Two single-point mutations shift the ligand selectivity of a pheromone receptor between two closely related moth species

Functional Investigation of the Receptor to the Major Pheromone Component in the C-Strain and the R-Strain of the Fall Armyworm Spodoptera frugiperda

The fall armyworm Spodoptera frugiperda (Lepidoptera: Noctuidae) is an important invasive polyphagous crop pest that has been invading the world since 2016. This species consists of two strains adapted to different host plants, the corn strain and the rice strain, which also exhibit differences in their mating behavior, pheromone composition, and pheromone receptor sequences. A way to monitor invasion and control this pest is the use of synthetic sex pheromones to trap adults and disturb the mate-finding process via the release of large amounts of pheromones in the air. However, the efficiency of these methods depends on the specificity and sensitivity of the corresponding pheromone receptors. Yet, only pheromone receptors of the corn strain have been characterized, and nothing is known about the specificity and sensitivity of the rice strain orthologues. To address this gap, we functionally expressed the receptors to the major sex pheromone component of the two strains in Drosophila olfactory sensory neurons and challenged them with a large panel of pheromone compounds using single-sensillum recordings. Although their sequences present subtle mutations, we revealed that they share similar response spectra and sensitivity. The implications of these results on pheromone-based pest management strategies are discussed.

Identification and functional characterization of chemosensory genes in olfactory and taste organs of Spodoptera litura (Lepidoptera: Noctuidae)

The tobacco cutworm Spodoptera litura is one of the most destructive polyphagous crop pests. Olfaction and taste play a crucial role in its host plant selection and sexual communication, but the expression profile of chemosensory genes remains unclear. In this study, we identified 185 chemosensory genes from 7 organs in S. litura by transcriptome sequencing, of which 72 genes were published for the first time, including 27 odorant receptors (ORs), 26 gustatory receptors (GRs), 1 ionotropic receptor (IR), 16 odorant-binding proteins (OBPs), and 2 chemosensory proteins (CSPs). Phylogenetic analyses revealed that ORs, IRs, OBPs, and sensory neuron membrane proteins (SNMPs) were mainly expressed in antennae and sequence-conserved among Noctuidae species. The most differentially expressed genes (DEGs) between sexes were ORs and OBPs, and no DEGs were found in GRs. GR transcripts were enriched in proboscis, and the expression of sugar receptors was the highest. Carbon dioxide receptors, sugar receptor-SliuGR6, and bitter GRs-SlituGR43 and SlituGR66 had higher sequence identities between Noctuidae species. CSPs were broadly expressed in various organs, and SlituCSP13 was a DEG in adult antennae. The functional analysis in the Drosophila OR67d expression system found that SlituOR50, a receptor highly expressed in female antennae, is selectively tuned to farnesyl acetate. The results provide a solid foundation for understanding the molecular mechanisms by which chemosensory genes operate to elicit behavioral responses in polyphagous insects.

Peripheral Coding of Sex Pheromones in the Tomato Leaf Miner, Phthorimaea absoluta (Meyrick)

Phthorimaea absoluta releases (E3,Z8,Z11)-tetradeca-3,8,11-trienyl acetate (E3,Z8,Z11-14:OAc) and (E3,Z8)-tetradeca-3,8-dienyl acetate (E3,Z8-14:OAc) with a ratio of 90:10 as the sex pheromone. However, how this pest uses pheromone receptors (PRs) to detect the two pheromone components is unknown. Here, we functionally characterize the PR repertoire of P. absoluta. First, we identified five putative PRs by transcriptome sequencing, i.e., PabsOR4, PabsOR8, PabsOR12a, PabsOR14, and PabsOR17. These receptors are predominantly expressed in the male antennae. Next, we expressed them in Drosophila OR67 neurons and investigated their responses. PabsOR14 and PabsOR8 selectively respond to the main component, E3,Z8,Z11-14:OAc with different sensitivities, while PabsOR17 is tuned to the minor component, E3,Z8-14:OAc. In addition, PabsOR4 weakly responds to both sex pheromone components. Moreover, PabsOR17 and PabsOR4 potently respond to a non-sex pheromone compound, (Z)-7-dodecenyl acetate (Z7-12:OAc). Lastly, we demonstrated that Z7-12:OAc can replace E3,Z8-14:OAc to attract virgin males. Our findings elucidate the peripheral coding of the sex pheromone in P. absoluta, providing a new perspective for controlling it.

Functional Characterization Supports Multiple Evolutionary Origins of Pheromone Receptors in Bark Beetles

Chemical communication using pheromones is thought to have contributed to the diversification and speciation of insects. The species-specific pheromones are detected by specialized pheromone receptors (PRs). Whereas the evolution and function of PRs have been extensively studied in Lepidoptera, only a few PRs have been identified in beetles, which limits our understanding of their evolutionary histories and physiological functions. To shed light on these questions, we aimed to functionally characterize potential PRs in the spruce bark beetle Ips typographus ("Ityp") and explore their evolutionary origins and molecular interactions with ligands. Males of this species release an aggregation pheromone comprising 2-methyl-3-buten-2-ol and (4S)-cis-verbenol, which attracts both sexes to attacked trees. Using two systems for functional characterization, we show that the highly expressed odorant receptor (OR) ItypOR41 responds specifically to (4S)-cis-verbenol, with structurally similar compounds eliciting minor responses. We next targeted the closely related ItypOR40 and ItypOR45. Whereas ItypOR40 was unresponsive, ItypOR45 showed an overlapping response profile with ItypOR41, but a broader tuning. Our phylogenetic analysis shows that these ORs are present in a different OR clade as compared to all other known beetle PRs, suggesting multiple evolutionary origins of PRs in bark beetles. Next, using computational analyses and experimental validation, we reveal two amino acid residues (Gln179 and Trp310) that are important for ligand binding and pheromone specificity of ItypOR41 for (4S)-cis-verbenol, possibly via hydrogen bonding to Gln179. Collectively, our results shed new light on the origins, specificity, and ligand binding mechanisms of PRs in beetles.

A conserved pheromone receptor in the American and the Asian palm weevils is also activated by host plant volatiles

The evolution of chemosensory receptors is key for the adaptation of animals to their environment. Recent knowledge acquired on the tri-dimensional structure of insect odorant receptors makes it possible to study the link between modifications in the receptor structure and evolution of response spectra in more depth. We investigated this question in palm weevils, several species of which are well-known invasive pests of ornamental or cultivated palm trees worldwide. These insects use aggregation pheromones to gather on their host plants for feeding and reproduction. An odorant receptor detecting the aggregation pheromone components was characterised in the Asian palm weevil Rhynchophorus ferrugineus. This study compared the response spectra of this receptor, RferOR1, and its ortholog in the American palm weevil R. palmarum, RpalOR1. Sequences of these two receptors exhibit more than 70 amino acid differences, but modelling of their 3D structures revealed that their putative binding pockets differ by only three amino acids, suggesting possible tuning conservation. Further functional characterization of RpalOR1 confirmed this hypothesis, as RpalOR1 and RferOR1 exhibited highly similar responses to coleopteran aggregation pheromones and chemically related molecules. Notably, we showed that R. ferrugineus pheromone compounds strongly activated RpalOR1, but we did not evidence any response to the R. palmarum pheromone compound rhynchophorol. Moreover, we discovered that several host plant volatiles also activated both pheromone receptors, although with lower sensitivity. This study not only reveals evolutionary conservation of odorant receptor tuning across the two palm weevil species, but also questions the specificity of pheromone detection usually observed in insects.

Characterization of the pheromone receptors in Mythimna loreyi reveals the differentiation of sex pheromone recognition in Mythimna species

Pheromone receptors (PRs) are key proteins in the molecular mechanism of pheromone recognition, and exploring the functional differentiation of PRs between closely related species helps to understand the evolution of moth mating systems. Pheromone components of the agricultural pest Mythimna loreyi have turned into (Z)-9-tetradecen-1-yl acetate (Z9-14:OAc), (Z)-7-dodecen-1-yl acetate (Z7-12:OAc), and (Z)-11-hexadecen-1-yl acetate, while the composition differs from that of M. separata in the genus Mythimna. To understand the molecular mechanism of pheromone recognition, we sequenced and analyzed antennal transcriptomes to identify 62 odorant receptor (OR) genes. The expression levels of all putative ORs were analyzed using differentially expressed gene analysis. Six candidate PRs were quantified and functionally characterized in the Xenopus oocytes system. MlorPR6 and MlorPR3 were determined to be the receptors of major and minor components Z9-14:OAc and Z7-12:OAc. MlorPR1 and female antennae (FA)-biased MlorPR5 both possessed the ability to detect pheromones of sympatric species, including (Z,E)-9,12-tetradecadien-1-ol, (Z)-9-tetradecen-1-ol, and (Z)-9-tetradecenal. Based on the comparison of PR functions between M. loreyi and M. separata, we analyzed the differentiation of pheromone recognition mechanisms during the evolution of the mating systems of 2 Mythimna species.

Discovery of Insect Attractants Based on the Functional Analyses of Female-Biased Odorant Receptors and Their Orthologs in Two Closely Related Species

Olfaction plays an instrumental role in host plant selection by phytophagous insects. Helicoverpa assulta and Helicoverpa armigera are two closely related moth species with different host plant ranges. In this study, we first comparatively analyzed the function of 11 female-biased odorant receptors (ORs) and their orthologs in the two species by the Drosophila T1 neuron expression system and then examined the electroantennography responses of the two species to the most effective OR ligands. Behavioral assays using a Y-tube olfactometer indicate that guaiene, the primary ligand of HassOR21-2 and HarmOR21-2, only attracts the females, while benzyl acetone, the main ligand of HassOR35 and HarmOR35, attracts both sexes of the two species. Oviposition preference experiments further confirm that guaiene and benzyl acetone are potent oviposition attractants for the mated females of both species. These findings deepen our understanding of the olfactory coding mechanisms of host plant selection in herbivorous insects and provide valuable attractants for managing pest populations.

Evolutionary shifts in pheromone receptors contribute to speciation in four Helicoverpa species

Male moths utilize their pheromone communication systems to distinguish potential mates from other sympatric species, which contributes to maintaining reproductive isolation and even drives speciation. The molecular mechanisms underlying the evolution of pheromone communication systems are usually studied between closely-related moth species for their similar but divergent traits associated with pheromone production, detection, and/or processing. In this study, we first identified the functional differentiation in two orthologous pheromone receptors, OR14b, and OR16, in four Helicoverpa species, Helicoverpa armigera, H. assulta, H. zea, and H. gelotopoeon. To understand the substrate response specificity of these two PRs, we performed all-atom molecular dynamics simulations of OR14b and OR16 based on AlphaFold2 structural prediction, and molecular docking, allowing us to predict a few key amino acids involved in substrate binding. These candidate residues were further tested and validated by site-directed mutagenesis and functional analysis. These results together identified two hydrophobic amino acids at positions 164 and 232 are the determinants of the response specificity of HarmOR14b and HzeaOR14b to Z9-14:Ald and Z9-16:Ald by directly interacting with the substrates. Interestingly, in OR16 orthologs, we found that position 66 alone determines the specific binding of Z11-16:OH, likely via allosteric interactions. Overall, we have developed an effective integrated method to identify the critical residues for substrate selectivity of ORs and elucidated the molecular mechanism of the diversification of pheromone recognition systems.

Predicted structure of odorant-binding protein 12 from Monochamus alternatus (Hope) suggests a mechanism of flexible odorant-binding

Odorant-binding proteins (OBPs) are thought to bind and deliver hydrophobic odorants from the environment to receptors on insect sensory neurons, and have been used to screen behaviorally active compounds of insects. In order to screen behaviorally active compounds for Monochamus alternatus by OBPs, we cloned full length of Obp12 coding sequence from M. alternatus and proved secretion property of MaltOBP12, then tested binding affinities of recombinant MaltOBP12 to 12 pine volatiles in vitro. We confirmed MaltOBP12 has binding affinities to 9 pine volatiles. The structure of MaltOBP12 and protein-ligand interactions were further analyzed by homology modeling, molecular docking, site-directed mutagenesis, and ligand-binding assays. These results demonstrated that the binding pocket of MaltOBP12 consists of several large aromatic and hydrophobic residues, and four aromatic residues (Tyr50, Phe109, Tyr112, Phe122) are essential for odorant-binding; ligands adopt extensive hydrophobic interactions with an overlapping subset of residues in the binding pocket. Finally, based on non-directional hydrophobic interactions, MaltOBP12 binds odorants flexibly. These findings will not only help us understand how OBPs flexibly bind odorants but also promote to screen of behaviourally active compounds by computer methods to prevent M. alternatus in the future.

A tale of two copies: Evolutionary trajectories of moth pheromone receptors

Pheromone communication is an essential component of reproductive isolation in animals. As such, evolution of pheromone signaling can be linked to speciation. For example, the evolution of sex pheromones is thought to have played a major role in the diversification of moths. In the crop pests Spodoptera littoralis and S. litura, the major component of the sex pheromone blend is (Z,E)-9,11-tetradecadienyl acetate, which is lacking in other Spodoptera species. It indicates that a major shift occurred in their common ancestor. It has been shown recently in S. littoralis that this compound is detected with high specificity by an atypical pheromone receptor, named SlitOR5. Here, we studied its evolutionary history through functional characterization of receptors from different Spodoptera species. SlitOR5 orthologs in S. exigua and S. frugiperda exhibited a broad tuning to several pheromone compounds. We evidenced a duplication of OR5 in a common ancestor of S. littoralis and S. litura and found that in these two species, one duplicate is also broadly tuned while the other is specific to (Z,E)-9,11-tetradecadienyl acetate. By using ancestral gene resurrection, we confirmed that this narrow tuning evolved only in one of the two copies issued from the OR5 duplication. Finally, we identified eight amino acid positions in the binding pocket of these receptors whose evolution has been responsible for narrowing the response spectrum to a single ligand. The evolution of OR5 is a clear case of subfunctionalization that could have had a determinant impact in the speciation process in Spodoptera species.

A mouthpart transcriptome for Spodoptera frugiperda adults: identification of candidate chemoreceptors and investigation of expression patterns

Moth mouthparts, consisting of labial palps and proboscis, not only are the feeding device but also are chemosensory organs for the detection of chemical signals from surrounding environment. Up to now, the chemosensory systems in the mouthpart of moths are largely unknown. Here, we performed systematic analyses of the mouthpart transcriptome of adult Spodoptera frugiperda (Lepidoptera: Noctuidae), a notorious pest that spreads worldwide. A total of 48 chemoreceptors, including 29 odorant receptors (ORs), 9 gustatory receptors (GRs), and 10 ionotropic receptors (IRs), were annotated. Further phylogenetic analyses with these genes and homologs from other insect species determined that specific genes, including ORco, carbon dioxide receptors, pheromone receptor, IR co-receptors, and sugar receptors, were transcribed in the mouthpart of S. frugiperda adults. Subsequently, expression profiling in different chemosensory tissues demonstrated that the annotated ORs and IRs were mainly expressed in S. frugiperda antennae, but one IR was also highly expressed in the mouthparts. In comparison, SfruGRs were mainly expressed in the mouthparts, but 3 GRs were also highly expressed in the antennae or the legs. Further comparison of the mouthpart-biased chemoreceptors using RT-qPCR revealed that the expression of these genes varied significantly between labial palps and proboscises. This study provides the first large-scale description of chemoreceptors in the mouthpart of adult S. frugiperda and provides a foundation for further functional studies of chemoreceptors in the mouthpart of S. frugiperda as well as of other moth species.

Moth sex pheromones affect interspecific competition among sympatric species and possibly population distribution by modulating pre-mating behavior

Premating behaviors mediated by pheromones play pivotal roles in animal mating choices. In natural populations of the striped stem borer Chilo suppressalis and the rice leaf roller Cnaphalocrocis medinalis in the rice field habitat, we discovered that Z11-16:Ald, a major component of the C. suppressalis pheromone, modulated the premating behavior of C. medinalis. Z11-16:Ald evoked a strong olfactory response in male antennae and strongly inhibited the sex pheromone trapping of male C. medinalis in the field. The functions of three C. medinalis sex pheromone receptor genes (CmedPR1-3) were verified through heterologous expression in Xenopus oocytes. CmedPR1 responded to Z11-18:OH and Z11-18:Ald, as well as the interspecific pheromone compound Z11-16:Ac of sympatric species; CmedPR2 responded to Z13-18:OH and Z13-18:Ald, as well as the sex pheromone compounds Z11-16:Ald and Z9-16:Ald of sympatric species; and CmedPR3 responded to Z11-18:OH and Z13-18:OH, as well as the interspecific pheromones Z11-16:OH, Z9-16:Ald, Z11-16:Ac, and Z11-16:Ald of sympatric species. Thus, CmedPR2 and CmedPR3 share the ligand Z11-16:Ald, which is not a component of the C. medinalis sex pheromone. Therefore, the sex pheromones of interspecific species affected the input of neural signals by stimulating the sex pheromone receptors on the antennae of male C. medinalis moths, thereby inhibiting the olfactory responses of the male moths to the sex pheromones. Our results demonstrate chemical communication among sympatric species in the rice field habitat, the recognition of intra- and interspecific sex pheromones by olfactory receptors, and how insect premating behaviors are modulated to possibly affect resource partitioning.

Drosophila olfaction: past, present and future

Among the many wonders of nature, the sense of smell of the fly Drosophila melanogaster might seem, at first glance, of esoteric interest. Nevertheless, for over a century, the 'nose' of this insect has been an extraordinary system to explore questions in animal behaviour, ecology and evolution, neuroscience, physiology and molecular genetics. The insights gained are relevant for our understanding of the sensory biology of vertebrates, including humans, and other insect species, encompassing those detrimental to human health. Here, I present an overview of our current knowledge of D. melanogaster olfaction, from molecules to behaviours, with an emphasis on the historical motivations of studies and illustration of how technical innovations have enabled advances. I also highlight some of the pressing and long-term questions.

Single amino acid residue mediates reciprocal specificity in two mosquito odorant receptors

The southern house mosquito, Culex quinquefasciatus, utilizes two odorant receptors, CquiOR10 and CquiOR2, narrowly tuned to oviposition attractants and well conserved among mosquito species. They detect skatole and indole, respectively, with reciprocal specificity. We swapped the transmembrane (TM) domains of CquiOR10 and CquiOR2 and identified TM2 as a specificity determinant. With additional mutations, we showed that CquiOR10A73L behaved like CquiOR2. Conversely, CquiOR2L74A recapitulated CquiOR10 specificity. Next, we generated structural models of CquiOR10 and CquiOR10A73L using RoseTTAFold and AlphaFold and docked skatole and indole using RosettaLigand. These modeling studies suggested space-filling constraints around A73. Consistent with this hypothesis, CquiOR10 mutants with a bulkier residue (Ile, Val) were insensitive to skatole and indole, whereas CquiOR10A73G retained the specificity to skatole and showed a more robust response than the wildtype receptor CquiOR10. On the other hand, Leu to Gly mutation of the indole receptor CquiOR2 reverted the specificity to skatole. Lastly, CquiOR10A73L, CquiOR2, and CquiOR2L74I were insensitive to 3-ethylindole, whereas CquiOR2L74A and CquiOR2L74G gained activity. Additionally, CquiOR10A73G gave more robust responses to 3-ethylindole than CquiOR10. Thus, we suggest the specificity of these receptors is mediated by a single amino acid substitution, leading to finely tuned volumetric space to accommodate specific oviposition attractants.

Comparison and Functional Analysis of Odorant-Binding Proteins and Chemosensory Proteins in Two Closely Related Thrips Species, Frankliniella occidentalis and Frankliniella intonsa (Thysanoptera: Thripidae) Based on Antennal Transcriptome Analysis

Two closely related thrips species, Frankliniella occidentalis and Frankliniella intonsa, are important pests on agricultural and horticultural crops. They have several similarities, including occurrence patterns, host range, and aggregation pheromone compounds. However, there are very few reports about the chemosensory genes and olfactory mechanisms in these two species. To expand our knowledge of the thrips chemosensory system, we conducted antennal transcriptome analysis of two thrips species, and identified seven odorant-binding proteins (OBPs) and eight chemosensory proteins (CSPs) in F. occidentalis, as well as six OBPs and six CSPs in F. intonsa. OBPs and CSPs showed high sequence identity between the two thrips species. The RT-qPCR results showed that the orthologous genes FoccOBP1/3/4/5/6, FintOBP1/3/4/6, FoccCSP1/2/3, and FintCSP1/2 were highly expressed in male adults. Molecular docking results suggested that orthologous pairs FoccOBP4/FintOBP4, FoccOBP6/FintOBP6, and FoccCSP2/FintCSP2 might be involved in transporting the major aggregation pheromone compound neryl (S)-2-methylbutanoate, while orthologous pairs FoccOBP6/FintOBP6, FoccCSP2/FintCSP2, and FoccCSP3/FintCSP3 might be involved in transporting the minor aggregation pheromone compound (R)-lavandulyl acetate. These results will provide a fundamental basis for understanding the molecular mechanisms of pheromone reception in the two thrips species.

Structural model for ligand binding and channel opening of an insect gustatory receptor

Insect gustatory receptors play roles in sensing tastants, such as sugars and bitter substances. We previously demonstrated that the BmGr9 silkworm gustatory receptor is a d-fructose–gated ion channel receptor. However, the molecular mechanism of how d-fructose could initiate channel opening were unclear. Herein, we present a structural model for a channel pore and a d-fructose–binding site in BmGr9. Since the membrane topology and oligomeric state of BmGr9 appeared to be similar to those of an insect odorant receptor coreceptor, Orco, we constructed a structural model of BmGr9 based on the cryo-EM Orco structure. Our site-directed mutagenesis data suggested that the transmembrane region 7 forms channel pore and controls channel gating. This model also suggested that a pocket formed by transmembrane helices 2 to 4 and 6 binds d-fructose. Using mutagenesis experiments in combination with docking simulations, we were able to determine the potent binding mode of d-fructose. Finally, based on these data, we propose a conformational change that leads to channel opening upon d-fructose binding. Taken together, these findings detail the molecular mechanism by which an insect gustatory receptor can be activated by its ligand molecule.

Mutagenesis of the odorant receptor co-receptor (Orco) reveals severe olfactory defects in the crop pest moth Helicoverpa armigera

BACKGROUND

Odorant receptors (ORs) as odorant-gated ion channels play a crucial role in insect olfaction. They are formed by a heteromultimeric complex of the odorant receptor co-receptor (Orco) and a ligand-selective Or. Other types of olfactory receptor proteins, such as ionotropic receptors (IRs) and some gustatory receptors (GRs), are also involved in the olfactory system of insects. Orco as an obligatory subunit of ORs is highly conserved, providing an opportunity to systematically evaluate OR-dependent olfactory responses.

RESULTS

Herein, we successfully established a homozygous mutant (Orco^-/-) of Helicoverpa armigera, a notorious crop pest, using the CRISPR/Cas9 gene editing technique. We then compared the olfactory response characteristics of wild type (WT) and Orco^-/- adults and larvae. Orco^-/- males were infertile, while Orco^-/- females were fertile. The lifespan of Orco^-/- females was longer than that of WT females. The expressions of most Ors, Irs, and other olfaction-related genes in adult antennae of Orco^-/- moths were not obviously affected, but some of them were up- or down-regulated. In addition, there was no change in the neuroanatomical phenotype of Orco^-/- moths at the level of the antennal lobe (including the macroglomerular complex region of the male). Using EAG and SSR techniques, we discovered that electrophysiological responses of Orco^-/- moths to sex pheromone components and many host plant odorants were absent. The upwind flight behaviors toward sex pheromones of Orco^-/- males were severely reduced in a wind tunnel experiment. The oviposition selectivity of Orco^-/- females to the host plant (green pepper) has completely disappeared, and the chemotaxis toward green pepper was also lost in Orco^-/- larvae.

CONCLUSIONS

Our study indicates that OR-mediated olfaction is essential for pheromone communication, oviposition selection, and larval chemotaxis of H. armigera, suggesting a strategy in which mate searching and host-seeking behaviors of moth pests could be disrupted by inhibiting or silencing Orco expression.

The Genetic Basis of Gene Expression Divergence in Antennae of Two Closely Related Moth Species, Helicoverpa armigera and Helicoverpa assulta

The closely related species Helicoverpa armigera (H. armigera) and Helicoverpa assulta (H. assulta) have different host plant ranges and share two principal components of sex pheromones but with reversed ratios. The antennae are the main olfactory organ of insects and play a crucial role in host plant selection and mate seeking. However, the genetic basis for gene expression divergence in the antennae of the two species is unclear. We performed an allele-specific expression (ASE) analysis in the antennal transcriptomes of the two species and their F₁ hybrids, examining the connection between gene expression divergence and phenotypic differences. The results show that the proportion of genes classified as all cis was higher than that of all trans in males and reversed in females. The contribution of regulatory patterns to gene expression divergence in males was less than that in females, which explained the functional differentiation of male and female antennae. Among the five groups of F₁ hybrids, the fertile males from the cross of H. armigera female and H. assulta male had the lowest proportion of misexpressed genes, and the inferred regulatory patterns were more accurate. By using this group of F₁ hybrids, we discovered that cis-related regulations play a crucial role in gene expression divergence of sex pheromone perception-related proteins. These results are helpful for understanding how specific changes in the gene expression of olfactory-related genes can contribute to rapid evolutionary changes in important olfactory traits in closely related moths.

Protocol to identify ligands of odorant receptors using two-electrode voltage clamp combined with the Xenopus oocytes heterologous expression system

Two-electrode voltage clamp (TEVC) combined with the Xenopus laevis oocytes heterologous expression system is a powerful electrophysiological tool widely used to study the properties of many transmembrane proteins. Here, we describe a protocol using this combined approach to identify the ligands of odorant receptors that form ligand-gated ion channels. We detail the procedures for site-directed mutagenesis, oocyte microinjection, and TEVC recording. This protocol can also be used to identify the key residues and illustrate the structure-function relationships of these proteins. For complete details on the use and execution of this protocol, please refer to Cao et al. (2021).

Behavioural regulator and molecular reception of a double-edge-sword hunter beetle

BACKGROUND

The black carabid beetle Calosoma maximoviczi is a successful predator that serves as both a beneficial insect and a severe threat to economic herbivores. Its hunting technique relies heavily on olfaction, but the underlying mechanism has not been studied. Here, we report the electrophysiological, ecological and molecular traits of bioactive components identified from a comprehensive panel of natural odorants in the beetle-prey-plant system. The aim of this work was to investigate olfactory perceptions and their influence on the behaviours of C. maximoviczi.

RESULTS

Among the 200 identified volatiles, 18 were concentrated in beetle and prey samples, and 14 were concentrated in plants. Insect feeding damage to plants led to a shift in the emission fingerprint. Twelve volatiles were selected using successive electrophysiological tests. Field trials showed that significant sex differences existed when trapping with a single chemical or chemical mixture. Expression profiles indicated that sex-biased catches were related to the expression of 15 annotated CmaxOBPs and 40 CmaxORs across 12 chemosensory organs. In silico evaluations were conducted with 16 CmaxORs using modelling and docking. Better recognition was predicted for the pairs CmaxOR5-(Z)-3-hexenyl acetate, CmaxOR6-β-caryophyllene, CmaxOR18-(E)-β-ocimene and CmaxOR18-tetradecane, with higher binding affinity and a suitable binding pocket. Lastly, 168Y in CmaxOR6 and 142Y in CmaxOR18 were predicted as key amino acid residues for binding β-caryophyllene and tetradecane, respectively.

CONCLUSION

This work provides an example pipeline for de novo investigation in C. maximoviczi baits and the underlying olfactory perceptions. The results will benefit the future development of trapping-based integrated pest management strategies and the deorphanization of odorant receptors in ground beetles. © 2022 Society of Chemical Industry.

Functional analysis of pheromone receptor repertoire in the fall armyworm, Spodoptera frugiperda

BACKGROUND

The fall armyworm, Spodoptera frugiperda (J. E. Smith), is a polyphagous moth species that is spreading all around the globe. It uses (Z)-9-tetradecenyl acetate (Z9-14:Ac) and (Z)-7-dodecenyl acetate (Z7-12:Ac) (100:3.9) as essential sex pheromone components. However, our understanding of the molecular basis of pheromone detection of S. frugiperda is still incomplete.

RESULTS

Herein, we identified six PRs, i.e. SfruOR6, 11, 13, 16, 56, and 62, by transcriptome sequencing. Subsequently, we heterologously expressed them in Drosophila OR67d neurons and determined their response spectra with a large panel of sex pheromones and analogs. Among them, SfruOR13-expressing neurons strongly respond to the major sex pheromone component Z9-14:Ac, but also comparably to (Z,E)-9,12-tetradecadienyl acetate (Z9,E12-14:Ac) and weakly to (Z)-9-dodecenyl acetate (Z9-12:Ac). Both SfruOR56 and SfruOR62 are specifically tuned to the minor sex pheromone component Z7-12:Ac with varying intensities and sensitivities. In addition, SfruOR6 is activated only by Z9,E12-14:Ac, and SfruOR16 by both (Z)-9-tetradecenol (Z9-14:OH) and (Z)-9-tetradecenal (Z9-14:Ald). However, the OR67d neurons expressing SfruOR11 remain silent to all compounds tested, a phenomenon commonly found in the OR11 clade of Noctuidae species. Next, using single sensillum recording, we characterized four sensilla types on the antennae of males, namely A, B, C and D types that are tuned to the ligands of PRs, thereby confirming that S. frugiperda uses both SfruOR56 and SfruOR62 to detect Z7-12:Ac. Finally, using wind tunnel assay, we demonstrate that both Z9,E12-14:Ac and Z9-14:OH act as antagonists to the sex pheromone.

CONCLUSION

We have deorphanized five PRs and characterized four types of sensilla responsible for the detection of pheromone compounds, providing insights into the peripheral encoding of sex pheromones in S. frugiperda.

Enantiomeric Discrimination in Insects: The Role of OBPs and ORs

Olfaction is a complex recognition process that is critical for chemical communication in insects. Though some insect species are capable of discrimination between compounds that are structurally similar, little is understood about how this high level of discrimination arises. Some insects rely on discriminating between enantiomers of a compound, demonstrating an ability for highly selective recognition. The role of two major peripheral olfactory proteins in insect olfaction, i.e., odorant-binding proteins (OBPs) and odorant receptors (ORs) has been extensively studied. OBPs and ORs have variable discrimination capabilities, with some found to display highly specialized binding capability, whilst others exhibit promiscuous binding activity. A deeper understanding of how odorant-protein interactions induce a response in an insect relies on further analysis such as structural studies. In this review, we explore the potential role of OBPs and ORs in highly specific recognition, specifically enantiomeric discrimination. We summarize the state of research into OBP and OR function and focus on reported examples in the literature of clear enantiomeric discrimination by these proteins.

Sex Pheromone Receptors of Lepidopteran Insects

The sex pheromone receptors (SPRs) of Lepidopteran insects play important roles in chemical communication. In the sex pheromone detection processes, sex pheromone molecule (SPM), SPR, co-receptor (Orco), pheromone binding protein (PBP), sensory neuron membrane protein (SNMP), and pheromone degradation enzyme (PDE) play individual and cooperative roles. Commonly known as butterfly and moth, the Lepidopteran insects are widely distributed throughout the world, most of which are pests. Comprehensive knowledge of the SPRs of Lepidopteran insects would help the development of sex lure technology and the sex communication pathway research. In this review, we summarized SPR/Orco information from 10 families of Lepidopteran insects from corresponding studies. According to the research progress in the literature, we speculated the evolution of SPRs/Orcos and phylogenetically analyzed the Lepidopteran SPRs and Orcos with the neighbor-joining tree and further concluded the relationship between the cluster of SPRs and their ligands; we analyzed the predicted structural features of SPRs and gave our prediction results of SPRs and Orcos with Consensus Constrained TOPology Prediction (CCTOP) and SwissModel; we summarized the functional characterization of Lepidopteran SPRs and SPR-ligand interaction and then described the progress in the sex pheromone signaling pathways and metabotropic ion channel. Further studies are needed to work out the cryo-electron microscopy (EM) structure of SPR and the SPR-ligand docking pattern in a biophysical perspective, which will directly facilitate the understanding of sex pheromone signal transduction pathways and provide guidance in the sex lure technology in field pest control.

Comparison of functions of pheromone receptor repertoires in Helicoverpa armigera and Helicoverpa assulta using a Drosophila expression system

Helicoverpa armigera and H. assulta are sympatric closely related species sharing two sex pheromone components, (Z)-11-hexadecenal (Z11-16:Ald) and (Z)-9-hexadecenal (Z9-16:Ald) but in opposite ratios, 97:3 and 3:97 respectively. This feature makes them a feasible model for studying the evolution of pheromone coding mechanisms of lepidopteran insects. Despite a decade-long study to deorphanize the pheromone receptor (PR) repertoires of the two species, the comparison of the function of all PR orthologs between the two species is incomplete. Moreover, the ligands of OR14 and OR15 have so far not been found, likely due to the missing of the active ligand(s) in the compound panel and/or incompatibility of heterologous expression systems used. In the present study, we expressed the PR repertoires of both Helicoverpa species in Drosophila T1 neurons to comparatively study the function of PRs. Among those PRs, OR13, OR6, and OR14 of both species are functionally conserved and narrowly tuned, and the T1 neurons expressing each of them respond to Z11-16:Ald, (Z)-9-hexadecenol (Z9-16:OH), and (Z)-11-hexadecenyl acetate (Z11-16:Ac), respectively. While HarmOR16-expressing neurons respond strongly to (Z)-9-tetradecenal (Z9-14:Ald) and (Z)-11-hexadecenol (Z11-16:OH), the neurons expressing HassOR16 mainly respond to Z9-14:Ald and also weakly respond to (Z)-9-tetradecenol (Z9-14:OH). Moreover, HarmOR14b-expressing neurons are activated by Z9-14:Ald, whereas HassOR14b-expressing neurons are sensitive to Z9-16:Ald, Z9-14:Ald, and (Z)-9-hexadecenol (Z9-16:OH). In addition, HarmOR15-expressing neurons are selectively responsive to Z9-14:Ald. However, the Drosophila T1 neurons expressing either HarmOR11 or HassOR11 are silent to all of the compounds tested. In summary, except for OR11, we have deorphanized all the PRs of these two Helicoverpa species using a Drosophila expression system and a large panel of pheromone compounds, thereby providing a valuable reference for parsing the code of peripheral coding of pheromones.

Drosophila sechellia: A Genetic Model for Behavioral Evolution and Neuroecology

Defining the mechanisms by which animals adapt to their ecological niche is an important problem bridging evolution, genetics, and neurobiology. We review the establishment of a powerful genetic model for comparative behavioral analysis and neuroecology, Drosophila sechellia. This island-endemic fly species is closely related to several cosmopolitan generalists, including Drosophila melanogaster, but has evolved extreme specialism, feeding and reproducing exclusively on the noni fruit of the tropical shrub Morinda citrifolia. We first describe the development and use of genetic approaches to facilitate genotype/phenotype associations in these drosophilids. Next, we survey the behavioral, physiological, and morphological adaptations of D. sechellia throughout its life cycle and outline our current understanding of the genetic and cellular basis of these traits. Finally, we discuss the principles this knowledge begins to establish in the context of host specialization, speciation, and the neurobiology of behavioral evolution and consider open questions and challenges in the field.

Functional characterization of an olfactory receptor in the Oriental fruit fly, Bactrocera dorsalis, that responds to eugenol and isoeugenol

Most insects utilize a diverse array of olfactory cues for host finding and reproduction behaviors. Males of several Dacini fruit fly species (Tephritidae) are selectively attracted to certain phytochemicals to acquire sex pheromone precursors. Despite the importance of olfaction, only a limited number of olfactory receptors (ORs) in Dacini fruit flies have been characterized. In this study, I focused on the OR94b-2 subfamily, because a clade consisting of these homologs was distinctly localized from other OR clades in a constructed phylogenetic tree. To deorphanize the ORs, I used plant essential oils to screen ligands. Using heterologous expression in Xenopus oocytes, I analyzed the functional properties of BdorOR94b-2 and ZcucOR94b-2 from the Oriental fruit fly, Bactrocera dorsalis, and the melon fruit fly, Zeugodacus cucurbitae, respectively. I found that cinnamon leaf oil evoked responses in oocytes expressing BdorOR94b-2 and the corresponding co-receptor BdorORCO. Gas chromatography-mass spectrometry analysis indicated that eugenol is a major component of cinnamon leaf oil. In accordance with the response to cinnamon leaf oil containing eugenol, BdorOR94b-2/BdorORCO revealed a weak but significant response to eugenol. BdorOR94b-2/BdorORCO also responded to isoeugenol, but not to other related aromatic semiochemicals such as known male-specific attractants and sex pheromones. In contrast, ZcucOR94b-2/ZcucORCO did not respond to any of the phenolic compounds tested, including eugenol analogs. Therefore, BdorOR94b-2/BdorORCO is narrowly tuned to eugenol analogs, whereas ZcucOR94b-2/ZcucORCO likely binds to other compound(s).

Molecular reconstruction of recurrent evolutionary switching in olfactory receptor specificity

Olfactory receptor repertoires exhibit remarkable functional diversity, but how these proteins have evolved is poorly understood. Through analysis of extant and ancestrally reconstructed drosophilid olfactory receptors from the Ionotropic receptor (Ir) family, we investigated evolution of two organic acid-sensing receptors, Ir75a and Ir75b. Despite their low amino acid identity, we identify a common ‘hotspot’ in their ligand-binding pocket that has a major effect on changing the specificity of both Irs, as well as at least two distinct functional transitions in Ir75a during evolution. Moreover, we show that odor specificity is refined by changes in additional, receptor-specific sites, including those outside the ligand-binding pocket. Our work reveals how a core, common determinant of ligand-tuning acts within epistatic and allosteric networks of substitutions to lead to functional evolution of olfactory receptors.

A single point mutation causes one-way alteration of pheromone receptor function in two Heliothis species

The sex pheromone processing system of moths has been a major focus of research on olfaction and speciation, as it is highly specific and closely related to reproductive isolation. The two noctuid moths Heliothis virescens and Heliothis subflexa have been used as a model for deciphering the mechanisms underlying differentiation in pheromone communication, but no information exist regarding the functions of the pheromone receptors (PRs) of H. subflexa. Here, we functionally characterized all candidate PRs of H. subflexa, and found that only the response profile of OR6 differed between the two species. Through domain swapping and site-directed mutation followed by functional characterization, we identified a critical amino acid in OR6 caused a one-way alteration in specificity. This result suggests HsubOR6 evolved from an ancestral OR6 gene with a HvirOR6-like function and implies that the evolutionary direction of the receptor specificity was from the H. virescens-like pattern to H. subflexa-like pattern.

The structural basis of odorant recognition in insect olfactory receptors

Olfactory systems must detect and discriminate amongst an enormous variety of odorants1. To contend with this challenge, diverse species have converged on a common strategy in which odorant identity is encoded through the combinatorial activation of large families of olfactory receptors1-3, thus allowing a finite number of receptors to detect a vast chemical world. Here we offer structural and mechanistic insight into how an individual olfactory receptor can flexibly recognize diverse odorants. We show that the olfactory receptor MhOR5 from the jumping bristletail4 Machilis hrabei assembles as a homotetrameric odorant-gated ion channel with broad chemical tuning. Using cryo-electron microscopy, we elucidated the structure of MhOR5 in multiple gating states, alone and in complex with two of its agonists-the odorant eugenol and the insect repellent DEET. Both ligands are recognized through distributed hydrophobic interactions within the same geometrically simple binding pocket located in the transmembrane region of each subunit, suggesting a structural logic for the promiscuous chemical sensitivity of this receptor. Mutation of individual residues lining the binding pocket predictably altered the sensitivity of MhOR5 to eugenol and DEET and broadly reconfigured the receptor's tuning. Together, our data support a model in which diverse odorants share the same structural determinants for binding, shedding light on the molecular recognition mechanisms that ultimately endow the olfactory system with its immense discriminatory capacity.

Odorant Receptors for Detecting Flowering Plant Cues Are Functionally Conserved across Moths and Butterflies

Odorant receptors (ORs) are essential for plant–insect interactions. However, despite the global impacts of Lepidoptera (moths and butterflies) as major herbivores and pollinators, little functional data are available about Lepidoptera ORs involved in plant-volatile detection. Here, we initially characterized the plant-volatile-sensing function(s) of 44 ORs from the cotton bollworm Helicoverpa armigera, and subsequently conducted a large-scale comparative analysis that establishes how most orthologous ORs have functionally diverged among closely related species whereas some rare ORs are functionally conserved. Specifically, our systematic analysis of H. armigera ORs cataloged the wide functional scope of the H. armigera OR repertoire, and also showed that HarmOR42 and its Spodoptera littoralis ortholog are functionally conserved. Pursuing this, we characterized the HarmOR42-orthologous ORs from 11 species across the Glossata suborder and confirmed the HarmOR42 orthologs form a unique OR lineage that has undergone strong purifying selection in Glossata species and whose members are tuned with strong specificity to phenylacetaldehyde, a floral scent component common to most angiosperms. In vivo studies via HarmOR42 knockout support that HarmOR42-related ORs are essential for host-detection by sensing phenylacetaldehyde. Our work also supports that these ORs coevolved with the tube-like proboscis, and has maintained functional stability throughout the long-term coexistence of Lepidoptera with angiosperms. Thus, beyond providing a rich empirical resource for delineating the precise functions of H. armigera ORs, our results enable a comparative analysis of insect ORs that have apparently facilitated and currently sustain the intimate adaptations and ecological interactions among nectar feeding insects and flowering plants.

Comparative Genomics Provide Insights Into Function and Evolution of Odorant Binding Proteins in Cydia pomonella

Insect olfaction is vital for foraging, mating, host-seeking, and avoidance of predators/pathogens. In insects, odorant binding proteins (OBPs) are involved in transporting hydrophobic odor molecules from the external environment to receptor neurons. The codling moth, Cydia pomonella, one of the most destructive insect fruit pests, causes enormous economic losses. However, little is known about the number, variety, gains and losses, and evolution of OBP genes in C. pomonella. Here we report the identification of 40 OBPs in C. pomonella, most (75%) of which are classic OBPs, using genomic and transcriptomic analyses. Two OBP genes were lost in C. pomonella relative to possible distant ancestor in Lepidoptera lineage based on an analysis of gene gains and losses. The phylogenetic tree and chromosome location showed that the expansion of OBP genes mainly resulted from tandem duplications, as the CpomGOBP2 gene was duplicated twice along with loss of CpomPBPB. Two positive selection sites of the CpomGOBP1 gene were identified while other OBP genes evolved under purifying selection. Our results provide fundamental knowledge of OBP genes allowing further study of their function in C. pomonella.

Identification and Functional Characterization of Two Putative Pheromone Receptors in the Potato Tuber Moth, Phthorimaea operculella

Pheromones are a kind of signal produced by an animal that evoke innate responses in conspecifics. In moth, pheromone components can be detected by specialized olfactory receptor neurons (OSNs) housed in long sensilla trichoids on the male antennae. The pheromone receptors (PRs) located in the dendrite membrane of OSNs are responsible for pheromone sensing in most Lepidopteran insects. The potato tuber moth Phthorimaea operculella is a destructive pest of Solanaceae crops. Although sex attractant is widely used in fields to monitor the population of P. operculella, no study has been reported on the mechanism the male moth of P. operculella uses to recognize sex pheromone components. In the present study, we cloned two pheromone receptor genes PopeOR1 and PopeOR3 in P. operculella. The transcripts of them were highly accumulated in the antennae of male adults. Functional analysis using the heterologous expression system of Xenopus oocyte demonstrated that these two PR proteins both responded to (E, Z)-4,7–13: OAc and (E, Z, Z)-4,7,10–13: OAc, the key sex pheromone components of P. operculella, whilst they responded differentially to these two ligands. Our findings for the first time characterized the function of pheromone receptors in gelechiid moth and could promote the olfactory based pest management of P. operculella in the field.

Olfactory coding of intra- and interspecific pheromonal messages by the male Mythimna separata in North China

Moths often use multi-component pheromones with fixed ratios to keep intraspecific communication and interspecific isolation. Unusually, the Oriental armyworm Mythimna separata in North China use only Z11-16:Ald as the essential component of its sex pheromone to find mates. To understand how this species keeps behavioral isolation from other species sharing Z11-16:Ald as a major pheromone component, we study the olfactory coding of intra- and interspecific pheromonal messages in the males of M. separata. Firstly, we functionally characterized the long trichoid sensilla in male antennae by single sensillum recording. Two types of sensilla were classified: the A type sensilla responded to Z11-16:Ald and Z9-14:Ald, and the B type sensilla mainly to Z9-14:Ald, and also to Z11-16:Ac, Z11-16:OH, and Z9-16:Ald. Next, we examined the glomerulus responses in the antennal lobes to these compounds by using in vivo optical imaging. The results showed that among the three subunits of the macroglomerular complex (MGC), Z11-16:Ald activated the cumulus, Z9-14:Ald activated the dorso-anterior and the cumulus, Z11-16:OH and Z11-16:Ac activated the dorso-anterior and dorso-posterior, respectively. However, Z9-16:Ald activated an ordinary glomerulus. Thirdly, we tested the behavioral responses of the males to these compounds in the wind tunnel. Addition of Z9-14:Ald at the ratio of 1:10 greatly reduced the attractiveness of Z11-16:Ald, addition of Z9-16:Ald or Z11-16:OH at the ratio of 1:1 also had behavioral antagonistic effects, while addition of Z11-16:Ac had no effect on the attractiveness of Z11-16:Ald. Finally, we used antennal transcriptome data and the Xenopus expression system to identify the receptor of Z9-14:Ald in M. separata. The Xenopus oocytes co-expressing MsepOR2 and MsepORco showed a strong response to Z9-14:Ald. Two-color fluorescence in situ hybridization validated that the cells expressing MsepOR2 and MsepOR3, tuned to Z9-14:Ald and Z11-16:Ald respectively, were localized in the different sensilla of male antennae. Comparing the sex pheromone communication channel of the related species, our results suggest that the conserved olfactory pathways for behavioral antagonists play a crucial role in behavioral isolation of noctuid species.

The Molecular Basis of Host Selection in a Crucifer-Specialized Moth

Glucosinolates (GSs) are sulfur-containing secondary metabolites characteristic of cruciferous plants [1, 2]. Their breakdown products, isothiocyanates (ITCs), are released following tissue disruption by insect feeding or other mechanical damages [3, 4]. ITCs repel and are toxic to generalist herbivores, while specialist herbivores utilize the volatile ITCs as key signals for localizing host plants [5, 6]. However, the molecular mechanisms underlying detection of ITCs remain open. Here, we report that in the diamondback moth Plutella xylostella, a crucifer specialist, ITCs indeed drive the host preference for Arabidopsis thaliana, and the two olfactory receptors Or35 and Or49 are essential for this behavior. By performing gene expression analyses, we identified 12 (out of 59 in total) female-biased Ors, suggesting their possible involvement in oviposition choice. By ectopically expressing these Ors in Xenopus oocytes and screening their responses with 49 odors (including 13 ITCs, 25 general plant volatiles, and 11 sex pheromone components), we found that Or35 and Or49 responded specifically to three ITCs (iberverin, 4-pentenyl ITC, and phenylethyl ITC). The same ITCs also exhibited highest activity in electroantennogram recordings with female antennae and were the strongest oviposition stimulants. Knocking out either Or35 or Or49 via CRISPR-Cas9 resulted in a reduced oviposition preference for the ITCs, while double Or knockout females lost their ITC preference completely and were unable to choose between wild-type A. thaliana and a conspecific ITC knockout plant. We hence conclude that the ITC-based oviposition preference of the diamondback moth for its host A. thaliana is governed by the cooperation of two highly specific olfactory receptors.

Identification and characterization of troponin genes in Locusta migratoria

Troponin complex comprises three subunits, namely troponin C (TpnC), troponin I (TpnI) and troponin T (TpnT), and regulates the contraction of striated muscle. We found that the locust Locusta migratoria genome has one TpnT gene (LmTpnT), one TpnI gene (LmTpnI) and three TpnC genes (LmTpnC1, LmTpnC2 and LmTpnC3). Through alternative splicing, LmTpnT and LmTpnI potentially encode two and eight isoforms, respectively. The flight muscle and the jump muscle of L. migratoria express an identical LmTpnT isoform, but different LmTpnC isoforms and LmTpnI isoforms. LmTpnC2 and LmTpnC3 both contain highly conserved residues essential for calcium binding in the EF-hand II and IV, thus belonging two-site isoform. LmTpnC1 contains non-conserved substitutions in the EF-hand II and all highly conserved residues for calcium binding in the EF-hand IV. Mutagenesis and tyrosine fluorescence spectroscopic analysis show that both the EF-hand II and IV of LmTpnC1 can serve as calcium-binding site. Therefore, all three LmTpnC isoforms belong to two-site isoform. This is in contrast to the situation in the insect with asynchronous flight muscle, which expresses both one-site isoform and two-site isoform of TpnC. Those results suggest that the origination of insect asynchronous flight muscle is associated with the emergence of one-site isoform of TpnC.

A moth odorant receptor highly expressed in the ovipositor is involved in detecting host-plant volatiles

Antennae are often considered to be the nostrils of insects. Here, we sequenced the transcriptome of the pheromone gland-ovipositor complex of Helicoverpa assulta and discovered that an odorant receptor (OR) gene, HassOR31, had much higher expression in the ovipositor than in antennae or other tissues. To determine whether the ovipositor was involved in odorant detection, we co-expressed HassOR31 and its co-receptor, HassORco, in a Xenopus oocyte model system, and demonstrated that the OR was responsive to 12 plant odorants, especially Z-3-hexenyl butyrate. These odorants elicited electrophysiological responses of some sensilla in the ovipositor, and HassOR31 and HassORco were co-expressed within ovipositor sensilla. Two oviposition preference experiments showed that female moths lacking antennae still preferentially selected oviposition sites containing plant volatiles. We suggest that the expression of HassOR31 in the ovipositor of H. assulta helps females to determine precise egg-laying sites in host plants.

When most insects reproduce they lay eggs that hatch into juveniles known as larvae. To provide good sources of food for the larvae, the adult insects have to carefully select where to lay the eggs. Host plants produce specific sets of chemicals known as odorants that the adult insects are able to smell using proteins called odorant receptors.

It is generally thought that odorant receptors in the antennae on the head are responsible for guiding adult insects to good egg-laying sites. However, recent studies have reported that odorant receptors are also present in the egg-laying organs of several different species of moth. It remains unclear what role these odorant receptors may play in egg-laying.

The oriental tobacco budworm (Helicoverpa assulta) is considered a serious pest in agriculture. The adult moths lay their eggs on a narrow range of plants in the nightshade family including tobacco and hot pepper. Li et al. have now investigated the odorant receptors of H. assulta and found that one gene for an odorant receptor called HassOR31 was expressed much more in the egg-laying organs of the moths than in the antennae. Further experiments showed that this receptor was tuned to respond to 12 odorants that also stimulated responses in the egg-laying organ of H. assulta. Together these findings suggest that this odorant receptor in the egg-laying organ helps the moths find suitable host plants to lay their eggs on.

The work of Li et al. may help us understand how H. assulta evolved to lay its eggs on specific members of the nightshade family and lead to new methods of controlling this pest. An insect’s sense of smell guides many other behaviors including finding food, mates and avoiding enemies. Therefore, these findings may inspire researchers to investigate whether odorant receptors in the antennae or other organs guide these behaviors.

An Orphan Pheromone Receptor Affects the Mating Behavior of Helicoverpa armigera

The Lepidoptera is the second largest insect order, which has the most extensive knowledge of sex pheromones and mechanisms of pheromone communication since the identification of the first insect pheromone in Bombyx mori. In the past 15 years, pheromone receptors have been identified and functionally characterized in many moth species. HarmOR14 is a typical pheromone receptor of Helicoverpa armigera which showed no response to the tested pheromones in Xenopus oocyte expression system, but its orthologous gene in Heliothis virescens, HvirOR14 could be activated by pheromones in the same expression system. To assess the possible functions of OR14 in vivo, in this study, we knocked out this gene using CRISPR/Cas9 system and compared the mating behaviors and EAG response to pheromones between the wild type and mutant strains. Our results showed that OR14 mutants did not affect the mating rate or the EAG response to pheromones but could prolong the mating duration and change the mating time in undefined manners, which extends our understanding to this kind of pheromone receptors.

Evolution of olfactory circuits in insects

Recent years have seen an explosion of interest in the evolution of neural circuits. Comparison of animals from different families, orders, and phyla reveals fascinating variation in brain morphology, circuit structure, and neural cell types. However, it can be difficult to connect the complex changes that occur across long evolutionary distances to behavior. Luckily, these changes accumulate through processes that should also be observable in recent time, making more tractable comparisons of closely related species relevant and complementary. Here, we review several decades of research on the evolution of insect olfactory circuits across short evolutionary time scales. We describe two well-studied systems, Drosophila sechellia flies and Heliothis moths, in detailed case studies. We then move through key types of circuit evolution, cataloging examples from other insects and looking for general patterns. The literature is dominated by changes in sensory neuron number and tuning at the periphery-often enhancing neural response to odorants with new ecological or social relevance. However, changes in the way olfactory information is processed by central circuits is clearly important in a few cases, and we suspect the development of genetic tools in non-model species will reveal a broad role for central circuit evolution. Moving forward, such tools should also be used to rigorously test causal links between brain evolution and behavior.

Distinct Roles of Cuticular Aldehydes as Pheromonal Cues in Two Cotesia Parasitoids

Cuticular compounds (CCs) that cover the surface of insects primarily serve as protection against entomopathogens, harmful substances, and desiccation. However, CCs may also have secondary signaling functions. By studying the role of CCs in intraspecific interactions, we may advance our understanding of the evolution of pheromonal communication in insects. We previously found that the gregarious parasitoid, Cotesia glomerata (L.), uses heptanal as a repellent pheromone to help avoid mate competition among sibling males, whereas another cuticular aldehyde, nonanal, is part of the female-produced attractive sex pheromone. Here, we show that the same aldehydes have different pheromonal functions in a related solitary parasitoid, Cotesia marginiventris (Cresson). Heptanal enhances the attractiveness of the female's sex pheromone, whereas nonanal does not affect a female's attractiveness. Hence, these common aldehydes are differentially used by the two Cotesia species to mediate, synergistically, the attractiveness of the main constituents of their respective sex pheromones. The specificity of the complete sex pheromone blend is apparently regulated by two specific, less volatile compounds, which evoke strong electroantennographic (EAG) responses. This is the first demonstration that volatile CCs have evolved distinct pheromonal functions to aid divergent mating strategies in closely related species. We discuss the possibility that additional compounds are involved in attraction and that, like the aldehydes, they are likely oxidative products of unsaturated cuticular hydrocarbons.

Molecular and Functional Evolution at the Odorant Receptor Or22 Locus in Drosophila melanogaster

Insect odorant receptor (Or) genes determine the responses of sensory neurons that mediate critical behaviors. The Drosophila melanogaster Or22 locus represents an interesting example of molecular evolution, with high levels of sequence divergence and copy number variation between D. melanogaster and other Drosophila species, and a corresponding high level of variability in the responses of the neuron it controls, ab3A. However, the link between Or22 molecular and functional diversity has not been established. Here, we show that several naturally occurring Or22 variants generate major shifts in neuronal response properties. We determine the molecular changes that underpin these response shifts, one of which represents a chimeric gene variant previously suggested to be under natural selection. In addition, we show that several alternative molecular genetic mechanisms have evolved for ensuring that where there is more than one gene copy at this locus, only one functional receptor is generated. Our data thus provide a causal link between the striking levels of phenotypic neuronal response variation found in natural populations of D. melanogaster and genetic variation at the Or22 locus. Since neuronal responses govern animal behavior, we predict that Or22 may be a key player in underlying one or more olfactory-driven behaviors of significant adaptive importance.

Functional characterization of pheromone receptors in the moth Athetis dissimilis (Lepidoptera: Noctuidae)

Sex pheromones are crucial for communication between females and males in moths, and pheromone receptors (PRs) play a key role in peripheral coding of sex pheromones. During the last decade, many PR candidates have been identified based on transcriptome sequencing and bioinformatic analysis, but their detailed functions remain mostly unknown. Here, focusing on four PR candidates of Athetis dissimilis (AdisOR1, AdisOR6, AdisOR11 and AdisOR14) identified in a previous study, we first cloned the full-length cDNAs and determined the tissue expression profiles by quantitative real-time PCR (qPCR). The results revealed that expression of three of these genes were male antennae-specific, while AdisOR11 was similar in expression between male and female antennae. Furthermore, the expression level of AdisOR1 was much higher than those of the other three genes. Then, functional analysis was conducted using Xenopus oocyte system. AdisOR1 responded strongly to the sex pheromone component Z9-14:OH and the potential pheromone component Z9,E12-14:OH, suggesting its important role in the sex pheromone perception; AdisOR14 showed specificity for Z9,E12-14:OH; while AdisOR6 and AdisOR11 did not respond to any of the pheromone components and analogs tested. Taken together, this study contributes to elucidate the molecular mechanism of sex pheromone reception and provides potential targets for development of OR based pest control techniques in A. dissimilis.

Expression patterns and colocalization of two sensory neurone membrane proteins in Ectropis obliqua Prout, a geometrid moth pest that uses Type-II sex pheromones

Sensory neurone membrane proteins (SNMPs) function as essential cofactors for insect sex pheromone detection. In this study, we report two SNMPs in Ectropis obliqua Prout, a serious geometrid pest that produces typical Type-II sex pheromones. Sequence alignments and phylogenetic analyses showed that EoblSNMP1 and EoblSNMP2 belong to two distinct SNMP subfamilies. Quantitative real-time PCR suggested that EoblSNMP1 was male antennae-biased, whereas EoblSNMP2 was highly expressed on male antennae but was also expressed on female antennae and other chemosensory tissues. Additionally, EoblSNMP1 and EoblSNMP2 differed in their developmental expression profiles. In situ hybridization revealed that EoblSNMP1 was sensilla trichodea I specific, whereas EoblSNMP2 was expressed in sensilla trichodea I and the sensilla basiconica; furthermore, EoblSNMP1 and EoblSNMP2 were co-expressed in sensilla trichodea I but in different cells. This study suggests that EoblSNMP1 is functionally distinct from EoblSNMP2 in E. obliqua; EoblSNMP1 may specifically contribute to the recognition of sex pheromones, whereas EoblSNMP2 exhibits multiple olfactory roles. Our findings comprehensively reveal the expression patterns of SNMPs in a lepidopteran species that uses Type-II sex pheromones, providing new insights into the functional evolution of SNMPs from lepidopteran moths with Type-I sex pheromones to those with Type-II sex pheromones.

Functional characterization of four sex pheromone receptors in the newly discovered maize pest Athetis lepigone

Chemoreception systems play a crucial role in regulating key behavioral activities of insects, such as mating, oviposition, and foraging. Odorant receptors (ORs) trigger the transduction of chemical signals into electric signals, and are involved in the corresponding responses associated with odorant guidance behaviors. Pheromone receptors (PRs) of male adult insects are generally thought to function in the recognition of female sex pheromones, and are also important molecular targets for the development of behavioral inhibitors and insecticides. In this study, we successfully expressed and functionally analyzed four AlepPRs of Athetis lepigone in Xenopus oocytes using the two-electrode voltage-clamp method. The results demonstrated that AlepOR3 responded exclusively to the sex pheromone compound of A. lepigone, (Z)-7-dodecenyl acetate (Z7-12:Ac) (EC₅₀ = 8.830 × 10^-6 M), while AlepOR4 responded to all five compounds [(Z7-12:Ac, (Z)-8-dodecenyl acetate (Z8-12:Ac), (Z)-9-tetradecenyl acetate (Z9-14:Ac), (Z,E)-9,11-tetradecadienyl acetate (Z9,E11-14:Ac), and (Z,E)-9,12-tetradecadienyl acetate (Z9,E12-14:Ac)] and had a higher response to Z9-14:Ac (EC₅₀ = 2.243 × 10^-5 M) than to Z7-12:Ac. However, AlepOR6 displayed a significantly higher response to a non-pheromone of A. lepigone, Z9,E12-14:Ac (EC₅₀ = 7.145 × 10^-6 M), than to the other four compounds. AlepOR5 displayed no responses to any of the pheromone compounds of A. lepigone, but responded exclusively to (Z)-11-hexadecenyl acetate (Z11-16:Ac) (EC₅₀ = 7.870 × 10^-6 M), a sex pheromone compound of other Noctuidae species. These findings can help explore the molecular mechanisms of sex pheromone recognition in A. lepigone and other moths, and develop broad-spectrum behavioral inhibitors and insecticides against different maize moths in future.

An odorant receptor mediates the attractiveness of cis-jasmone to Campoletis chlorideae, the endoparasitoid of Helicoverpa armigera

Parasitic wasps rely on olfaction to locate their hosts in complex chemical environments. Odorant receptors (ORs) function together with well-conserved odorant coreceptors (ORcos) to determine the sensitivity and specificity of olfactory reception. Campoletis chlorideae (Hymenoptera: Ichneunmonidae) is a solitary larval endoparasitoid of the cotton bollworm, Helicoverpa armigera, and some other noctuid species. To understand the molecular basis of C. chlorideae's olfactory reception, we sequenced the transcriptome of adult male and female heads (including antennae) and identified 211 OR transcripts, with 95 being putatively full length. The tissue expression profiles, as assessed by reverse-transcription PCR, showed that seven ORs were expressed only or more highly in female antennae. Their functions were analysed using the Xenopu slaevis oocyte expression system and two-electrode voltage-clamp recordings. CchlOR62 was tuned to cis-jasmone, which was attractive to female C. chlorideae adults and H. armigera larvae in the subsequent behavioural assays. Further bioassays using caged plants showed that the parasitism rate of H. armigera larvae by C. chlorideae on cis-jasmone-treated tobacco plants was higher than on the control plants. Thus, cis-jasmone appears to be an important infochemical involved in the interactions of plants, H. armigera and C. chlorideae, and CchlOR62 mediates the attractiveness of cis-jasmone to C. chlorideae.

Insect Pheromone Receptors - Key Elements in Sensing Intraspecific Chemical Signals

Pheromones are chemicals that serve intraspecific communication. In animals, the ability to detect and discriminate pheromones in a complex chemical environment substantially contributes to the survival of the species. Insects widely use pheromones to attract mating partners, to alarm conspecifics or to mark paths to rich food sources. The various functional roles of pheromones for insects are reflected by the chemical diversity of pheromonal compounds. The precise detection of the relevant intraspecific signals is accomplished by specialized chemosensory neurons housed in hair-like sensilla located on the surface of body appendages. Current data indicate that the extraordinary sensitivity and selectivity of the pheromone-responsive neurons (PRNs) is largely based on specific pheromone receptors (PRs) residing in their ciliary membrane. Besides these key elements, proper ligand-induced responses of PR-expressing neurons appear to generally require a putative co-receptor, the so-called "sensory neuron membrane protein 1" (SNMP1). Regarding the PR-mediated chemo-electrical signal transduction processes in insect PRNs, ionotropic as well as metabotropic mechanisms may be involved. In this review, we summarize and discuss current knowledge on the peripheral detection of pheromones in the olfactory system of insects with a focus on PRs and their specific role in the recognition and transduction of volatile intraspecific chemical signals.

Proceeding From in vivo Functions of Pheromone Receptors: Peripheral-Coding Perception of Pheromones From Three Closely Related Species, Helicoverpa armigera, H. assulta, and Heliothis virescens

Three closely related species, Helicoverpa armigera, H. assulta, and Heliothis virescens from Lepidoptera Noctuidae, are used as a model system for exploring sexual communication and species isolation. Pheromone receptors (PRs) previously discovered in model moth species include seven in H. armigera, six in H. assulta, and six in H. virescens. PRs named OR6, OR13, and OR16 among these species were found to be functional, characterized by an in vitro Xenopus oocytes system. Using an in vivo transgenic fly system, functional assays of OR6, OR13, and OR16 clades from three closely related Noctuidae species showed that OR13 function was highly conserved, whereas OR6 and OR16 exhibited functional divergence. Similar results were produced from assays in the Xenopus oocytes system. Combined with earlier behavioral results and electrophysiological recordings, we found corresponding relationships among pheromones, PRs, and neurons at the periphery sensory system of each species. Our results provide vital information at the neuronal and molecular level, shedding insight into the sexual communication of closely related species in Lepidoptera.

Cryo-EM structure of the insect olfactory receptor Orco

The olfactory system must recognize and discriminate amongst an enormous variety of chemicals in the environment. To contend with such diversity, insects have evolved a family of odorant-gated ion channels comprised of a highly conserved co-receptor (Orco) and a divergent odorant receptor (OR) that confers chemical specificity. Here, we present the single-particle cryo-electron microscopy structure of an Orco homomer from the parasitic fig wasp Apocrypta bakeri at 3.5 Å resolution, providing structural insight into this receptor family. Orco possesses a novel channel architecture, with four subunits symmetrically arranged around a central pore that diverges into four lateral conduits that open to the cytosol. The Orco tetramer has few inter-subunit interactions within the membrane and is bound together by a small cytoplasmic anchor domain. The minimal sequence conservation among ORs maps largely to the pore and anchor domain, shedding light on how the architecture of this receptor family accommodates its remarkable sequence diversity and facilitates the evolution of odour tuning.