Discovery of Chemosensory Genes in the Oriental Fruit Fly, Bactrocera dorsalis

Rapid visual detection assay for Bactrocera dorsalis (Hendel) using recombinase polymerase amplification and CRISPR/Cas12b

The oriental fruit fly Bactrocera dorsalis (Hendel) is considered as a quarantine pest in many countries and regions. Challenges remain in distinguishing this species with morphological similarities, especially in relevant development stages. In recent years, CRISPR/Cas12b genetic diagnostics has seen rapid advancements and offers an efficient tool for the identification of pathogens, viruses, and other genetic targets. Here we developed a new and rapid visual detection assay of B. dorsalis using recombinase polymerase amplification (RPA) and the CRISPR/Cas12b system. The system can detect different developmental stages of B. dorsalis within 30-35 min at 43 ℃ and the results are easily observed by the naked eye based on the color change in the tube during the reaction. The specificity and high sensitivity of this method was demonstrated, allowing for detection from 3.2 pg µL- 1 of DNA. With crude DNA, this diagnostic system successfully identified B. dorsalis by holding the reaction tubes in the hand. Our study demonstrates that RPA-CRISPR/Cas12b visualization system is effective to detect B. dorsalis rapidly and accurately. This approach can be applied for monitoring and identification of other pests in border and relevant locations, preventing biological invasions and ensuring pest control.

Template-based modeling of insect odorant receptors outperforms AlphaFold3 for ligand binding predictions

Insects rely on odorant receptors (ORs) to detect and respond to volatile environmental cues, so the ORs are attracting increasing interest as potential targets for pest control. However, experimental analysis of their structures and functions faces significant challenges. Computational methods such as template-based modeling (TBM) and AlphaFold3 (AF3) could facilitate the structural characterisation of ORs. This study first showed that both models accurately predicted the structural fold of MhOR5, a jumping bristletail OR with known experimental 3D structures, although accuracy was higher in the extracellular region of the protein and binding mode of their cognate ligands with TBM. The two approaches were then compared for their ability to predict the empirical binding evidence available for OR-odorant complexes in two economically important fruit fly species, Bactrocera dorsalis and B. minax. Post-simulation analyses including binding affinities, complex and ligand stability and receptor-ligand interactions (RLIs) revealed that TBM performed better than AF3 in discriminating between binder and non-binder complexes. TBM's superior performance is attributed to hydrophobicity-based helix-wise multiple sequence alignment (MSA) between available insect OR templates and the ORs for which the binding data were generated. This MSA identified conserved residues and motifs which could be used as anchor points for refining the alignments.

Ionotropic Receptor 8a (Ir8a) Plays an Important Role in Acetic Acid Perception in the Oriental Fruit Fly, Bactrocera dorsalis

Bactrocera dorsalis is one of the major invasive pests worldwide. The acetic acid-enriched sweet bait trapping is an important method for monitoring and controlling this fly. Several studies showed that acetic acid is perceived by ionotropic receptors (IRs). Thus, we annotated 65 IR genes in the B. dorsalis genome. We also investigated the IRs involved in acetic acid perception in this fly by behavioral, electrophysiological, and molecular methods. As the results indicated, the antennae are the main olfactory organs to sense acetic acid. Among the antennal IRs showed acetic acid-induced expression profiles, IR8a was proven to perceive acetic acid by CRISPR/Cas9-mediated mutagenesis. Additionally, calcium imaging showed that IR64a and IR75a are potential acetic acid receptors respectively co-expressed with IR76b and IR8a. This study represents the first comprehensive characterization of IRs in B. dorsalis at the whole-genome level, revealing the significant role of IRs in acetic acid perception.

Reproductive behavior of fruit flies: courtship, mating, and oviposition

Many species of the Tephritidae family are invasive and cause huge damage to agriculture and horticulture, owing to their reproductive characteristics. In this review, we have summarized the existing studies on the reproductive behavior of Tephritidae, particularly those regarding the genes and external factors that are associated with courtship, mating, and oviposition. Furthermore, we outline the issues that still need to be addressed in fruit fly reproduction research. The review highlights the implications for understanding the reproductive behavior of fruit flies and discusses methods for their integrated management and biological control. © 2023 Society of Chemical Industry.

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.

An Antenna-Enriched Chemosensory Protein Plays Important Roles in the Perception of Host Plant Volatiles in Bactrocera dorsalis (Diptera: Tephritidae)

Olfaction plays indispensable roles in insect behavior such as host location, foraging, oviposition, and avoiding predators. Chemosensory proteins (CSPs) can discriminate the hydrophobic odorants and transfer them to the odorant receptors. Presently, CSPs have been identified in many insect species. However, their presence and functions remain unknown in Bactrocera dorsalis, a destructive and invasive insect pest in the fruit and vegetable industry. Here, we annotated eight CSP genes in the genome of B. dorsalis. The results of quantitative real-time polymerase chain reaction (RT-qPCR) showed that BdorCSP3 was highly expressed in the antennae. Molecular docking and in vitro binding assays showed that BdorCSP3 had a good binding ability to host volatiles methyl eugenol (ME, male-specific attractant) and β-caryophyllene (potential female attractant). Subsequently, CRISPR/Cas9 was used to generate BdorCSP3-/- mutants. Electroantennograms (EAGs) and behavioral assays revealed that male mutants significantly reduced the preference for ME, while female mutants lost their oviposition preference to β-caryophyllene. Our data indicated that BdorCSP3 played important roles in the perception of ME and β-caryophyllene. The results not only expanded our knowledge of the olfaction perception mechanism of insect CSPs but also provided a potential molecular target for the control of B. dorsalis.

Insights on reproduction-related genes in the striped fruit fly, Zeugodacus scutellata (Hendel) (Diptera: Tephritidae)

The striped fruit fly, Zeugodacus scutellata is a significant pest in East and Southeast Asia by damaging Cucurbitaceae blossoms and fruits. To control this pest, a novel strategy to suppress the gene(s) associated with sexually dimorphic phenotypes has been devised and implemented in a laboratory scale. However, comprehensive transcriptomic analysis related to this sex differentiation of Z. scutellata was necessary to determine effective target genes for the genetic control. We performed de novo assembly of the transcript obtained by paired-end sequencing using an Illumina HiSeq platform and let to 217,967 unigenes (i.e., unique genes) with a minimum length of 200 bp. The female produced 31, 604, 442 reads with 97.93% of Q20, 94.76% of Q30, and the male produced 130, 592, 828 reads with 97.93% of Q20 and 94.76 of Q30%. The differentially expressed genes were used to predict genetic factors associated with sex differentiation, which included Rho1, extra-macrochaetae (emc), hopscotch (hop), doublesex (dsx), sex-lethal (sxl), transformer-2 (tra-2), testis-specific serine/threonine-protein kinase (tssk1), tektin1 (tkt1) and 2 (tkt2), odorant binding proteins (OBPs), fruitless (fru), vitellogenin receptor, and hormone receptors in Z. scutellata. In addition, this transcriptome analysis provides the additional gene associated with sex determination and mating behaviors, which would be applied to develop a novel sterile insect technique against Z. scutellata.

Recapitulation of the embryonic transcriptional program in holometabolous insect pupae

Holometabolous insects are predominantly motionless during metamorphosis, when no active feeding is observed and the body is enclosed in a hardened cuticle. These physiological properties as well as undergoing processes resemble embryogenesis, since at the pupal stage organs and systems of the imago are formed. Therefore, recapitulation of the embryonic expression program during metamorphosis could be hypothesized. To assess this hypothesis at the transcriptome level, we have performed a comprehensive analysis of the developmental datasets available in the public domain. Indeed, for most datasets, the pupal gene expression resembles the embryonic rather than the larval pattern, interrupting gradual changes in the transcriptome. Moreover, changes in the transcriptome profile during the pupa-to-imago transition are positively correlated with those at the embryo-to-larvae transition, suggesting that similar expression programs are activated. Gene sets that change their expression level during the larval stage and revert it to the embryonic-like state during the metamorphosis are enriched with genes associated with metabolism and development.

Identification of Candidate Chemosensory Gene Families by Head Transcriptomes Analysis in the Mexican Fruit Fly, Anastrepha ludens Loew (Diptera: Tephritidae)

Insect chemosensory systems, such as smell and taste, are mediated by chemosensory receptor and non-receptor protein families. In the last decade, many studies have focused on discovering these families in Tephritidae species of agricultural importance. However, to date, there is no information on the Mexican fruit fly Anastrepha ludens Loew, a priority pest of quarantine importance in Mexico and other countries. This work represents the first effort to identify, classify and characterize the six chemosensory gene families by analyzing two head transcriptomes of sexually immature and mature adults of A. ludens from laboratory-reared and wild populations, respectively. We identified 120 chemosensory genes encoding 31 Odorant-Binding Proteins (OBPs), 5 Chemosensory Proteins (CSPs), 2 Sensory Neuron Membrane Proteins (SNMPs), 42 Odorant Receptors (ORs), 17 Ionotropic Receptors (IRs), and 23 Gustatory Receptors (GRs). The 120 described chemosensory proteins of the Mexican fruit fly significantly contribute to the genetic databases of insects, particularly dipterans. Except for some OBPs, this work reports for the first time the repertoire of olfactory proteins for one species of the genus Anastrepha, which provides a further basis for studying the olfactory system in the family Tephritidae, one of the most important for its economic and social impact worldwide.

Identification and functional analysis of a chemosensory protein from Bactrocera minax (Diptera: Tephritidae)

BACKGROUND

Olfaction has an indispensable role in insect behavior, enabling location of suitable host plants and oviposition sites, finding mates and evasion of natural enemies. Chemosensory proteins (CSPs) function to screen external odorants and transport them to olfactory receptor neurons, thereby increasing the sensitivity of the olfactory system. At present, CSP genes have been identified in many insect species, but there are relatively few studies on the function of CSP, especially in Tephritidae.

RESULTS

In this study, we sequenced and analyzed 12 transcriptomes of Bactrocera minax and identified five CSP genes. The results of polymerase chain reactions with reverse transcription showed that BminCSP3 was highly expressed only in antennae. Results from competitive binding experiments showed that BminCSP3 has good binding ability to citral compared with 23 other volatile organic compounds. The docking model with citral showed hydrogen bond formation with residues (ARG97); however, no hydrogen bonds were formed in the docking of five other ligands (furfuryl alcohol, linalool, cis-3-hexenyl acetate, (R)-(+)-limonene and (+)-carvone). Electroantennogram (EAG) analyses revealed that citral was active in B. minax at the antennal level, and the EAG response value of female adults was significantly higher than that of male adults. Furthermore, the results of behavioral bioassays showed that females were significantly attracted to citral.

CONCLUSION

Our results suggest that BminCSP3 plays an important role in the recognition of citral by B. minax adults. © 2022 Society of Chemical Industry.

Raspberry ketone diet supplement reduces attraction of sterile male Queensland fruit fly to cuelure by altering expression of chemoreceptor genes

Sterile male Queensland fruit fly, Bactrocera tryoni (Froggatt), fed as immature adults on the plant compound raspberry ketone (RK), show a reduced attraction to cuelure, a synthetic analogue of RK used as an attractant in Male Annihilation Technique. We hypothesized the reduced attraction of RK-fed adult males to cuelure may be a consequence of altered expression of chemoreception genes. A Y-tube olfactometer assay with RK-fed and RK-unfed sterile B. tryoni males tested the subsequent behavioural response to cuelure. Behavioral assays confirmed a significant decrease in attraction of RK-fed sterile males to cuelure. RK-fed, non-responders (to cue-lure) and RK-unfed, responders (to cue-lure) males were sampled and gene expression compared by de novo RNA-seq analysis. A total of 269 genes in fly heads were differentially expressed between replicated groups of RK-fed, cuelure non-responders and RK-unfed, cuelure responders. Among them, 218 genes including 4 chemoreceptor genes were up regulated and 51 genes were down regulated in RK-fed, cuelure non-responders. De novo assembly generated many genes with unknown functions and no significant BLAST hits to homologues in other species. The enriched and suppressed genes reported here, shed light on the transcriptional changes that affect the dynamics of insect responses to chemical stimuli.

Tephritid Fruit Fly Semiochemicals: Current Knowledge and Future Perspectives

The Dipteran family Tephritidae (true fruit flies) comprises more than 5000 species classified in 500 genera distributed worldwide. Tephritidae include devastating agricultural pests and highly invasive species whose spread is currently facilitated by globalization, international trade and human mobility. The ability to identify and exploit a wide range of host plants for oviposition, as well as effective and diversified reproductive strategies, are among the key features supporting tephritid biological success. Intraspecific communication involves the exchange of a complex set of sensory cues that are species- and sex-specific. Chemical signals, which are standing out in tephritid communication, comprise long-distance pheromones emitted by one or both sexes, cuticular hydrocarbons with limited volatility deposited on the surrounding substrate or on the insect body regulating medium- to short-distance communication, and host-marking compounds deposited on the fruit after oviposition. In this review, the current knowledge on tephritid chemical communication was analysed with a special emphasis on fruit fly pest species belonging to the Anastrepha, Bactrocera, Ceratitis, and Rhagoletis genera. The multidisciplinary approaches adopted for characterising tephritid semiochemicals, and the real-world applications and challenges for Integrated Pest Management (IPM) and biological control strategies are critically discussed. Future perspectives for targeted research on fruit fly chemical communication are highlighted.

Ultrastructure of the Olfactory Sensilla across the Antennae and Maxillary Palps of Bactrocera dorsalis (Diptera: Tephritidae)

Simple Summary

The environmentally friendly methods have been employed to control the serious pest, Bactrocera dorsalis, based on chemical communications. However, their olfaction mechanism has not been unveiled. In this study, the ultrastructure of the sensilla on the antennae and maxillary palps was examined by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The results showed that three types of olfactory sensilla (trichodea, basiconica and coeloconica) and two types of non-olfactory sensilla (chaetica and microtrichia) located on the antennae. These findings will benefit the olfactory research and the integrated management of this pest.

Abstract

The sensilla on the antennae and maxillary palps are the most important olfactory organs, via which the insect can perceive the semiochemicals to adjust their host seeking and oviposition behaviors. The oriental fruit fly, Bactrocera dorsalis (Hendel) (Diptera: Tephritidae), is a major agricultural quarantine pest infesting more than 250 different fruits and vegetables. However, the sensilla involved in olfaction have not been well documented even though a variety of control practices based on chemical communication have already been developed. In this study, the ultrastructure of the sensilla, especially the olfactory sensilla on the antennae and maxillary palps of both males and females, were investigated with field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). Three types of olfactory sensillum types including trichodea, basiconica, and coeloconica, and two non-olfactory sensilla including both chaetica and microtrichia, were observed. Each of these three types of olfactory sensilla on the antennae of B. dorsalis were further classified into two subtypes according to the morphology and number of receptor cells. For the first time, the pores on the sensilla trichodea and basiconica cuticular wall were observed in this species, suggesting they are involved in semiochemical perception. This study provides new information on B. dorsalis olfaction, which can be connected to other molecular, genetic, and behavioral research to construct an integral olfactory system model for this species.

Transcriptome Characterization and Expression Analysis of Chemosensory Genes in Chilo sacchariphagus (Lepidoptera Crambidae), a Key Pest of Sugarcane

Insect chemoreception involves many families of genes, including odourant/pheromone binding proteins (OBP/PBPs), chemosensory proteins (CSPs), odourant receptors (ORs), ionotropic receptors (IRs), and sensory neuron membrane proteins (SNMPs), which play irreplaceable roles in mediating insect behaviors such as host location, foraging, mating, oviposition, and avoidance of danger. However, little is known about the molecular mechanism of olfactory reception in Chilo sacchariphagus, which is a major pest of sugarcane. A set of 72 candidate chemosensory genes, including 31 OBPs/PBPs, 15 CSPs, 11 ORs, 13 IRs, and two SNMPs, were identified in four transcriptomes from different tissues and genders of C. sacchariphagus. Phylogenetic analysis was conducted on gene families and paralogs from other model insect species. Quantitative real-time PCR (qRT-PCR) showed that most of these chemosensory genes exhibited antennae-biased expression, but some had high expression in bodies. Most of the identified chemosensory genes were likely involved in chemoreception. This study provides a molecular foundation for the function of chemosensory proteins, and an opportunity for understanding how C. sacchariphagus behaviors are mediated via chemical cues. This research might facilitate the discovery of novel strategies for pest management in agricultural ecosystems.

Recent Advancements in Studies on Chemosensory Mechanisms Underlying Detection of Semiochemicals in Dacini Fruit Flies of Economic Importance (Diptera: Tephritidae)

Dacini fruit flies mainly contain two genera, Bactrocera and Zeugodacus, and include many important pests of fruits and vegetables. Their life cycle is affected by various environmental cues. Among them, multiple characteristic semiochemicals have remarkable effects on their reproductive and host-finding behaviors. Notably, floral fragrances released from so-called fruit fly orchids strongly attract males of several Dacini fruit fly species. Focusing on the strong attraction of male flies to particular chemicals, natural and synthetic lures have been used for pest management. Thus, the perception of semiochemicals is important to understand environmental adaptation in Dacini fruit flies. Since next-generation sequencers are available, a large number of chemosensory-related genes have been identified in Dacini fruit flies, as well as other insects. Furthermore, recent studies have succeeded in the functional analyses of olfactory receptors in response to semiochemicals. Thus, characterization of molecular components required for chemoreception is under way. However, the mechanisms underlying chemoreception remain largely unknown. This paper reviews recent findings on peripheral mechanisms in the perception of odors in Dacini fruit flies, describing related studies in other dipteran species, mainly the model insect Drosophilamelanogaster. Based on the review, important themes for future research have also been discussed.

A First Glimpse of the Mexican Fruit Fly Anastrepha ludens (Diptera: Tephritidae) Antenna Morphology and Proteome in Response to a Proteinaceous Attractant

Anastrepha ludens is a key pest of mangoes and citrus from Texas to Costa Rica but the mechanisms of odorant perception in this species are poorly understood. Detection of volatiles in insects occurs mainly in the antenna, where molecules penetrate sensillum pores and link to soluble proteins in the hemolymph until reaching specific odor receptors that trigger signal transduction and lead to behavioral responses. Scrutinizing the molecular foundation of odorant perception in A. ludens is necessary to improve biorational management strategies against this pest. After exposing adults of three maturity stages to a proteinaceous attractant, we studied antennal morphology and comparative proteomic profiles using nano-LC-MS/MS with tandem mass tags combined with synchronous precursor selection (SPS)-MS3. Antennas from newly emerged flies exhibited dense agglomerations of olfactory sensory neurons. We discovered 4618 unique proteins in the antennas of A. ludens and identified some associated with odor signaling, including odorant-binding and calcium signaling related proteins, the odorant receptor co-receptor (Orco), and putative odorant-degrading enzymes. Antennas of sexually immature flies exhibited the most upregulation of odor perception proteins compared to mature flies exposed to the attractant. This is the first report where critical molecular players are linked to the odor perception mechanism of A. ludens.

Comparative genomic and transcriptomic analyses of chemosensory genes in the citrus fruit fly Bactrocera (Tetradacus) minax

The citrus fruit fly Bactrocera (Tetradacus) minax is a major and devastating agricultural pest in Asian subtropical countries. Previous studies have shown that B. minax interacts with plant hosts via the efficient chemosensory system. However, the molecular components of the B. minax chemosensory system have not been well characterized. Herein, we identified a total of 25 putative odorant-binding receptors (OBPs), 4 single-copy chemosensory proteins (CSPs) and 53 candidate odorant receptors (ORs) using a newly generated whole-genome dataset for B. minax. This study significantly extended the chemosensation-related gene profiles (particularly, OBPs and ORs) in six other tephritid species. Comparative transcriptome analysis of adult B. minax and Bactrocera dorsalis showed that there were 14 highly expressed OBPs (FPKM > 100) in B. dorsalis and 7 highly expressed ones in B. minax. The expression level of CSP3 gene and CSP4 gene was higher in B. dorsalis than that in B. minax. Comparative genomic and transcriptomic analyses of chemosensory genes in the citrus fruit fly B. minax provided new insights for preventive control of this agriculture important pest and closely related species.

Identification and Expression Profiling of Chemosensory Genes in Hermetia illucens via a Transcriptomic Analysis

The black soldier fly, Hermetia illucens, is a cosmopolitan insect of the family Stratiomyidae (Diptera). Chemosensory genes encode proteins involved directly in the detection of odorants. In this study, we sequenced the antennal transcriptome of H. illucens adults to identify chemosensory genes. Putative unigenes encoding 27 odorant binding proteins (OBPs), five chemosensory proteins (CSPs), 70 odorant receptors (ORs), 25 ionotropic receptors (IRs), 10 gustatory receptors (GRs) and two sensory neuron membrane proteins (SNMPs) were identified. Tissue-specific expression profiles of the identified OBPs, CSPs and SNMPs were investigated using RT-PCR. Eight OBPs (HillOBP1-2, 9, 11-14, and 17), one CSP (HillCSP5) and one SNMP (HillSNMP1) were predominantly expressed in antennae. Further real-time quantitative PCR analyses revealed that the antennae-enriched unigenes also exhibited significant differences in expression between males and females. Among the sex-biased unigenes, six ORs showed female-biased expression, suggesting that these genes might participate in female-specific behaviors such as oviposition site searching. Sixteen ORs and two OBPs showed male-biased expression, indicating that they may play key roles in the detection of female sex pheromones. Our study is the first attempt to delineate the molecular basis of chemoreception in H. illucens. Our data provide useful information for comparative studies on the differentiation and evolution of Dipteran chemosensory gene families.

Identification of olfactory genes and functional analysis of BminCSP and BminOBP21 in Bactrocera minax

Insects possess highly developed olfactory systems which play pivotal roles in its ecological adaptations, host plant location, and oviposition behavior. Bactrocera minax is an oligophagous tephritid insect whose host selection, and oviposition behavior largely depend on the perception of chemical cues. However, there have been very few reports on molecular components related to the olfactory system of B. minax. Therefore, the transcriptome of B. minax were sequenced in this study, with 1 candidate chemosensory protein (CSP), 21 candidate odorant binding proteins (OBPs), 53 candidate odorant receptors (ORs), 29 candidate ionotropic receptors (IRs) and 4 candidate sensory neuron membrane proteins (SNMPs) being identified. After that, we sequenced the candidate olfactory genes and performed phylogenetic analysis. qRT-PCR was used to express and characterize 9 genes in olfactory and non-olfactory tissues. Compared with GFP-injected fly (control), dsOBP21-treated B. minax and dsCSP-treated B. minax had lower electrophysiological response to D-limonene (attractant), suggesting the potential involvement of BminOBP21 and BminCSP genes in olfactory perceptions of the fly. Our study establishes the molecular basis of olfaction, tributary for further functional analyses of chemosensory processes in B. minax.

Candidates for chemosensory genes identified in the Chinese citrus fly, Bactrocera minax, through a transcriptomic analysis

Background

The males of many Bactrocera species (Diptera: Tephritidae) respond strongly to plant-derived chemicals (male lures) and can be divided into cue lure/raspberry ketone (CL/RK) responders, methyl eugenol (ME) responders and non-responders. Representing a non-responders, Bactrocera minax display unique olfactory sensory characteristics compared with other Bactrocera species. The chemical senses of insects mediate behaviors that are associated with survival and reproduction. Here, we report the generation of transcriptomes from antennae and the rectal glands of both male and female adults of B. minax using Illumina sequencing technology, and annotated gene families potentially responsible for chemosensory.

Results

We developed four transcriptomes from different tissues of B. minax and identified a set of candidate genes potentially responsible for chemosensory by analyzing the transcriptomic data. The candidates included 40 unigenes coding for odorant receptors (ORs), 30 for ionotropic receptors (IRs), 17 for gustatory receptors (GRs), three for sensory neuron membrane proteins (SNMPs), 33 for odorant-binding proteins (OBPs), four for chemosensory proteins (CSPs). Sex- and tissue-specific expression profiles for candidate chemosensory genes were analyzed via transcriptomic data analyses, and expression profiles of all ORs and antennal IRs were investigated by real-time quantitative PCR (RT-qPCR). Phylogenetic analyses were also conducted on gene families and paralogs from other insect species together.

Conclusions

A large number of chemosensory genes were identified from transcriptomic data. Identification of these candidate genes and their expression profiles in various tissues provide useful information for future studies towards revealing their function in B. minax.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-6022-5) contains supplementary material, which is available to authorized users.

Involvement of the Antennal and Maxillary Palp Structures in Detection and Response to Methyl Eugenol by Male Bactrocera dorsalis (Diptera: Tephritidae)

The oriental fruit fly, Bactrocera dorsalis (Handel) is one of the most destructive pests of fruits. The discovery of methyl eugenol (ME) as a potent male attractant for this species has led to its successful use in area-wide fruit fly control programs such as male annihilation. While the antenna is recognized as primarily responsible for male flies' detection of attractants such as ME, little is known of the involvement of the maxillary palp. Using behavioral assays involving males with intact and ablated antennae and maxillary palp structures, we seek to ascertain the relative involvement of the maxillary palp in the ability of the male fly to detect ME. In cage bioassays (distance of ≤40 cm from the source), >97% of unmodified males will normally show a response to ME. Here, we showed that 17.6% of males with their antennae ablated were still attracted to ME versus 75.0% of males with their palps ablated. However, none of the antennae-ablated males were able to detect ME over a distance of >100 cm. Furthermore, wind tunnel bioassays showed that maxillary palp-ablated males took a significantly longer time compared to unablated males to successfully detect and eventually feed on ME. These results suggest that although the antennae are necessary for detection of ME over longer distances, at shorter distances, both antennae and maxillary palps are also involved in detecting ME. Hence, those palps may play a larger role than previously recognized in maneuvering males toward lure sources over shorter ranges.

Glomerular Organization in the Antennal Lobe of the Oriental Fruit Fly Bactrocera dorsalis

The oriental fruit fly, Bactrocera dorsalis is one of the most destructive pests of horticultural crops in tropical and subtropical Asia. The insect relies heavily on its olfactory system to select suitable hosts for development and reproduction. To understand the neural basis of its odor-driven behaviors, it is fundamental to characterize the anatomy of its olfactory system. In this study, we investigated the anatomical organization of the antennal lobe (AL), the primary olfactory center, in B. dorsalis, and constructed a 3D glomerular atlas of the AL based on synaptic antibody staining combined with computerized 3D reconstruction. To facilitate identification of individual glomeruli, we also applied mass staining of olfactory sensory neurons (OSNs) and projection neurons (PNs). In total, 64 or 65 glomeruli are identifiable in both sexes based on their shape, size, and relative spatial relationship. The overall glomerular volume of two sexes is not statistically different. However, eight glomeruli are sexually dimorphic: four (named AM2, C1, L2, and L3) are larger in males, and four are larger in females (A3, AD1, DM3, and M1). The results from anterograde staining, obtained by applying dye in the antennal lobe, show that three typical medial, media lateral, and lateral antennal-lobe tracts form parallel connections between the antennal lobe and protocerebrum. In addition to these three tracts, we also found a transverse antennal-lobe tract. Based on the retrograde staining of the calyx in the mushroom body, we also characterize the arrangement of roots and cell body clusters linked to the medial antennal-lobe tracts. These data provide a foundation for future studies on the olfactory processing of host odors in B. dorsalis.

BdorOR88a Modulates the Responsiveness to Methyl Eugenol in Mature Males of Bactrocera dorsalis (Hendel)

Insect attractants are important prevention tools for managing populations of the Oriental fruit fly, Bactrocera dorsalis (Hendel), which is a highly destructive agricultural pest with health implications in tropical and subtropical countries. Methyl eugenol (ME) is still considered the gold standard of B. dorsalis attractants. Mature male flies use their olfactory system to detect ME, but the molecular mechanism underlying their olfactory detection of ME largely remains unclear. Here, we showed that ME activates the odorant receptors OR63a-1 and OR88a in mature B. dorsalis males antennae by RNA-Seq and qRT-PCR analysis. Interestingly, ME only elicited robust responses in the BdorOR88a/BdorOrco-expressing Xenopus oocytes, thus suggesting that BdorOR88a is necessary for ME reception and tropism in B. dorsalis. Next, our indoor behavioral assays demonstrated that BdorOR63a-1 knockdown had no significant effects on ME detection and tropism. By contrast, reducing the BdorOR88a transcript levels led to a significant decrease in the males' responsiveness to ME. Taken together, our results gave novel insight in the understanding of the olfactory background to the Oriental fruit fly's attraction toward ME.

Identification and expression profiles of novel odorant binding proteins and functional analysis of OBP99a in Bactrocera dorsalis

Odorant-binding proteins (OBPs) in insects are essential for mating and oviposition host selection. How these OBPs respond to different hosts at the mRNA level and their effects on behavior remain poorly characterized. The oriental fruit fly Bactrocera dorsalis is a highly invasive agricultural pest with an extremely broad host range and high fecundity. Based on our previously constructed B. dorsalis transcriptome, six OBPs that were differentially expressed during three different physiological adult stages were identified. A phylogenetic tree was constructed to illustrate the relationships of these six OBPs with OBP sequences from three other dipteran species (Drosophila melanogaster, Anopheles gambiae, and Ceratitis capitata). The spatiotemporal expression profiles of the six OBPs were analyzed using quantitative real-time PCR. Our results revealed that OBP19c, OBP44a, OBP99a, and OBP99d were abundantly expressed from the prepupa stage to the adult stage, and most of the OBPs were mainly expressed in the head, wings, and antennae. The expression levels of these OBPs were upregulated when female flies were exposed to their preferred hosts. Silencing OBP99a resulted fewer eggs being laid compared with the control group when the females were exposed to their preferred host, that is, banana, whereas more eggs were laid when a non-preferred host, that is, tomato, was used. Furthermore, silencing OBP99a led to sexually dimorphic mating behavior. dsOBP99a-injected males dramatically reduced courtship, whereas enhanced courtship was observed in the treated females. These data indicate that OBPs may participate in different biological processes of B. dorsalis. Our study will provide insight into the molecular mechanism of chemoreception and help develop ecologically friendly pest-control strategies.

Screening for key genes and transcription factors in ankylosing spondylitis by RNA-Seq

Ankylosing spondylitis (AS) is a chronic inflammatory arthritis and autoimmune disease, the etiology and pathogenesis of which remain largely unknown. In the present study, blood samples were harvested from patients with AS and from healthy volunteers as a normal control (NC) for RNA-sequencing. Differentially expressed genes (DEGs) in the AS group compared with the NC group were identified, and gene ontology (GO) term and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses were subsequently performed. Protein-protein interaction (PPI) network and AS-specific transcriptional regulatory network construction was performed for the DEGs. A total of 503 DEGs, including 338 upregulated and 165 downregulated DEGs, were identified in patients with AS compared with the NC group. Three upregulated DEGs identified, interferon-induced protein with tetratricopeptide repeats (IFIT)1, IFIT3 and radical S-adenosyl methionine domain containing (RSAD)2, are interferon (IFN)-stimulated genes that serve a role in the IFN signaling pathway. The most significantly enriched GO term was response to other organisms. Osteoclast differentiation was a significantly enriched pathway for eight DEGs [High affinity immunoglobulin gamma Fc receptor (FCGR)1A, FCGR2B, four and a half LIM domains 2, integrin β3, signal transducer and activator of transcription 2 (STAT2), suppressor of cytokine signaling 3 (SOCS3), leukocyte immunoglobulin like receptor (LILR)A4 and LILRA6]. The six hub genes in the PPI network constructed were interferon-stimulated gene 15, heat shock protein β1, microtubule-associated proteins 1A/1B light chain 3A, IFIT1, IFIT3 and SOCS3. POU domain class 2 transcription factor 1 (1-Oct) and ecotropic virus integration site-1 (Evi-1) were identified as two important transcription factors (TFs) in AS according to the AS-specific transcriptional regulatory network constructed. In addition, IFIT1 and IFIT3 were identified as targets of 1-Oct. The results of the present study indicate that osteoclast differentiation, the IFN signaling pathway and genes associated with these two signaling pathways, particularly FCGR2B, STAT2, SOCS3, IFIT1 and IFIT3, may serve a role in AS. In addition, Evi-1 and 1-Oct may be two important TFs associated with AS. These results may provide a basis for elucidating the underlying mechanisms of and developing novel treatments for AS.

Olfactory Plasticity: Variation in the Expression of Chemosensory Receptors in Bactrocera dorsalis in Different Physiological States

Changes in physiological conditions could influence the perception of external odors, which is important for the reproduction and survival of insect. With the alteration of physiological conditions, such as, age, feeding state, circadian rhythm, and mating status, insect can modulate their olfactory systems accordingly. Ionotropic, gustatory, and odorant receptors (IR, GR, and ORs) are important elements of the insect chemosensory system, which enable insects to detect various external stimuli. In this study, we investigated the changes in these receptors at the mRNA level in Bactrocera dorsalis in different physiological states. We performed transcriptome analysis to identify chemosensory receptors: 21 IRs, 12 GRs, and 43 ORs were identified from B. dorsalis antennae, including almost all previously known chemoreceptors in B. dorsalis and a few more. Quantitative real-time polymerase chain reaction analysis revealed the effects of feeding state, mating status and time of day on the expression of IR, GR, and OR genes. The results showed that expression of chemosensory receptors changed in response to different physiological states, and these changes were completely different for different types of receptors and between male and female flies. Our study suggests that the expressions of chemosensory receptors change to adapt to different physiological states, which may indicate the significant role of these receptors in such physiological processes.

Odorant-binding proteins expression patterns in recently diverged species of Anastrepha fruit flies

We studied two species of closely related South American fruit flies, Anastrepha fraterculus and Anastrepha obliqua which, despite being able to interbreed, still show some ecological and reproductive differences. Because part of these differences, such as host and mate preferences, may be related to olfactory perception, we focused our investigation on the differential expression of Odorant-binding protein (OBP) gene family, which participate in initial steps of the olfactory signal transduction cascade. We investigated patterns of expression of eight OBP genes by qPCR in male and female head tissues of both species. The expression patterns of these OBPs suggest that some OBP genes are more likely involved with the location of food resources, while others seem to be associated with mate and pheromone perception. Furthermore, the expression patterns obtained at different reproductive stages indicate that OBP expression levels changed significantly after mating in males and females of both species. All eight OBP genes analyzed here showed significant levels of differential expression between A. fraterculus and A. obliqua, suggesting that they may hold important roles in their olfactory perception differences, and consequently, may potentially be involved in their differentiation.

Differential expression of chemosensory-protein genes in midguts in response to diet of Spodoptera litura

While it has been well characterized that chemosensory receptors in guts of mammals have great influence on food preference, much remains elusive in insects. Insect chemosensory proteins (CSPs) are soluble proteins that could deliver chemicals to olfactory and gustatory receptors. Recent studies have identified a number of CSPs expressed in midgut in Lepidoptera insects, which started to reveal their roles in chemical recognition and stimulating appetite in midgut. In this study, we examined expression patterns in midgut of 21 Spodoptera litura CSPs (SlitCSPs) characterized from a previously reported transcriptome, and three CSPs were identified to be expressed highly in midgut. The orthologous relationships between midgut expressed CSPs in S. litura and those in Bombyx mori and Plutella xylostella also suggest a conserved pattern of CSP expression in midgut. We further demonstrated that the expression of midgut-CSPs may change in response to different host plants, and SlitCSPs could bind typical chemicals from host plant in vitro. Overall, our results suggested midgut expressed SlitCSPs may have functional roles, likely contributing to specialization and adaption to different ecosystems. Better knowledge of this critical component of the chemsensation signaling pathways in midguts may improve our understanding of food preference processes in a new perspective.

BdorOBP83a-2 Mediates Responses of the Oriental Fruit Fly to Semiochemicals

The oriental fruit fly, Bactrocera dorsalis (Diptera: Tephritidae), is one of the most destructive pests throughout tropical and subtropical regions in Asia. This insect displays remarkable changes during different developmental phases in olfactory behavior between sexually immature and mated adults. The olfactory behavioral changes provide clues to examine physiological and molecular bases of olfactory perception in this insect. We comparatively analyzed behavioral and neuronal responses of B. dorsalis adults to attractant semiochemicals, and the expression profiles of antenna chemosensory genes. We found that some odorant-binding proteins (OBPs) were upregulated in mated adults in association with their behavioral and neuronal responses. Ligand-binding assays further showed that one of OBP83a orthologs, BdorOBP83a-2, binds with high affinity to attractant semiochemicals. Functional analyses confirmed that the reduction in BdorOBP83a-2 transcript abundance led to a decrease in neuronal and behavioral responses to selected attractants. This study suggests that BdorOBP83a-2 mediates behavioral responses to attractant semiochemicals and could be a potential efficient target for pest control.

Molecular Characterization and Sex Distribution of Chemosensory Receptor Gene Family Based on Transcriptome Analysis of Scaeva pyrastri

Chemosensory receptors play key roles in insect behavior. Thus, genes encoding these receptors have great potential for use in integrated pest management. The hover fly Scaeva pyrastri (L.) is an important pollinating insect and a natural enemy of aphids, mainly distributed in the Palearctic and Nearctic regions. However, a systematic identification of their chemosensory receptor genes in the antennae has not been reported. In the present study, we assembled the antennal transcriptome of S. pyrastri by using Illumina sequencing technology. Analysis of the transcriptome data identified 60 candidate chemosensory genes, including 38 for odorant receptors (ORs), 16 for ionotropic receptors (IRs), and 6 for gustatory receptors (GRs). The numbers are similar to those of other Diptera species, suggesting that we were able to successfully identify S. pyrastri chemosensory genes. We analyzed the expression patterns of all genes by using reverse transcriptase PCR (RT-PCR), and found that some genes exhibited sex-biased or sex-specific expression. These candidate chemosensory genes and their tissue expression profiles provide information for further studies aimed at fully understanding the molecular basis behind chemoreception-related behaviors in S. pyrastri.

Transcriptome and Expression Patterns of Chemosensory Genes in Antennae of the Parasitoid Wasp Chouioia cunea

Chouioia cunea Yang is an endoparasitic wasp that attacks pupae of Hyphantria cunea (Drury), an invasive moth species that severely damages forests in China. Chemosensory systems of insects are used to detect volatile chemical odors such as female sex pheromones and host plant volatiles. The antennae of parasite wasps are important for host detection and other sensory-mediated behaviors. We identified and documented differential expression profiles of chemoreception genes in C. cunea antennae. A total of 25 OBPs, 80 ORs, 10 IRs, 11 CSP, 1 SNMPs, and 17 GRs were annotated from adult male and female C. cunea antennal transcriptomes. The expression profiles of 25 OBPs, 16 ORs, and 17 GRs, 5 CSP, 5 IRs and 1 SNMP were determined by RT-PCR and RT-qPCR for the antenna, head, thorax, and abdomen of male and female C. cunea. A total of 8 OBPs, 14 ORs, and 8 GRs, 1 CSP, 4 IRs and 1 SNMPs were exclusively or primarily expressed in female antennae. These female antennal-specific or dominant expression profiles may assist in locating suitable host and oviposition sites. These genes will provide useful targets for advanced study of their biological functions.

Identification and preliminary characterization of chemosensory perception-associated proteins in the melon fly Bactrocera cucurbitae using RNA-seq

An investigation into proteins involved in chemosensory perception in the melon fly, Bactrocera cucurbitae (Diptera: Tephritidae) is described here using a newly generated transcriptome dataset. The melon fly is a major agricultural pest, widely distributed in the Asia-Pacific region and some parts of Africa. For this study, a transcriptome dataset was generated using RNA extracted from 4-day-old adult specimens of the melon fly. The dataset was assembled and annotated via Gene Ontology (GO) analysis. Based on this and similarity searches to data from other species, a number of protein sequences putatively involved in chemosensory reception were identified and characterized in the melon fly. This included the highly conserved "Orco" along with a number of other less conserved odorant binding protein sequences. In addition, several sequences representing putative ionotropic and gustatory receptors were also identified. This study provides a foundation for future functional studies of chemosensory proteins in the melon fly and for making more detailed comparisons to other species. In the long term, this will ultimately help in the development of improved tools for pest management.