to filtering by the UV-absorbing cornea. The set of photoreceptors represents the retinal substrate for putative trichromatic color vision.

Variation in opsin genes correlates with signalling ecology in North American fireflies

Genes underlying signal reception should evolve to maximize signal detection in a particular environment. In animals, opsins, the protein component of visual pigments, are predicted to evolve according to this expectation. Fireflies are known for their bioluminescent mating signals. The eyes of nocturnal species are expected to maximize the detection of conspecific signal colours emitted in the typical low-light environment. This is not expected for species that have transitioned to diurnal activity in bright daytime environments. Here, we test the hypothesis that opsin gene sequence plays a role in modifying firefly eye spectral sensitivity. We use genome and transcriptome sequencing in four firefly species, transcriptome sequencing in six additional species and targeted gene sequencing in 28 other species to identify all opsin genes present in North American fireflies and to elucidate amino acid sites under positive selection. We also determine whether amino acid substitutions in opsins are linked to evolutionary changes in signal mode, signal colour and light environment. We find only two opsins, one long wavelength and one ultraviolet, in all firefly species and identify 25 candidate sites that may be involved in determining spectral sensitivity. In addition, we find elevated rates of evolution at transitions to diurnal activity, and changes in selective constraint on long wavelength opsin associated with changes in light environment. Our results suggest that changes in eye spectral sensitivity are at least partially due to opsin sequence. Fireflies continue to be a promising system in which to investigate the evolution of signals, receptors and signalling environments.

A route from darkness to light: emergence and evolution of luciferase activity in AMP-CoA-ligases inferred from a mealworm luciferase-like enzyme

The origin of luciferases and of bioluminescence is enigmatic. In beetles, luciferases seem to have evolved from AMP-CoA-ligases. How the new oxygenase luminogenic function originated from AMP-ligases leading to luciferases is one of the most challenging mysteries of bioluminescence. Comparison of the cloned luciferase-like enzyme from the nonluminescent Zophobas morio mealworm and beetle luciferases showed that the oxygenase activity may have emerged as a stereoselective oxidative drift with d-luciferin, a substrate that cannot be easily thioesterified to CoA as in the case of the l-isomer. While the overall kcat displayed by beetle luciferases is orders of magnitude greater than that of the luciferase-like enzyme, the respective oxidation rates and quantum yields of bioluminescence are roughly similar, suggesting that the rate constant of the AMP-ligase activity exerted on the new d-luciferin substrate in beetle protoluciferases was the main enzymatic property that suffered optimization during the evolution of luciferases. The luciferase-like enzyme and luciferases boost the rate of luciferyl-adenylate chemiluminescent oxidation by factors of 10(6) and 10(7), respectively, as compared to the substrate spontaneous oxidation in buffer. A similar enhancement of luciferyl-adenylate chemiluminescence is provided by nucleophilic aprotic solvents, implying that the peptide bonds in the luciferin binding site of beetle luciferase could provide a similar catalytically favorable environment. These data suggest that the luciferase-like enzyme and other similar AMP-ligases are potential alternative oxygenases. Site-directed mutagenesis studies of the luciferase-like enzyme and the red light-producing luciferase of Phrixotrix hirtus railroadworm confirm here a critical role for T/S345 in luciferase function. Mutations such as I327T/S in the luciferase-like enzyme, which simultaneously increases luciferase activity and promotes blue shifts in the emission spectrum, could have been critical for evolving functional bioluminescence from red-emitting protoluciferases. Through the combination of I327T/S mutations and N-terminal fusion, the luminescence activity of this enzyme was increased to visible levels, with the development of a totally new orange-emitting luciferase. These results open the possibility of engineering luciferase activity in a set of AMP-CoA-ligases.

Luciferase from Fulgeochlizus bruchi (Coleoptera:Elateridae), a Brazilian click-beetle with a single abdominal lantern: molecular evolution, biological function and comparison with other click-beetle luciferases

Bioluminescent click-beetles emit a wide range of bioluminescence colors (λ(Max) = 534-594 nm) from thoracic and abdominal lanterns, which are used for courtship. Only the luciferases from Pyrophorus and Pyrearinus species were cloned and sequenced. The Brazilian Fulgeochlizus bruchi click-beetle, which inhabits the Central-west Cerrado (Savannas), is noteworthy because, differently from other click-beetles, the adult stage displays only a functional abdominal lantern, which produces a bright green bioluminescence for sexual attraction purposes, and lacks functional thoracic lanterns. We cloned the cDNA for the abdominal lantern luciferase of this species. Notably, the primary sequence of this luciferase showed slightly higher identity with the green emitting dorsal lantern luciferases of the Pyrophorus genus instead of the abdominal lanterns luciferases. This luciferase displays a blue-shifted spectrum (λ(Max) = 540 nm), which is pH-insensitive from pH 7.5 to 9.5 and undergoes a slight red shift and broadening above this pH; the lowest K(M) for luciferin among studied click-beetle luciferases, and the highest optimum pH (9.0) ever reported for a beetle luciferase. At pH 9.0, the K(M) for luciferin increases, showing a decrease of affinity for this substrate, despite the higher activity. The slow luminescence decay rate of F. bruchi luciferase in vitro reaction could be an adaptation of this luciferase for the long and sustained in vivo luminescence display of the click-beetle during the courtship, and could be useful for in vivo intracellular imaging.

Evidence of red sensitive photoreceptors in Pygopleurus israelitus (Glaphyridae: Coleoptera) and its implications for beetle pollination in the southeast Mediterranean

A very well-documented case of flower-beetle interaction is the association in the Mediterranean region between red bowl-shaped flowers and beetles of the family Glaphyridae. The present study examines the visual mechanisms by which Pygopleurus israelitus (Glaphyridae: Scarabaeoidea: Coleoptera) would perceive the colors of flowers they visit by characterizing the spectral sensitivity of its photoreceptors. Our measurements revealed the presence of three types of photoreceptors, maximally sensitive in the UV, green and red areas of the spectrum. Using color vision space diagrams, we calculated the distribution of beetle-visited flower colors in the glaphyrid and honeybee color space and evaluated whether chromatic discrimination differs between the two types of pollinators. Respective color loci in the beetle color space are located on one side of the locus for green foliage background, whereas in the honeybee the flower color loci surround the locus occupied by green foliage. Our results represent the first evidence of a red sensitive photoreceptor in a flower-visiting coleopteran species, highlighting Glaphyridae as an interesting model group to study the role of pollinators in flower color evolution.