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

The larva and adult of Helicoverpa armigera use differential gustatory receptors to sense sucrose

Almost all herbivorous insects feed on plants and use sucrose as a feeding stimulant, but the molecular basis of their sucrose reception remains unclear. Helicoverpa armigera as a notorious crop pest worldwide mainly feeds on reproductive organs of many plant species in the larval stage, and its adult draws nectar. In this study, we determined that the sucrose sensory neurons located in the contact chemosensilla on larval maxillary galea were 100-1000 times more sensitive to sucrose than those on adult antennae, tarsi, and proboscis. Using the Xenopus expression system, we discovered that Gr10 highly expressed in the larval sensilla was specifically tuned to sucrose, while Gr6 highly expressed in the adult sensilla responded to fucose, sucrose and fructose. Moreover, using CRISPR/Cas9, we revealed that Gr10 was mainly used by larvae to detect lower sucrose, while Gr6 was primarily used by adults to detect higher sucrose and other saccharides, which results in differences in selectivity and sensitivity between larval and adult sugar sensory neurons. Our results demonstrate the sugar receptors in this moth are evolved to adapt toward the larval and adult foods with different types and amounts of sugar, and fill in a gap in sweet taste of animals.

Odorant receptor co-receptors affect expression of tuning receptors in Drosophila

Insects detect odorants using two large families of heteromeric receptors, the Odorant Receptors (ORs) and Ionotropic Receptors (IRs). Most OR and IR genes encode odorant-binding "tuning" subunits, whereas four (Orco, Ir8a, Ir25a, and Ir76b) encode co-receptor subunits required for receptor function. Olfactory neurons are thought to degenerate in the absence of Orco in ants and bees, and limited data suggest this may happen to some olfactory neurons in Drosophila fruit flies as well. Here, we thoroughly examined the role of co-receptors on olfactory neuron survival in Drosophila. Leveraging knowledge that olfactory neuron classes are defined by the expression of different tuning receptors, we used tuning receptor expression in antennal transcriptomes as a proxy for the survival of distinct olfactory neuron classes. Consistent with olfactory neuron degeneration, expression of many OR-family tuning receptors is decreased in Orco mutants relative to controls, and transcript loss is progressive with age. The effects of Orco are highly receptor-dependent, with expression of some receptor transcripts nearly eliminated and others unaffected. Surprisingly, further studies revealed that olfactory neuron classes with reduced tuning receptor expression generally survive in Orco mutant flies. Furthermore, there is little apoptosis or neuronal loss in the antenna of these flies. We went on to investigate the effects of IR family co-receptor mutants using similar approaches and found that expression of IR tuning receptors is decreased in the absence of Ir8a and Ir25a, but not Ir76b. As in Orco mutants, Ir8a-dependent olfactory neurons mostly endure despite near-absent expression of associated tuning receptors. Finally, we used differential expression analysis to identify other antennal genes whose expression is changed in IR and OR co-receptor mutants. Taken together, our data indicate that odorant co-receptors are necessary for maintaining expression of many tuning receptors at the mRNA level. Further, most Drosophila olfactory neurons persist in OR and IR co-receptor mutants, suggesting that the impact of co-receptors on neuronal survival may vary across insect species.

Transcriptome-wide analysis uncovers regulatory elements of the antennal transcriptome repertoire of bumblebee at different life stages

Bumblebees are crucial pollinators, providing essential ecosystem services and global food production. The success of pollination services relies on the interaction between sensory organs and the environment. The antenna functions as a versatile multi-sensory organ, pivotal in mediating chemosensory/olfactory information, and governs adaptive responses to environmental changes. Despite an increasing number of RNA-sequencing studies on insect antenna, comprehensive antennal transcriptome studies at the different life stages were not elucidated systematically. Here, we quantified the expression profile and dynamics of coding/microRNA genes of larval head and antennal tissues from early- and late-stage pupa to the adult of Bombus terrestris as suitable model organism among pollinators. We further performed Pearson correlation analyses on the gene expression profiles of the antennal transcriptome from larval head tissue to adult stages, exploring both positive and negative expression trends. The positively correlated coding genes were primarily enriched in sensory perception of chemical stimuli, ion transport, transmembrane transport processes and olfactory receptor activity. Negatively correlated genes were mainly enriched in organic substance biosynthesis and regulatory mechanisms underlying larval body patterning and the formation of juvenile antennal structures. As post-transcriptional regulators, miR-1000-5p, miR-13b-3p, miR-263-5p and miR-252-5p showed positive correlations, whereas miR-315-5p, miR-92b-3p, miR-137-3p, miR-11-3p and miR-10-3p exhibited negative correlations in antennal tissue. Notably, based on the inverse expression relationship, positively and negatively correlated microRNA (miRNA)-mRNA target pairs revealed that differentially expressed miRNAs predictively targeted genes involved in antennal development, shaping antennal structures and regulating antenna-specific functions. Our data serve as a foundation for understanding stage-specific antennal transcriptomes and large-scale comparative analysis of transcriptomes in different insects.

A single mutation in the mosquito (Aedes aegypti) olfactory receptor 8 causes loss of function to 1-octen-3-ol

The host-seeking behavior of mosquitoes have long been established to be primarily odor-mediated. In this process, olfactory receptors (Ors) play a critical role. 1-Octen-3-ol is a common volatile compound that is attractive to hematophagous arthropods such as mosquitos. The olfactory receptor 8 (AaOr8) on the tip of the stylet and maxillary palp of Aedes aegypti is tuned to 1-octen-3-ol, which is required for mosquitoes to quickly find blood vessels from a vertebrate host. However, little is known about the interaction of AaOr8 with 1-octen-3-ol which was studied in vivo and in silico in this study. The molecular binding poses and energies between ligands and the receptor were investigated. Three mutants of AaOr8 were cloned and compared with in vivo calcium imaging utilizing heterologous expression systems. As a result, our findings imply that a genetic disruption including targeted modification of Ors genes may be used to reduce mosquito bites.

The olfactory system of Pieris brassicae caterpillars: from receptors to glomeruli

The olfactory system of adult lepidopterans is among the best described neuronal circuits. However, comparatively little is known about the organization of the olfactory system in the larval stage of these insects. Here, we explore the expression of olfactory receptors and the organization of olfactory sensory neurons in caterpillars of Pieris brassicae, a significant pest species in Europe and a well-studied species for its chemical ecology. To describe the larval olfactory system in this species, we first analyzed the head transcriptome of third-instar larvae (L3) and identified 16 odorant receptors (ORs) including the OR coreceptor (Orco), 13 ionotropic receptors (IRs), and 8 gustatory receptors (GRs). We then quantified the expression of these 16 ORs in different life stages, using qPCR, and found that the majority of ORs had significantly higher expression in the L4 stage than in the L3 and L5 stages, indicating that the larval olfactory system is not static throughout caterpillar development. Using an Orco-specific antibody, we identified all olfactory receptor neurons (ORNs) expressing the Orco protein in L3, L4, and L5 caterpillars and found a total of 34 Orco-positive ORNs, distributed among three sensilla on the antenna. The number of Orco-positive ORNs did not differ among the three larval instars. Finally, we used retrograde axon tracing of the antennal nerve and identified a mean of 15 glomeruli in the larval antennal center (LAC), suggesting that the caterpillar olfactory system follows a similar design as the adult olfactory system, although with a lower numerical redundancy. Taken together, our results provide a detailed analysis of the larval olfactory neurons in P. brassicae, highlighting both the differences as well as the commonalities with the adult olfactory system. These findings contribute to a better understanding of the development of the olfactory system in insects and its life-stage-specific adaptations.

Opsin mutants alter host plant selection by color vision in the nocturnal invasive pest Tuta absoluta

In insects, vision is crucial in finding host plants, but its role in nocturnal insects is largely unknown. Vision involves responses to specific spectra of photon wavelengths and opsins plays an important role in this process. Long-wavelength sensitive opsin (LW opsin) and blue-sensitive opsin (BL opsin) are main visual opsin proteins and play important in behavior regulation.We used CRISPR/Cas9 technology to mutate the long-wavelength-sensitive and blue wavelength-sensitive genes and explored the role of vision in the nocturnal invasive pest Tuta absoluta. Light wave experiments revealed that LW2(-/-) and BL(-/-) mutants showed abnormal wavelength tropism. Both LW2 and BL mutations affected the preference of T. absoluta for the green environment. Mutations in LW2 and BL are necessary to inhibit visual attraction. The elimination of LW2 and BL affected the preference of leaf moths for green plants, and mutations in both induced a preference in moths for white plants. Behavioral changes resulting from LW2(-/-) and BL(-/-) mutants were not affected by sense of smell, further supporting the regulatory role of vision in insect behavior. To the best of our knowledge, this is the first study to reveal that vision, not smell, plays an important role in the host-seeking behavior of nocturnal insects at night, of which LW2 and BL opsins are key regulatory factors. These study findings will drive the development of the "vision-ecology" theory.

Comparative transcriptomic analysis of chemoreceptors in two sympatric scarab beetles, Hylamorpha elegans and Brachysternus prasinus

Chemoreception through odorant receptors (ORs), ionotropic receptors (IRs) and gustatory receptors (GRs) represents the functions of key proteins in the chemical ecology of insects. Recent studies have identified chemoreceptors in coleopterans, facilitating the evolutionary analysis of not only ORs but also IRs and GRs. Thus, Cerambycidae, Tenebrionidae and Curculionidae have received increased attention. However, knowledge of the chemoreceptors from Scarabaeidae is still limited, particularly for those that are sympatric. Considering the roles of chemoreceptors, this analysis could shed light on evolutionary processes in the context of sympatry. Therefore, the aim of this study was to identify and compare the repertoires of ORs, GRs and IRs between two sympatric scarab beetles, Hylamorpha elegans and Brachysternus prasinus. Here, construction of the antennal transcriptomes of both scarab beetle species and analyses of their phylogeny, molecular evolution and relative expression were performed. Thus, 119 new candidate chemoreceptors were identified for the first time, including 17 transcripts for B. prasinus (1 GR, 3 IRs and 13 ORs) and 102 for H. elegans (22 GRs, 14 IRs and 66 ORs). Orthologs between the two scarab beetle species were found, revealing specific expansions as well as absence in some clades. Purifying selection appears to have occurred on H. elegans and B. prasinus ORs. Further efforts will be focused on target identification to characterize kairomone and/or pheromone receptors.

Identification of candidate chemosensory genes in Bactrocera cucurbitae based on antennal transcriptome analysis

The melon fly, Bactrocera cucurbitae (Coquillett) (Tephritidae: Diptera), is an invasive pest that poses a significant threat to agriculture in Africa and other regions. Flies are known to use their olfactory systems to recognise environmental chemical cues. However, the molecular components of the chemosensory system of B. cucurbitae are poorly characterised. To address this knowledge gap, we have used next-generation sequencing to analyse the antenna transcriptomes of sexually immature B. cucurbitae adults. The results have identified 160 potential chemosensory genes, including 35 odourant-binding proteins (OBPs), one chemosensory protein (CSP), three sensory neuron membrane proteins (SNMPs), 70 odourant receptors (ORs), 30 ionotropic receptors (IRs), and 21 gustatory receptors (GRs). Quantitative real-time polymerase chain reaction quantitative polymerase chain reaction was used to validate the results by assessing the expression profiles of 25 ORs and 15 OBPs. Notably, high expression levels for BcucOBP5/9/10/18/21/23/26 were observed in both the female and male antennae. Furthermore, BcucOROrco/6/7/9/13/15/25/27/28/42/62 exhibited biased expression in the male antennae, whereas BcucOR55 showed biased expression in the female antennae. This comprehensive investigation provides valuable insights into insect olfaction at the molecular level and will, thus, help to facilitate the development of enhanced pest management strategies in the future.

Identification of 2,4-Di-tert-butylphenol as a Novel Agonist for Insect Odorant Receptors

Odorant molecules interact with odorant receptors (ORs) lining the pores on the surface of the sensilla on an insect's antennae and maxillary palps. This interaction triggers an electrical signal that is transmitted to the insect's nervous system, thereby influencing its behavior. Orco, an OR coreceptor, is crucial for olfactory transduction, as it possesses a conserved sequence across the insect lineage. In this study, we focused on 2,4-di-tert-butylphenol (DTBP), a single substance present in acetic acid bacteria culture media. We applied DTBP to oocytes expressing various Drosophila melanogaster odor receptors and performed electrophysiology experiments. After confirming the activation of DTBP on the receptor, the binding site was confirmed through point mutations. Our findings confirmed that DTBP interacts with the insect Orco subunit. The 2-heptanone, octanol, and 2-hexanol were not activated for the Orco homomeric channel, but DTBP was activated, and the EC50 value was 13.4 ± 3.0 μM. Point mutations were performed and among them, when the W146 residue changed to alanine, the Emax value was changed from 1.0 ± 0 in the wild type to 0.0 ± 0 in the mutant type, and all activity was decreased. Specifically, DTBP interacted with the W146 residue of the Orco subunit, and the activation manner was concentration-dependent and voltage-independent. This molecular-level analysis provides the basis for novel strategies to minimize pest damage. DTBP, with its specific binding to the Orco subunit, shows promise as a potential pest controller that can exclusively target insects.

Genome-wide identification of candidate chemosensory receptors in the bean bug Riptortus pedestris (Hemiptera: Alydidae) and the functional verification of its odorant receptor co-receptor (Orco) in recognizing aggregation pheromone

A sophisticated and sensitive olfactory system plays a vital role in the survival and reproduction of insects. Chemosensory receptors are indispensable for the molecular recognition and discrimination of semiochemicals. Riptortus pedestris is a notorious pest of legume plants, resulting in yield losses and quality decreases in soybeans. It is well accepted that R. pedestris highly relies on its olfactory system in detecting aggregation pheromones, host volatiles, and pesticides; however, little research focused on its chemosensory receptors. In the present study, we identified 237 odorant receptors (ORs), 42 gustatory receptors (GRs), and 31 ionotropic receptors (IRs) from the reported genome of R. pedestris, and analyzed their phylogenetic relationship with other hemipteran species. Through the results of RNA-seq and real-time quantitative PCR (qRT-PCR), we found that RpedORs displayed different expression levels in the antennae of R. pedestris at different development stages. To further verify the function of odorant receptor co-receptor (Orco), an obligate and unique insect OR, we silenced RpedOrco by RNA interference (RNAi) method. The results showed that silencing RpedOrco could significantly impair the response to aggregation pheromone in R. pedestris, indicating that RpedOrco plays an essential role in odorant detection. Our results can provide the theoretical foundations for revealing the olfactory recognition mechanism of R. pedestris and help explore and develop novel olfactory-based agents against this pest.

RNAi verifications on olfactory defects of an essential biocontrol agent Agasicles hygrophila (Coleoptera: Chrysomelidae) regarding mating and host allocation

Alligator weed Alternanthera philoxeroides is a perennial, worldwide pernicious weed. The beetle Agasicles hygrophila is considered to be a classical biological agent used to control A. philoxeroides. In the insect peripheral olfactory system, the odorant receptor co-receptor (ORco) plays an important function in the perception of odors in insects. However, the function of ORco in the mating and host-finding behaviors of A. hygrophila remains unclear. In this study, we characterized the odorant receptor co-receptor of A. hygrophila (AhygOrco). Real-time quantitative PCR (qRT–PCR) showed that AhygOrco was predominantly expressed in the antennae of both male and female adults, and the difference between male and female antennae was not significant. The RNA interference (RNAi) results showed that compared to the control, the injection of AhygOrco dsRNA strongly reduced the expression of AhygOrco by 90% in male beetles and 89% in female beetles. The mate-seeking and feeding behavior of AhygOrco-silenced beetles were significantly inhibited. Male adults were significantly less successful in finding a mate compared to the control group. Furthermore, host allocation abilities toward A. philoxeroides of both adults were significantly repressed. These results indicated that AhygOrco is associated with A. hygrophila feeding and mate-seeking and that inhibition of AhygOrco expression is one of the causes of reduced host and mate recognition in A. hygrophila. Meanwhile, the study provides support for exploring gene functions based on RNAi.

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.