Chemosensory coding by neurons in the coeloconic sensilla of the Drosophila antenna

Olfactory receptor and circuit evolution promote host specialization

The evolution of animal behaviour is poorly understood^1,2. Despite numerous correlations of behavioural and nervous system divergence, demonstration of the genetic basis of interspecific behavioural differences remains rare^3–5. Here, we develop a novel neurogenetic model, Drosophila sechellia, a close cousin of D. melanogaster^6,7 that displays profound behavioural changes linked to its extreme specialisation on noni fruit^8–16. Using calcium imaging, we identify D. sechellia olfactory pathways detecting host volatiles. Mutational analysis indicates roles for different olfactory receptors in long- and short-range attraction to noni. Cross-species allele transfer demonstrates that tuning of one of these receptors is important for species-specific host-seeking. We identify the molecular determinants of this functional change, and characterise their evolutionary origin and behavioural significance. Through circuit tracing in the D. sechellia brain, we find that receptor adaptations are accompanied by increased sensory pooling onto interneurons and novel central projection patterns. This work reveals the accumulation of molecular, physiological and anatomical traits linked to behavioural divergence, and defines a powerful model for investigating nervous system evolution and speciation.

Observations on the Ultrastructure of Antennal Sensilla of Adult Glenea cantor (Cerambycidae: Lamiinae)

The external morphology and distribution of antennal sensilla of Glenea cantor Fabricius were studied with scanning electron microscopy. The antennae of G. cantor were observed to be filiform, consisting of scape, pedicel, and flagellum (nine flagellomeres). Four distinct types of sensory receptors were observed, including sensilla chaetica, sensilla trichodea, sensilla basiconica, and Böhm bristles. Three morphological subtypes of sensilla chaetica were found on the antennae, and sensilla trichodea were also categorized into three morphological subtypes. Sensilla basiconica was grouped into two morphological subtypes that were found on subsegments F2-F9 of the flagellum, and Böhm bristles were only found at the intersegmental joints between the scape and the head and between the scape and the pedicel. The antennae of male and female adults were similar in shape, length, and diameter. However, the length, diameter, distribution, and number of each of the four distinct types of sensilla on the males were significantly different from those on females. The types, lengths, diameters, numbers, and distributions of these sensilla were described, and their possible functions were also discussed. The results indicated that the base and end of an antennal segment have a similar sensillum density, but the middle section sensor density is significantly greater, especially for olfactory and gustatory sensilla, possibly because the joints are more involved in mechanical sensing. The density of sensors is closely related to its sensing function; so, future studies on the biology of olfaction and sexual communication in G. cantor will be facilitated by these observations.

Chemosensation and Evolution of Drosophila Host Plant Selection

The ability to respond to chemosensory cues is critical for survival of most organisms. Among insects, Drosophila melanogaster has the best characterized olfactory system, and the availability of genome sequences of 30 Drosophila species provides an ideal scenario for studies on evolution of chemosensation. Gene duplications of chemoreceptor genes allow for functional diversification of the rapidly evolving chemoreceptor repertoire. Although some species of the genus Drosophila are generalists for host plant selection, rapid evolution of olfactory receptors, gustatory receptors, odorant-binding proteins, and cytochrome P450s has enabled diverse host specializations of different members of the genus. Here, I review diversification of the chemoreceptor repertoire among members of the genus Drosophila along with co-evolution of detoxification mechanisms that may have enabled occupation of diverse host plant ecological niches.

Role of chemical cues in cabbage stink bug host plant selection

The cabbage stink bugs of the genus Eurydema, encompassing several oligophagous species, such as Eurydema oleracea (L.), are known to be important pests of cabbage, broccoli, and other cole crops in Europe. Despite their economic importance, the knowledge regarding the role of chemical cues in host plant selection of these species is very limited. The present investigation on E. oleracea at the adult stage revealed the use of olfaction in host plant selection of this species and demonstrated with behavioural tests that E. oleracea preferred feeding on wild Eruca sativa, rather than on Brassica oleracea. Moreover, ultrastructural data revealed the antennal sensilla of E. oleracea, encompassing single walled and double walled olfactory sensilla, and electroantennographic recordings revealed their sensitivity to several host plant VOCs from E. sativa and B. oleracea. The data shown in the present research may be useful in the development of semiochemical-based strategies or trap crops for the control of this pest in the field.

Aedes aegypti Ir8a mutant female mosquitoes show increased attraction to standing water

The detection of water sources is crucial for insects such as mosquitoes to avoid desiccation and survive. In addition, mosquitoes use humidity cues to successfully navigate the environment to find a suitable oviposition site. Previous studies have implicated some members of the ionotropic receptor family in humidity sensing by Drosophila. Here, we investigate if IR8a co-receptor mediates water detection in Aedes aegypti mosquitoes. Using a simple behavioral assay, we examined the attraction of Ir8a mutant mosquitoes to standing water. Ir8a mutant mosquitoes were able to discriminate between traps containing water and those without as well as wild-type and heterozygous control females. Surprisingly, the female mutants were more robustly drawn to standing water than control mosquitoes. Further investigation revealed that the increased behavioral attraction to water is likely not mediated by a metabolic need or an activity defect.

Robust olfactory responses in the absence of odorant binding proteins

Odorant binding proteins (Obps) are expressed at extremely high levels in the antennae of insects, and have long been believed essential for carrying hydrophobic odorants to odor receptors. Previously we found that when one functional type of olfactory sensillum in Drosophila was depleted of its sole abundant Obp, it retained a robust olfactory response (Larter et al., 2016). Here we have deleted all the Obp genes that are abundantly expressed in the antennal basiconic sensilla. All of six tested sensillum types responded robustly to odors of widely diverse chemical or temporal structure. One mutant gave a greater physiological and behavioral response to an odorant that affects oviposition. Our results support a model in which many sensilla can respond to odorants in the absence of Obps, and many Obps are not essential for olfactory response, but that some Obps can modulate olfactory physiology and the behavior that it drives.

Amplification of Drosophila Olfactory Responses by a DEG/ENaC Channel

Insect olfactory receptors operate as ligand-gated ion channels that directly transduce odor stimuli into electrical signals. However, in the absence of any known intermediate transduction steps, it remains unclear whether and how these ionotropic inputs are amplified in olfactory receptor neurons (ORNs). Here, we find that amplification occurs in the Drosophila courtship-promoting ORNs through Pickpocket 25 (PPK25), a member of the degenerin/epithelial sodium channel family (DEG/ENaC). Pharmacological and genetic manipulations indicate that, in Or47b and Ir84a ORNs, PPK25 mediates Ca²⁺-dependent signal amplification via an intracellular calmodulin-binding motif. Additionally, hormonal signaling upregulates PPK25 expression to determine the degree of amplification, with striking effects on male courtship. Together, these findings advance our understanding of sensory neurobiology by identifying an amplification mechanism compatible with ionotropic signaling. Moreover, this study offers new insights into DEG/ENaC activation by highlighting a novel means of regulation that is likely conserved across species.

The olfactory coreceptor IR8a governs larval feces-mediated competition avoidance in a hawkmoth

Finding a suitable oviposition site is a challenging task for a gravid female moth. At the same time, it is of paramount importance considering the limited capability of most caterpillars to relocate to alternative host plants. The hawkmoth, Manduca sexta, oviposits on solanaceous plants. Larvae hatching on a plant that is already attacked by conspecific caterpillars face food competition. Here, we show that feces from conspecific caterpillars are sufficient to deter a female M. sexta from ovipositing on a plant. Furthermore, we not only identify the responsible compound in the feces but also localize the population of sensory neurons that governs the female’s avoidance. Hence, our work increases the understanding of how animals cope with a competitive environment.

Finding a suitable oviposition site is a challenging task for a gravid female moth. At the same time, it is of paramount importance considering the limited capability of most caterpillars to relocate to alternative host plants. The hawkmoth, Manduca sexta (Sphingidae), oviposits on solanaceous plants. Larvae hatching on a plant that is already attacked by conspecific caterpillars can face food competition, as well as an increased exposure to predators and induced plant defenses. Here, we show that feces from conspecific caterpillars are sufficient to deter a female M. sexta from ovipositing on a plant and that this deterrence is based on the feces-emitted carboxylic acids 3-methylpentanoic acid and hexanoic acid. Using a combination of genome editing (CRISPR-Cas9), electrophysiological recordings, calcium imaging, and behavioral analyses, we demonstrate that ionotropic receptor 8a (IR8a) is essential for acid-mediated feces avoidance in ovipositing hawkmoths.

Tools in the Investigation of Volatile Semiochemicals on Insects: From Sampling to Statistical Analysis

The recognition of volatile organic compounds (VOCs) involved in insect interactions with plants or other organisms is essential for constructing a holistic comprehension of their role in ecology, from which the implementation of new strategies for pest and disease vector control as well as the systematic exploitation of pollinators and natural enemies can be developed. In the present paper, some of the general methods employed in this field are examined, focusing on their available technologies. An important part of the investigations conducted in this context begin with VOC collection directly from host organisms, using classical extraction methods, by the employment of adsorption materials used in solid-phase micro extraction (SPME) and direct-contact sorptive extraction (DCSE) and, subsequently, analysis through instrumental analysis techniques such as gas chromatography (GC), nuclear magnetic resonance (NMR) and mass spectrometry (MS), which provide crucial information for determining the chemical identity of volatile metabolites. Behavioral experiments, electroantennography (EAG), and biosensors are then carried out to define the semiochemicals with the best potential for performing relevant functions in ecological relationships. Chemical synthesis of biologically-active VOCs is alternatively performed to scale up the amount to be used in different purposes such as laboratory or field evaluations. Finally, the application of statistical analysis provides tools for drawing conclusions about the type of correlations existing between the diverse experimental variables and data matrices, thus generating models that simplify the interpretation of the biological roles of VOCs.

Morphological and functional heterogeneity in olfactory perception between antennae and maxillary palps in the pumpkin fruit fly, Bactrocera depressa

The morphology and ultrastructure of the olfactory sensilla on the antennae and maxillary palps were investigated through scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and their responses to five volatile compounds were measured using electroantenogram (EAG) and electropalpogram (EPG) techniques in the pumpkin fruit fly, Bactrocera depressa (Shiraki; Diptera: Tephritidae). Male and female B. depressa displayed distinct morphological types of olfactory sensilla in the antennae and maxillary palps, with predominant populations of trichoid, basiconic, and coeloconic sensilla. Basiconic sensilla, the most abundant type of olfactory sensilla in the antennae, could be further classified into two different types. In contrast, the maxillary palps exhibited predominant populations of a single type of curved basiconic sensilla. High-resolution SEM observation revealed the presence of multiple nanoscale wall-pores on the cuticular surface of trichoid and basiconic sensilla, indicating that their primary function is olfactory. In contrast, coeloconic sensilla displayed several longitudinal grooves around the sensillum peg. The TEM observation of individual antennal olfactory sensilla indicates that the basiconic sensilla are thin-walled, while the trichoid sensilla are thick-walled. The profile of EAG responses of male B. depressa was different from their EPG response profile, indicating that the olfactory function of maxillary palps is different from that of antennae in this species. The structural and functional variation in the olfactory sensilla between antennae and maxillary palps suggests that each plays an independent role in the perception of olfactory signals in B. depressa.

Labial and maxillary palp recordings of the Asian longhorned beetle, Anoplophora glabripennis, reveal olfactory and hygroreceptive capabilities

Electrophysiological recordings from the labial and maxillary palps of the Asian longhorned beetle, Anoplophora glabripennis, revealed their ability to detect several volatile chemicals, including water vapor and acetic acid. The results indicate that these appendages may play a large role in this beetle's assessment of its immediate environment. A. glabripennis is a highly destructive, invasive pest that feeds preferentially on maple - but accepts many other tree species - in North America, warranting USDA quarantine zones and an eradication program. While control and sampling techniques are being developed for this insect, a better understanding of its sensory capabilities is helpful. Electropalpograms (EPGs) revealed that both the maxillary and labial palps are highly sensitive to changes in humidity, indicating the presence of hygroreceptors and the likely important role of humidity in such things as feeding and finding water or oviposition sites. Strong EPG responses to a narrow set of volatile chemicals indicate that olfactory sensory neurons (OSNs) on the palps may be tuned to a small number of volatile compounds. The types of odorant molecules eliciting responses indicate that there are likely both odorant receptors (ORs) as well as ionotropic receptors (IRs) expressed on the OSNs, enabling palp OSNs to be able to respond to acids and aldehydes such as acetic acid and butyraldehyde. There were no significant EPG responses to this species' trail-sex pheromone components, which may indicate that the trail pheromone is primarily perceived via gustatory receptors contacting the substrate. These results indicate that the palps have a role in the beetle's assessment of its immediate environment underfoot, and that the sampling of surface odors and humidity via mouth parts may be important to this species' success.

Food-derived volatiles enhance consumption in Drosophila melanogaster

Insects use multiple sensory modalities when searching for and accepting a food source, in particular odor and taste cues. Food-derived odorants are generally involved in mediating long- and short-range attraction. Taste cues, in contrast, act directly by contact with the food source, promoting the ingestion of nutritious food and the avoidance of toxic substances. It is possible, however, that insects integrate information from these sensory modalities during the process of feeding itself. Here, using a simple feeding assay, we investigated whether odors modulate food consumption in the fruit fly Drosophila melanogaster We found that the presence of both single food-derived odorants and complex odor mixtures enhanced consumption of an appetitive food. Feeding enhancement depended on the concentration and the chemical identity of the odorant. Volatile cues alone were sufficient to mediate this effect, as feeding was also increased when animals were prevented from contacting the odor source. Both males and females, including virgin females, increased ingestion in the presence of food-derived volatiles. Moreover, the presence of food-derived odorants significantly increased the consumption of food mixtures containing aversive bitter compounds, suggesting that flies integrate diverse olfactory and gustatory cues to guide feeding decisions, including situations in which animals are confronted with stimuli of opposite valence. Overall, these results show that food-derived olfactory cues directly modulate feeding in D. melanogaster, enhancing ingestion.

The molecular and cellular basis of olfactory response to tsetse fly attractants

Dipteran or "true" flies occupy nearly every terrestrial habitat, and have evolved to feed upon a wide variety of sources including fruit, pollen, decomposing animal matter, and even vertebrate blood. Here we analyze the molecular, genetic and cellular basis of odor response in the tsetse fly Glossina morsitans, which feeds on the blood of humans and their livestock, and is a vector of deadly trypanosomes. The G. morsitans antenna contains specialized subtypes of sensilla, some of which line a sensory pit not found in the fruit fly Drosophila. We characterize distinct patterns of G. morsitans Odor receptor (GmmOr) gene expression in the antenna. We devise a new version of the "empty neuron" heterologous expression system, and use it to functionally express several GmmOrs in a mutant olfactory receptor neuron (ORN) of Drosophila. GmmOr35 responds to 1-hexen-3-ol, an odorant found in human emanations, and also alpha-pinene, a compound produced by malarial parasites. Another receptor, GmmOr9, which is expressed in the sensory pit, responds to acetone, 2-butanone and 2-propanol. We confirm by electrophysiological recording that neurons of the sensory pit respond to these odorants. Acetone and 2-butanone are strong attractants long used in the field to trap tsetse. We find that 2-propanol is also an attractant for both G. morsitans and the related species G. fuscipes, a major vector of African sleeping sickness. The results identify 2-propanol as a candidate for an environmentally friendly and practical tsetse attractant. Taken together, this work characterizes the olfactory system of a highly distinct kind of fly, and it provides an approach to identifying new agents for controlling the fly and the devastating diseases that it carries.

Variant Ionotropic Receptors are Expressed in the Antennae of Anopheles sinensis (Diptera: Culicidae)

Mosquitoes transmit many harmful diseases that seriously threaten public health. The mosquito's olfactory system is of great significance for host selection. Inotropic receptors (IRs) and olfactory receptors (ORs) have been demonstrated to be capable of odorant molecular recognition. Analyzing the molecular principles of mosquito olfaction facilitates the development of prevention and therapy techniques. Advances in the understanding of IRs have been seriously inadequate compared to those of ORs. Here, we provide evidence that 35 Anopheles sinensis IR (AsIR) genes are expressed, 7 of which are in the antennae and 2 have expression levels that are upregulated with a blood meal. A homologous analysis of the sequences showed that AsIRs are a subfamily of ionotropic glutamate receptors (iGLURs). This is the first that time IRs have been identified in Anopheles sinensis in vitro. The ultrastructure of the antennae supports the theory that diverse sensilla are distributed in the antennae. The results here may facilitate the revelation of the regulation mechanism in AsIRs, which could mitigate the transmission of diseases by mosquitoes.

Antennal scales improve signal detection efficiency in moths

The elaborate bipectinate antennae of male moths are thought to increase their sensitivity to female sex pheromones, and so should be favoured by selection. Yet simple filamentous antennae are the most common structure among moths. The stereotypic arrangements of scales on the surface of antennae may resolve this paradox. We use computational fluid dynamics techniques to model how scales on the filamentous antennae of moths affect the passage of different particles in the airflow across the flagellum in both small and large moths. We found that the scales provide an effective solution to improve the efficacy of filamentous antennae, by increasing the concentration of nanoparticles, which resemble pheromones, around the antennae. The smaller moths have a greater increase in antennal efficiency than larger moths. The scales also divert microparticles, which resemble dust, away from the antennal surface, thereby reducing contamination. The positive correlations between antennal scale angles and sensilla number across Heliozelidae moths are consistent with the predictions of our model.

Evaluation of the DREAM Technique for a High-Throughput Deorphanization of Chemosensory Receptors in Drosophila

In the vinegar fly Drosophila melanogaster, the majority of olfactory receptors mediating the detection of volatile chemicals found in their natural habitat have been functionally characterized (deorphanized) in vivo. In this process, receptors have been assigned ligands leading to either excitation or inhibition in the olfactory sensory neuron where they are expressed. In other, non-drosophilid insect species, scientists have not yet been able to compile datasets about ligand-receptor interactions anywhere near as extensive as in the model organism D. melanogaster, as genetic tools necessary for receptor deorphanization are still missing. Recently, it was discovered that exposure to artificially high concentrations of odorants leads to reliable alterations in mRNA levels of interacting odorant receptors in mammals. Analyzing receptor expression after odorant exposure can, therefore, help to identify ligand-receptor interactions in vivo without the need for other genetic tools. Transfer of the same methodology from mice to a small number of receptors in D. melanogaster resulted in a similar trend, indicating that odorant exposure induced alterations in mRNA levels are generally applicable for deorphanization of interacting chemosensory receptors. Here, we evaluated the potential of the DREAM (Deorphanization of receptors based on expression alterations in mRNA levels) technique for high-throughput deorphanization of chemosensory receptors in insect species using D. melanogaster as a model. We confirmed that in some cases the exposure of a chemosensory receptor to high concentration of its best ligand leads to measureable alterations in mRNA levels. However, unlike in mammals, we found several cases where either confirmed ligands did not induce alterations in mRNA levels of the corresponding chemosensory receptors, or where gene transcript-levels were altered even though there is no evidence for a ligand-receptor interaction. Hence, there are severe limitations to the suitability of the DREAM technique for deorphanization as a general tool to characterize olfactory receptors in insects.

Evolution of coeloconic sensilla in the peripheral olfactory system of Drosophila mojavensis

Populations inhabiting habitats with different environmental conditions, such as climate, resource availability, predation, competition, can undergo selection for traits that are adaptive in one habitat and not the other, leading to divergence between populations. Changes in the olfactory systems of insects that rely on different host plants, for example, can occur in response to differences in sensory stimuli between habitats. In this study, we investigate the evolution of host preference by characterizing the coeloconic sensilla in Drosophila mojavensis, a species that breeds on different necrotic cacti across its geographic range. These cactus species differ in the volatile chemicals they emit, a primary sensory cue for host plant discrimination. Analysis of odor-evoked responses identified four coeloconic sensilla that were qualitatively similar to those of Drosophila melanogaster, but varied in the breadth and strength of their olfactory sensory neuron responses to some acids and amines. Variation in responses to certain odorants among D. mojavensis populations was also observed. Compared to D. melanogaster, there was a lack of sensitivity of antennal coeloconic type 3 (ac3) sensilla to primary ligands of OR35a across all populations. Consistent with this result was a lack of detectable Or35a gene expression. Using a comparative approach, we then examined odor specificity of ac3 sensilla for seven additional Drosophila species, and found that OR35a-like sensitivity may be limited to the melanogaster subgroup. The variation in specificity that was observed among species is not clearly attributable to the degree of ecological specialization, nor to the ecological niche.

Membrane Proteins Mediating Reception and Transduction in Chemosensory Neurons in Mosquitoes

Mosquitoes use chemical cues to modulate important behaviors such as feeding, mating, and egg laying. The primary chemosensory organs comprising the paired antennae, maxillary palps and labial palps are adorned with porous sensilla that house primary sensory neurons. Dendrites of these neurons provide an interface between the chemical environment and higher order neuronal processing. Diverse proteins located on outer membranes interact with chemicals, ions, and soluble proteins outside the cell and within the lumen of sensilla. Here, we review the repertoire of chemosensory receptors and other membrane proteins involved in transduction and discuss the outlook for their functional characterization. We also provide a brief overview of select ion channels, their role in mammalian taste, and potential involvement in mosquito taste. These chemosensory proteins represent targets for the disruption of harmful biting behavior and disease transmission by mosquito vectors.

Humidity response depends on the small soluble protein Obp59a in Drosophila

Hygrosensation is an essential sensory modality that is used to find sources of moisture. Hygroreception allows animals to avoid desiccation, an existential threat that is increasing with climate change. Humidity response, however, remains poorly understood. Here we find that humidity-detecting sensilla in the Drosophila antenna express and rely on a small protein, Obp59a. Mutants lacking this protein are defective in three hygrosensory behaviors, one operating over seconds, one over minutes, and one over hours. Remarkably, loss of Obp59a and humidity response leads to an increase in desiccation resistance. Obp59a is an exceptionally well-conserved, highly localized, and abundantly expressed member of a large family of secreted proteins. Antennal Obps have long been believed to transport hydrophobic odorants, and a role in hygroreception was unexpected. The results enhance our understanding of hygroreception, Obp function, and desiccation resistance, a process that is critical to insect survival.

Some insects have a sense – called hygroreception – that allows them to detect changing levels of moisture in the air. These insects use this sense to avoid becoming too dry, or to find food or places to lay their eggs. In many species, including the fruit fly Drosophila melanogaster, the antennae are important for hygroreception. Cells in the antennae produce lots of small proteins called odorant binding proteins, or Obps for short. These proteins are believed mostly to help the antennae to detect various chemical signals in the air, but it was not known if any of these proteins were also involved in hygroreception.

Obp59a is an odorant binding protein that is found in the parts of the antennae that sense moisture, and Sun et al. set out to establish whether it has a role in hygroreception in the fruit fly. A closer look confirmed that Obp59a proteins were indeed found specifically in the moisture-sensitive parts of the antennae, the hygroreceptive sensilla. Further experiments showed that flies without Obp59a could not respond properly to changing humidity over periods of seconds, minutes and hours. These results indicated that Obp59a is important for insect hygroreception.

Perhaps unexpectedly, these mutant flies were also more resistant to drying out. Sun et al. suggest that, because flies without Obp59a struggle with hygroreception, they may also become more cautious to avoid becoming too dry. Further experiments could now test this hypothesis. Since insects like mosquitoes use hygroreception to find their human hosts or choose where to lay their eggs, Obp59a may become a useful target for controlling insect-borne infections. Also, understanding insect hygroreception may yield new insights into how climate change will affect insect populations around the world.

Antennal Olfactory Physiology and Behavior of Males of the Ponerine Ant Harpegnathos saltator

In comparison to the large amount of study on the communication abilities of females in ant societies and their associated chemical ecology and sensory physiology, such study of male ants has been largely ignored; accordingly, little is known about their olfactory sensory capabilities. To address this, we explored peripheral odor sensitivities in male Harpegnathos saltator by measuring the electrophysiological activity of olfactory sensory neurons within antennal trichoid and coeloconic sensilla using an extracellular recording technique. In an initial trial of 46 compounds, sensilla trichodea responded strongly to two alarm pheromone components, while a limited number of non-hydrocarbon odorants elicited strong responses in sensilla coeloconica. Both sensillar types responded indifferently to 31 cuticular hydrocarbons (CHCs) and synthetic long-chain hydrocarbons (HCs) typically found on insect cuticle. In a search for sensilla responding to CHCs and other compounds, we found some sensilla that responded to synthetic HCs and CHCs from virgin queen postpharyngeal glands that are potentially used in close range mate recognition. Olfactometer bioassays of male ants to 15 non-HCs correlated sensory responsiveness to the respective behavioral responses. Comparing olfactory responses between H. saltator males and females, we found that sensilla coeloconica and basiconica of workers showed greater responses and broader selectivity to all compounds. The rarity of CHC-responding trichoid sensilla in Harpegnathos males suggests a more specific role in sexual communication compared to that in females, which use CHCs in a broader communication context.

The Two Main Olfactory Receptor Families in Drosophila, ORs and IRs: A Comparative Approach

Most insect species rely on the detection of olfactory cues for critical behaviors for the survival of the species, e.g., finding food, suitable mates and appropriate egg-laying sites. Although insects show a diverse array of molecular receptors dedicated to the detection of sensory cues, two main types of molecular receptors have been described as responsible for olfactory reception in Drosophila, the odorant receptors (ORs) and the ionotropic receptors (IRs). Although both receptor families share the role of being the first chemosensors in the insect olfactory system, they show distinct evolutionary origins and several distinct structural and functional characteristics. While ORs are seven-transmembrane-domain receptor proteins, IRs are related to the ionotropic glutamate receptor (iGluR) family. Both types of receptors are expressed on the olfactory sensory neurons (OSNs) of the main olfactory organ, the antenna, but they are housed in different types of sensilla, IRs in coeloconic sensilla and ORs in basiconic and trichoid sensilla. More importantly, from the functional point of view, they display different odorant specificity profiles. Research advances in the last decade have improved our understanding of the molecular basis, evolution and functional roles of these two families, but there are still controversies and unsolved key questions that remain to be answered. Here, we present an updated review on the advances of the genetic basis, evolution, structure, functional response and regulation of both types of chemosensory receptors. We use a comparative approach to highlight the similarities and differences among them. Moreover, we will discuss major open questions in the field of olfactory reception in insects. A comprehensive analysis of the structural and functional convergence and divergence of both types of receptors will help in elucidating the molecular basis of the function and regulation of chemoreception in insects.

Chemosensory Transmembrane Protein Families in the Coffee White Stemborer, Xylotrechus quadripes (Coleoptera: Cerambycidae)

The coffee white stemborer, Xylotrechus quadripes Chevrolat (Coleoptera: Cerambycidae), feeds primarily on Coffea arabica L. (Gentianales: Rubiaceae) with its egg, larva, and pupa being developed within the trunk. The detection of chemosensory-related cues linked to adult mating, host seeking, and recognition is driven by three chemoreceptor gene repertoires of odorant (ORs), gustatory (GRs), and ionotropic (IRs) receptors as well as sensory neuron membrane proteins (SNMPs). Yet, information on these genes involved in chemoreception is unavailable in X. quadripes and relatively poor in the cerambycid beetles. Here, we presented the identification of four chemosensory transmembrane proteins from the antennal transcriptome of X. quadripes, including 33 ORs, five GRs, 18 IRs, and four SNMPs. Phylogenetic analysis classified the ORs into groups 1, 2, 3, 7, and olfactory coreceptor (Orco), showing three potential candidates (OR13, OR17, and OR21) for the sensing of male sex pheromones. The IRs were clustered into 10 orthologous groups, with additional copies for IR41a, IR64a, and IR75 clades. Four SNMPs were distributed in four independent clades, possibly representing a complete set in this species. Expression profiles revealed that all the genes were highly expressed in antennae, suggesting their olfactory roles. In addition, most of the genes showed the expression in nonantennal tissues including thoraxes, abdomens, wings, and legs, suggesting their involvement in nonchemosensory functions. Of notice, a highly conserved coreceptor IR25a displayed male-biased expression in the antennae, as the first presence in the cerambycid beetles. This study has established reference resources for understanding the mechanisms underlying the interactions between/within this beetle and its host plants.

Genome-wide analysis of ionotropic receptor gene repertoire in Lepidoptera with an emphasis on its functions of Helicoverpa armigera

The functions of the Ionotropic Receptor (IR) family have been well studied in Drosophila melanogaster, but only limited information is available in Lepidoptera. Here, we conducted a large-scale genome-wide analysis of the IR gene repertoire in 13 moths and 16 butterflies. Combining a homology-based approach and manual efforts, totally 996 IR candidates are identified including 31 pseudogenes and 825 full-length sequences, representing the most current comprehensive annotation in lepidopteran species. The phylogeny, expression and sequence characteristics classify Lepidoptera IRs into three sub-families: antennal IRs (A-IRs), divergent IRs (D-IRs) and Lepidoptera-specific IRs (LS-IRs), which is distinct from the case of Drosophila IRs. In comparison to LS-IRs and D-IRs, A-IRs members share a higher degree of protein identity and are distinguished into 16 orthologous groups in the phylogeny, showing conservation of gene structure. Analysis of selective forces on 27 orthologous groups reveals that these lepidopteran IRs have evolved under strong purifying selection (dN/dS≪1). Most notably, lineage-specific gene duplications that contribute primarily to gene number variations across Lepidoptera not only exist in D-IRs, but are present in the two other sub-families including members of IR41a, 76b, 87a, 100a and 100b. Expression profiling analysis reveals that over 80% (21/26) of Helicoverpa armigera A-IRs are expressed more highly in antennae of adults or larvae than other tissues, consistent with its proposed function in olfaction. However, some are also detected in taste organs like proboscises and legs. These results suggest that some A-IRs in H. armigera likely bear a dual function with their involvement in olfaction and gustation. Results from mating experiments show that two HarmIRs (IR1.2 and IR75d) expression is significantly up-regulated in antennae of mated female moths. However, no expression difference is observed between unmated female and male adults, suggesting an association with female host-searching behaviors. Our current study has greatly extended the IR gene repertoire resource in Lepidoptera, and more importantly, identifies potential IR candidates for olfactory, gustatory and oviposition behaviors in the cotton bollworm.

Hygrosensation: Feeling Wet and Cold

Identification of ionotropic receptors required for hygrosensation in Drosophila supports the notion that hygrosensory neurons across insects share common morphological and anatomical features. This further advances the field by uncovering central circuits that respond to both humidity and temperature.

Signaling Mode of the Broad-Spectrum Conserved CO2 Receptor Is One of the Important Determinants of Odor Valence in Drosophila

Odor detection involves hundreds of olfactory receptors from diverse families, making modeling of hedonic valence of an odorant difficult, even in Drosophila melanogaster where most receptors have been deorphanised. We demonstrate that a broadly tuned heteromeric receptor that detects CO₂ (Gr21a, Gr63a) and other odorants is a key determinant of valence along with a few members of the Odorant receptor family in a T-maze, but not in a trap assay. Gr21a and Gr63a have atypically high amino acid conservation in Dipteran insects, and they use both inhibition and activation to convey positive or negative valence for numerous odorants. Inhibitors elicit a robust Gr63a-dependent attraction, while activators, strong aversion. The attractiveness of inhibitory odorants increases with increasing background CO₂ levels, providing a mechanism for behavior modulation in odor blends. In mosquitoes, valence is switched and activation of the orthologous receptor conveys attraction. Reverse chemical ecology enables the identification of inhibitory odorants to reduce attraction of mosquitoes to skin.

Access to the odor world: olfactory receptors and their role for signal transduction in insects

The sense of smell enables insects to recognize and discriminate a broad range of volatile chemicals in their environment originating from prey, host plants and conspecifics. These olfactory cues are received by olfactory sensory neurons (OSNs) that relay information about food sources, oviposition sites and mates to the brain and thus elicit distinct odor-evoked behaviors. Research over the last decades has greatly advanced our knowledge concerning the molecular basis underlying the reception of odorous compounds and the mechanisms of signal transduction in OSNs. The emerging picture clearly indicates that OSNs of insects recognize odorants and pheromones by means of ligand-binding membrane proteins encoded by large and diverse families of receptor genes. In contrast, the mechanisms of the chemo-electrical transduction process are not fully understood; the present status suggests a contribution of ionotropic as well as metabotropic mechanisms. In this review, we will summarize current knowledge on the peripheral mechanisms of odor sensing in insects focusing on olfactory receptors and their specific role in the recognition and transduction of odorant and pheromone signals by OSNs.

The multidimensional ionotropic receptors of Drosophila melanogaster

Ionotropic receptors (IRs), which form ion channels, can be categorized into conserved 'antennal IRs', which define the first olfactory receptor family of insects, and species-specific 'divergent IRs', which are expressed in gustatory receptor neurones. These receptors are located primarily in cell bodies and dendrites, and are highly enriched in the tips of the dendritic terminals that convey sensory information to higher brain centres. Antennal IRs play important roles in odour and thermosensation, whereas divergent IRs are involved in other important biological processes such as taste sensation. Some IRs are known to play specific biological roles in the perception of various molecules; however, many of their functions have not yet been defined. Although progress has been made in this field, many functions and mechanisms of these receptors remain unknown. In this review, we provide a comprehensive summary of the current state of knowledge in this field.

Population differences in olfaction accompany host shift in Drosophila mojavensis

Evolutionary shifts in plant-herbivore interactions provide a model for understanding the link among the evolution of behaviour, ecological specialization and incipient speciation. Drosophila mojavensis uses different host cacti across its range, and volatile chemicals emitted by the host are the primary cue for host plant identification. In this study, we show that changes in host plant use between distinct D. mojavensis populations are accompanied by changes in the olfactory system. Specifically, we observe differences in olfactory receptor neuron specificity and sensitivity, as well as changes in sensillar subtype abundance, between populations. Additionally, RNA-seq analyses reveal differential gene expression between populations for members of the odorant receptor gene family. Hence, alterations in host preference are associated with changes in development, regulation and function at the olfactory periphery.

Molecular characterization of tsetse's proboscis and its response to Trypanosoma congolense infection

Tsetse flies (Glossina spp.) transmit parasitic African trypanosomes (Trypanosoma spp.), including Trypanosoma congolense, which causes animal African trypanosomiasis (AAT). AAT detrimentally affects agricultural activities in sub-Saharan Africa and has negative impacts on the livelihood and nutrient availability for the affected communities. After tsetse ingests an infectious blood meal, T. congolense sequentially colonizes the fly’s gut and proboscis (PB) organs before being transmitted to new mammalian hosts during subsequent feedings. Despite the importance of PB in blood feeding and disease transmission, little is known about its molecular composition, function and response to trypanosome infection. To bridge this gap, we used RNA-seq analysis to determine its molecular characteristics and responses to trypanosome infection. By comparing the PB transcriptome to whole head and midgut transcriptomes, we identified 668 PB-enriched transcripts that encoded proteins associated with muscle tissue, organ development, chemosensation and chitin-cuticle structure development. Moreover, transcripts encoding putative mechanoreceptors that monitor blood flow during tsetse feeding and interact with trypanosomes were also expressed in the PB. Microscopic analysis of the PB revealed cellular structures associated with muscles and cells. Infection with T. congolense resulted in increased and decreased expression of 38 and 88 transcripts, respectively. Twelve of these differentially expressed transcripts were PB-enriched. Among the transcripts induced upon infection were those encoding putative proteins associated with cell division function(s), suggesting enhanced tissue renewal, while those suppressed were associated with metabolic processes, extracellular matrix and ATP-binding as well as immunity. These results suggest that PB is a muscular organ with chemosensory and mechanosensory capabilities. The mechanoreceptors may be point of PB-trypanosomes interactions. T. congolense infection resulted in reduced metabolic and immune capacity of the PB. The molecular knowledge on the composition and putative functions of PB forms the foundation to identify new targets to disrupt tsetse’s ability to feed and parasite transmission.

Tsetse flies are economically important insects responsible for transmitting African trypanosomes, which cause debilitating and fatal diseases in humans and animals in sub-Saharan Africa. In the tsetse vector, trypanosomes undergo complex developmental processes in the midgut, culminating with the generation of mammalian infective forms in the salivary glands for Trypanosoma brucei spp. and in the proboscis (PB) for Trypanosoma congolense and Trypanosoma vivax. Molecular studies on tsetse’s PB, and its interactions with trypanosomes, are limited. We used RNA-seq analysis to obtain molecular information on the putative products associated with tsetse’s PB and characterized PB responses to infection with T. congolense. Based on the predicted putative protein profile, the PB appears to be a muscular organ with mechanoreceptors and may have the capacity to sense and respond to chemical cues. Parasite infections of the PB lead to decreased expression of genes whose products are associated with metabolic and immune functions. These data provide insights into tsetse-trypanosome interactions in the PB organ and identify potential candidate targets that can be further explored to develop biotechnological strategies to reduce transmission of trypanosomes by tsetse flies.

Current source density mapping of antennal sensory selectivity reveals conserved olfactory systems between tephritids and Drosophila

Ecological specialization of insects involves the functional and morphological reshaping of olfactory systems. Little is known about the degree to which insect sensitivity to odorant compounds is conserved between genera, tribes, or families. Here we compared the olfactory systems of six tephritid fruit fly species spanning two tribes and the distantly related Drosophila melanogaster at molecular, functional, and morphological levels. Olfaction in these flies is mediated by a set of olfactory receptors (ORs) expressed in different functional classes of neurons located in distinct antennal regions. We performed a phylogenetic analysis that revealed both family-specific OR genes and putative orthologous OR genes between tephritids and Drosophila. With respect to function, we then used a current source density (CSD) analysis to map activity across antennae. Functional maps mirrored the intrinsic structure of antennae observed with scanning electron microscopy. Together, the results revealed partial conservation of the olfactory systems between tephritids and Drosophila. We also demonstrate that the mapping of olfactory responses is necessary to decipher antennal sensory selectivity to olfactory compounds. CSD analysis can be easily applied to map antennae of other species and therefore enables the rapid deriving of olfactory maps and the reconstructing of the target organisms' history of evolution.

Distinct Subfamilies of Odorant Binding Proteins in Locust (Orthoptera, Acrididae): Molecular Evolution, Structural Variation, and Sensilla-Specific Expression

Odorant binding proteins (OBPs) play an important role in insect olfaction, facilitating transportation of odorant molecules in the sensillum lymph. While most of the researches are concentrated on Lepidopteran and Dipteran species, our knowledge about Orthopteran species is still very limited. In this study, we have investigated OBPs of the desert locust Schistocerca gregaria, a representative Orthopteran species. We have identified 14 transcripts from a S. gregaria antennal transcriptome encoding SgreOBPs, and recapitulated the phylogenetic relationship of SgreOBPs together with OBPs from three other locust species. Two conserved subfamilies of classic OBPs have been identified, named I-A and II-A, exhibiting both common and subfamily-specific amino acid motifs. Distinct evolutionary features were observed for subfamily I-A and II-A OBPs. Surface topology and interior cavity were elucidated for OBP members from the two subfamilies. Antennal topographic expression revealed distinct sensilla- and cellular- specific expression patterns for SgreOBPs from subfamily I-A and II-A. These findings give first insight into the repertoire of locust OBPs with respect to their molecular and evolutionary features as well as their expression in the antenna, which may serve as an initial step to unravel specific roles of distinct OBP subfamilies in locust olfaction.

Olfactory Ionotropic Receptors in Mosquito Aedes albopictus (Diptera: Culicidae)

Ionotropic glutamate receptors (iGluRs) are a conserved family of ligand-gated ion channels that primarily function to mediate neuronal communication at synapses. A variant subfamily of iGluRs, the ionotropic receptors (IRs), was recently identified in insects and proved with the function in odorant recognition. Ionotropic receptors participate in a distinct olfactory signaling pathway that is independent of olfactory receptors activity. In the present study, we identify 102 putative IR genes, dubbed as AalbIr genes, in mosquito Aedes albopictus (Skuse) by in silico comparative sequence analysis. Among AalbIr genes, 19 show expression in the female antenna by RT-PCR. These putative olfactory AalbIRs share four conservative hydrophobic domains of amino acids, similar to the transmembrane and ion channel pore regions found in conventional iGluRs. To determine the potential function of these olfactory AalbIRs in host-seeking, we compared their transcript expression levels in the antennae of blood-fed females with that of non-blood-fed females by quantitative real-time RT-PCR. Three AalbIr genes showed downregulation when the mosquito finished a bloodmeal. These results may help to improve our understanding of the IR-mediated olfactory signaling in mosquitoes.

The Antioxidative, Antiaging, and Hepatoprotective Effects of Alkali-Extractable Polysaccharides by Agaricus bisporus

The aim of this work was designed to investigate the antioxidant, antiaging, and hepatoprotective effects of alkali-extractable polysaccharides (AlAPS) and their three purified fractions (AlAPS-1, AlAPS-2, and AlAPS-3) from Agaricus bisporus in D-galactose induced aging mice. For in vitro antioxidant analysis, both AlAPS and its fractions exhibited moderate reducing power, Fe²⁺-chelating activities, and potent scavenging activities on hydroxyl and 1,1-diphenyl-2-picrylhydrazyl (DPPH) radicals. The in vivo results demonstrated that the polysaccharides, especially AlAPS-2, showed potential antiaging and hepatoprotective effects by enhancing the antioxidant status, decreasing serum hepatic enzyme activities, and improving the lipid metabolism. This study suggested that the polysaccharides extracted and purified from A. bisporus could be exploited as a potent dietary supplement to attenuate aging and prevent age-related diseases.

Chemosensory sensitivity reflects reproductive status in the ant Harpegnathos saltator

Insects communicate with pheromones using sensitive antennal sensilla. Although trace amounts of pheromones can be detected by many insects, context-dependent increased costs of high sensitivity might lead to plasticity in sensillum responsiveness. We have functionally characterized basiconic sensilla of the ant Harpegnathos saltator for responses to general odors in comparison to cuticular hydrocarbons which can act as fertility signals emitted by the principal reproductive(s) of a colony to inhibit reproduction by worker colony members. When released from inhibition workers may become reproductive gamergates. We observed plasticity in olfactory sensitivity after transition to reproductive status with significant reductions in electrophysiological responses to several long-chained cuticular hydrocarbons. Although gamergates lived on average five times longer than non-reproductive workers, the shift to reproductive status rather than age differences matched the pattern of changes in olfactory sensitivity. Decreasing sensillum responsiveness to cuticular hydrocarbons could potentially reduce mutually inhibitory or self-inhibitory effects on gamergate reproduction.

Two Olfactory Pathways to Detect Aldehydes on Locust Mouthpart

Sensing chemical cues is crucial for insects through their olfactory systems to adapt the environments. The receptors employed in insect olfactory system belong to the Odorant Receptor (ORs) and Ionotropic Receptor (IRs) families. In general, ORs and IRs are present in distinct olfactory sensory neurons and function independently. Here, we present evidence that in locust, the abundant host plant odor Hexanal is detected by both IR- and OR-expressing neurons. Use of the palp opening response (POR) as a simple behavioral paradigm in conjunction with RNA interference (RNAi) revealed that these two pathways are both needed for the detection of Hexanal. Two-color fluorescence in situ hybridization showed that OR2 and odorant-binding protein 1 (obp1) were co-localized in palps sensilla basiconica. Obp2a and IR8a were co-localized as well, but associated with sensilla chaetica on the palps. Furthermore, both OR2- and obp1-knockdowns showed reduced POR responses to Hexanal and E-2-Hexenal, and the same was true for Hexanal with IR8a- and obp2a-knockdowns. Detection to E-2-Hexenal was independent of IR8a-mediated gene silencing. Besides, Hexanal and E-2-Hexenal evoked dose-dependent responses in palp basiconica via extracellular recordings. Our results indicate that both OR and IR pathways are involved in the detection of one aldehyde.

Antennal responses to volatile organic compounds in a stonefly

Notwithstanding their long antennae, especially in the adult stage, Plecoptera represent a poorly studied group as regards their sensory structures and their ability to perceive olfactory stimuli is so far totally unknown. A recent investigation on the antenna of Dinocras cephalotes revealed two kinds of putative olfactory sensilla (multiporous single walled sensilla and digitated double walled sensilla). The present electrophysiological study (electroantennography, EAG), in which male and female antennae of D. cephalotes (Plecoptera, Perlidae) have been stimulated with a set of generic odours belonging to different functional groups, shows that Plecoptera can perceive olfactory cues through their antennal sensilla. Indeed, although many chemicals did not elicit any response, high EAG activity has been recorded in response to pentanoic acid, propanal, butyric acid, propionic acid, isoamylamine and ammonia. The response was very similar in both sexes even if propanal elicited a response only in males. EAG dose-responses curves in both males and females showed that EAG responses were similar in males and females and generally increased in amplitude with increasing doses of the chemicals. Behavioural, neuroanatomical and molecular investigations on Plecoptera olfaction, could be particularly interesting not only to increase the knowledge of the adult stonefly behaviour but also to better understand the adaptation of the olfactory sensory system in aquatic insects moving from land to water.

Measuring activity in olfactory receptor neurons in Drosophila: Focus on spike amplitude

Olfactory responses at the receptor level have been thoroughly described in Drosophila melanogaster by electrophysiological methods. Single sensilla recordings (SSRs) measure neuronal activity in intact individuals in response to odors. For sensilla that contain more than one olfactory receptor neuron (ORN), their different spontaneous spike amplitudes can distinguish each signal under resting conditions. However, activity is mainly described by spike frequency. Some reports on ORN response dynamics studied two components in the olfactory responses of ORNs: a fast component that is reflected by the spike frequency and a slow component that is observed in the LFP (local field potential, the single sensillum counterpart of the electroantennogram, EAG). However, no apparent correlation was found between the two elements. In this report, we show that odorant stimulation produces two different effects in the fast component, affecting spike frequency and spike amplitude. Spike amplitude clearly diminishes at the beginning of a response, but it recovers more slowly than spike frequency after stimulus cessation, suggesting that ORNs return to resting conditions long after they recover a normal spontaneous spike frequency. Moreover, spike amplitude recovery follows the same kinetics as the slow voltage component measured by the LFP, suggesting that both measures are connected. These results were obtained in ab2 and ab3 sensilla in response to two odors at different concentrations. Both spike amplitude and LFP kinetics depend on odorant, concentration and neuron, suggesting that like the EAG they may reflect olfactory information.

Organization and function of Drosophila odorant binding proteins

Odorant binding proteins (Obps) are remarkable in their number, diversity, and abundance, yet their role in olfactory coding remains unclear. They are widely believed to be required for transporting hydrophobic odorants through an aqueous lymph to odorant receptors. We construct a map of the Drosophila antenna, in which the abundant Obps are mapped to olfactory sensilla with defined functions. The results lay a foundation for an incisive analysis of Obp function. The map identifies a sensillum type that contains a single abundant Obp, Obp28a. Surprisingly, deletion of the sole abundant Obp in these sensilla does not reduce the magnitude of their olfactory responses. The results suggest that this Obp is not required for odorant transport and that this sensillum does not require an abundant Obp. The results further suggest a novel role for this Obp in buffering changes in the odor environment, perhaps providing a molecular form of gain control.

DOI:http://dx.doi.org/10.7554/eLife.20242.001

Insects use their sense of smell to find mates, to find food and – in the case of insects that transmit diseases such as malaria and Zika – to find us. If we can understand how insect scent detection works at the molecular and cellular level, we may be able to devise new ways of manipulating the insects’ sense of smell and prevent them from finding us.

Insects contain a family of proteins called odorant binding proteins that are intriguing in several ways. They are numerous (there are 52 kinds in the fruit fly Drosophila), they are diverse and some are made in remarkably large amounts in the antennae. Fine hair-like structures known as olfactory sensilla protrude from the surface of the antennae. Odorant binding proteins are widely believed to carry odorant molecules through the fluid inside the sensilla to olfactory neurons, which then send signals that trigger the insect’s response to the scent.

Larter et al. have now mapped the most abundant odorant binding proteins to the various olfactory sensilla of Drosophila. This revealed that a type of sensillum known as ab8 contained only one abundant odorant binding protein, called Obp28a. Unexpectedly, Larter et al. found that ab8 sensilla that are deprived of this protein respond strongly to odorant molecules. This result suggests that Obp28a is not required to transport odorants to the neurons in ab8; indeed, it appears that these neurons do not require an abundant odorant binding protein in order to respond to a scent. Instead, Obp28a helps to moderate the effects of sudden changes in the level of an odorant in the environment, so that concentrated odors do not trigger too large a response from the olfactory neurons.

The details of the role that Obp28a plays in olfactory sensilla remain to be investigated in future studies, and the map created by Larter et al. also lays a foundation for studying the roles of other odorant binding proteins. The discovery that Obp28a is not needed to transport odorant molecules also raises questions about how insects are able to detect smells. Many odorant molecules repel water, so how do these molecules travel through the fluid in the sensilla if odorant binding proteins are not needed to transport them?

DOI:http://dx.doi.org/10.7554/eLife.20242.002

Olfactory receptor pseudo-pseudogenes

Pseudogenes are generally considered to be non-functional DNA sequences that arise through nonsense or frame-shift mutations of protein-coding genes. Although certain pseudogene-derived RNAs have regulatory roles, and some pseudogene fragments are translated, no clear functions for pseudogene-derived proteins are known. Olfactory receptor families contain many pseudogenes, which reflect low selection pressures on loci no longer relevant to the fitness of a species. Here we report the characterization of a pseudogene in the chemosensory variant ionotropic glutamate receptor repertoire of Drosophila sechellia, an insect endemic to the Seychelles that feeds almost exclusively on the ripe fruit of Morinda citrifolia. This locus, D. sechellia Ir75a, bears a premature termination codon (PTC) that appears to be fixed in the population. However, D. sechellia Ir75a encodes a functional receptor, owing to efficient translational read-through of the PTC. Read-through is detected only in neurons and is independent of the type of termination codon, but depends on the sequence downstream of the PTC. Furthermore, although the intact Drosophila melanogaster Ir75a orthologue detects acetic acid-a chemical cue important for locating fermenting food found only at trace levels in Morinda fruit-D. sechellia Ir75a has evolved distinct odour-tuning properties through amino-acid changes in its ligand-binding domain. We identify functional PTC-containing loci within different olfactory receptor repertoires and species, suggesting that such 'pseudo-pseudogenes' could represent a widespread phenomenon.

The antenna of horse stomach bot flies: morphology and phylogenetic implications (Oestridae, Gasterophilinae: Gasterophilus Leach)

Antennae are among the most elaborate sensory organs in adult flies, and they provide rich information for phylogenic studies. The antennae of five out of eight species of Gasterophilus Leach (G. haemorrhoidalis (Linnaeus), G. intestinalis (De Geer), G. nasalis (Linnaeus), G. nigricornis (Loew) and G. pecorum (Fabricius)), were examined using scanning electron microscopy. The general morphology, including distribution, type, size, and ultrastructure of antennal sensilla were presented, and the definition of auriculate sensilla and sensory pits were updated and clarified. Eighteen antennal characters were selected to construct the first species-level phylogeny of this genus. The monophyly of Gasterophilus was supported by the presence of coeloconic sensilla III on the antennal arista. The species-level cladogram showed G. pecorum branching off at the base, and the remaining species forming the topology (G. intestinalis+ (G. haemorrhoidalis+ (G. nasalis+ G. nigricornis))). Our research shows the importance of the antennal ultrastructure as a reliable source for phylogenetic analysis.

Distinct combinations of variant ionotropic glutamate receptors mediate thermosensation and hygrosensation in Drosophila

Ionotropic Receptors (IRs) are a large subfamily of variant ionotropic glutamate receptors present across Protostomia. While these receptors are most extensively studied for their roles in chemosensory detection, recent work has implicated two family members, IR21a and IR25a, in thermosensation in Drosophila. Here we characterize one of the most evolutionarily deeply conserved receptors, IR93a, and show that it is co-expressed and functions with IR21a and IR25a to mediate physiological and behavioral responses to cool temperatures. IR93a is also co-expressed with IR25a and a distinct receptor, IR40a, in a discrete population of sensory neurons in the sacculus, a multi-chambered pocket within the antenna. We demonstrate that this combination of receptors is required for neuronal responses to dry air and behavioral discrimination of humidity differences. Our results identify IR93a as a common component of molecularly and cellularly distinct IR pathways important for thermosensation and hygrosensation in insects.

DOI:http://dx.doi.org/10.7554/eLife.17879.001

Intracellular regulation of the insect chemoreceptor complex impacts odour localization in flying insects

Flying insects are well known for airborne odour tracking and have evolved diverse chemoreceptors. While ionotropic receptors (IRs) are found across protostomes, insect odorant receptors (ORs) have only been identified in winged insects. We therefore hypothesized that the unique signal transduction of ORs offers an advantage for odour localization in flight. Using Drosophila, we found expression and increased activity of the intracellular signalling protein PKC in antennal sensilla following odour stimulation. Odour stimulation also enhanced phosphorylation of the OR co-receptor Orco in vitro, while site-directed mutation of Orco or mutations in PKC subtypes reduced the sensitivity and dynamic range of OR-expressing neurons in vivo, but not IR-expressing neurons. We ultimately show that these mutations reduce competence for odour localization of flies in flight. We conclude that intracellular regulation of OR sensitivity is necessary for efficient odour localization, which suggests a mechanistic advantage for the evolution of the OR complex in flying insects.

Cloning and expression profile of ionotropic receptors in the parasitoid wasp Microplitis mediator (Hymenoptera: Braconidae)

Ionotropic receptors (IRs) mainly detect the acids and amines having great importance in many insect species, representing an ancient olfactory receptor family in insects. In the present work, we performed RNAseq of Microplitis mediator antennae and identified seventeen IRs. Full-length MmedIRs were cloned and sequenced. Phylogenetic analysis of the Hymenoptera IRs revealed that ten MmedIR genes encoded "antennal IRs" and seven encoded "divergent IRs". Among the IR25a orthologous groups, two genes, MmedIR25a.1 and MmedIR25a.2, were found in M. mediator. Gene structure analysis of MmedIR25a revealed a tandem duplication of IR25a in M. mediator. The tissue distribution and development specific expression of the MmedIR genes suggested that these genes showed a broad expression profile. Quantitative gene expression analysis showed that most of the genes are highly enriched in adult antennae, indicating the candidate chemosensory function of this family in parasitic wasps. Using immunocytochemistry, we confirmed that one co-receptor, MmedIR8a, was expressed in the olfactory sensory neurons. Our data will supply fundamental information for functional analysis of the IRs in parasitoid wasp chemoreception.

Neuroethology of Olfactory-Guided Behavior and Its Potential Application in the Control of Harmful Insects

Harmful insects include pests of crops and storage goods, and vectors of human and animal diseases. Throughout their history, humans have been fighting them using diverse methods. The fairly recent development of synthetic chemical insecticides promised efficient crop and health protection at a relatively low cost. However, the negative effects of those insecticides on human health and the environment, as well as the development of insect resistance, have been fueling the search for alternative control tools. New and promising alternative methods to fight harmful insects include the manipulation of their behavior using synthetic versions of "semiochemicals", which are natural volatile and non-volatile substances involved in the intra- and/or inter-specific communication between organisms. Synthetic semiochemicals can be used as trap baits to monitor the presence of insects, so that insecticide spraying can be planned rationally (i.e., only when and where insects are actually present). Other methods that use semiochemicals include insect annihilation by mass trapping, attract-and- kill techniques, behavioral disruption, and the use of repellents. In the last decades many investigations focused on the neural bases of insect's responses to semiochemicals. Those studies help understand how the olfactory system detects and processes information about odors, which could lead to the design of efficient control tools, including odor baits, repellents or ways to confound insects. Here we review our current knowledge about the neural mechanisms controlling olfactory responses to semiochemicals in harmful insects. We also discuss how this neuroethology approach can be used to design or improve pest/vector management strategies.

Differential Electrophysiological Responses to Odorant Isotopologues in Drosophilid Antennae

Olfaction presents the ultimate challenge to molecular recognition as thousands of molecules have to be recognized by far fewer olfactory receptors. We have presented evidence that Drosophila readily distinguish odorants based on their molecular vibrations using a battery of behavioral assays suggesting engagement of a molecular vibration-sensing component. Here we interrogate electrophysiologically the antennae of four Drosophilids and demonstrate conserved differential response amplitudes to aldehydes, alcohols, ketones, nitriles, and their deuterated isotopologues. Certain deuterated odorants evoked larger electroantennogram (EAG) amplitudes, while the response to the normal odorant was elevated in others. Significantly, benzonitrile isotopologues were not distinguishable as predicted. This suggests that isotopologue-specific EAG amplitudes result from differential activation of specific olfactory receptors. In support of this, odorants with as few as two deuteria evoke distinct EAG amplitudes from their normal isotopologues, and this is independent of the size of the deuterated molecule. Importantly, we find no evidence that these isotopologue-specific amplitudes depend on perireceptor mechanisms or other pertinent physical property of the deuterated odorants. Rather, our results strongly suggest that Drosophilid olfactory receptors are activated by molecular vibrations differentiating similarly sized and shaped odorants in vivo, yielding sufficient differential information to drive behavioral choices.

Antioxidant and anti-aging activities of polysaccharides from Calocybe indica var. APK2

The crude polysaccharides were extracted from the fruiting bodies of Calocybe indica (CIP). The antioxidant activities of CIP were evaluated both in vitro and in vivo. Chemical characteristics of the polysaccharides were investigated. In in vitro antioxidant assay, CIP showed noticeable 2,2-diphenyl-1-picryl-hydrazyl (DPPH), hydroxyl radical scavenging activities, reducing power and lipid peroxidation inhibition. Chemical analysis showed the presence of carbohydrate, protein and the FTIR spectra revealed the presence of general characteristic absorption peak of the polysaccharides. For in vivo antioxidant activity, two different doses of CIP were orally administrated over a period of 6 weeks in a d-galactose (d-gal) induced aged mice model. Significantly lowered activities of superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), levels of glutathione (GSH) and elevated malondialdehyde (MDA) levels were observed in brain and serum of d-galactose induced rats, when compared to control rats. Administration of CIP significantly raised the activities of SOD, CAT, GPx, levels of GSH and lowered the levels of MDA in mice brain and serum in a dose-dependent manner. The results suggested that CIP had potent antioxidant activity and could minimize the occurrence of age-associated disorders associated with involvement of free radicals.