Olfactory Sensilla Research Articles

Three-dimensional reconstruction of insect chemosensory sensillum

The olfactory system is involved in food and mate recognition in insects. However, 3D structures of chemosensory sensilla in insects are unexplored yet. Here, the internal structures of an olfactory sensillum on the antenna of the brown planthopper, Nilaparvata lugens (Hemiptera: Delphacidae), one of the most important rice pests, are examined and imaged using focused ion beam scanning electron microscopy. Based on these images, a 3D structure is reconstructed in this study. We find that the trichoid olfactory sensillum possesses a multiporous wall encircling a lumen with one sensory cell. Besides, there are three accessory cells (ACs) and a glia cell with different cell contents surrounding the sensory cell. The abundant tubular membrane structures in the tormogen cell suggest its role in secreting proteins like odorant binding proteins into the receptor lymph, while three auxiliary cells with simpler cellular content closely enfold the sensory cell, probably to prevent leaking of the receptor lymph into the surrounding epidermis. In the sensory cell, the microtubules and two tandem basal bodies at the base of the microtubules are also reconstructed. They are considered as a propulsive engine to ensure dendrite vibration or spinning in the receptor lymph, so that the proteins and odorant molecules move faster in the receptor lymph, which improves recognition of environmental odors and enables the insect to immediately respond to this information.

Foraging in the darkness: Highly selective tuning of below‐ground larval olfaction to Brassicaceae volatiles in striped flea beetle

AbstractThe olfactory system of above‐ground insects is among the best described perceptual architectures. However, remarkably little is known about how below‐ground insects navigate in the dark for foraging. Here, we investigated host plant preferences, olfactory sensilla and characterise olfactory proteins in below‐ground larvae of the striped flea beetle (SFB) Phyllotreta striolata Fabricius (Coleoptera: Chrysomelidae). Both the adults and larvae of this coleopteran pest cause serious damage to Brassicaceous crops above and below ground, respectively. To elucidate the role of olfactory system in host location of below‐ground larvae, we initially demonstrated that SFB larvae distinctly favoured Brassicaceae over other plant families by two‐choice behavioural bioassay. Subsequently, scanning electron microscopy of sensilla in SFB larval head showed a significant reduction in the number of olfactory sensilla in larvae compared with adults. However, essential olfactory sensilla such as sensilla basiconica are underscoring the indispensability of the larval olfactory system. We selected four larval‐specific odorant binding proteins for functional validation from our previous transcriptome data. Functional studies revealed that PstrOBP23 exhibits robust binding affinity to 24 volatiles of Brassicaceae plants, including seven isothiocyanate compounds. This suggests a pivotal role of PstrOBP23 in the foraging behaviour of the larvae below the ground. Moreover, two ligands displaying strong binding capacity exhibit apparent attractive or repellent activity towards SFB larvae. Our findings provide a crucial insight into the olfactory system of below‐ground larvae in SFB, highlighting the highly selective tuning of larvae specific OBP to host plant volatiles. These results offer potential avenues for developing effective pest control strategies against SFB.

Functional Morphology and Ultrastructure of the Peripheral Antennal Sensillar System of Graphosoma italicum (Müller, 1766) (Insecta: Hemiptera: Pentatomidae).

The antennae of the shield bug Graphosoma italicum (Müller, 1766) were examined through scanning and transmission electron microscopy to reveal their general morphology, as well as the antennal sensilla's distribution, size, and ultrastructure of their dendrites and function. The antennae comprise five antennomeres (one scape, two pedicels, and two flagellomeres). Different lengths of chaetic mechanosensilla (Ch1-Ch4) exist on all antennomeres, and several highly sensitive campaniform sensilla are embedded in the exoskeleton and measure cuticular strain. One pair of peg sensilla, the typical proprioceptive, is only on the proximal edge of the first pedicel and directed to the distal edge of the scapus. The antennal flagellum possesses two subtypes of trichoid and basiconic sensilla, each with one type of coeloconic olfactory sensilla. The distinctive characteristics of G. italicum are also apparent in two subtypes of coeloconic sensilla embedded in different cavities on both antennomeres of the flagellum, probably with a thermo-hypersensitive function. All studied morphological types of the sensilla and their function were supported by ultrastructural elements. The long and thin trichoid sensilla type 2 (TrS2) with an olfactive function was the most abundant sensilla localized on both flagellomeres. The peripheral antennal sensilla system consists of six main types of sensilla divided into twelve subtypes.

Specialized structure and function of the apical extracellular matrix at sense organs

Apical extracellular matrix (aECM) covers every surface of the body and exhibits tissue-specific structures that carry out specialized functions. This is particularly striking at sense organs, where aECM forms the interface between sensory neurons and the environment, and thus plays critical roles in how sensory stimuli are received. Here, we review the extraordinary adaptations of aECM across sense organs and discuss how differences in protein composition and matrix structure assist in sensing mechanical forces (tactile hairs, campaniform sensilla, and the tectorial membrane of the cochlea); tastes and smells (uniporous gustatory sensilla and multiporous olfactory sensilla in insects, and salivary and olfactory mucus in vertebrates); and light (cuticle-derived lenses in arthropods and mollusks). We summarize the power of using C. elegans, in which defined sense organs associate with distinct aECM, as a model for understanding the tissue-specific structural and functional specializations of aECM. Finally, we synthesize results from recent studies in C. elegans and Drosophila into a conceptual framework for aECM patterning, including mechanisms that involve transient cellular or matrix scaffolds, mechanical pulling or pushing forces, and localized secretion or endocytosis.

Functional types of long trichoid sensilla responding to sex pheromone components in Plutella xylostella.

Sex pheromones, which consist of multiple components in specific ratios promote intraspecific sexual communications of insects. Plutella xylostella (L.) is a worldwide pest of cruciferous vegetables, the mating behavior of which is highly dependent on its olfactory system. Long trichoid sensilla on male antennae are the main olfactory sensilla that can sense sex pheromones. However, the underlying mechanisms remain unclear. In this study, 3 sex pheromone components from sex pheromone gland secretions of P. xylostella female adults were identified as Z11-16:Ald, Z11-16:Ac, and Z11-16:OH in a ratio of 9.4 : 100 : 17 using gas chromatography - mass spectrometry and gas chromatography with electroantennographic detection. Electrophysiological responses of 581 and 385 long trichoid sensilla of male adults and female adults, respectively, to the 3 components were measured by single sensillum recording. Hierarchical clustering analysis showed that the long trichoid sensilla were of 6 different types. In the male antennae, 52.32%, 5.51%, and 1.89% of the sensilla responded to Z11-16:Ald, Z11-16:Ac, and Z11-16:OH, which are named as A type, B type, and C type sensilla, respectively; 2.93% named as D type sensilla responded to both Z11-16:Ald and Z11-16:Ac, and 0.34% named as E type sensilla were sensitive to both Z11-16:Ald and Z11-16:OH. In the female antennae, only 7.53% of long trichoid sensilla responded to the sex pheromone components, A type sensilla were 3.64%, B type and C type sensilla were both 0.52%, D type sensilla were 1.30%, and 1.56% of the sensilla responded to all 3 components, which were named as F type sensilla. The responding long trichoid sensilla were located from the base to the terminal of the male antennae and from the base to the middle of the female antennae. The pheromone mixture (Z11-16:Ald : Z11-16:Ac : Z11-16:OH=9.4 : 100 : 17) had a weakly repellent effect on female adults of P. xylostella. Our results lay the foundation for further studies on sex pheromone communications in P. xylostella.

Females smell differently: characteristics and significance of the most common olfactory sensilla of female silkmoths.

In the silkmoth Bombyx mori, the role of male sensilla trichodea in pheromone detection is well established. Here we study the corresponding female sensilla, which contain two olfactory sensory neurons (OSNs) and come in two lengths, each representing a single physiological type. Only OSNs in medium trichoids respond to the scent of mulberry, the silkworm's exclusive host plant, and are more sensitive in mated females, suggesting a role in oviposition. In long trichoids, one OSN is tuned to (+)-linalool and the other to benzaldehyde and isovaleric acid, both odours emitted by silkworm faeces. While the significance of (+)-linalool detection remains unclear, isovaleric acid repels mated females and may therefore play a role in avoiding crowded oviposition sites. When we examined the underlying molecular components of neurons in female trichoids, we found non-canonical co-expression of Ir8a, the co-receptor for acid responses, and ORco, the co-receptor of odorant receptors, in long trichoids, and the unexpected expression of a specific odorant receptor in both trichoid sensillum types. In addition to elucidating the function of female trichoids, our results suggest that some accepted organizational principles of the insect olfactory system may not apply to the predominant sensilla on the antenna of female B. mori.

Comparison of chemosensilla in three phytoseiid mites with different feeding habits

Phytoseiidae comprised over 2000 described species with different feeding habits. However, the mechanisms of food selection were virtually unknown in most phytoseiid mites. To explore the potential mechanism of odor perception, we compared the type and number of chemosensilla on the tarsi of legs and pedipalps by scanning and transmission electron microscopy in three representative predatory mites, i.e., Phytoseiulus persimilis (specialized predator of Tetranychus), Neoseiulus californicus (selective predator of tetranychid mites), and Amblyseius swirskii (generalist predator). All three species had two types of putatively olfactory sensilla, with a porous vs. grooved surface on the tip of the first pair of legs. Of these, five porous setae were found in each of the three species of predatory mites studied. However, only one grooved seta was found at the tip of the first pair of legs in P. persimilis, and two in A. swirskii and N. californicus. In addition, the pedipalp has nine grooved setae, which were hypothesized to be contact chemosensilla in each of the three species. No porous setae were found on either pedipalp or the other three pairs of legs, i.e., leg II, III and IV. Additionally, we provided a new chaetotactic notation for the tarsus of leg I by considering the type and number of setae based on our observation of the three species of Phytoseiidae studied.

Comparison of Olfactory Sensilla Structure in Littoral and Deep-water Amphipods from the Baikal Region

Comparison of Olfactory Sensilla Structure in Littoral and Deep-water Amphipods from the Baikal Region

Effects of pesticide-adjuvant combinations used in almond orchards on olfactory responses to social signals in honey bees (Apis mellifera)

Exposure to agrochemical sprays containing pesticides and tank-mix adjuvants has been implicated in post-bloom mortality, particularly of brood, in honey bee colonies brought into California almond orchards for pollination. Although adjuvants are generally considered to be biologically inert, some adjuvants have exhibited toxicity and sublethal effects, including decreasing survival rates of next-generation queens. Honey bees have a highly developed olfactory system to detect and discriminate among social signals. To investigate the impact of pesticide-adjuvant combinations on honey bee signal perception, we performed electroantennography assays to assess alterations in their olfactory responsiveness to the brood ester pheromone (BEP), the volatile larval pheromone β-ocimene, and the alarm pheromone 2-heptanone. These assays aimed to uncover potential mechanisms underlying changes in social behaviors and reduced brood survival after pesticide exposure. We found that combining the adjuvant Dyne-Amic with the fungicide Tilt (propiconazole) and the insecticide Altacor (chlorantraniliprole) synergistically enhanced olfactory responses to three concentrations of BEP and as well exerted dampening and compensatory effects on responses to 2-heptanone and β-ocimene, respectively. In contrast, exposure to adjuvant alone or the combination of fungicide and insecticide had no effect on olfactory responses to BEP at most concentrations but altered responses to β-ocimene and 2-heptanone. Exposure to Dyne-Amic, Altacor, and Tilt increased BEP signal amplitude, indicating potential changes in olfactory receptor sensitivity or sensilla permeability to odorants. Given that, in a previous study, next-generation queens raised by nurses exposed to the same treated pollen experienced reduced survival, these new findings highlight the potential disruption of social signaling in honey bees and its implications for colony reproductive success.

OLFACTORY RESPONSES OF AULACOPHORA FOVEICOLLIS LUCAS – AN ECOLOGICALLY IMPORTANT PEST - FROM WEST BENGAL, INDIA

Pumpkin, Cucurbita maxima, is severely harmed by Aulacophora foveicollis Lucas (Coleoptera: Chrysomelidae), as a major insect pest of the plant. Based on Scanning electron microscopic analysis, compared to mechanosensilla, prevalence of antennal olfactory sensilla, was observed. The number of olfactory sensilla in the circumferential band of IXth antennal segment was higher in female species. Stimulation of the sensilla present on the antennae of A. foveicollis was analyzed by electroantennogram (EAG) at four different concentrations viz. 10.0, 5.0, 1.0, and 0.1 mg/ml in both sexes. The male species displayed greatest peak amplitude at 5mg/mL for heneicosane, whereas the female species displayed maximum peak amplitude at 10.0 mg/mL for 2-methyl phenol followed by 5.0 mg/mL for 1,4-dimethoxybenzene. However, pooled EAG data revealed that GLV elicited the strongest reactions, followed by aliphatic chemicals, oxygenated monoterpenes, hydrocarbon monoterpenes, and sesqueterpenes. These findings univocally shows that the pest's antennal responses clearly displayed sexual dimorphism.

Comparison of Olfactory Sensilla Structure in Littoral and Deep-Water Amphipods from the Baikal Region

The structure of olfactory sensilla was compared in amphipods from the Baikal region, namely in nine Baikal species (Ommatogammarus carneolus melanophthalmus Bazikalova, 1945, O. flavus (Dybowsky, 1874), O. albinus (Dybowsky, 1874), Eulimnogammarus verrucosus (Gerstfeldt, 1858), E. vittatus (Dybowsky, 1874), E. cyaneus (Dybowsky, 1874), Acanthogammarus godlewskii (Dybowsky, 1874), Pallasea cancelloides (Gerstfeldt, 1858) and Gmelinoides fasciatus (Stebbing, 1899)) and a Holarctic species Gammarus lacustris Sars, 1863. The results of the study demonstrate the comparison of aesthetasc lengths and numbers, as well as SEM images of the aesthetascs for Baikal amphipods for the first time. Our results confirm and complement the previously obtained data on the number of aesthetascs per segment for three deep-water Baikal amphipod species of the genus Ommatogammarus. Within the studied deep-water amphipods with different habitat depth ranges, Ommatogammarus albinus (Dybowsky, 1874), inhabiting maximum habitat depth, had the highest number of aesthetascs. The size and number of aesthetascs did not differ in males and females of the littoral Eulimnogammarus verrucosus (Gerstfeldt, 1858) and deep-water species. The littoral amphipods were shown to have equipped with more mechanoreceptors antennae I than deep-water amphipods.

Gain control in olfactory receptor neurons and the detection of temporal fluctuations in odor concentration.

The ability of the cockroach to locate an odor source in still air suggests that the temporal dynamic of odor concentration in the slowly expanding stationary plume alone is used to infer odor source distance and location. This contradicts with the well-established view that insects use the wind direction as the principle directional cue. This contribution highlights the evidence for, and likely functional relevance of, the capacity of the cockroach's olfactory receptor neurons to detect and process-from one moment to the next-not only a succession of odor concentrations but also the rates at which concentration changes. This presents a challenge for the olfactory system because it must detect and encode the temporal concentration dynamic in a manner that simultaneously allows invariant odor recognition. The challenge is met by a parallel representation of odor identity and concentration changes in a dual pathway that starts from olfactory receptor neurons located in two morphologically distinct types of olfactory sensilla. Parallel processing uses two types of gain control that simultaneously allocate different weight to the instantaneous odor concentration and its rate of change. Robust gain control provides a stable sensitivity for the instantaneous concentration by filtering the information on fluctuations in the rate of change. Variable gain control, in turn, enhances sensitivity for the concentration rate according to variations in the duration of the fluctuation period. This efficiently represents the fluctuation of concentration changes in the environmental context in which such changes occur.

Characterization of olfactory sensory neurons in the striped ambrosia beetle Trypodendron lineatum.

Introduction: The striped ambrosia beetle Trypodendron lineatum (Coleoptera, Curculionidae, Scolytinae) is a major forest pest in the Holarctic region. It uses an aggregation pheromone and host and non-host volatiles to locate suitable host trees, primarily stressed or dying conifer trees. The beetles bore into the xylem and inoculate spores of their obligate fungal mutualist Phialophoropsis ferruginea inside their excavated egg galleries, with the fungus serving as the main food source for the developing larvae. Olfactory sensory neuron (OSN) responses to pheromones and host volatiles are poorly understood in T. lineatum and other ambrosia beetles, and nothing is known about potential responses to fungal volatiles. Methods: We screened responses of OSNs present in 170 antennal olfactory sensilla using single sensillum recordings (SSR) and 57 odor stimuli, including pheromones, host and non-host compounds, as well as volatiles produced by P. ferruginea and fungal symbionts of other scolytine beetles. Results and Discussion: Thirteen OSN classes were characterized based on their characteristic response profiles. An OSN class responding to the aggregation pheromone lineatin was clearly the most abundant on the antennae. In addition, four OSN classes responded specifically to volatile compounds originating from the obligate fungal mutualist and three responded to non-host plant volatiles. Our data also show that T. lineatum has OSN classes tuned to pheromones of other bark beetles. Several OSN classes showed similar response profiles to those previously described in the sympatric bark beetle Ips typographus, which may reflect their shared ancestry.

Identification of a Female-Produced Sex Attractant Pheromone of the Winter Firefly, Photinus corruscus Linnaeus (Coleoptera: Lampyridae)

Firefly flashes are well-known visual signals used by these insects to find, identify, and choose mates. However, many firefly species have lost the ability to produce light as adults. These “unlighted” species generally lack developed adult light organs, are diurnal rather than nocturnal, and are believed to use volatile pheromones acting over a distance to locate mates. While cuticular hydrocarbons, which may function in mate recognition at close range, have been examined for a handful of the over 2000 extant firefly species, no volatile pheromone has ever been identified. In this study, using coupled gas chromatography - electroantennographic detection, we detected a single female-emitted compound that elicited antennal responses from wild-caught male winter fireflies, Photinus corruscus. The compound was identified as (1S)-exo-3-hydroxycamphor (hydroxycamphor). In field trials at two sites across the species’ eastern North American range, large numbers of male P. corruscus were attracted to synthesized hydroxycamphor, verifying its function as a volatile sex attractant pheromone. Males spent more time in contact with lures treated with synthesized hydroxycamphor than those treated with solvent only in laboratory two-choice assays. Further, using single sensillum recordings, we characterized a pheromone-sensitive odorant receptor neuron in a specific olfactory sensillum on male P. corruscus antennae and demonstrated its sensitivity to hydroxycamphor. Thus, this study has identified the first volatile pheromone and its corresponding sensory neuron for any firefly species, and provides a tool for monitoring P. corruscus populations for conservation and further inquiry into the chemical and cellular bases for sexual communication among fireflies.

Antennal and maxillary palp morphology, and sensillar equipment, of the spruce bark beetle predators, Medetera signaticornis and Medetera infumata (Diptera: Dolichopodidae)

Many long-legged Medetera flies are natural enemies of bark beetle pests, which they detect using olfactory cues, likely through olfactory sensilla on the antennae and maxillary palps. Morphological characterisation of olfactory sensilla among insects can provide a basis for future taxonomic, phylogenetic or electrophysiological studies. Scanning electron microscopy was used to describe the morphology of olfactory organs and sensillar equipment of Medetera signaticornis and M. infumata. Three different olfactory sensillum types were found in both fly species, sensilla trichodea, s. basiconica and grooved pegs. Based on size and wall structure, s. trichodea and s. basiconica were categorised into different subtypes. Sharp-tipped curved s. trichodea, and small, large and thin s. basiconica were found on the antennal postpedicel of M. signaticornis adults, while grooved s. basiconica were found in M. infumata. The density of sharp-tipped long s. trichodea was significantly higher in males compared to females, and in M. signaticornis compared to M. infumata. Long-grooved s. basiconica were found grouped in a small pit on the maxillary palps of both species. Comparison of our results with the limited available ecological data suggests that differences in numbers of specific sensillum types may reflect adaptations related to olfactory-driven behaviours such as host seeking.

Identification and analysis of odorant receptors expressed in the two main olfactory organs, antennae and palps, of Schistocerca americana

Locusts depend upon their sense of smell and provide useful models for understanding olfaction. Extending this understanding requires knowledge of the molecular and structural organization of the olfactory system. Odor sensing begins with olfactory receptor neurons (ORNs), which express odorant receptors (ORs). In insects, ORNs are housed, in varying numbers, in olfactory sensilla. Because the organization of ORs within sensilla affects their function, it is essential to identify the ORs they contain. Here, using RNA sequencing, we identified 179 putative ORs in the transcriptomes of the two main olfactory organs, antenna and palp, of the locust Schistocerca americana. Quantitative expression analysis showed most putative ORs (140) are expressed in antennae while only 31 are in the palps. Further, our analysis identified one OR detected only in the palps and seven ORs that are expressed differentially by sex. An in situ analysis of OR expression suggested ORs are organized in non-random combinations within antennal sensilla. A phylogenetic comparison of OR predicted protein sequences revealed homologous relationships among two other Acrididae species. Our results provide a foundation for understanding the organization of the first stage of the olfactory system in S. americana, a well-studied model for olfactory processing.

Antennal sensory structures of Phenacoccus solenopsis (Hemiptera: Pseudococcidae)

BackgroundThe cotton mealybug Phenacoccus solenopsis Tinsley (Hemiptera: Pseudococcidae) is one of the most devastating sap-sucking pests of cultivated plants. The success of P. solenopsis is attributable to its ecological resilience and insecticide resistance, making its control extremely difficult and expensive. Thus, alternative safe approaches are needed to prevent the pest population from reaching the economic threshold. One of these novel approaches is based on the fact that chemical communication via the olfactory system drives critical behaviors required for the survival and development of the species. This knowledge can be useful for controlling insect pests using traps based on semiochemicals. The antennae of insects are an invaluable model for studying the fundamentals of odor perception. Several efforts have been made to investigate the histological and ultrastructural organization of the olfactory organs, such as the antennae and maxillary palps, in many insect species. However, studies on the antennal sensory structures of Phenacoccus species are lacking. Furthermore, although enormous progress has been made in understanding the antennal structures of many mealybug species, the olfactory sensilla in the antennae of P. solenopsis have not yet been described. In this study, we describe, for the first time, the morphology and distribution of the antennal sensilla in male and female P. solenopsis using scanning electron microscopy.ResultsOur results revealed that the entire antennae length and the number of flagellar segments were different between the sexes. Eight morphological types of sensilla were identified on male antennae: trichoid sensilla, chaetic sensilla (three subtypes), basiconic sensilla (two subtypes), and campaniform sensilla (two subtypes). Six morphological types of sensilla were found on female antennae. Sensilla chaetica of subtype 2 and campaniform sensilla of subtype 1 were distributed only on male antennae, suggesting that these sensilla are involved in the recognition of female sex pheromones. The subtype 1 of sensilla chaetica was significantly more abundant on female antennae than on male ones, while subtype 3 was only located on the terminal flagellar segment of the antenna in both sexes.ConclusionsThis study provides insightful information for future electrophysiological and behavioral studies on chemical communication in insects, particularly the cotton mealybug, P. solenopsis that could help in developing new strategies for controlling this economically important insect species.

Recording from Fly Olfactory Sensilla.

Olfactory systems detect and discriminate an enormous diversity of volatile environmental stimuli and provide important paradigms to investigate how sensory cues are represented in the brain. Key stimulus-coding events occur in peripheral olfactory sensory neurons, which typically express a single olfactory receptor-from a large repertoire encoded in the genome-with a defined ligand-response profile. These receptors convert odor ligand recognition into spatial and temporal patterns of neural activity that are transmitted to, and interpreted in, central brain regions. Drosophila provides an attractive model to study olfactory coding because it possesses a relatively simple peripheral olfactory system that displays many organizational parallels to those of vertebrates. Moreover, nearly all olfactory sensory neurons have been molecularly characterized and are accessible for physiological analysis, as they are exposed on the surface of sensory organs (antennae and maxillary palps) housed in specialized hairs called sensilla. This protocol describes how to perform recordings of odor-evoked activity from Drosophila olfactory sensilla, covering the basics of sample preparation, setting up the electrophysiology rig, assembling an odor stimulus-delivery device, and data analysis. The methodology can be used to characterize the ligand-recognition properties of most olfactory sensory neurons and the role of olfactory receptors (and other molecular components) in signal transduction.

Activation of pheromone-sensitive olfactory neurons by plant volatiles in the moth Agrotis ipsilon does not occur at the level of the pheromone receptor protein

In moths, mate finding relies on female-emitted sex pheromones that the males have to decipher within a complex environmental odorant background. Previous studies have shown that interactions of both sex pheromones and plant volatiles can occur in the peripheral olfactory system, and that some plant volatiles can activate the pheromone-specific detection pathway. In the noctuid moth Agrotis ipsilon, plant volatiles such as heptanal activate the receptor neurons tuned to the pheromone component (Z)7-12:OAc. However, the underlying mechanisms remain totally unknown. Following the general rule that states that one olfactory receptor neuron usually expresses only one type of receptor protein, a logic explanation would be that the receptor protein expressed in (Z)7-12:OAc-sensitive neurons recognizes both pheromone and plant volatiles. To test this hypothesis, we first annotated odorant receptor genes in the genome of A. ipsilon and we identified a candidate receptor putatively tuned to (Z)7-12:OAc, named AipsOR3. Then, we expressed it in Drosophila olfactory neurons and determined its response spectrum to a large panel of pheromone compounds and plant volatiles. Unexpectedly, the receptor protein AipsOR3 appeared to be very specific to (Z)7-12:OAc and was not activated by any of the plant volatiles tested, including heptanal. We also found that (Z)7-12:OAc responses of Drosophila neurons expressing AipsOR3 were not affected by a background of heptanal. As the Drosophila olfactory sensilla that house neurons in which AipsOR3 was expressed contain other olfactory proteins – such as odorant-binding proteins – that may influence its selectivity, we also expressed AipsOR3 in Xenopus oocytes and confirmed its specificity and the lack of activation by plant volatiles. Altogether, our results suggest that a still unknown second odorant receptor protein tuned to heptanal and other plant volatiles is expressed in the (Z)7-12:OAc-sensitive neurons of A. ipsilon.

Microstructures at the distal tip of ant chemosensory sensilla

Ants and other eusocial insects emit and receive chemical signals to communicate important information within the colony. In ants, nestmate recognition, task allocation, and reproductive distribution of labor are largely mediated through the detection of cuticular hydrocarbons (CHCs) that cover the exoskeleton. With their large size and limited volatility, these CHCs are believed to be primarily detected through direct contact with the antennae during behavioral interactions. Here we first use scanning electron microscopy to investigate the unique morphological features of CHC-sensitive basiconic sensilla of two ant species, the black carpenter ant Camponotus pennsylvanicus and the Indian jumping ant Harpegnathos saltator. These basiconic sensilla possess an abundance of small pores typical of most insect olfactory sensilla, but also have a large concave depression at the terminal end. Basiconic sensilla are enriched at the distal segments of the antennae in both species, which aligns with their proposed role in contact chemosensation of CHCs. A survey of these sensilla across additional ant species shows varied microstructures at their tips, but each possess surface textures that would also increase sensory surface area. These unique ant chemosensory sensilla represent yet another example of how specialized structures have evolved to serve the functional requirements of eusocial communication.