Silencing the odorant coreceptor (Orco) disrupts sex pheromonal communication and feeding responses in Blattella germanica: toward an alternative target for controlling insect-transmitted human diseases

Female contact sex pheromone recognition in the German cockroach (Blattella germanica) is mediated by two male antennae-enriched sensory neuron membrane proteins

BACKGROUND

The German cockroach Blattella germanica is a notorious urban health pest that has developed resistance to multiple pesticides. Thus, novel non-lethal pest control agents are urgently needed. Olfaction interference via disruption of sex pheromone recognition-related genes offers a promising approach. The German cockroach has a unique courtship behavior in which female adults emit contact sex pheromones (CSPs) in response to antennal touching, which subsequently triggers distinctive male sex behavioral responses. Due to the limited volatility of CSPs, the molecular mechanisms underlying their recognition and the specific olfactory pathways activated remain poorly defined. Although the odorant receptor coreceptor (Orco) is critical for most insect olfaction, sensory neuron membrane proteins (SNMPs), in particular SNMP1, also play crucial roles in sex pheromone recognition in moths and flies. While multiple SNMP1 homologs have been identified in multiple insect species, they have yet to be fully functionally characterized in cockroaches.

RESULTS

In this study, RNA-interference (RNAi)-mediated knockdown of BgerOrco reduced both the electrophysiology responses and courtship behaviors of males, indicating CSP perception proceeds via an olfaction pathway. Similar RNAi knockdown of BgerSNMP1e and BgerSNMP1d, which are predominantly expressed in male antennae, revealed critical roles in perceiving the major component of the Blattella germanica CSP blend. Unlike BgerSNMP1e, BgerSNMP1d was also found to function in the perception of the minor CSP component. Molecular docking analyses revealed no differences in the binding affinities of BgerSNMP1d for the major and minor CSP components, whereas the binding affinities of BgerSNMP1e displayed clear selectivity for the major component.

CONCLUSION

Our results show that the olfactory pathway is critical for CSP recognition and that two male-enriched SNMP genes, BgerSNMP1e and BgerSNMP1d, are crucial factors mediating the male response to CSP stimulation in German cockroaches. This study lays a foundation for studying the mechanisms of CSP recognition and provides novel molecular targets with potential to be exploited as disruptors of courtship behavior. © 2024 Society of Chemical Industry.

The function of OforOrco in the allogrooming behavior of Odontotermes formosanus (Shiraki) induced by Serratia marcescens Bizio (SM1)

Termites are consistently confronted with a complex microbial environment. In addition to the role of their innate immune system in resisting pathogen infection, social immune behavior also plays a significant role in helping termites withstand the stress caused by pathogenic microorganisms. The allogrooming behavior among different individuals is commonly observed in termites, and it plays a crucial role in the social immune interaction network. In the case of Odontotermes formosanus (Shiraki), Orco is specifically involved in detecting pheromones and volatile chemicals released by termites to communicate with each other. Nonetheless, the function of Orco in the social immunity remains unreported in O. formosanus. Consequently, in this study, we recorded the allogrooming behavior of O. formosanus workers under SM1 stress. The results indicated a significant increase in allogrooming behavior due to SM1 infection. The allogrooming behavior of workers under SM1 stress was significantly increased after the addition of soldiers. Compared with pronotum group treated by SM1, SM1 treatment of workers' heads significantly reduced the allogrooming behavior among workers. In addition, we found that SM1 could greatly increase the expression of OforOrco. Furthermore, interfering with OforOrco could markedly reduce the allogrooming behavior among workers under SM1 stress, and increase the mortality of worker under SM1 stress. This study demonstrated the significant role of OforOrco in the social immunity of O. formosanus, which offers a theoretical foundation for the advancement of research on termite RNA biopesticides, and the integration of RNA interference (RNAi) with pathogens. This study is valuable for elucidating the social immune behavior and interaction network of termites.

Silencing the odorant co-receptor (Orco) in Anoplophora glabripennis disrupts responses to pheromones and host volatiles

Insects rely primarily on a robust and precise olfactory recognition system to detect chemicals and environmental signals. Olfaction is mediated mainly by various odorant receptors (ORs) expressed on olfactory neurons. The odorant co-receptor (Orco) is a highly conserved and obligatory subunit of ORs, and its combination with conventional ORs to form ligand-gated ion channel heterodimeric complexes plays a crucial role in odor recognition. Anoplophora glabripennis Is a major quarantinable pest that affects broadleaved tree species worldwide. Odorant binding proteins (OBPs) and ORs have been identified in the A. glabripennis genome and the binding properties of some OBPs and their cognate ligands have been clarified. The role of the OR-mediated recognition pathway, however, remains largely uncharacterized. Here, we cloned and sequenced the full-length Orco gene sequence of A. glabripennis and performed structural characterization of the protein. We found that AglaOrco has high sequence homology with Orco from other orders of insects, and that it is highly conserved. Spatio-temporal differential expression analysis revealed that AglaOrco is highly expressed in adult antennae, and that expression at the sexually mature stage is significantly higher than at other developmental stages. There was no significant difference in expression between sexes. Silence AglaOrco using RNAi revealed that expression levels of AglaOrco mRNA fell significantly in both males and females at 72 h post-injection of 5 μg of dsOrco, with no obvious effect on expression of most other olfactory-related genes; however, some were up-or downregulated. For example, silenced Orco-expressing males and females showed a significant reduction in antennal potential responses to the odorants 3-carene, Ocimene, and 4-heptyloxy-1-butanol. Overall, the data suggest that AglaOrco plays an important role in mediating olfactory perception in A. glabripennis, and also identifies potential target genes for environmentally friendly pest control strategies.

Identification of candidate chemosensory genes in the antennal transcriptome of Monolepta signata

In the polyphagous insect Monolepta signata (M. signata) (Coleoptera: Chrysomelidae), antennae are important for olfactory reception used during feeding, mating, and finding a suitable oviposition site. Based on NextSeq 6000 Illumina sequencing, we assembled the antennal transcriptome of mated M. signata and described the first chemosensory gene repertoire expressed in this species. The relative expression levels of some significant chemosensory genes were conducted by quantitative real-time PCR. We identified 114 olfactory-related genes based on the antennal transcriptome database of M. signata, including 21 odorant binding proteins (OBPs), six chemosensory proteins (CSPs), 46 odorant receptors (ORs), 15 ionotropic receptors (IRs), 23 gustatory receptors (GRs) and three sensory neuron membrane proteins (SNMPs). Blastp best hit and phylogenetic analyses showed that most of the chemosensory genes had a close relationship with orthologs from other Coleoptera species. Overall, this study provides a foundation for elucidating the molecular mechanism of olfactory recognition in M. signata as well as a reference for the study of chemosensory genes in other species of Coleoptera.

RNAi of yellow-y, required for normal cuticle pigmentation, impairs courtship behavior and oviposition in the German cockroach (Blattella germanica)

The insect cuticle plays a key role in maintaining the insect's physiological function and behavior. Herein, the yellow-y protein is required to produce black melanin, and is expressed in a pattern that correlates with the distribution of this pigment. However, yellow-y can also have other functions, for instance, in insect behavior, but not much is known. In this study, we have studied the yellow-y gene in one important model and pest species, namely the German cockroach (Blattella germanica), which is to our knowledge the first time reported. In essence, we identified the yellow-y gene (BgY-y) and characterized its function by using RNA interference (RNAi). Silencing of BgY-y gene led to different developmental abnormalities (body weight and wings) in both genders. Specifically, there was an abundant decrease in melanin, turning the body color in pale yellow and the cuticle softer and more transparent. Interestingly, we also observed that the knockdown of BgY-y impaired the male cockroaches to display a weaker response to female-emitted contact sex pheromones, and also that the oviposition ability was weakened in the RNAi females. This study comprehensively analyzed the biological functions of the yellow-y gene in German cockroaches from the perspectives of development, body color, courtship behavior and oviposition, and as a consequence, this may opens new avenues to explore it as a novel pest control gene.

Interactive parallel sex pheromone circuits that promote and suppress courtship behaviors in the cockroach

Many animals use multicomponent sex pheromones for mating, but the specific function and neural processing of each pheromone component remain unclear. The cockroach Periplaneta americana is a model for studying sex pheromone communication, and an adult female emits major and minor sex pheromone components, periplanone-B and -A (PB and PA), respectively. Attraction and courtship behaviors (wing-raising and abdominal extension) are strongly expressed when adult males are exposed to PB but weakly expressed when they are exposed to PA. When major PB is presented together with minor PA, behaviors elicited by PB were impaired, indicating that PA can both promote and suppress courtship behaviors depending on the pheromonal context. In this study, we identified the receptor genes for PA and PB and investigated the effects of knocking down each receptor gene on the activities of PA- and PB-responsive sensory neurons (PA- and PB-SNs), and their postsynaptic interneurons, and as well as effects on courtship behaviors in males. We found that PB strongly and PA weakly activate PB-SNs and their postsynaptic neurons, and activation of the PB-processing pathway is critical for the expression of courtship behaviors. PA also activates PA-SNs and the PA-processing pathway. When PA and PB are simultaneously presented, the PB-processing pathway undergoes inhibitory control by the PA-processing pathway, which weakens the expression of courtship behaviors. Our data indicate that physiological interactions between the PA- and PB-processing pathways positively and negatively mediate the attraction and courtship behaviors elicited by sex pheromones.

Neuroregulation of foraging behavior mediated by the olfactory co-receptor Orco in termites

Olfaction is critical for survival because it allows animals to look for food and detect pheromonal cues. Neuropeptides modulate olfaction and behaviors in insects. While how the neuroregulation of olfactory recognition affects foraging behavior in termites is still unclear. Here, we analyzed the change after silencing the olfactory co-receptor gene (Orco) and the neuropeptide Y gene (NPY), and then investigated the impact of olfactory recognition on foraging behavior in Odontotermes formosanus under different predation pressures. The knockdown of Orco resulted in the reduced Orco protein expression in antennae and the decreased EAG response to trail pheromones. In addition, NPY silencing led to the damaged ability of olfactory response through downregulating Orco expression. Both dsOrco- and dsNPY-injected worker termites showed significantly reduced walking activity and foraging success. Additionally, we found that 0.1 pg/cm trail pheromone and nestmate soldiers could provide social buffering to relieve the adverse effect of predator ants on foraging behavior in worker termites with the normal ability of olfactory recognition. Our orthogonal experiments further verified that Orco/NPY genes are essential in manipulating termite olfactory recognition during foraging under different predation pressures, suggesting that the neuroregulation of olfactory recognition plays a crucial role in regulating termite foraging behavior.

Inundative practice for screening siRNA management candidates against a notorious predatory beetle using olfactory silencing

Calosoma maximoviczi, a predatory pest beetle, poses a significant threat to wild silk farm production due to its predation on wild silkworms. Given the coexistence of this species with beneficial silkworms in the farm orchards, chemical pesticides are not an ideal solution for controlling its population. In this study, we employed a comprehensive multi-target RNA interference (RNAi) approach to disrupt the olfactory perception of C. maximoviczi through independently silencing 16 odorant receptors (ORs) in the respective genders. Specifically, gene-specific siRNAs were designed to target a panel of ORs, allowing us to investigate the specific interactions between odorant receptors and ligands within this species. Our investigation led to identifying four candidate siOR groups that effectively disrupted the beetle's olfactory tracking of various odorant ligands associated with different trophic levels. Furthermore, we observed sex-specific differences in innate RNAi responses reflected by subsequent gene expression, physiological and behavioral consequences, underscoring the complexity of olfactory signaling and emphasizing the significance of considering species/sex-specific traits when implementing pest control measures. These findings advance our understanding of olfactory coding patterns in C. maximoviczi beetles and establish a foundation for future research in the field of pest management strategies.

The Orco gene involved in recognition of host plant volatiles and sex pheromone in the chive maggot Bradysia odoriphaga

The insect olfactory recognition system plays a crucial role in the feeding and reproductive behaviors of insects. The odorant receptor co-receptor (Orco), as an obligatory chaperone, is critical for odorant recognition by way of forming heteromeric complexes with conventional odorant receptors (ORs). To investigate the biological functions of Orco in perceiving host plant volatiles and sex pheromone, the Orco gene was identified from the chive maggot Bradysia odoriphaga transcriptome data. Multiple sequence alignment reveals that BodoOrco exhibits an extremely high sequence identity with Orcos from other dipteran insects. The expression of BodoOrco is significantly higher in adults than in larvae and pupae, and the BodoOrco gene is primarily expressed in the antennae of both sexes. Furthermore, the Y-tube assay indicated that knockdown of BodoOrco leads to significant reductions in B. odoriphaga adults' response to all tested host plant volatiles. The dsOrco-treated unmated male adults show less attraction to unmated females and responded slowly compared with dsGFP control group. These results indicated that BodoOrco is involved in recognition of sex pheromone and host plant volatiles in B. odoriphaga and has the potential to be used as a target for the design of novel active compounds for developing ecofriendly pest control strategies.

The vitellogenin receptor gene contributes to mating and host-searching behaviors in parasitoid wasps

Vitellogenin receptor (VgR) is essential to vitellogenin uptaking and dominates ovary maturation in insects. However, the function of VgR in parasitoid wasps is largely unknown. Here, we applied the Drosophila parasitoid Leptopilina boulardi as a study model to investigate the function of VgR in parasitoids. Despite the conserved sequence characteristics with other insect VgRs, we found L. boulardi VgR (LbVgR) gene was highly expressed in head but lower in ovary. In addition, we found that LbVgR had no effects on ovary development, but participated in host-searching behavior of female L. boulardi and mating behavior of male L. boulardi. Comparative transcriptome analysis further revealed LbVgR might play crucial roles in regulating the expression of some important chemoreception genes to adjust the parasitoid behaviors. These results will broaden our knowledge of the function of VgR in insects, and contribute to develop advanced pest management strategies using parasitoids as biocontrol agents.

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.

Orco mutagenesis causes deficiencies in olfactory sensitivity and fertility in the migratory brown planthopper, Nilaparvata lugens

BACKGROUND

The migratory brown planthopper (BPH), Nilaparvata lugens (Hemiptera: Delphacidae), is the most destructive pest affecting rice plants in Asia and feeds exclusively on rice. Studies have investigated the olfactory response of BPHs to the major rice volatile compounds in rice. The insect olfactory co-receptor (Orco) is a crucial component of the olfactory system and is essential for odorant detection. Functional analysis of the Orco gene in BPHs would aid in the identification of their host preference.

RESULTS

We identified the BPH Orco homologue (NlOrco) by Blast searching the BPH transcriptome with the Drosophila Orco gene sequence. Spatiotemporal analysis indicated that NlOrco is first expressed in the later egg stage, and is expressed mainly in the antennae in adult females. A NlOrco-knockout line (NlOrco-/- ) was generated through clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9-mediated mutagenesis. The NlOrco-/- mutants showed no response to rice volatile compounds and consequently no host-plant preference. In addition, NlOrco-/- mutants exhibited extended nymphal duration and impaired fecundity compared with wild-type BPHs.

CONCLUSION

Our findings indicated that BPHs exhibit strong olfactory responses to major rice volatile compounds and suggest that NlOrco is required for the maximal fitness of BPHs. Our results may facilitate the identification of potential target genes or chemical compounds for BPH control applications. © 2022 Society of Chemical Industry.

Functional characterization of tyrosine melanin genes in the white-backed planthopper and utilization of a spray-based nanoparticle-wrapped dsRNA technique for pest control

As a significant pest of rice the white-backed planthopper (WBPH) Sogatella furcifera is a focus of pest management. However, traditional chemical-based control methods risk the development of pesticide resistance as well as severe ecological repercussions. Although nanoparticle-encapsulated dsRNAs provide a promising alternative method for sustainable pest management, gene targets specific to WBPH have yet to be optimized. Genes in the tyrosine-melanin pathway impact epidermal melanization and sclerotization, two processes essential for insect development and metabolism, have been proposed as good candidate targets for pest management. Seven genes (aaNAT, black, DDC, ebony, tan, TH, and yellow-y) in this group were identified from WBPH genome and functionally characterized by using RNAi for their impact on WBPH body color, development, and mortality. Knockdown of SfDDC, Sfblack, SfaaNAT, and Sftan caused cuticles to turn black, whereas Sfyellow-y and Sfebony knockdown resulted in yellow coloration. SfTH knockdown resulted in pale-colored bodies and high mortality. Additionally, an Escherichia coli expression system for large-scale dsRNA production was coupled with star polycation nanoparticles to develop a sprayable RNAi method targeting SfTH that induced high WBPH mortality rates on rice seedlings. These findings lay the groundwork for the development of large-scale dsRNA nanoparticle sprays as a WBPH control method.

Enhancement of Pathogen Toxicity by Feeding Reticulitermes chinensis Snyder Sonicated Bacteria Expressing Double-Stranded RNA That Interferes with Olfaction

Reticulitermes chinensis Snyder is a serious pest in China, and the odorant receptor co-receptor gene RcOrco plays a crucial role in olfaction. However, the function of RcOrco in the resistance of termites to entomopathogens has not been reported. We constructed dsRcOrco-HT115 engineered bacteria based on the RcOrco sequence from the full-length transcriptome data of R. chinensis. The engineered bacteria expressed dsRNA of RcOrco. Sonication was used to inactivate the dsRNA-HT115 strain and obtain a large amount of dsRcOrco. The dsRcOrco produced using this method overcame the problem that genetically engineered bacteria could not be applied directly and improved its effectiveness against termites. Bioassays using the dsRcOrco generated using this method showed that dsRcOrco significantly increased the toxicity of the bacterial and fungal pathogens to R. chinensis. The present study showed, for the first time, the function of Orco in termite resistance to pathogens, and the results provide a theoretical basis for the development and application of termite RNA biopesticides.

Knockout of the odorant receptor co-receptor, orco, impairs feeding, mating and egg-laying behavior in the fall armyworm Spodoptera frugiperda

The olfactory transduction system of insects is involved in multiple behavioral processes such as foraging, mating, and egg-laying behavior. In the insect olfactory receptor neurons (ORNs), the odorant receptor co-receptor (Orco) is an obligatory component that is required for dimerization with odorant receptors (ORs) to form a ligand-gated ion channel complex. The ORs/Orco heteromeric complex plays a crucial role in insect olfaction. To explore the function of OR-mediated olfaction in the physiological behavior of the fall armyworm, Spodoptera frugiperda, we applied CRISPR/Cas9 genome editing to mutate its Orco gene and constructed a homozygous mutant strain of Orco (Orco^-/-) by genetic crosses. Electroantennogram (EAG) analysis showed that the responses of Orco^-/- male moths to two universal sex pheromones, Z9-14: Ac and Z7-12: Ac, were abolished. We found that Orco^-/- males cannot successfully mate with female moths. An oviposition preference assay confirmed that Orco^-/- female moths had a reduced preference for the optimal host plant maize. A larval feeding assay revealed that the time for Orco^-/- larvae to locate the food source was significantly longer than in the wild-type. Overall, in the absence of Orco, the OR-dependent olfactory behavior was impaired in both larval and adult stages. Our results confirm that Orco is essential for multiple behavioral processes related to olfaction in the fall armyworm.

The odorant receptor co-receptor gene contributes to mating and host-searching behaviors in parasitoid wasps

BACKGROUND

Biological control of pest insects by parasitoid wasps is an effective and environmentally friendly strategy compared with the use of synthetic pesticides. Successful courtship and host-search behaviors of parasitoid wasps are important for biological control efficiency and are often mediated by chemical odorant cues. The odorant receptor co-receptor (Orco) gene has an essential role in the perception of odors in insects. However, the function of Orco in the mating and host-searching behaviors of parasitoid wasps remains underexplored.

RESULTS

We identified the full-length Orco genes of four Drosophila parasitoid species in the genus Leptopilina, namely L. heterotoma, L. boulardi, L. syphax and L. drosophilae. Sequence alignment and membrane-topology analysis showed that Leptopilina Orcos had similar amino acid sequences and topology structures. Phylogenetic analysis revealed that Leptopilina Orcos were highly conserved. Furthermore, the results of quantitative real-time polymerase chain reactions showed that all four Orco genes had a typical antennae-biased tissue expression pattern. After knockdown of Orco in these different parasitoid species, we found that Orco-deficient male parasitoid wasps, but not females, lost their courtship ability. Moreover, Orco-deficient female parasitoid wasps presented impaired host-searching performance and decreased oviposition rates.

CONCLUSION

Our study demonstrates that Orcos are essential in the mating and host-searching behaviors of parasitoid wasps. To our knowledge, this is the first time that the functions of Orco genes have been characterized in parasitoid wasps, which broadens our understanding of the chemoreception basis of parasitoid wasps and contributes to developing advanced pest management strategies. © 2022 Society of Chemical Industry.

Diet Influences the Gut Microbial Diversity and Olfactory Preference of the German Cockroach Blattella germanica

The gut microbiota of insects has been proven to play a role in the host's nutrition and foraging. The German cockroach, Blattella germanica, is an important vector of various pathogens and causes severe allergic reactions in humans. Food bait is an effective and frequently used method of controlling this omnivorous insect. Thus, understanding the relationships among diet, gut microbiota, and olfactory preferences could be useful for optimizing this management strategy. In this study, B. germanica was exposed to different foods, i.e., high-fat diet, high-protein diet, high-starch diet, and dog food (as control). Then their gut microbial and olfactory responses were investigated. 16S rRNA gene sequencing confirmed that the gut microbiota significantly differed across the four treatments, especially in relation to bacteria associated with the metabolism and digestion of essential components. Behavioral tests and the antenna electrophysiological responses showed that insects had a greater preference for other types of diets compared with their long-term domesticated diet. Moreover, continuously providing a single-type diet could change almost all the OR genes' expression of B. germanica, especially BgORco, which was significantly repressed compared to control. These results indicate that diet can shape the gut microbiota diversity and drive the olfactory preference of B. germanica. The association between gut microbiota profiles and diets can be utilized in managing B. germanica according to their olfactory preference.

Silencing the odorant receptor co-receptor impairs olfactory reception in a sensillum-specific manner in the cockroach

Insects detect odors via a large variety of odorant receptors (ORs) expressed in olfactory sensory neurons (OSNs). The insect OR is a heteromeric complex composed of a ligand-specific receptor and the co-receptor (ORco). In this study, we identified the ORco gene of the cockroach, Periplaneta americana (PameORco), and performed RNAi-based functional analysis of PameORco. All OSNs in the basiconic sensilla expressed PameORco and received a large variety of odors including sex pheromones. In trichoid sensilla, a PameORco-positive OSN was consistently paired with a PameORco-negative OSN tuned to acids. In adult cockroaches injected with PameORco dsRNA at the nymphal stage, the expression of PameORco, odor receptions via ORs, and its central processing were strongly suppressed. These results provide new insights into the molecular basis of olfactory reception in the cockroach. The long-lasting and irreversible effects of PameORco RNAi would be an effective method for controlling the household pest.

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Whole sequence of ORco in the American cockroach (PameORco) was characterized

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PameORco expressed in olfactory sensory neurons in a sensillar type-specific manner

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RNAi chronically and irreversibly suppressed the PameORco expression beyond molts

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PameORco was essential for receptions of sex pheromones and general odors

Molecular characterization and expression variation of the odorant receptor co-receptor in the Formosan subterranean termite

Subterranean termites live in underground colonies with a division of labor among castes (i.e., queens and kings, workers, and soldiers). The function of social colonies relies on sophisticated chemical communication. Olfaction, the sense of smell from food, pathogens, and colony members, plays an important role in their social life. Olfactory plasticity in insects can be induced by long- and short-term environmental perturbations, allowing adaptive responses to the chemical environment according to their physiological and behavioral state. However, there is a paucity of information on the molecular basis of olfaction in termites. In this study, we identified an ortholog encoding the odorant receptor co-receptor (Orco) in the Formosan subterranean termite, Coptotermes formosanus, and examined its expression variation across developmental stages and in response to social conditions. We found that C. formosanus Orco showed conserved sequence and structure compared with other insects. Spatial and temporal analyses showed that the Orco gene was primarily expressed in the antennae, and it was expressed in eggs and all postembryonic developmental stages. The antennal expression of Orco was upregulated in alates (winged reproductives) compared with workers and soldiers. Further, the expression of Orco decreased in workers after starvation for seven days, but it was not affected by the absence of soldiers or different group sizes. Our study reveals the molecular characteristics of Orco in a termite, and the results suggest a link between olfactory sensitivity and nutritional status. Further studies are warranted to better understand the role of Orco in olfactory plasticity and behavioral response.

RNAi-Mediated Screening of Selected Target Genes Against Culex quinquefasciatus (Diptera: Culicidae)

Culex quinquefasciatus, a member of the Culex pipiens complex, is widespread in Saudi Arabia and other parts of the world. It is a vector for lymphatic filariasis, Rift Valley fever, and West Nile virus. Studies have shown the deleterious effect of RNA interference (RNAi)-mediated knockdown of various lethal genes in model and agricultural pest insects. RNAi was proposed as a tool for mosquito control with a focus on Aedes aegypti and Anopheles gambiae. In this study, we examined the effect of RNAi of selected target genes on both larval mortality and adult emergence of Cx. quinquefasciatus through two delivery methods: soaking and nanoparticles. Ten candidate genes were selected for RNAi based on their known lethal effect in other insects. Disruption of three genes, chitin synthase-1, inhibitor of apoptosis 1, and vacuolar adenosine triphosphatase, resulted in the highest mortality among the selected genes using the two treatment methods. Silencing the other seven genes resulted in a medium to low mortality in both assays. These three genes are also active against a wide range of insects and could be used for RNAi-based mosquito control in the future.

The Odorant Binding Protein, SiOBP5, Mediates Alarm Pheromone Olfactory Recognition in the Red Imported Fire Ant, Solenopsis invicta

Olfaction is crucial in mediating various behaviors of social insects such as red imported fire ants, Solenopsis invicta Buren. Olfactory receptor (OR) complexes consist of odor-specific ORs and OR co-receptors (Orco). Orcos are highly conserved across insect taxa and are widely co-expressed with ORs. Odorant binding proteins (OBPs) can transport semiochemicals to ORs as protein carriers and thus constitute the first molecular recognition step in insect olfaction. In this study, three OBP genes highly expressed in S. invicta antenna, OBP1, OBP5, OBP6, and Orco were partially silenced using RNA interference (RNAi). RNAi SiOBP5- and Orco-injected ants showed significantly lower EAG (electroantennography) responses to fire ant alarm pheromones and the alkaloid, 2,4,6-trimethylpyridine than water- or GFP-injected ants 72 h post injection. Subsequent qRT-PCR analysis demonstrated that the transcript level of the OBP1, OBP5, OBP6, and Orco significantly decreased 72 h after ants were injected with dsRNAs; however, there were no transcript level or EAG changes in ants fed dsRNAs. Our results suggest that S. invicta Orco and SiOBP5 are crucial to fire ants for their responses to alarm pheromones. RNAi knocking down SiOBP5 can significantly disrupt alarm pheromone communication, suggesting that disrupting SiOBP5 and Orcos could be potentially useful in the management of red imported fire ants.

Functional analysis of the odorant receptor coreceptor in odor detection in Grapholita molesta (lepidoptera: Tortricidae)

The olfactory system must detect and discriminate various semiochemicals in the environment. In response to such diversity, insects have evolved a family of odorant-gated ion channels composed of a common receptor (coreceptor, Orco) and a ligand-binding tuning odorant receptor (OR) that confers odour specificity. This study aims to examine the expression pattern of Orco gene of Grapholita molesta (GmolOrco) and to elucidate the role of GmolOrco in detecting G. molesta sex pheromone and green leaf volatiles by using gene silencing via RNA interference (RNAi) coupled antennal electrophysiological (EAG). Multiple sequence alignment showed that GmolOrco shared high sequence similarities with the Orco ortholog of lepidopterans. The results of real-time quantitative PCR detection demonstrated that GmolOrco was predominantly expressed in adult antennae and had the highest expression quantity in adult period among the different developmental stages. Compared with the noninjected controls, GmolOrco expression in GmolOrcodouble-stranded RNA (dsRNA)-injected males was reduced to 39.92% and that in females was reduced to 40.43%. EAG assays showed that the responses of GmolOrco-dsRNA injected males to sex pheromones (Z)-8-dodecenyl acetate (Z8-12:OAc) and (Z)-8-dodecenyl alcohol (Z8-12:OH) were significantly reduced, and the GmolOrco-dsRNA-injected female to green leaf volatile (Z)-3-hexenyl acetate also significantly declined. We inferred that Orco-mediated olfaction was different in male and female G. molesta adults and was mainly involved in recognizing the sex pheromones released by female moths.