Non-palm Plant Volatile α-Pinene Is Detected by Antenna-Biased Expressed Odorant Receptor 6 in the Rhynchophorus ferrugineus (Olivier) (Coleoptera: Curculionidae)

Identification of chemosensory genes and antennal sensilla in Nassophasis sp. (Coleoptera: Rhynchophorinae)

Phytophagous insects rely on plant volatiles to select and locate hosts for feeding or reproduction and their olfactory system is essential for detecting plant volatiles. The stem-boring pest, Nassophasis sp. damages Dendrobium and causes economic losses. Currently, there are no effective methods for its control. However, understanding the morphological and molecular basis of its olfactory system may identify new pathways for their management and control. In this study, we observed the stemborer's antennal sensilla using scanning electron microscopy, and transcriptome sequencing was undertaken to annotate and analyze its chemosensory genes. Results showed that the antennal morphology is similar between males and females, with five types of antennal sensilla observed: sensilla chaetica (SC), sensilla trichodea (ST), sensilla brush (SB), sensilla basiconica (SBA) and sensilla gemmiformium (SG). Sexual dimorphism was not observed in sensilla type, but in the length of SBA and SG. A total of 70 olfactory-related genes were annotated, including 16 odorant binding proteins (OBP), 5 chemosensory proteins (CSPs), 26 olfactory receptors (ORs), 9 gustatory receptors (GRs), 10 ionotropic receptors (IRs), and 4 sensory neuron membrane proteins (SNMPs). Most genes were highly expressed and 14 of these genes were only expressed in the head, and 7 genes in the abdomen. This study provides a theoretical basis for the olfactory perception of Nassophasis sp. and a scientific basis for developing new pest control strategies.

Candidate membrane protein gene families related to chemoreception in a wood-boring beetle, Pharsalia antennata Gahan (Coleoptera: Cerambycidae)

The longhorned beetles are key players for the maintenance of biodiversity in the terrestrial ecosystem. As xylophagous cerambycid insects in Coleoptera, the beetles have evolved specialized olfactory and gustatory systems to recognize chemical cues in the surrounding habitats. Despite over 36,000 described species in the Cerambycidae family including a wood-boring pest Pharsalia antennata, only a limited number of them (<1 %) have been characterized regarding their chemical ecology at the molecular level. Here, we surveyed four membrane protein gene families in P. antennata related to chemoreception through transcriptomics, phylogenetics and expression profiling analyses. In total, 144 genes encoding 72 odorant receptors (ORs), 33 gustatory receptors (GRs), 23 ionotropic receptors (IRs), four sensory neuron membrane proteins (SNMPs) and 12 ionotropic glutamate receptors (iGluRs) were harvested from the transcriptome of multiple tissues including antennae and legs of both sexes. The lineage-specific expansion of PantORs possibly implied a diverse range of host plants in this beetle, supporting this correlation between the host range and olfactory receptor repertoire sizes across cerambycid species. Further phylogenetic analysis revealed that Group 2 was contributed mainly to the large OR gene repertoire in P. antennata, representing 18 genes in Group 2A and eight in Group 2B. On the other hand, some key chemosensory genes were identified by applying a phylogenetics approach, such as PantOR21 close to the 2-phenylethanol receptor in Megacyllene caryae, three carbon dioxide GRs and seven Antennal IRs (A-IRs) clades. We also determined sex- and tissue-specific expression profiles of 69 chemosensory genes, revealing the high expression of most PantORs in antennae. Noticeably, 10 sex-biased genes (six PantORs, three PantIRs and PantSNMP1a) were presented in antennae, five sex-biased PantGRs in legs and 39 sex-biased genes (15 PantORs, 13 PantGRs, eight PantIRs and three PantSNMPs) in abdomens. These findings have greatly enhanced our knowledge about the chemical ecology of P. antennata and identify candidate molecular targets for mediating smell and taste of this beetle.

Circabidian rhythm of sex pheromone reception in a scarab beetle

Animals have endogenous clocks that regulate their behavior and physiology. These clocks rely on environmental cues (time givers) that appear approximately every 24 h due to the Earth's rotation; thus, most insects exhibit a circadian rhythm. One notable exception is the scarab beetle, Holotrichia parallela, a severe agricultural pest in China, Japan, South Korea, and India. Females emerge from the soil every other night, reach the canopy of host plants, evert an abdominal gland, and release a pheromone bouquet comprising l-isoleucine methyl ester (LIME) and l-linalool. To determine whether this circa'bi'dian rhythm affects the olfactory system, we aimed to identify H. parallela sex pheromone receptor(s) and study their expression patterns. We cloned 14 odorant receptors (ORs) and attempted de-orphanizing them in the Xenopus oocyte recording system. HparOR14 gave robust responses to LIME and smaller responses to l-linalool. Structural modeling, tissue expression profile, and RNAi treatment followed by physiological and behavioral studies support that HparOR14 is a sex pheromone receptor-the first of its kind discovered in Coleoptera. Examination of the HparOR14 transcript levels throughout the adult's life showed that on sexually active days, gene expression was significantly higher in the scotophase than in the photophase. Additionally, the HparOR14 expression profile showed a circabidian rhythm synchronized with the previously identified pattern of sex pheromone emission. 48 h of electroantennogram recordings showed that responses to LIME were abolished on non-calling nights. In contrast, responses to the green leaf volatile (Z)-3-henexyl acetate remained almost constant throughout the recording period.

Taxonomy, Biology, Symbionts, Omics, and Management of Rhynchophorus Palm Weevils (Coleoptera: Curculionidae: Dryophthorinae)

Palm weevils, Rhynchophorus spp., are destructive pests of native, ornamental, and agricultural palm species. Of the 10 recognized species, two of the most injurious species, Rhynchophorus ferrugineus and Rhynchophorus palmarum, both of which have spread beyond their native range, are the best studied. Due to its greater global spread and damage to edible date industries in the Middle East, R. ferrugineus has received more research interest. Integrated pest management programs utilize traps baited with aggregation pheromone, removal of infested palms, and insecticides. However, weevil control is costly, development of resistance to insecticides is problematic, and program efficacy can be impaired because early detection of infestations is difficult. The genome of R. ferrugineus has been sequenced, and omics research is providing insight into pheromone communication and changes in volatile and metabolism profiles of weevil-infested palms. We outline how such developments could lead to new control strategies and early detection tools.

Host plant recognition by two odorant-binding proteins in Rhynchophorus ferrugineus (Coleoptera: Curculionidae)

BACKGROUND

Rhynchophorus ferrugineus, the red palm weevil (RPW), is a key pest that attacks many economically important palm species and that has evolved a sensitive and specific olfactory system to seek palm hosts. Odorant-binding proteins (OBPs) not only play crucial roles in its olfactory perception process but are also important molecular targets for the development of new approaches for pest management.

RESULTS

Analysis of the tissue expression profiles of RferOBP8 and RferOBP11 revealed that these two Rhynchophorus ferrugineus odorant binding proteins (RferOBPs) exhibited high expression in the antennae and showed sexual dimorphism. We analyzed the volatiles of seven host plants by gas chromatography-mass spectrometry and screened 13 potential ligands by molecular docking. The binding affinity of two recombinant OBPs to aggregation pheromones and 13 palm odorants was tested by fluorescence competitive binding assays. The results revealed that eight tested palm volatiles and ferrugineol have high binding affinities with RferOBP8 or RferOBP11. Behavioral trials showed that these eight odor compounds could elicit an attraction response in adult RPW. RNA interference analysis indicated that the reduction in the expression levels of the two RferOBPs led to a decrease in behavioral responses to these volatiles.

CONCLUSION

These results suggest that RferOBP8 and RferOBP11 are involved in mediating the responses of RPW to palm volatiles and to aggregation pheromones and may play important roles in RPW host-seeking. This study also provides a theoretical foundation for the promising application of novel molecular targets in the development of new behavioral interference strategies for RPW management in the future. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Chemosensory Protein 2 of Male Athetis lepigone Is Involved in the Perception of Sex Pheromones and Maize Volatiles

In moths, the interactions between chemosensory proteins (CSPs) and sex pheromones have yet to be comprehensively investigated. Here, we examined the function of AlepCSP2 in male Athetis lepigone based on protein expression, molecular docking, site-directed mutagenesis, fluorescence competitive binding analyses, and RNA interference (RNAi) experiments. We found that AlepCSP2 showed strong binding affinity for two sex pheromones and five maize volatiles and that binding was optimal under neutral conditions. Furthermore, we identified six amino acids as being key residues involved in the interaction between AlepCSP2 and multiple ligands. Further RNAi showed that siCSP2 males displayed consistently lower electroantennography responses to two sex pheromones and three maize volatiles at different dosages tested, and the mating rate also decreased significantly by 37.50%. These findings will contribute to characterizing the binding mechanisms of moth CSPs to sex pheromones and host volatiles and also identify unique targets for developing novel pest behavior disruptors.

Genome-Wide Identification of the Odorant Receptor Gene Family and Revealing Key Genes Involved in Sexual Communication in Anoplophora glabripennis

Insects use a powerful and complex olfactory recognition system to sense odor molecules in the external environment to guide behavior. A large family of odorant receptors (ORs) mediates the detection of pheromone compounds. Anoplophora glabripennis is a destructive pest that harms broad-leaved tree species. Although olfactory sensation is an important factor affecting the information exchange of A. glabripennis, little is known about the key ORs involved. Here, we identified ninety-eight AglaORs in the Agla2.0 genome and found that the AglaOR gene family had expanded with structural and functional diversity. RT-qPCR was used to analyze the expression of AglaORs in sex tissues and in adults at different developmental stages. Twenty-three AglaORs with antennal-biased expression were identified. Among these, eleven were male-biased and two were female-biased and were more significantly expressed in the sexual maturation stage than in the post-mating stage, suggesting that these genes play a role in sexual communication. Relatively, two female-biased AglaORs were overexpressed in females seeking spawning grounds after mating, indicating that these genes might be involved in the recognition of host plant volatiles that may regulate the selection of spawning grounds. Our study provides a theoretical basis for further studies into the molecular mechanism of A. glabripennis olfaction.

Prediction of a conserved pheromone receptor lineage from antennal transcriptomes of the pine sawyer genus Monochamus (Coleoptera: Cerambycidae)

Longhorned beetles (Cerambycidae) are a diverse family of wood-boring insects, many species of which produce volatile pheromones to attract mates over long distances. The composition and structure of the pheromones remain constant across many cerambycid species, and comparative studies of those groups could, therefore, reveal the chemoreceptors responsible for pheromone detection. Here, we use comparative transcriptomics to identify a candidate pheromone receptor in the large and economically important cerambycid genus Monochamus, males of which produce the aggregation-sex pheromone 2-(undecyloxy)-ethanol ("monochamol"). Antennal transcriptomes of the North American species M. maculosus, M. notatus, and M. scutellatus revealed 60-70 odorant receptors (ORs) in each species, including four lineages of simple orthologs that were highly conserved, highly expressed in both sexes, and upregulated in the flagellomeres where olfactory sensilla are localized. Two of these orthologous lineages, OR29 and OR59, remained highly expressed and conserved when we included a re-annotation of an antennal transcriptome of the Eurasian congener M. alternatus. OR29 is also orthologous to a characterized pheromone receptor in the cerambycid Megacyllene caryae, suggesting it as the most likely candidate for a monochamol receptor and highlighting its potential as a conserved lineage of pheromone receptors within one of the largest families of beetles.

Expression Profiling and Functional Analysis of Candidate Odorant Receptors in Galeruca daurica

Simple Summary

Odorant receptors (ORs) play an important role in the olfactory system in insects. However, there is no functional research on the odorant receptors of Galeruca daurica. In this study, 21 OR genes were identified from the transcriptome database of G. daurica adults. Most GdauORs were mainly expressed in antennae, and the expression levels of GdauORs in adults were affected by age. When GdauOR4, GdauOR15, and GdauORco were silenced by RNAi, the electrophysiological responses to host plant volatiles were significantly decreased.

Abstract

Galeruca daurica (Joannis) is an oligophagous pest in the grasslands of Inner Mongolia, China, which feed mainly on Allium spp. Odorant receptors (ORs) play an important role in the olfactory system in insects, and function together with olfactory co-receptor (ORco). In this study, 21 OR genes were identified from the transcriptome database of G. daurica adults, and named GdauOR1-20 and GdauORco. The expression profiles were examined by RT-qPCR and RNA interference (RNAi) and electroantennogram (EAG) experiments were conducted to further identify the olfactory functions of GdauOR4, GdauOR11, GdauOR15, and GdauORco. It was found that 15 GdauORs (OR1, OR3-6, OR8, OR11-13, OR15, OR17-20, and ORco) were mainly expressed in antennae, and the expression levels of GdauORs in adults were affected by age. When GdauOR4, GdauOR15, and GdauORco were silenced by RNAi, the electrophysiological responses to host plant volatiles were significantly decreased in G. daurica. This study lays a necessary foundation for clarifying the mechanism on finding host plants in G. daurica.

Odorant receptor orthologues in conifer-feeding beetles display conserved responses to ecologically relevant odors

Insects are able to detect a plethora of olfactory cues using a divergent family of odorant receptors (ORs). Despite the divergent nature of this family, related species frequently express several evolutionarily conserved OR orthologues. In the largest order of insects, Coleoptera, it remains unknown whether OR orthologues have conserved or divergent functions in different species. Using HEK293 cells, we addressed this question through functional characterization of two groups of OR orthologues in three species of the Curculionidae (weevil) family, the conifer‐feeding bark beetles Ips typographus L. (“Ityp”) and Dendroctonus ponderosae Hopkins (“Dpon”) (Scolytinae), and the pine weevil Hylobius abietis L. (“Habi”; Molytinae). The ORs of H. abietis were annotated from antennal transcriptomes. The results show highly conserved response specificities, with one group of orthologues (HabiOR3/DponOR8/ItypOR6) responding exclusively to 2‐phenylethanol (2‐PE), and the other group (HabiOR4/DponOR9/ItypOR5) responding to angiosperm green leaf volatiles (GLVs). Both groups of orthologues belong to the coleopteran OR subfamily 2B, and share a common ancestor with OR5 in the cerambycid Megacyllene caryae, also tuned to 2‐PE, suggesting a shared evolutionary history of 2‐PE receptors across two beetle superfamilies. The detected compounds are ecologically relevant for conifer‐feeding curculionids, and are probably linked to fitness, with GLVs being used to avoid angiosperm nonhost plants, and 2‐PE being important for intraspecific communication and/or playing a putative role in beetle–microbe symbioses. To our knowledge, this study is the first to reveal evolutionary conservation of OR functions across several beetle species and hence sheds new light on the functional evolution of insect ORs.

The Genome of Rhyzopertha dominica (Fab.) (Coleoptera: Bostrichidae): Adaptation for Success

The lesser grain borer, Rhyzopertha dominica (F.) (Coleoptera: Bostrichidae), is a major global pest of cereal grains. Infestations are difficult to control as larvae feed inside grain kernels, and many populations are resistant to both contact insecticides and fumigants. We sequenced the genome of R. dominica to identify genes responsible for important biological functions and develop more targeted and efficacious management strategies. The genome was assembled from long read sequencing and long-range scaffolding technologies. The genome assembly is 479.1 Mb, close to the predicted genome size of 480.4 Mb by flow cytometry. This assembly is among the most contiguous beetle assemblies published to date, with 139 scaffolds, an N₅₀ of 53.6 Mb, and L₅₀ of 4, indicating chromosome-scale scaffolds. Predicted genes from biologically relevant groups were manually annotated using transcriptome data from adults and different larval tissues to guide annotation. The expansion of carbohydrase and serine peptidase genes suggest that they combine to enable efficient digestion of cereal proteins. A reduction in the copy number of several detoxification gene families relative to other coleopterans may reflect the low selective pressure on these genes in an insect that spends most of its life feeding internally. Chemoreceptor genes contain elevated numbers of pseudogenes for odorant receptors that also may be related to the recent ontogenetic shift of R. dominica to a diet consisting primarily of stored grains. Analysis of repetitive sequences will further define the evolution of bostrichid beetles compared to other species. The data overall contribute significantly to coleopteran genetic research.