Olfactory reception in invertebrates

Structural Basis and Recognition Mechanism of Host-Derived Volatiles by Olfactory Proteins in the Agricultural Pest Bactrocera correcta (Diptera: Tephritidae)

Host-derived volatiles play a critical role in mediating plant-insect interactions. Bactrocera correcta is a destructive pest of fruit crops. In this study, we investigated the recognition mechanisms of three key volatiles─β-caryophyllene, ethyl decanoate, and hexyl hexanoate─derived from the host fruits of B. correcta. Using transcriptomic analysis, fluorescence binding assays, molecular docking, and molecular dynamics simulations, we identified BcorOBP19d-2 as a key odorant-binding protein that binds multiple volatiles and facilitates their stabilization and transport. Odorant receptors (BcorOR7a-13, BcorOR74a-3, and BcorOR7a-3) selectively recognize these volatiles, with hydrophobic interactions as the primary driving force for binding. β-Caryophyllene exhibited the highest binding specificity with BcorOR7a-13, ethyl decanoate demonstrated the strongest binding affinity with BcorOR74a-3, and hexyl hexanoate showed moderate stability with BcorOR7a-3. These findings provide structural insights into volatile recognition in polyphagous insects and offer a basis for developing attractants or repellents for pest management.

Identification of Candidate Olfactory Genes in the Antennal Transcriptome of Loxostege sticticalis Trapped by Three Different Sex Pheromone Blends

Insects sense intraspecific or interspecific information about the chemical substances in the habitat through the sensitive olfactory system to carry out foraging, mating, oviposition, and other activities. The antennae serve as the primary olfactory organs in insects. The olfactory process involves the participation of many proteins, such as odorant-binding proteins (OBPs) and odorant receptors (ORs), but ORs play a central role in olfactory specificity and sensitivity. The beet webworm, Loxostege sticticalis, is an omnivorous agricultural pest that endangers crops and poses a significant risk to the agricultural and animal husbandry production in northern China. In this study, Illumina sequencing was conducted on the antennal transcriptome of male L. sticticalis trapped by three different sex pheromone blends. A total of 10,320 DEGs were identified, from which 46 candidate olfactory genes were selected for further analysis. These candidate olfactory genes comprise 13 odorant receptors, 6 ionotropic receptors (IRs), 3 gustatory receptors (GRs), 12 odorant-binding proteins, and 13 chemosensory proteins (CSPs). In summary, we analyzed the antennal transcriptome of male L. sticticalis trapped by three different sex pheromone blends and identified several candidate olfactory genes. This discovery offers a foundation for further molecular-level investigations into the olfactory system of L. sticticalis.

The olfactory pathway in the peracarid crustacean Parhyale hawaiensis (Malacostraca): new insights into the evolution of olfactory processing in Pancrustacea

Our current understanding of the functional morphology of olfactory systems in arthropods largely relies on information obtained in hexapods. Existing analyses of the olfactory pathway in crustacean representatives have suggested that these animals share several corresponding anatomical elements with hexapod olfactory systems but that the latter likely feature a different olfactory wiring logic from receptor to olfactory glomerulus. This study sets out to further explore the diversity of arthropod olfactory systems by presenting a detailed morphological analysis of the peripheral and central olfactory pathways in an emerging model system, the peracarid crustacean Parhyale hawaiensis (Malacostraca). These animals feature all neuronal elements that characterize malacostracan crustacean's olfactory systems, and the simplicity of this animal's olfactory system provided the unique opportunity to quantify the numbers of olfactory sensilla and associated sensory neurons, olfactory interneurons and olfactory glomeruli. These data showed that the number of those neuronal elements is highly variable across individuals, contrasting with more stable numbers of neuronal elements in hexapod olfactory systems that typically are characterized by olfactory glomeruli with individual identities and constant numbers. We discuss the possible steps needed for an evolutionary transformation of a malacostracan crustacean type of olfactory system into a hexapod type.

Understanding the Effects of Three Carbohydrate Feeds on the Health of Apis mellifera by Transcriptome Analysis

At present, there is no clear consensus on the impact of carbohydrate feeds on bee colony health, and comprehensive research and evaluation in this context is lacking. To comprehensively and objectively examine the health status of honeybees after consuming those carbohydrates from multiple perspectives, experimental techniques, including high-throughput sequencing of the transcriptome, proboscis extension reflex (PER), and measuring bee growth parameters were employed. This study showed that compared with honey, feeding high fructose syrup (HFS) resulted in a decrease in the survival rate and body weight of bees, while sucrose decreased the learning and memory ability of bees. After feeding on honey, the main antimicrobial peptides including abaecin, apidaecin1, hymenoptin, and defensin in bees, are all upregulated in expression. The 14 DEGs significantly enriched in the axonal regeneration pathway were all downregulated in the sucrose group and HFS group. This study demonstrated that the expression of multiple genes involved in oxidative phosphorylation was downregulated in bees fed with HFS, moreover, HFS also affected the biosynthesis of unsaturated fatty acids. These effects may lead to energy and metabolic disorders (including fatty acids), thereby inhibiting the growth and development of bees. Sucrose can decrease the learning and memory ability of bees, which may be due to the downregulation of genes related to learning and memory in the axonal regeneration pathway. Honey can upregulate antimicrobial peptides and other immune-related proteins, activating the bee's immune system and boosting bees' immunity to pathogens.

The Periodic Feeding Frequency of the Juvenile Tropical Rock Lobster (Panulirus ornatus) in the Examination of Chemo-Attract Diet Performance and Colour-Contrast Preference

Significant research investment into tropical rock lobster (TRL) aquaculture production methods has led to a rapidly developing industry in Vietnam and, more recently, in Australia. The need for an effective formulated feed has been highlighted for both industries; however, feed intake has been a consistent limitation. Visual and chemical cues regulating feed recognition and consumption are expected to yield valuable data, leading to increased feed intake. Lobsters were placed in white- and grey-coloured enclosures to examine the effect of background colour on their feeding behaviour in terms of feeding occurrence and response time. The impact of background colour on TRL feeding behaviour found no statistically significant differences between TRL in white and grey enclosures, suggesting grayscale contrast does not directly affect feeding behaviour. Experiment 2 studied the effects of coloured feeding zones on feeding response in white enclosures. Yellow feeding zones led to a decreased feeding time (473 ± 443 s) and increased time spent feeding (168 ± 1832 s) compared to other colours, possibly due to the yellow-blue chromaticity (b*) contrast. Experiment 3 examined chemo-attractants (glycine, taurine and inositol) and their influence on the feeding behaviour of TRL, but no increased responses were observed. Experiments two and three assessed TRL feeding activity in morning and evening periods, highlighting their nocturnal behaviour, with more feeding occurring in the evening. This project enhances our understanding of photoreceptive and chemoreceptive factors affecting TRL feeding behaviour with formulated feed. It also reveals the potential for background colour changes to enhance marketable colours in commercial settings. Additionally, the study confirmed the effective use of animal tracking software (EthoVision XT) for lobster species tracking in future behavioural trials.

Innexin expression and localization in the Drosophila antenna indicate gap junction or hemichannel involvement in antennal chemosensory sensilla

Odor detection in insects is largely mediated by structures on antennae called sensilla, which feature a strongly conserved architecture and repertoire of olfactory sensory neurons (OSNs) and various support cell types. In Drosophila, OSNs are tightly apposed to supporting cells, whose connection with neurons and functional roles in odor detection remain unclear. Coupling mechanisms between these neuronal and non-neuronal cell types have been suggested based on morphological observations, concomitant physiological activity during odor stimulation, and known interactions that occur in other chemosensory systems. For instance, it is not known whether cell-cell coupling via gap junctions between OSNs and neighboring cells exists, or whether hemichannels interconnect cellular and extracellular sensillum compartments. Here, we show that innexins, which form hemichannels and gap junctions in invertebrates, are abundantly expressed in adult drosophilid antennae. By surveying antennal transcriptomes and performing various immunohistochemical stainings in antennal tissues, we discover innexin-specific patterns of expression and localization, with a majority of innexins strongly localizing to glial and non-neuronal cells, likely support and epithelial cells. Finally, by injecting gap junction-permeable dye into a pre-identified sensillum, we observe no dye coupling between neuronal and non-neuronal cells. Together with evidence of non-neuronal innexin localization, we conclude that innexins likely do not conjoin neurons to support cells, but that junctions and hemichannels may instead couple support cells among each other or to their shared sensillum lymph to achieve synchronous activity. We discuss how coupling of sensillum microenvironments or compartments may potentially contribute to facilitate chemosensory functions of odor sensing and sensillum homeostasis.

Binding characteristics and structural dynamics of two general odorant-binding proteins with plant volatiles in the olfactory recognition of Glyphodes pyloalis

Glyphodes pyloalis Walker (Lepidoptera: Pyralidae) is the most destructive pest, causing severe damage to mulberry production in China's sericulture industry. The insecticide application in mulberry orchards poses a significant risk of poisoning to Bombyx mori. Shifting from insecticides to odor attractants is a beneficial alternative, but not much data is available on the olfactory system of G. pyloalis. We identified 114 chemosensory genes from the antennal transcriptome database of G. pyloalis, with 18 odorant-binding protein (OBP) and 17 chemosensory protein (CSP) genes significantly expressed in the antennae. Ligand-binding assays for two antennae-biased expressed general odorant-binding proteins (GOBPs) showed high binding affinities of GOBP1 to hexadecanal, β-ionone, and 2-ethylhexyl acrylate, while GOBP2 exhibited binding to 4-tert-octylphenol, benzyl benzoate, β-ionone, and farnesol. Computational simulations indicated that van der Waal forces predominantly contributed to the binding free energy in the binding processes of complexes. Among them, Phe12 of GOBP1 and Phe19 of GOBP2 were demonstrated to play crucial roles in their bindings to plant volatiles using site-directed mutagenesis experiments. Moreover, hexadecanal and β-ionone attracted G. pyloalis male moths in the behavioral assays, while none of the candidate plant volatiles significantly affected female moths. Our findings provide a comprehensive understanding of the molecular mechanisms underlying olfactory recognition in G. pyloalis, setting the groundwork for novel mulberry pests control strategies based on insect olfaction.

Identification of odorant-binding proteins and functional analysis of antenna-specific BhorOBP28 in Batocera horsfieldi (Hope)

BACKGROUND

The important wood-boring pest Batocera horsfieldi has evolved a sensitive olfactory system to locate host plants. Odorant-binding proteins (OBPs) are thought to play key roles in olfactory recognition. Therefore, exploring the physiological function of OBPs could facilitate a better understanding of insect chemical communications.

RESULTS

In this research, 36 BhorOBPs genes were identified via transcriptome sequencing of adults' antennae from B. horsfieldi, and most BhorOBPs were predominantly expressed in chemosensory body parts. Through fluorescence competitive binding and fluorescence quenching assays, the antenna-specific BhorOBP28 was investigated and displayed strong binding affinities forming stable complexes with five volatiles, including (+)-α-Pinene, (+)-Limonene, β-Pinene, (-)-Limonene, and (+)-Longifolene, which could also elicit conformation changes when they were interacting with BhorOBP28. Batocera horsfieldi females exhibited a preference for (-)-Limonene, and a repellent response to (+)-Longifolene. Feeding dsOBP19 produced by a bacteria-expressed system with a newly constructed vector could lead to the knockdown of BhorOBP28, and could further impair B. horsfieldi attraction to (-)-Limonene and repellent activity of (+)-Longifolene. The analysis of site-directed mutagenesis revealed that Leu7, Leu72, and Phe121 play a vital role in selectively binding properties of BhorOBP28.

CONCLUSION

By modeling the molecular mechanism of olfactory recognition, these results demonstrate that BhorOBP28 is involved in the chemoreception of B. horsfieldi. The bacterial-expressed dsRNA delivery system gains new insights into potential population management strategies. Through the olfactory process concluded that discovering novel behavioral regulation and environmentally friendly control options for B. horsfieldi in the future. © 2024 Society of Chemical Industry.

Interaction between females and males grapevine moth Lobesia botrana modifies further mating preference

During reproduction, females may boost their fitness by being selective based on direct material benefits provided by the males, such as nuptial gifts. In Lepidoptera, male provides a spermatophore containing nutrients. However, virgin males produce a bigger spermatophore, containing spermatozoa and nutrients, allowing higher female fertility. Lepidoptera females that could detect the sexual status of males may thus prefer a male without previous mating experience (i.e. a virgin male). This mate selection could be achieved by the use of chemical indices, such as sexual pheromones and cuticular compounds, known to be possibly exchanged during reproduction, and which can be indicators of a previous mating experience and known to be possibly sources of information exchanged. In this study, we experimentally presented Lobesia botrana virgin males with females in order for them to be exposed to females' natural sexual pheromones or cuticular compounds. 12 or 48 h after the exposure of males to either females' sexual pheromones or cuticular compounds, these males were confronted to naïve females, which have a choice between them or a virgin non-exposed males. We highlighted that, despite producing a spermatophore of similar volume, all exposed virgin males were less likely to mate with females 12 h after exposure, while after 48 h of exposure this is only the case for virgin males exposed to sexual pheromones. L. botrana females may thus discriminate male sexual experience based on chemical cues (either from cues transferred directly from females to males, or from changes in the cuticular or pheromone males' profile) indicating past mating experiences. Mating duration was longer for males exposed to sexual pheromones after 12 h only, and for males exposed to cuticular compounds after 48 h only. Pheromones signal might be more persistent over time and seems to more easily gather information for males. The physiological reasoning behind this result still needs to be investigated.

The Plant Volatile-Sensing Mechanism of Insects and Its Utilization

Plants and insects are engaged in a tight relationship, with phytophagous insects often utilizing volatile organic substances released by host plants to find food and egg-laying sites. Using plant volatiles as attractants for integrated pest management is vital due to its high efficacy and low environmental toxicity. Using naturally occurring plant volatiles combined with insect olfactory mechanisms to select volatile molecules for screening has proved an effective method for developing plant volatile-based attractant technologies. However, the widespread adoption of this technique is still limited by the lack of a complete understanding of molecular insect olfactory pathways. This paper first describes the nature of plant volatiles and the mechanisms of plant volatile perception by insects. Then, the attraction mechanism of plant volatiles to insects is introduced with the example of Cnaphalocrocis medinalis. Next, the progress of the development and utilization of plant volatiles to manage pests is presented. Finally, the functions played by the olfactory system of insects in recognizing plant volatiles and the application prospects of utilizing volatiles for green pest control are discussed. Understanding the sensing mechanism of insects to plant volatiles and its utilization will be critical for pest management in agriculture.

Key Amino Acid Residues Involved in Binding Interactions between Bactrocera minax Odorant-Binding Protein 3 (BminOBP3) and Undecanol

Insect odorant-binding proteins (OBPs) are significant in binding and transporting odorants to specific receptors. Our previous study demonstrated that BminOBP3 exhibited a strong affinity with undecanol. However, the binding mechanism between them remains unknown. Here, using homology modeling and molecular docking, we found that the C-terminus (I116-P122), especially the hydrogenbonds formed by the last three amino acid residues (V120, F121, and P122) of the C-terminus, is essential for BminOBP3's ligand binding. Mutant binding assays showed that the mutant T-OBP3 that lacks C-terminus (I116-P122) displayed a significant decrease in affinity to undecanol (Ki = 19.57 ± 0.45) compared with that of the wild-type protein BminOBP3 (Ki = 11.59 ± 0.51). In the mutant 3D2a that lacks F121 and P122 and the mutant V120A in which V120 was replaced by alanine, the bindings to undecanol were completely abolished. In conclusion, the C-terminus plays a crucial role in the binding interactions between BminOBP3 and undecanol. Based on the results, we discussed the ligand-binding process of BminOBP3.

Antennae-enriched expression of candidate odorant degrading enzyme genes in the turnip aphid, Lipaphis erysimi

Aphids heavily rely on their olfactory system for foraging behavior. Odorant-degrading enzymes (ODEs) are essential in preserving the olfactory acuity of aphids by removing redundant odorants in the antennae. Certain enzymes within this group stand out as being enriched and/or biased expressed in the antennae, such as carboxylesterases (CXEs), cytochrome P450 (CYPs), glutathione S-transferases (GSTs), and UDP-glycosyltransferases (UGTs). Here, we performed a comparative transcriptome analysis of antennae and body tissue to isolate the antennal ODE genes of turnip aphid Lipaphis erysimi. A dataset of one CXE, seven CYPs, two GSTs, and five UGTs enriched in the antennae was identified and subjected to sequence analysis. Furthermore, qRT-PCR analyses showed that 13 ODE genes (LeCXE6, LeCYP4c1, LeCYP6a2, LeCYP6a13, LeCYP6a14.2, LeCYP6k1, LeCYP18a1, LeGST1, LeUGT1-7, LeUGT2B7, LeUGT2B13, LeUGT2C1.1, and LeUGT2C1.2) were specifically or significantly elevated in antennal tissues. Among these antennae-enriched ODEs, LeCYP4c1, LeCYP6a2, LeCYP6a13, LeCYP6a14.2, LeCYP18a1, LeUGT2B7, and LeUGT2B13 were found to exhibit significantly higher expression levels in alate aphids compared to apterous and nymph aphids, suggesting their putative role in detecting new host plant location. The results presented in this study highlight the identification and expression of ODE genes in L. erysimi, paving the path to investigate their functional role in odorant degradation during the olfactory processes.

Modulation of the NO-cGMP pathway has no effect on olfactory responses in the Drosophila antenna

Olfaction is a crucial sensory modality in insects and is underpinned by odor-sensitive sensory neurons expressing odorant receptors that function in the dendrites as odorant-gated ion channels. Along with expression, trafficking, and receptor complexing, the regulation of odorant receptor function is paramount to ensure the extraordinary sensory abilities of insects. However, the full extent of regulation of sensory neuron activity remains to be elucidated. For instance, our understanding of the intracellular effectors that mediate signaling pathways within antennal cells is incomplete within the context of olfaction in vivo. Here, with the use of optical and electrophysiological techniques in live antennal tissue, we investigate whether nitric oxide signaling occurs in the sensory periphery of Drosophila. To answer this, we first query antennal transcriptomic datasets to demonstrate the presence of nitric oxide signaling machinery in antennal tissue. Next, by applying various modulators of the NO-cGMP pathway in open antennal preparations, we show that olfactory responses are unaffected by a wide panel of NO-cGMP pathway inhibitors and activators over short and long timescales. We further examine the action of cAMP and cGMP, cyclic nucleotides previously linked to olfactory processes as intracellular potentiators of receptor functioning, and find that both long-term and short-term applications or microinjections of cGMP have no effect on olfactory responses in vivo as measured by calcium imaging and single sensillum recording. The absence of the effect of cGMP is shown in contrast to cAMP, which elicits increased responses when perfused shortly before olfactory responses in OSNs. Taken together, the apparent absence of nitric oxide signaling in olfactory neurons indicates that this gaseous messenger may play no role as a regulator of olfactory transduction in insects, though may play other physiological roles at the sensory periphery of the antenna.

Two Antenna-Enriched Carboxylesterases Mediate Olfactory Responses and Degradation of Ester Volatiles in the German Cockroach Blattella germanica

Insects have evolved an extremely sensitive olfactory system that is essential for a series of physiological and behavioral activities. Some carboxylesterases (CCEs) comprise a major subfamily of odorant-degrading enzymes (ODEs) playing a crucial role in odorant signal inactivation to maintain the odorant receptor sensitivity. In this study, 93 CCEs were annotated in the genome of the German cockroach Blattella germanica, a serious urban pest. Phylogenetic and digital tissue expression pattern analyses identified two antenna-enriched CCEs, BgerCCE021e3 and BgerCCE021d1, as candidate ODEs. RNA interference (RNAi)-mediated knockdown of BgerCCE021e3 and BgerCCE021d1 resulted in partial anosmia with experimental insects exhibiting reduced attraction to ester volatile resources and slower olfactory responses than controls. Furthermore, enzymatic conversion of geranyl acetate by crude male antennal extracts from BgerCCE021e3 and BgerCCE021d1 RNAi insects was also significantly reduced. Our results provide evidence for CCE function in German cockroach olfaction and provide a basis for further exploring behavioral inhibitors that target olfactory-related CCEs.

Transfer of knowledge from model organisms to evolutionarily distant non-model organisms: The coral Pocillopora damicornis membrane signaling receptome

With the ease of gene sequencing and the technology available to study and manipulate non-model organisms, the extension of the methodological toolbox required to translate our understanding of model organisms to non-model organisms has become an urgent problem. For example, mining of large coral and their symbiont sequence data is a challenge, but also provides an opportunity for understanding functionality and evolution of these and other non-model organisms. Much more information than for any other eukaryotic species is available for humans, especially related to signal transduction and diseases. However, the coral cnidarian host and human have diverged over 700 million years ago and homologies between proteins in the two species are therefore often in the gray zone, or at least often undetectable with traditional BLAST searches. We introduce a two-stage approach to identifying putative coral homologues of human proteins. First, through remote homology detection using Hidden Markov Models, we identify candidate human homologues in the cnidarian genome. However, for many proteins, the human genome alone contains multiple family members with similar or even more divergence in sequence. In the second stage, therefore, we filter the remote homology results based on the functional and structural plausibility of each coral candidate, shortlisting the coral proteins likely to have conserved some of the functions of the human proteins. We demonstrate our approach with a pipeline for mapping membrane receptors in humans to membrane receptors in corals, with specific focus on the stony coral, P. damicornis. More than 1000 human membrane receptors mapped to 335 coral receptors, including 151 G protein coupled receptors (GPCRs). To validate specific sub-families, we chose opsin proteins, representative GPCRs that confer light sensitivity, and Toll-like receptors, representative non-GPCRs, which function in the immune response, and their ability to communicate with microorganisms. Through detailed structure-function analysis of their ligand-binding pockets and downstream signaling cascades, we selected those candidate remote homologues likely to carry out related functions in the corals. This pipeline may prove generally useful for other non-model organisms, such as to support the growing field of synthetic biology.

Stringent in-silico identification of putative G-protein-coupled receptors (GPCRs) of the entomopathogenic nematode Heterorhabditis bacteriophora

The infective juveniles (IJs) of entomopathogenic nematode (EPN) Heterorhabditis bacteriophora find and infect their host insects in heterogeneous soil ecosystems by sensing a universal host cue (CO2) or insect/plant-derived odorants, which bind to various sensory receptors, including G protein-coupled receptors (GPCRs). Nematode chemosensory GPCRs (NemChRs) bind to a diverse set of ligands, including odor molecules. However, there is a lack of information on the NemChRs in EPNs. Here we identified 21 GPCRs in the H. bacteriophora genome sequence in a triphasic manner, combining various transmembrane detectors and GPCR predictors based on different algorithms, and considering inherent properties of GPCRs. The pipeline was validated by reciprocal BLAST, InterProscan, GPCR-CA, and NCBI CDD search. Functional classification of predicted GPCRs using Pfam revealed the presence of four NemChRs. Additionally, GPCRs were classified into various families based on the reciprocal BLAST approach into a frizzled type, a secretin type, and 19 rhodopsin types of GPCRs. Gi/o is the most abundant kind of G-protein, having a coupling specificity to all the fetched GPCRs. As the 21 GPCRs identified are expected to play a crucial role in the host-seeking behavior, these might be targeted to develop novel insect-pest management strategies by tweaking EPN IJ behavior, or to design novel anthelminthic drugs. Our new and stringent GPCR detection pipeline may also be used to identify GPCRs from the genome sequence of other organisms.

Identification and expression profile analysis of chemosensory genes in pine needle gall midge, Thecodiplosis japonensis (Diptera: Cecidomyiidae)

Insects have highly specialized and sensitive olfactory systems involving several chemosensory genes to locate their mates and hosts or escape from predators. Pine needle gall midge, Thecodiplosis japonensis (Diptera: Cecidomyiidae), has invaded China since 2016 and caused serious damage. Till now, there is no environmentally friendly measure to control this gall midge. Screening molecules with high affinity to target odorant-binding protein to develop highly efficient attractants is a potential pest management method. However, the chemosensory genes in T. japonensis are still unclear. We identified 67 chemosensory-related genes in the transcriptomes of antennae, including 26 OBPs, 2 CSPs, 17 ORs, 3 SNMPs, 6 GRs, and 13 IRs, using high throughput sequencing. Phylogenetic analysis of these six chemosensory gene families among Dipteran was performed to classify and predict the functions. The expression profiles of OBPs, CSPs and ORs were validated by quantitative real-time PCR. 16 of the 26 OBPs were biased expressed in antennae. TjapORco and TjapOR5 were highly expressed in the antenna of unmated male and female adults. The functions of related OBPs and ORs genes were also discussed. These results provide a basis for the functional research on chemosensory genes at the molecular level.

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

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

An odorant binding protein mediates Bactrocera dorsalis olfactory sensitivity to host plant volatiles and male attractant compounds

Odorant-binding proteins (OBPs) are believed to play critical roles in host-seeking behavior. However, little attention was paid to its different functions in male and female. The antenna-specific OBP gene from Bactrocera dorsalis, BdorOBP13, was cloned and its expression profile was examined. The results showed that BdorOBP13 was exclusively expressed in male and female adults, which exhibited a high transcript level in antennae. After injection of BdorOBP13 dsRNA, its transcript level in males and females decreased significantly. Electrophysiological responses of RNAi-injected flies to, methyl eugenol (male attractant) and γ-octalactone (female attractant) decreased significantly. However, no significant changes in the electrophysiological response were observed in RNAi-injected flies to benzothiazole, (+),dipentene, and ethyl tiglate. The behavioral bioassay showed that males treated with RNAi significantly reduced their preference to methyl eugenol, while RNAi-injected females showed a significantly lower preference to γ-octalactone, suggesting that BdorOBP13 may have different functions between males and females: it may be involved in the detection of methyl eugenol in males but is involved in the detection of γ-octalactone in females. These findings improve our understanding of insect OBPs and their roles in insect chemosensation, which may provide us with new molecular targets in the management of B. dorsalis.

Invertebrate neurones, genomes, phenotypic and target-based screening; their contributions to the search for new chemical leads and new molecular targets for the control of pests, parasites and disease vectors

Insect-borne diseases of humans, animals and plants can be devastating. The direct damage to crops by insect and nematode pests can also severely reduce crop yields and threaten harvests. Parasitic nematodes can impair human health and the health of farm livestock. Effective control for all such pests, vectors and pathogens is required as the economic and health burden can be substantial. Insecticides, nematicides and anthelmintics have been at the forefront of control and will remain important in the immediate future, even as we explore new and more sustainable methods to maintain the necessary disease control and the growth in food supply. Many important chemicals deployed for the control of invertebrate disease vectors and pathogens of humans, agricultural crops and farm livestock are active on ion channels, resulting in rapid actions. Understanding their modes of action has been accelerated by studies on the physiology of identifiable invertebrate excitable cells. Nematode and insect genetic model organisms and comparative genomics have contributed to defining the molecular targets of insecticides and anthelmintics, facilitating target-based screening. Automated phenotyping, which allows high-throughput screening of chemical libraries for new and re-purposed compounds, has been increasingly deployed in the search for new molecules of interest. With a growing world population to be fed and a 20-49% loss of global harvest to pests, we need to maintain control of the pests, parasites and pathogens that threaten global food supply and global health.

Identification of Genes Associated with Crest Cushion Development in the Chinese Crested Duck

The crest trait is a specific and widely distributed phenotype in birds. However, the shape and function vary in different species of birds. To understand the mechanism of crest formation, the present study used RNA sequencing and weighted gene co-expression network analysis (WGCNA) to identify the crest-cushion-associated genes in the Chinese crested (CC) duck. As a result, 28, 40, 32, 33, and 126 differentially expressed genes (DEGs) were identified between CC and cherry valley (CV) ducks at the embryonic days (E)15, E22, E28, D7 (7 days old), and D42 stages, respectively. In addition, the results of WGCNA show that 3697 (turquoise module), 485 (green-yellow module), 687 (brown module), 205 (red module), and 1070 (yellow module) hub genes were identified in the E15, E22, E28, D7, and D42 stages, respectively. Based on the results of DEGs and WGCNA Venn analysis, three, two, zero, one, and seven genes were found to be associated with crest cushion formation at the E15, E22, E28, D7, and D42 stages, respectively. The expression of all the associated genes and some DEGs was verified by real-time quantitative polymerase chain reaction. In conclusion, this study provided an approach revealing the molecular mechanisms underlying the crested trait development.

Seed choice in ground beetles is driven by surface-derived hydrocarbons

Ground beetles (Coleoptera: Carabidae) are among the most prevalent biological agents in temperate agroecosystems. Numerous species function as omnivorous predators, feeding on both pests and weed seeds, yet the sensory ecology of seed perception in omnivorous carabids remains poorly understood. Here, we explore the sensory mechanisms of seed detection and discrimination in four species of omnivorous carabids: Poecilus corvus, Pterostichus melanarius, Harpalus amputatus, and Amara littoralis. Sensory manipulations and multiple-choice seed feeding bioassays showed olfactory perception of seed volatiles as the primary mechanism used by omnivorous carabids to detect and distinguish among seeds of Brassica napus, Sinapis arvensis, and Thlaspi arvense (Brassicaceae). Seed preferences differed among carabid species tested, but the choice of desirable seed species was generally guided by the olfactory perception of long chain hydrocarbons derived from the seed coat surface. These olfactory seed cues were essential for seed detection and discrimination processes to unfold. Disabling the olfactory appendages (antennae and palps) of carabid beetles by ablation left them unable to make accurate seed choices compared to intact beetles.

Ground beetles are generalist predators of various arable weed seeds. Sensory manipulations and multiple-choice feeding bioassays show that seed choice is stimulated by volatile chemicals derived from the epicuticular lipids on the seed coat.

Identification and characterization of olfactory receptor genes and olfactory perception in rapa whelk Rapana venosa (Valenciennes, 1846) during larval settlement and metamorphosis

The rapa whelk Rapana venosa, an economically important marine fishery resource in China but a major invader all over the world, changes from a phytophagous to a carnivorous form following settlement and metamorphosis. However, the low settlement and metamorphosis rates (<1%) of larvae limit the abundance of R. venosa. This critical step (settlement and metamorphosis) remains poorly characterized but may be related to how larvae perceive the presence of shellfish, their new source of food. Here, we report that larvae may use olfactory perception to sense shellfish. Olfactory receptor (OR) genes are involved in odor sensing in animals. We identified a total of 463 OR genes, which could be grouped into nine clades based on phylogenetic analysis. When assessing the attraction of larvae at different developmental stages to oyster odor, R. venosa showed active settlement and metamorphosis behavior only at the J4 stage (competent larva, 1000-1500 μm shell length) and in the presence of shellfish odor at the same time. Expression of OR gene family members differed between stage 2 (four-spiral whorl stage) and stage 1 (single- to three-spiral whorl stage), indicating significant changes in the olfactory system during larval development. These findings broaden our understanding of olfactory perception, settlement, and metamorphosis in gastropods and can be used to improve R. venosa harvesting, as well as the sustainable development and utilization of this resource.

Find My Way to You: A Comparative Study of Antennal Sensilla and Olfactory Genes in Slug Moth With Different Diet Ranges (Lepidoptera: Limacodidae)

Insects and plants that provide them with foods have coexisted for several hundred million years, which leads to various defense approaches and insect-feeding strategies. The host plant provides insects with food sources, shelter materials, and oviposition sites for phytophagous insects. However, they need to find the most suitable host plants in complicated plant communities. The antenna is the main sensory organ of insects, housing different types of sensilla dedicated to detecting chemical cues, motion, humidity, and temperature. Phytophagous insects with different diets may possess various adaptations in their olfactory system. We selected three species of slug moth (Narosoideus flavidorsalis, Chalcoscelides castaneipars, and Setora postornata) with different diet breadths to detect the structural diversity of antennal sensilla using the scanning electron microscope. A total of nine types of sensilla were identified in these three species, in which two types of sensilla (sensilla uniporous peg and sensilla furcatea) were the first found and reported in Limacodidae. By comparing the number of sensilla types, there was a trend of gradually decreasing the number of sensory types with the gradual expansion of feeding habitats. To better understand the vital roles of olfactory proteins in localizing host plants, we investigated the chemosensory proteins in the antennal transcriptomes of N. flavidorsalis and S. postornata. However, there was no significant correlation between the number of olfactory genes and the increase of antennal sensilla types. Combining antennal morphology, transcriptome analysis, and the prediction of suitable areas, we better understood the olfactory systems with different feeding preferences, which will provide new prospects for plant–insect interactions and population control methods.

Identification and characterization of olfactory genes in the parasitoid wasp Diadegma semiclausum (Hellén) (Hymenoptera: Ichneumonidae)

Diadegma semiclausum is an important parasitoid wasp and widely used in the biological control of the diamondback moth, Plutella xylostella, one of the most destructive pests of cruciferous plants. Insect olfactory system is critical in guiding behaviors including feeding, mating, and oviposition, in which odorant binding proteins (OBPs) and odorant receptors (ORs) are two key components. However, limited attention has been paid to D. semiclausum olfactory genes. In this study, a transcriptome sequencing was performed on the RNA samples extracted from D. semiclausum male and female adult antennae. A total of 17 putative OBP and 67 OR genes were annotated and further compared to OBPs and ORs from P. xylostella, and other hemipteran parasitoid species. The expression patterns of D. semiclausum OBPs between male and female antennae were examined using reverse transcription polymerase chain reaction (RT-PCR) and quantitative real-time PCR. Six OBPs (DsemOBP 6, 7, 8, 9, 10, and 14) demonstrated significantly higher expression levels in females than in males, which may assist in female D. semiclausum host-seeking and oviposition behaviors. This study advances our understanding of the olfactory system of D. semiclausum at the molecular level and paves the way for future functional studies aiming at increasing the efficacy to control P. xylostella by using D. semiclausum.

The molecular identification, odor binding characterization, and immunolocalization of odorant-binding proteins in Liriomyza trifolii

The Liriomyza trifolii is a highly invasive polyphagia pest. Understanding the physiological functions of odorant binding proteins (OBPs) in the chemical communication of L. trifolii can lead to effective pest management strategies. Seven full-length OBPs were identified by transcriptome screening of L. trifolii adults. Bioinformatics analyses classified the seven OBPs into two subfamilies (six classic OBPs, one minus-C OBP). The analysis of their expression in different development stages revealed that LtriOBP5 was highly expressed in the larval stage, LtriOBP4 in the pupa stage, and LtriOBP1, 2, 3, 6, 7 in the adult stage; the expression levels were higher in male adults than in females. The analysis of different tissues showed high expression of LtriOBP1, 3, 6, 7 in the antennae, which were selected for in vitro purification. To explore the ligand compounds of OBPs, fluorescence competitive binding experiments were performed. Immunofluorescence localization revealed that LtriOBP1, 3, 6, 7 showed strong binding abilities to plant volatiles and were located in the antennae, implying that LtriOBP1, 3, 6, 7 may play key roles in olfaction, such as host location. LtriOBP6 and LtriOBP7 had strong binding abilities to specific herbivore-induced plant volatiles, suggesting LtriOBP6 and LtriOBP7 may also play critical roles in chemoreception. This study provides preliminary exploration of the olfactory perception mechanism of L. trifolii, which can be used as a basis to design insect behavior regulators and develop highly effective insecticides using mixture of ligands and known pesticides.

Functional Interaction Between Drosophila Olfactory Sensory Neurons and Their Support Cells

Insects detect volatile chemicals using antennae, which house a vast variety of olfactory sensory neurons (OSNs) that innervate hair-like structures called sensilla where odor detection takes place. In addition to OSNs, the antenna also hosts various support cell types. These include the triad of trichogen, tormogen, and thecogen support cells that lie adjacent to their respective OSNs. The arrangement of OSN supporting cells occurs stereotypically for all sensilla and is widely conserved in evolution. While insect chemosensory neurons have received considerable attention, little is known about the functional significance of the cells that support them. For instance, it remains unknown whether support cells play an active role in odor detection, or only passively contribute to homeostasis, e.g., by maintaining sensillum lymph composition. To investigate the functional interaction between OSNs and support cells, we used optical and electrophysiological approaches in Drosophila. First, we characterized the distribution of various supporting cells using genetic markers. By means of an ex vivo antennal preparation and genetically-encoded Ca²⁺ and K⁺ indicators, we then studied the activation of these auxiliary cells during odor presentation in adult flies. We observed acute responses and distinct differences in Ca²⁺ and K⁺ fluxes between support cell types. Finally, we observed alterations in OSN responses upon thecogen cell ablation in mature adults. Upon inducible ablation of thecogen cells, we notice a gain in mechanical responsiveness to mechanical stimulations during single-sensillum recording, but a lack of change to the neuronal resting activity. Taken together, these results demonstrate that support cells play a more active and responsive role during odor processing than previously thought. Our observations thus reveal that support cells functionally interact with OSNs and may be important for the extraordinary ability of insect olfactory systems to dynamically and sensitively discriminate between odors in the turbulent sensory landscape of insect flight.

Chemosensory Proteins (CSPs) in the Cotton Bollworm Helicoverpa armigera

Simple Summary

The insect chemosensory system is crucial in regulating insect behaviors. Chemosensory proteins (CSPs) are a family of small, soluble proteins conventionally known to transport odorant molecules in insect chemosensory system. Besides chemosensation, CSPs have been reported to play important roles in development, nutrient metabolism, and insecticide resistance. Therefore, identification and characterization of previously unknown CSPs will be valuable for further investigation of this protein family. The cotton bollworm, Helicoverpa armigera (Hübner) is among the most serious insect pests in various agricultural and horticultural crops. In this study, 27 CSP genes were identified from H. armigera genome and transcriptome sequences, and their expression patterns were further examined by using transcriptomic data obtained from different tissues and stages. The results demonstrate that H. armigera CSP genes are highly expressed in both chemosensory and non-chemosensory tissues. Moreover, a new recombinant expression method was developed that can significantly increase H. armigera CSP expression levels as soluble proteins in Escherichia coli. This study improves our understanding of insect CSPs and developed a new approach to highly express recombinant CSPs, which can be expanded to examine CSPs in other species for functional characterization.

Abstract

Chemosensory proteins (CSPs) are a family of small, soluble proteins that play a crucial role in transporting odorant and pheromone molecules in the insect chemosensory system. Recent studies reveal that they also function in development, nutrient metabolism and insecticide resistance. In-depth and systematic characterization of previously unknown CSPs will be valuable to investigate more detailed functionalities of this protein family. Here, we identified 27 CSP genes from the genome and transcriptome sequences of cotton bollworm, Helicoverpa armigera (Hübner). The expression patterns of these genes were studied by using transcriptomic data obtained from different tissues and stages. The results demonstrate that H. armigera CSP genes are not only highly expressed in chemosensory tissues, such as antennae, mouthparts, and tarsi, but also in the salivary glands, cuticle epidermis, and hind gut. HarmCSP6 and 22 were selected as candidate CSPs for expression in Escherichia coli and purification. A new method was developed that significantly increased the HarmCSP6 and 22 expression levels as soluble recombinant proteins for purification. This study advances our understanding of insect CSPs and provides a new approach to highly express recombinant CSPs in E. coli.

Design and Synthesis of Fluorophore-Tagged Disparlure Enantiomers to Study Pheromone Enantiomer Discrimination in the Pheromone-Binding Proteins from the Gypsy Moth, Lymantria dispar

Fluorescent analogues of the gypsy moth sex pheromone (+)-disparlure (1) and its enantiomer (-)-disparlure (ent-1) were designed, synthesized, and characterized. The fluorescently labelled analogues 6-FAM (+)-disparlure and 1a 6-FAM (-)-disparlure ent-1a were prepared by copper-catalyzed azide-alkyne cycloaddition of disparlure alkyne and 6-FAM azide. These fluorescent disparlure analogues 1a and ent-1a were used to measure disparlure binding to two pheromone-binding proteins from the gypsy moth, LdisPBP1 and LdisPBP2. The fluorescence binding assay showed that LdisPBP1 has a stronger affinity for 6-FAM (-)-disparlure ent-1a, whereas LdisPBP2 has a stronger affinity for 6-FAM (+)-disparlure 1a, consistent with findings from previous studies with disparlure enantiomers. The 6-FAM disparlure enantiomers appeared to be much stronger ligands for LdisPBPs, with binding constants (K_d) in the nanomolar range, compared to the fluorescent reporter 1-NPN (which had K_d values in the micromolar range). Fluorescence competitive binding assays were used to determine the displacement constant (K_i) for the disparlure enantiomers in competition with fluorescent disparlure analogues binding to LdisPBP1 and LdisPBP2. The K_i data show that disparlure enantiomers can effectively displace the fluorescent disparlure from the binding pocket of LdisPBPs and, therefore, occupy the same binding site.

The biology of seed discrimination and its role in shaping the foraging ecology of carabids: A review

Species of carabid (ground) beetles are among the most important postdispersal weed seed predators in temperate arable lands. Field studies have shown that carabid beetles can remove upwards of 65%-90% of specific weed seeds shed in arable fields each year. Such data do not explain how and why carabid predators go after weed seeds, however. It remains to be proven that weed seed predation by carabids is a genuine ecological interaction driven by certain ecological factors or functional traits that determine interaction strength and power predation dynamics, bringing about therefore a natural regulation of weed populations. Along these lines, this review ties together the lines of evidence around weed seed predation by carabid predators. Chemoperception rather than vision seems to be the primary sensory mechanism guiding seed detection and seed selection decisions in carabid weed seed predators. Selection of weed seeds by carabid seed predators appears directed rather than random. Yet, the nature of the chemical cues mediating detection of different seed species and identification of the suitable seed type among them remains unknown. Selection of certain types of weed seeds cannot be predicted based on seed chemistry per se in all cases, however. Rather, seed selection decisions are ruled by sophisticated behavioral mechanisms comprising the assessment of both chemical and physical characteristics of the seed. The ultimate selection of certain weed seed types is determined by how the chemical and physical properties of the seed match with the functional traits of the predator in terms of seed handling ability. Seed density, in addition to chemical and physical seed traits, is also an important factor that is likely to shape seed selection decisions in carabid weed seed predators. Carabid responses to seed density are rather complex as they are influenced not only by seed numbers but also by trait-based suitability ranks of the different seed types available in the environment.

Identification and characterization of odorant binding proteins in the diamondback moth, Plutella xylostella

Odorant binding proteins (OBPs) are a group of soluble proteins functioning as odorant carriers in insect antennae, mouth parts and other chemosensory organs. However, multiple insect OBPs have been detected in other tissues and various functions have been proposed. Therefore, a detailed expression profile including stages, tissues and sexes where OBPs are expressed will assist in building the links to their potential functions, enhancing the functional studies of insect OBPs. Here, we identified 39 putative OBP genes from its genome and transcriptome sequences of diamondback moth (DBM), Plutella xylostella. The expression patterns of identified PxylOBPs were further investigated from eggs, larvae, pupae, virgin adults, mated adults, larval midgut, larval heads, adult antennae, adult heads and adult tarsi. Moreover, P. xylostella larvae and adults with and without host plants for 5 h were utilized to study the interactions between OBP expression and host plants. The results showed that most PxylOBPs were highly expressed in male and female adult antennae. The expression levels of certain PxyOBPs could be regulated by mating activities and feeding host plants. This study advances our knowledge of P. xylostella OBPs, which may help develop new strategies for more environmentally sustainable management of P. xylostella.

Pain, Smell, and Taste in Adults: A Narrative Review of Multisensory Perception and Interaction

Every day our sensory systems perceive and integrate a variety of stimuli containing information vital for our survival. Pain acts as a protective warning system, eliciting a response to remove harmful stimuli; it may also be a symptom of an illness or present as a disease itself. There is a growing need for additional pain-relieving therapies involving the multisensory integration of smell and taste in pain modulation, an approach that may provide new strategies for the treatment and management of pain. While pain, smell, and taste share common features and are strongly linked to emotion and cognition, their interaction has been poorly explored. In this review, we provide an overview of the literature on pain modulation by olfactory and gustatory substances. It includes adult human studies investigating measures of pain threshold, tolerance, intensity, and/or unpleasantness. Due to the limited number of studies currently available, we have structured this review as a narrative in which we comment on experimentally induced and clinical pain separately on pain–smell and pain–taste interaction. Inconsistent study findings notwithstanding, pain, smell, and taste seem to interact at both the behavioral and the neural levels. Pain intensity and unpleasantness seem to be affected more by olfactory substances, whereas pain threshold and tolerance are influenced by gustatory substances. Few pilot studies to date have investigated these effects in clinical populations. While the current results are promising for the future, more evidence is needed to elucidate the link between the chemical senses and pain. Doing so has the potential to improve and develop novel options for pain treatment.

Functional properties of insect olfactory receptors: ionotropic receptors and odorant receptors

The majority of insect olfactory receptors belong to two distinct protein families, the ionotropic receptors (IRs), which are related to the ionotropic glutamate receptor family, and the odorant receptors (ORs), which evolved from the gustatory receptor family. Both receptor types assemble to heteromeric ligand-gated cation channels composed of odor-specific receptor proteins and co-receptor proteins. We here present in short the current view on evolution, function, and regulation of IRs and ORs. Special attention is given on how their functional properties can meet the environmental and ecological challenges an insect has to face.

Plant volatiles mediate evolutionary interactions between plants and tephritid flies and are evolutionarily more labile than non-volatile defenses

Studies show that plant defenses influence the host-use of herbivores and tend to be evolutionarily more labile than herbivore traits (e.g. feeding preferences). However, all previous studies have focused exclusively on non-volatile plant defenses thereby overlooking the roles of plant volatiles. We hypothesized that volatiles are equally important determinants of herbivore host-use and are evolutionarily more labile than herbivore traits. To test these hypotheses, the following experiments were conducted. We identified the volatiles and non-volatiles of 17 Asteraceae species and measured their relative contents. We also used a highly resolved bipartite trophic network of the 17 host species and 20 herbivorous (pre-dispersal seed predator) tephritid fly species to determine the evolutionary interactions between plants and herbivores. The chemical data showed that interspecific similarity in volatiles-but not non-volatiles and phylogenetic distance-significantly accounted for the herbivore community across the plant species; this implies that plant volatiles-but not non-volatile compounds and species identity-dictate plant-tephritid fly interactions. Moreover, we observed phylogenetic signal for non-volatiles but not for volatiles; therefore closely related herbivores do not necessarily use closely related host species with similar non-volatiles, but do tend to attack plants producing similar volatiles. Thus, plant volatiles are evolutionarily more labile than non-volatiles and herbivore traits associate with host use. These results show that the interactions between plants and herbivores are evolutionary asymmetric, shed light on the role of plant volatiles in plant-herbivore interactions, and highlight the need to include data for both volatiles and non-volatiles when investigating plant-animal interactions.

Structural evidence for pheromone discrimination by the pheromone binding protein 3 from Plutella xylostella

Insect pheromone binding proteins (PBPs) are believed to have a high degree of pheromone selectivity, acting as the first filter to discriminate specific pheromones from other volatile compounds. Herein, we provide evidence using homology-based model for the pheromone discrimination of Plutella xylostella pheromone binding protein 3 (PxPBP3). Combining molecular dynamics simulations and in vitro binding assays, two dominant sites are determined to be essential for the PxPBP3 to discriminate (Z)-11-hexadecenyl acetate (Hexadecenyl) from (Z)-11-hexadecenal (Hexadecenal). As the first key site for pheromone discrimination, Arg111 is indispensable to the PxPBP3-Hexadecenyl interaction. However, its importance in the binding of Hexadecenal to PxPBP3 is greatly reduced. A second site where pheromone discrimination occurs is a small loop (residues 34-38) in PxPBP3. It is shown that the hydrophobic strength provided by three hydrophobic residues (Phe34, Tyr37, and Trp38) in the small loop is significantly biased in the two complexes formed by PxPBP3 and the two pheromones. The discrimination capacity of PxPBP3 indicates that the P. xylostella pheromones may not share the same peri-receptor pathway, although they both show high affinity to PxPBP3.

Expression, purification and characterization of three odorant binding proteins from the diamondback moth, Plutella xylostella

Odorant binding proteins (OBPs) are critical components in insect olfactory systems where they bind, solubilize and transport odorant molecules to receptors. Here, we cloned three OBPs (PxylGOBP1, PxylGOBP2 and PxylOBP24) from the diamondback moth, Plutella xylostella, one of the most destructive pests of cruciferous crops. These three OBPs were expressed in Escherichia coli as recombinant proteins, purified and characterized by fluorescence binding assays with 39 ligands including sex pheromone and plant-derived chemical compounds. PxylGOBP1 and PxylGOBP2 showed significantly different binding affinities to theses ligands, suggesting distinct binding preferences of these two general odorant binding proteins. PxylOBP24 showed no or extremely low binding activities to selected ligands, suggesting it may be involved in non-olfactory functions. Circular dichroism spectral results demonstrated that PxylGOBP1 and PxylGOBP2 shared similar secondary structures while PxylOBP24 was significantly different. This study improves our knowledge of insect OBPs, which will assist in a better understanding of insect olfactory system and developing more environmentally friendly pest control strategies for P. xylostella.

Odorant-binding proteins involved in sex pheromone and host-plant recognition of the sugarcane borer Chilo infuscatellus (Lepidoptera: Crambidae)

BACKGROUND

Pheromone-binding proteins (PBPs) are responsible for transporting sex pheromones and general odorant-binding proteins (GOBPs) have been proposed to transport host-plant volatiles. A large number of OBPs have been identified from Lepidoptera species. However, olfactory molecular biology and physiology studies on PBP and GOBP in sugarcane pests are limited. Chilo infuscatellus is one of the most widely distributed pests in sugarcane-producing areas.

RESULTS

Three PBPs (CinfPBP1, CinfPBP2 and CinfPBP3) and two GOBPs (CinfGOBP1 and CinfGOBP2) were identified, and five olfactory gene transcripts were abundantly expressed in antennae of C. infuscatellus. Binding assays showed that CinfPBP1-3 exhibited strong binding affinity for the sex pheromone components Z11-16:OH and 16:OH of C. infuscatellus. Meanwhile, CinfGOBP1-2 had high binding affinity with host-plant volatiles from sugarcane (Saccharum officinarum). Field-trapping results suggested that four volatile components, octadecane, (Z)-3-hexen-1-ol, α-terpineol and hexadecane from host plants and sex pheromone mixed baits have synergistic roles in attracting C. infuscatellus adult moths.

CONCLUSION

Functional characterization of CinfPBPs and CinfGOBPs in C. infuscatellus could help us find new environmentally friendly alternatives to conventional pest control using pesticides in sugarcane fields. © 2020 Society of Chemical Industry.

Sex pheromones and olfactory proteins in Antheraea moths: A. pernyi and A. polyphemus (Lepidoptera: Saturniidae)

Olfaction is essential for regulating the physiological and behavioral actions of insects with specific recognition of various odors. Antheraea moths (Lepidoptera: Saturniidae) possess relatively large bodies and antennae so that they are good subjects for exploring molecular aspects of insect olfaction. Current knowledge of the molecular aspects of Antheraea olfaction is focused on the Chinese tussah silkmoth A. pernyi Guérin-Méneville and another species A. polyphemus (Cramer) in their pheromones, odorant-binding proteins (OBPs), odorant receptors (ORs), odorant receptor coreceptors (ORCOs), sensory neuron membrane proteins (SNMPs), and odorant-degrading enzymes (ODEs). The first insect OBP, SNMP, and ODE were identified from A. polyphemus. This review summarizes the principal findings associated with the olfactory physiology and its molecular components in the two Antheraea species. Three types of olfactory neurons may have specific ORs for three respective sex-pheromone components, with the functional sensitivity and specificity mediated by three respective OBPs. SNMPs and ODEs are likely to play important roles in sex-pheromone detection, inactivation, and degradation. Identification and functional analysis of the olfactory molecules remain to be further performed in A. pernyi, A. polyphemus, and other Antheraea species.

Molecular Principles of Insect Chemoreception

Chemoreception, an ability to perceive specific chemical stimuli, is one of the most evolutionarily ancient forms of interaction between living organisms and their environment. Chemoreception systems are found in organisms belonging to all biological kingdoms. In higher multicellular animals, chemoreception (along with photo- and mechanoreception) underlies the functioning of five traditional senses. Insects have developed a peculiar and one of the most sophisticated chemoreception systems, which exploits at least three receptor superfamilies providing perception of smell and taste, as well as chemical communication in these animals. The enormous diversity of physiologically relevant compounds in the environment has given rise to a wide-ranging repertoire of chemoreceptors of various specificities. Thus, in insects, they are represented by several structurally and functionally distinct protein classes and are encoded by hundreds of genes. In the current review, we briefly characterize the insect chemoreception system by describing the main groups of receptors that constitute it and putting emphasis on the peculiar architecture and mechanisms of functioning possessed by these molecules.

Convergent evolution of small molecule pheromones in Pristionchus nematodes

The small molecules that mediate chemical communication between nematodes-so-called 'nematode-derived-modular-metabolites' (NDMMs)-are of major interest because of their ability to regulate development, behavior, and life-history. Pristionchus pacificus nematodes produce an impressive diversity of structurally complex NDMMs, some of which act as primer pheromones that are capable of triggering irreversible developmental switches. Many of these NDMMs have only ever been found in P. pacificus but no attempts have been made to study their evolution by profiling closely related species. This study brings a comparative perspective to the biochemical study of NDMMs through the systematic MS/MS- and NMR-based analysis of exo-metabolomes from over 30 Pristionchus species. We identified 36 novel compounds and found evidence for the convergent evolution of complex NDMMs in separate branches of the Pristionchus phylogeny. Our results demonstrate that biochemical innovation is a recurrent process in Pristionchus nematodes, a pattern that is probably typical across the animal kingdom.

Methods for independently manipulating palatability and color in small insect prey

Understanding how the psychology of predators shapes the defenses of colorful aposematic prey has been a rich area of inquiry, with emphasis on hypothesis-driven experiments that independently manipulate color and palatability in prey to examine predator responses. Most of these studies focus on avian predators, despite calls to consider more taxonomically diverse predators. This taxonomic bias leaves gaps in our knowledge about the generalizability of current theory. Here we have adapted tools that have been successfully used with bird predators and scaled them down and tested them with smaller predators (Habronattus jumping spiders) and small insect prey (termites, milkweed bug nymphs, pinhead crickets, fruit flies). Specifically, we test (1) the application of denatonium benzoate (DB) to the surface of live termites, crickets, and fruit flies, and (2) the effectiveness of manipulating the palatability of milkweed bug nymphs through diet. We also test the effectiveness of combining these palatability manipulations with various color manipulations. Across several experiments, we confirm that our palatability manipulations are not detectable to the spiders before they attack (i.e., they do not produce aversive odors that spiders avoid), and show that unpalatable prey are indeed quickly rejected and spiders do not habituate to the taste with experience. We also investigate limitations of these techniques by assessing possible unintended effects on prey behavior and the risk of contact contamination when using DB-treated prey in experiments. While similar tools have been used to manipulate color and palatability with avian predators and relatively large insect prey, we show how these techniques can be effectively adapted for use with small invertebrate predators and prey.

Effects of advanced age on olfactory response of male and female Queensland fruit fly, Bactrocera tryoni (Froggatt) (Diptera: Tephritidae)

Olfaction is an essential sensory modality of insects which is known to vary with age. In short-lived insects odour response generally declines rapidly with increasing age, but how increasing age affects the olfactory response of long-lived insects is less known and there may be different life-time patterns of olfactory response. Here, we examine the effect of age on olfactory response and exploratory activity of a long-lived tephritid fruit fly, Bactrocera tryoni from sexual maturity (3 weeks) to advanced age (15 weeks). Males were tested against a male-specific attractant, cue-lure, which is associated with courtship and sexual selection in this species; while females were tested against guava-juice, a highly attractive oviposition host fruit odour. Trials were done in the laboratory using a Y-tube olfactometer at three weekly intervals. The probability of olfactory response of both males and females to tested odours declined with age. Males retained a constant attraction to cue-lure until 12 weeks of age, but then showed a significant drop in olfactory response at 15 weeks. However, females showed the highest attraction to guava-juice odour until six weeks of age and declined gradually thereafter. The change on odour response over time can be associated with an age-related change in initial locomotor activity for females as there was no change, over the life of the experiment, in selective female orientation to the odour source once flies started exploring within the olfactometer. However, for 15 week-old males, there was a simultaneous drop in both locomotor activity and selective olfactory orientation. The consistent attraction of male to cue-lure might be related to life-long reproductive activities of males, as males are thought to mate continuously during life. On the other hand, females' highest attraction to guava-juice odour in early life followed by a gradual decline might be linked with their oviposition rate which peaks in early life.

Holothurians have a reduced GPCR and odorant receptor-like repertoire compared to other echinoderms

Sea cucumbers lack vision and rely on chemical sensing to reproduce and survive. However, how they recognize and respond to environmental cues remains unknown. Possible candidates are the odorant receptors (ORs), a diverse family of G protein-coupled receptors (GPCRs) involved in olfaction. The present study aimed at characterizing the chemosensory GPCRs in sea cucumbers. At least 246 distinct GPCRs, of which ca. 20% putative ORs, were found in a transcriptome assembly of putative chemosensory (tentacles, oral cavity, calcareous ring, and papillae/tegument) and reproductive (ovary and testis) tissues from Holothuria arguinensis (57 ORs) and in the Apostichopus japonicus genome (79 ORs). The sea cucumber ORs clustered with those of sea urchin and starfish into four main clades of gene expansions sharing a common ancestor and evolving under purifying selection. However, the sea cucumber ORs repertoire was the smallest among the echinoderms and the olfactory receptor signature motif LxxPxYxxxxxLxxxDxxxxxxxxP was better conserved in cluster OR-l1 which also had more members. ORs were expressed in tentacles, oral cavity, calcareous ring, and papillae/tegument, supporting their potential role in chemosensing. This study is the first comprehensive survey of chemosensory GPCRs in sea cucumbers, and provides the molecular basis to understand how they communicate.

Aedes aegypti Odorant Binding Protein 22 selectively binds fatty acids through a conformational change in its C-terminal tail

Aedes aegypti is the primary vector for transmission of Dengue, Zika and chikungunya viruses. Previously it was shown that Dengue virus infection of the mosquito led to an in increased expression of the odorant binding protein 22 (AeOBP22) within the mosquito salivary gland and that siRNA mediated knockdown of AeOBP22 led to reduced mosquito feeding behaviors. Insect OBPs are implicated in the perception, storage and transport of chemosensory signaling molecules including air-borne odorants and pheromones. AeOBP22 is unusual as it is additionally expressed in multiple tissues, including the antenna, the male reproductive glands and is transferred to females during reproduction, indicating multiple roles in the mosquito life cycle. However, it is unclear what role it plays in these tissues and what ligands it interacts with. Here we present solution and X-ray crystallographic studies that indicate a potential role of AeOBP22 binding to fatty acids, and that the specificity for longer chain fatty acids is regulated by a conformational change in the C-terminal tail that leads to creation of an enlarged binding cavity that enhances binding affinity. This study sheds light onto the native ligands for AeOBP22 and provides insight into its potential functions in different tissues.

Antennal transcriptome analysis and expression profiles of putative chemosensory soluble proteins in Histia rhodope Cramer (Lepidoptera: Zygaenidae)

Histia rhodope Cramer (Lepidoptera: Zygaenidae) is one of the most destructive defoliators of landscape tree Bischofia polycarpa (Levl.) Airy Shaw in China stretching to other Southeast Asia regions. Olfactory genes, encoding proteins such as odorant carrier proteins believed to initiate olfactory signal transduction in insects, have been acknowledged to be novel targets for pest control. In this study, we established antennal transcriptome of H. rhodope and ultimately identified 19 odorant binding proteins (OBPs), 23 chemosensory proteins (CSPs) and 4 Niemann-Pick type C2 proteins (NPC2s). The 19 OBPs, 6 CSPs and 4 NPC2s were assessed to validate the differential expressions between sexes, and between olfactory and non-olfactory tissues. 8 OBPs and 2 CSPs exhibited male-biased antennae expression, while 6 OBPs, 2 CSPs and HrhoNPC2a exhibited female-biased antennae expression. Moreover, 17 OBPs, 4 CSPs and 2 NPC2s were predominantly expressed in the antennae compared with non-olfactory tissues. HrhoOBP1 and HrhoOBP8 were predominantly expressed in the antennae and heads, HrhoCSP8 and HrhoCSP14 were highly expressed in abdomens and legs, HrhoNPC2c was highly expressed in abdomens, while HrhoNPC2d was expressed in all tissues. Phylogenetic analysis revealed that most H. rhodope proteins were closely related to proteins from other moths. Moreover, compared with other nocturnal moths, acting as a diurnal moth, we found that H. rhodope may have lost a PBP gene. Our results provide important molecular information for further studies on olfactory mechanisms of H. rhodope.

The fire ant social supergene is characterized by extensive gene and transposable element copy number variation

In the fire ant Solenopsis invicta, a supergene composed of ~600 genes and having two variants, SB and Sb, regulates colony social form. In single queen colonies, all individuals carry only the SB allele, while in multiple queen colonies, some individuals carry the Sb allele. In this study, we characterized genes with copy number variation between SB and Sb-carrying individuals. We showed extensive acquisition of gene duplicates in the Sb genome, with some likely involved in polygyne-related phenotypes. We found 260 genes with copy number differences between SB and Sb, of which 239 have greater copy number in Sb. We observed transposable element (TE) accumulation on Sb, likely due to the accumulation of repetitive elements on the nonrecombining chromosome. We found a weak correlation between TE copy number and differential expression, suggesting some TEs may still be proliferating in Sb while many of the duplicated TEs have presumably been silenced. Among the 115 non-TE genes with higher copy in Sb, enzymes responsible for cuticular hydrocarbon synthesis were highly represented. These include a desaturase and an elongase, both potentially responsible for differential queen odour and likely beneficial for polygyne ants. These genes seem to have translocated into the supergene from other chromosomes and proliferated by multiple duplication events. While the presence of TEs in supergenes is well documented, little is known about duplication of non-TE genes and their possible adaptive role. Overall, our results suggest that gene duplications may be an important factor leading to monogyne and polygyne ant societies.