Systemic and sex-biased regulation of OBP expression under semiochemical stimuli

Residues of chlorpyrifos in the environment induce resistance in Aedes albopictus by affecting its olfactory system and neurotoxicity

Aedes albopictus, a virus-vector pest, is primarily controlled through the use of insecticides. In this study, we investigated the mechanisms of resistance in Ae. albopictus in terms of chlorpyrifos neurotoxicity to Ae. albopictus and its effects on the olfactory system. We assessed Ca2+-Mg2+-ATP levels, choline acetyltransferase (ChAT), Monoamine oxidase (MAO), odorant-binding proteins (OBPs), and olfactory receptor (OR7) gene expression in Ae. albopictus using various assays including Y-shaped tube experiments and DanioVision analysis to evaluate macromotor behavior. Our findings revealed that cumulative exposure to chlorpyrifos reduced the activity of neurotoxic Ca2+-Mg2+-ATPase and ChAT enzymes in Ae. albopictus to varying degrees, suppressed MAO-B enzyme expression, altered OBPs and OR7 expression patterns, as well as affected evasive response, physical mobility, and cumulative locomotor time under chlorpyrifos stress conditions for Ae. albopictus individuals. Consequently, these changes led to decreased feeding ability, reproductive capacity, and avoidance behavior towards natural enemies in Ae. albopictus populations exposed to chlorpyrifos stressors over time. To adapt to unfavorable living environments, Ae. albopictus may develop certain tolerance mechanisms against organophosphorus pesticides. This study provides valuable insights for guiding rational insecticide usage or dosage adjustments targeting the nervous system of Ae. albopictus.

Identification of the odorant binding proteins of Western Flower Thrips (Frankliniella occidentalis), characterization and binding analysis of FoccOBP3 with molecular modelling, molecular dynamics simulations and a confirmatory field trial

Olfactory systems are indispensable for insects as they, including Western Flower Thrips (Frankliniella occidentalis), use olfactory cues for ovipositing and feeding. F. occidentalis use odorant binding proteins (OBPs) to transport semiochemicals to odorant receptors to induce a behavioural response from the sensillum lymph of the insect's antennae. This study identifies four OBPs of F. occidentalis and analyses their expression at three stages of growth: larvae, adult males and adult females. Further, it investigates the presence of conserved motifs and their phylogenetic relationship to other insect species. Moreover, FoccOBP3 was in silico characterized to analyse its structure along with molecular docking and molecular dynamics simulations to understand its binding with semiochemicals of F. occidentalis. Molecular docking revealed the interactions of methyl isonicotinate, p-anisaldehyde and (S)-(-)-verbenone with FoccOBP3. Moreover, molecular dynamics simulations showed bonding stability of these ligands with FoccOBP3, and field trials validated that Lurem TR (commercial product) and p-anisaldehyde had greater attraction as compared to (S)-(-)-verbenone, given the compound's binding with FoccOBP3. The current study helps in understanding the tertiary structure and interaction of FoccOBP3 with lures using computational and field data and will help in the identification of novel lures of insects in the future, given the importance of binding with OBPs.Communicated by Ramaswamy H. Sarma.

Odorant-Binding and Chemosensory Proteins in Anthonomus eugenii (Coleoptera: Curculionidae) and Their Tissue Expression

The pepper weevil Anthonomus eugenii is one of the most damaging pests to the pepper crop. To offer alternative management strategies to insecticides, several studies have identified the semiochemicals that are involved in the pepper weevil's aggregation and mating behavior; however, there is no information on its perireceptor molecular mechanism, to date. In this study, bioinformatics tools were used to functionally annotate and characterize the A. eugenii head transcriptome and their probable coding proteins. We identified twenty-two transcripts belonging to families related to chemosensory processes, seventeen corresponding to odorant-binding proteins (OBP), and six to chemosensory proteins (CSP). All results matched with closely related Coleoptera: Curculionidae homologous proteins. Likewise, twelve OBP and three CSP transcripts were experimentally characterized by RT-PCR in different female and male tissues. The results by sex and tissue display the different expression patterns of the AeugOBPs and AeugCSPs; some are present in both sexes and all tissues, while others show expressions with higher specificity, which suggests diverse physiological functions in addition to chemo-detection. This study provides information to support the understanding of odor perception in the pepper weevil.

Transcriptome Analysis of Antennal Chemosensory Genes in Curculio Dieckmanni Faust. (Coleoptera: Curculionidae)

The olfactory system plays a key role in regulating insect behaviors, such as locating host plants, spawning sites, and mating partners and avoiding predators. Chemosensory genes are required for olfactory recognition in insects. Curculio dieckmanni Faust. (Coleoptera: Curculionidae) damages hazelnuts and causes severe economic losses. There are no effective control measures, but understanding the olfaction mechanisms of this insect could lead to a new approach for population management. However, the genes that perform chemosensory functions in C. dieckmanni are still unclear. Using high-throughput sequencing, we assembled the antennal transcriptome of C. dieckmanni and annotated the major chemosensory gene families. Of the chemosensory gene families, we found 23 odorant-binding proteins, 15 chemosensory proteins, 2 sensory neuron membrane proteins, 15 odorant receptors, 23 ionotropic receptors, and nine gustatory receptors. Using Blast sequence alignment and phylogenetic analysis, the sequences of these proteins were identified. Male- and female-specific chemosensory genes involved in odorant detection and recognition were validated by qRT-PCR. Among the chemosensory genes, we found significant differences in the expression of CdieOBP8, CdieOBP9, CdieOBP19, CdieOBP20, CdieOBP21, CdieCSP15, CdieOR13, and CdieOR15 between adult male and female C. dieckmanni. A total of 87 expressed chemosensory proteins were found in C. dieckmanni. Investigating these proteins will help reveal the molecular mechanism of odorant recognition in C. dieckmanni and may aid the development of novel control strategies for this species.

BarH1 regulates odorant-binding proteins expression and olfactory perception of Monochamus alternatus Hope

Insect odorant-binding proteins (OBPs) are a class of small soluble proteins that can be found in various tissues wherein binding and transport of small molecules are required. Thus, OBPs are not only involved in typical olfactory function by specific activities with odorants but also participate in other physiological processes in non-chemosensory tissues. To better understand the complex biological functions of OBPs, it is necessary to study the transcriptional regulation of their expression patterns. In this paper, an apparent gradient expression pattern of Obp19, that was highly and specifically expressed in antennae and played an essential role in the detection of camphene, was defined in the antennae of the Japanese pine sawyer. Further, the transcription factor BarH1, that also presented gradient expression pattern in antennae, was found to regulate expression of Obp19 directly through binding to its upstream DNA sequence. The condition of BarH1 gene silence, the gene expression levels of Obp19 significantly decreased. At the same time, additional olfactory genes also were regulated and thus influence camphene reception. These findings provide us an opportunity to incorporate Obps in the gene regulatory networks of insects, which contribute to a better understanding of the multiplicity and diversity of OBPs and the olfactory mediated behaviors.

CRISPR/Cas9 mutagenesis abolishes odorant-binding protein BdorOBP56f-2 and impairs the perception of methyl eugenol in Bactrocera dorsalis (Hendel)

Olfaction underpins many insect behaviors, such as foraging, host location, mating, and predator avoidance. In the first step of insect olfaction, odorant-binding proteins (OBPs) bind hydrophobic odorants and transport them to odorant receptors. Methyl eugenol (ME) is a powerful attractant for mature males of the oriental fruit fly Bactrocera dorsalis (Hendel), one of the most destructive fruit pests. The underlying molecular mechanism is unclear, but there is in vitro evidence that BdorOBP56f-2 is involved in ME perception. We used microscale thermophoresis to confirm that BdorOBP56f-2 directly binds ME with strong affinity in vitro. We then used CRISPR/Cas9 to knock out the BdorOBP56f-2 gene, allowing us to establish a homozygous mutant B. dorsalis line. The electroantennogram response and behavioral attraction to ME were significantly reduced in the mutant, providing in vivo evidence that BdorOBP56f-2 is necessary for efficient ME perception. Our results offer insight into the molecular mechanism of ME perception in B. dorsalis and provide a theoretical basis for the functional analysis of other OBPs.

Two classic OBPs modulate the responses of female Holotrichia oblita to three major ester host plant volatiles

Insects possess a fairly sophisticated olfactory system in their antennae to detect odorants essential for their survival and reproduction. Among them, insect first perceives odour sources by odorant-binding proteins (OBPs) to locate host-plants. Methyl salicylate, (Z)-3-hexenyl acetate and dibutyl phthalate are major volatile components of Ulmus pumila and Ricinus communis and elicit strong responses of the scarab beetle Holotrichia oblita adults. However, olfactory perception of the scarab beetle to these odorant compounds is unclear. In the current study, we cloned the OBP6 and OBP7 of H. oblita. The expression pattern shows that the two genes were highly expressed in the antennae of female beetles. Binding assays verified that the HoblOBP6 had a better binding affinity to methyl salicylate, and so did HoblOBP7 to (Z)-3-hexenyl acetate and dibutyl phthalate. The effect on the responses of female beetles to the three compounds was decreased significantly after these two genes were silenced by RNA interference. These results indicate that HoblOBP6 and HoblOBP7 are essential for female H. oblita perception of methyl salicylate, (Z)-3-hexenyl acetate and dibutyl phthalate. Our study provides important insights into the olfactory mechanism of female H. oblita to ester plant volatiles and could facilitate the development of potential pest control strategies in the field.

Host plants transfer induced regulation of the chemosensory genes repertoire in the alfalfa plant bug Adelphocoris lineolatus (Goeze)

The alfalfa plant bug Adelphocoris lineolatus, an economically important pest, has representative behavioral characteristics with host plants transfer. Olfactory system is essential for insects to perceive ever-changing chemical signals in the external environment, and chemosensory genes play crucial roles in signals reception and transduction. In this work, we compared the differences in chemosensory genes expression before and after host plants transfer by constructing 12 antennal transcriptomes of male and female bugs, respectively. The results showed that the expression levels of most chemosensory genes in A. lineolatus changed to adapt to the transformation of the hosts plant. More remarkable, female bugs had more up-regulated chemosensory genes than males. Differentially expressed genes (DEGs) analysis revealed three odorant binding proteins (OBPs), three chemosensory proteins (CSPs), eight odorant receptors (ORs) and one ionotropic receptor (IR) showed significant differences when the host plant transferred. There were complex characteristics of up- and down- regulated genes in male and female adults, among which OBP19 showed higher expression in females exposing to the new host plant alfalfa, suggesting this OBP may be associated with the localization of the oviposition site. The OR54 and OR82 were up-regulated in both genders, indicating their possible roles in recognizing some alfalfa-specific volatiles. These findings will provide valuable insights in biological functions of chemosensory genes in A. lineolatus and facilitate the development of new targets for novel strategies to control the alfalfa plant bug and other herbivores.

Mating-based regulation and ligand binding of an odorant-binding protein support the inverse sexual communication of the greater wax moth, Galleria mellonella (Lepidoptera: Pyralidae)

In moths, sex pheromones play a key role in mate finding. These chemicals are transported in the antennae by odorant-binding proteins (OBPs). Commonly, males encounter conspecific females; therefore, several OBPs are male-biased. Less is known, however, about how the olfactory system of moths has evolved toward inverse sexual communication, ie where females seek males. Therefore, the objective of this study was to identify the profile of OBPs and their expression patterns in the bee hive pest, Galleria mellonella, a moth that uses inverse sexual communication. Here, OBP-related transcripts were identified by an RNA Sequencing (RNA-Seq) approach and analysed through both Reverse Transcription Polymerase Chain Reaction (RT-PCR) in different tissues and quantitative real-time PCR for two states, virgin and postmating. Our results indicate that G. mellonella has 20 OBPs distributed amongst different tissues. Interestingly, 17 of the 20 OBPs were significantly down-regulated after mating in females, whereas only OBP7 was up-regulated. By contrast, 18 OBP transcripts were up-regulated in males after mating. Additionally, binding assays and structural simulations showed general odorant-binding protein 2 (GOBP2) was able to bind sex pheromone components and analogues. These findings suggest a possible role of OBPs, especially GOBPs, in the inverse sexual communication of G. mellonella, with gene expression regulated as a response to mating.

GOBP1 Plays a Key Role in Sex Pheromones and Plant Volatiles Recognition in Yellow Peach Moth, Conogethes punctiferalis (Lepidoptera: Crambidae)

Insects recognize odorous compounds using sensory neurons organized in olfactory sensilla. The process odor detection in insects requires an ensemble of proteins, including odorant binding proteins, olfactory receptors, and odor degrading enzymes; each of them are encoded by multigene families. Most functional proteins seem to be broadly tuned, responding to multiple chemical compounds with different, but mostly quite similar structures. Based on the hypothesis that insects recognize host volatiles by means of general odorant binding proteins (GOBPs), the current study aimed to characterize GOBPs of the yellow peach moth, Conogethes punctiferalis (Guenée). In oviposition preference tests, it was found that the yellow peach moth preferred volatiles from Prunus persica (peach) in finding their host plant. Exposure of the moth to volatiles from peaches affected the expression level of GOBP genes. Binding affinity of GOBPs from yellow peach moth was assessed for 16 host plant volatiles and 2 sex pheromones. The fluorescence ligand-binding assays revealed highest affinities for hexadecanal, farnesol, and limonene with K_D values of 0.55 ± 0.08, 0.35 ± 0.04, and 1.54 ± 0.39, respectively. The binding sites of GOBPs from yellow peach moth were predicted using homology modeling and characterized using molecular docking approaches. The results indicated the best binding affinity of both GOBP1 and GOBP2 for farnesol, with scores of −7.4 and −8.5 kcal/mol. Thus, GOBPs may play an important role in the process of finding host plants.

Functional characterization of odorant-binding proteins from the scarab beetle Holotrichia oblita based on semiochemical-induced expression alteration and gene silencing

With the advent of next-generation sequencing, it is now possible to rapidly identify the entire repertoire of olfactory genes likely to be involved in chemical communication of an insect species. It remains, however, a challenge to identify olfactory proteins, such as odorant receptors and odorant-binding proteins (OBPs), vis-à-vis the odorants they detect. It has been reported that exposing the olfactory system to a physiologically relevant odorant alters the transcript levels of odorant receptor(s) involved in the detection of the tested odorant. We applied this paradigm in an attempt to identify putative OBPs from the scarab beetle Holotrichia oblita involved in the reception of plant-derived kairomones. Twenty-nine OBP genes were identified in the H. oblita transcriptome, 20 of which were enriched in antennae compared with nonolfactory tissues. Of these, 2 OBP genes, HoblOBP13 and HoblOBP9, were upregulated upon exposure to one of the female attractants (E)-2-hexenol and phenethyl alcohol; none of the OBP transcripts changed upon exposure to methyl anthranilate, which does not attract H. oblita females. Binding assays showed that HoblOBP13 and HoblOBP9 have high affinity for (E)-2-hexenol and phenethyl alcohol, respectively. RNAi treatment showed that transcripts of both HoblOBP13 and HoblOBP9 declined in a time-course manner 24-72 h postinjection. OBP-dsRNA-treated female beetles showed significantly lower attraction to (E)-2-hexenol and phenethyl alcohol than did water-injected beetles and those treated with GFP-dsRNA. We, therefore, concluded that HoblOBP13 and HoblOBP9 are essential for H. oblita reception of the plant-derived kairomones (E)-2-hexenol and phenethyl alcohol.