Expression and functional characterization of odorant-binding protein genes in the endoparasitic wasp Cotesia vestalis

Odorant-binding protein CrufOBP1 in Cotesia ruficrus females plays a pivotal role in the detection of Spodoptera frugiperda larvae

Cotesia ruficrus presents a promising local natural enemy for controlling the invasive fall armyworm Spodoptera frugiperda in China. However, the mechanisms underlying how C. ruficrus locates its target pest remain unclear. In this study, we analyzed the expression patterns of 18 CrufOBPs across different developmental stages of C. ruficrus, and found that CrufOBP1 exhibited consistent and high expression levels in female adults. CrufOBP1 transcript was predominantly localized in sensilla placodea and sensilla trichodea on the antennae. Additionally, we confirmed the binding properties of CrufOBP1 protein to various cuticular compounds of S. frugiperda larvae. Subsequent electroantennogram and behavioral assays revealed that 1-(2-hydroxy-5-methylphenyl)-ethanone attracted female C. ruficrus, consequently increased the parasitism rate. However, upon silencing CrufOBP1, females exhibited reduced attraction towards 1-(2-hydroxy-5-methylphenyl)-ethanone, indicating the crucial role of CrufOBP1 in the chemoreception of C. ruficrus. These findings shed light on the kairomone-based mechanism employed by C. ruficrus to locate S. frugiperda larvae and hold a promise for the development of environmentally friendly pest management strategies.

Exploring the functional profiles of odorant binding proteins crucial for sensing key odorants in the new leaves of coconut palms in Rhynchophorus ferrugineus

The red palm weevil (RPW), Rhynchophorus ferrugineus (Curculionidae: Coleoptera) is a highly destructive global pest of coconut trees, with a preference for laying its eggs on new leaves. Females can identify where to lay eggs by using their sense of smell to detect specific odorants found in new leaves. In this study, we focused on the two odorants commonly found in new leaves by GC-MS: trans, trans-2,4-nonadienal and trans-2-nonenal. Our behavioral assays demonstrated a significant attraction of females to both of these odorants, with their electrophysiological responses being dose-dependent. Furthermore, we examined the expression patterns induced by these odorants in eleven RferOBP genes. Among them, RferOBP3 and RferOBP1768 exhibited the most significant and simultaneous upregulation. To further understand the role of these two genes, we conducted experiments with females injected with OBP-dsRNA. This resulted in a significant decrease in the expression of RferOBP3 and RferOBP1768, as well as impaired the perception of the two odorants. A fluorescence competitive binding assay also showed that both RferOBPs strongly bound to the odorants. Additionally, sequence analysis revealed that these two RferOBPs belong to the Minus-C family and possess four conserved cysteines. Molecular docking simulations showed strong interactions between these two RferOBPs and the odorant molecules. Overall, our findings highlight the crucial role of RferOBP3 and RferOBP1768 in the olfactory perception of the key odorants in coconut palm new leaves. This knowledge significantly improves our understanding of how RPW females locate sites for oviposition and lays the foundation for future research on the development of environmentally friendly pest attractants.

The antennal transcriptome analysis and characterizations of odorant-binding proteins in Megachile saussurei (Hymenoptera, Megachilidae)

BACKGROUND

Odorant-binding proteins (OBPs) are essential in insect's daily behaviors mediated by olfactory perception. Megachile saussurei Radoszkowski (Hymenoptera, Megachilidae) is a principal insect pollinating alfalfa (Medicago sativa) in Northwestern China. The olfactory function have been less conducted, which provides a lot of possibilities for our research.

RESULTS

Our results showed that 20 OBPs were identified in total. Multiple sequence alignment analysis indicated MsauOBPs were highly conserved with a 6-cysteine motif pattern and all belonged to the classic subfamily, coding 113-196 amino acids and sharing 41.32%-99.12% amino acid identity with known OBPs of other bees. Phylogenetic analysis indicated there were certain homologies existed among MsauOBPs and most sequences were clustered with that of Osmia cornuta (Hymenoptera, Megachilidae). Expression analysis showed the identified OBPs were mostly enriched in antennae instead of other four body parts, especially the MsauOBP2, MsauOBP3, MsauOBP4, MsauOBP8, MsauOBP11 and MsauOBP17, in which the MsauOBP2, MsauOBP4 and MsauOBP8 presented obvious tissue-biased expression pattern. Molecular docking results indicated MsauOBP4 might be the most significant protein in recognizing alfalfa flower volatile 3-Octanone, while MsauOBP13 might be the most crucial protein identifying (Z)-3-hexenyl acetate. It was also found the lysine was a momentous hydrophilic amino acid in docking simulations.

CONCLUSION

In this study, we identified and analyzed 20 OBPs of M. saussurei. The certain homology existed among these OBPs, while some degree of divergence could also be noticed, indicating the complex functions that different MsauOBPs performed. Besides, the M. saussurei and Osmia cornuta were very likely to share similar physiological functions as most of their OBPs were clustered together. MsauOBP4 might be the key protein in recognizing 3-Octanone, while MsauOBP13 might be the key protein in binding (Z)-3-hexenyl acetate. These two proteins might contribute to the alfalfa-locating during the pollination process. The relevant results may help determine the highly specific and effective attractants for M. saussurei in alfalfa pollination and reveal the molecular mechanism of odor-evoked pollinating behavior between these two species.

Identification and functional analysis of odorant-binding proteins of the parasitoid wasp Scleroderma guani reveal a chemosensory synergistic evolution with the host Monochamus alternatus

Scleroderma guani is a generalist ectoparasitoid of wood-boring insects. The chemosensory genes expressed in its antennae play crucial roles in host-seeking. In the present study, we identified 14 OBP genes for the first time from the antennae transcriptomes and genomic data of S. guani. The expression profiles of 14 OBPs were tested by RT-qPCR, and the RT-qPCR results showed that SguaOBP2/5/6/11/12/13 were specifically highly expressed in the female antennae. Then we performed ligand binding assays to test the interactions between six selected SguaOBPs with host specific chemical compounds from M. alternatus and pines. The binding results indicated that SguaOBP12 had a higher binding affinity with longifolene, β-caryophyllene, α-pinene, β-pinene, myrcene, butylated hydroxytoluene, and 3-carene. SguaOBP11 had a high or medium binding affinity with them. Furthermore, both SguaOBP11 and SguaOBP12 had a medium binding affinity with the aggregation pheromone of Monochamus species, 2-undecyloxy-1-ethanol. Finally, by using molecular docking and RNAi, we further explored the molecular interactions and behavioral functions of SguaOBP11 and SguaOBP12 with these vital odor molecules. Our study contributes to the further understanding of chemical communications between S. guani and its host, and further exploration for its role as a more effective biological control agent.

Odorant-Binding Protein 6 Contributes High Binding Affinity to Insecticides in a Parasitic Wasp Meteorus pulchricornis (Hymenoptera: Braconidae)

Meteorus pulchricornis is a preponderant parasitic wasp of various lepidopteran pests. The extensive application of broad-spectrum insecticides usually causes serious threats to the olfactory recognition of nontarget insects such as parasitoid wasps. However, the binding mechanism of odorant-binding proteins (OBPs) to insecticides in parasitoid wasps remains unknown. Herein, we find that the MpulOBP6 protein had a strong binding affinity to three insecticides (phoxim, chlorpyrifos, and chlorfenapyr). Results of computational simulations revealed that the hydrophobic interaction contributed by a mass of nonpolar amino acid residues was the primary driving force in the formation and stabilization of MpulOBP6-insecticide complexes. Among them, four residues (Met75, Val84, Phe121, and Pro122) and two residues (Val84 and Phe111) play an essential role in the binding of MpulOBP6 to phoxim and chlorfenapyr, respectively. Our findings could be instrumental to elucidate the effects of insecticide application toward the olfactory recognition of nontarget insects in the processes of agricultural production.

Identification and Comparative Expression Profiles of Candidate Olfactory Receptors in the Transcriptomes of the Important Egg Parasitoid Wasp Anastatus japonicus Ashmead (Hymenoptera: Eupelmidae)

Anastatus japonicus Ashmead is an egg parasitoid wasp important for the biological control of fruit crop pests. The olfaction of parasitoids is crucial to searching for host pests in fruit crops. In this study, we sequenced and analyzed the antennal and abdominal transcriptomes of A. japonicus to better understand the olfactory mechanisms in this species. A total of 201 putative olfactory receptor genes were identified, including 184 odorant receptors (ORs) and 17 ionotropic receptors (IRs). Then, we assayed the tissue-specific and sex-biased expression profiles of those genes based on the transcriptional levels. In total, 165 ORs and 15 IRs had upregulated expression in the antennae. The expression levels of 133 ORs, including odorant receptor co-receptor (AjapORco), and 10 IRs, including AjapIR8a, were significantly different between the female and male antennae. Our results provide valuable information for further studies on the molecular mechanisms of the olfactory system in A. japonicus.

The Chromosome-Level Genome of Hestina assimilis (Lepidoptera: Nymphalidae) Reveals the Evolution of Saprophagy-Related Genes in Brush-Footed Butterflies

Most butterflies feed on nectar, while some saprophagous butterflies forage on various non-nectar foods. To date, little is known about the genomic and molecular shifts associated with the evolution of the saprophagous feeding strategy. Here, we assembled the high-quality chromosome-level genome of Hestina assimilis to explore its saprophagous molecular and genetic mechanisms. This chromosome-level genome of H. assimilis is 412.82 Mb, with a scaffold N50 of 15.70 Mb. In total, 98.11% of contigs were anchored to 30 chromosomes. Compared with H. assimilis and other Nymphalidae butterflies, the genes of metabolism and detoxification experienced expansions. We annotated 80 cytochrome P450 (CYP) genes in the H. assimilis genome, among which genes belonging to the CYP4 subfamily were significantly expanded (p < 0.01). These P450 genes were unevenly distributed and mainly concentrated on chromosomes 6-9. We identified 33 olfactory receptor (OR), 20 odorant-binding protein (OBP), and six gustatory receptor (GR) genes in the H. assimilis genome, which were fewer than in the nectarivorous Danaus plexippus. A decreased number of OBP, OR, and GR genes implied that H. assimilis should resort less to olfaction and gustation than their nectarivorous counterparts, which need highly specialized olfactory and gustatory functions. Moreover, we found one site under positive selection occurred in residue 996 (phenylalanine) of GR genes exclusive to H. assimilis, which is conservative in most lineages. Our study provides support for the adaptive evolution of feeding habits in butterflies.

Plant volatile ligands for male-biased MmedOBP14 stimulate orientation behavior of the parasitoid wasp Microplitis mediator

As an important class of chemosensory-associated proteins, odorant binding proteins (OBPs) play a key role in the perception of olfactory signals for insects. Parasitoid wasp Microplitis mediator relies on its sensitive olfactory system to locate host larvae of Noctuidae and Geometridae. In the present study, MmedOBP14, a male-biased OBP in M. mediator, was functionally investigated. In fluorescence competitive binding assays, the recombinant MmedOBP14 showed strong binding abilities to five plant volatiles: β-ionone, 3,4-dimethylacetophenone, 4-ethylacetophenone, acetophenone and ocimene. Homology modeling and molecular docking results indicated that the binding sites of all five ligands were similar and concentrated in the binding pocket of MmedOBP14. Except acetophenone, the remaining four ligands at 1, 10 and 100 μg/μL caused strong antennal electrophysiological responses in adults M. mediator, and males showed more obvious EAG responses to most ligands than females. In behavioral trials, males were attracted by low concentrations of MmedOBP14 ligands, whereas high doses of β-ionone and acetophenone had a repellent effect on males. Moreover, 1 μg/μL of 3,4-dimethylacetophenone showed the strongest attractiveness to female wasps. These findings suggest that MmedOBP14 may play a more important role in the perception of plant volatiles for male wasps to locate habitat, supplement nutrition and search partners.

Transcriptome Analysis and Identification of Chemosensory Genes in Baryscapus dioryctriae (Hymenoptera: Eulophidae)

Baryscapus dioryctriae is a pupal endoparasitoid of many Pyralidae pests and has been used as a biocontrol agent against insect pests that heavily damage the cone and seed of the Korean pine. The olfactory system of wasps plays an essential role in sensing the chemical signals during their foraging, mating, host location, etc., and the chemosensory genes are involved in detecting and transducing these signals. Many chemosensory genes have been identified from the antennae of Hymenoptera; however, there are few reports on the chemosensory genes of Eulophidae wasps. In this study, the transcriptome databases based on ten different tissues of B. dioryctriae were first constructed, and 274 putative chemosensory genes, consisting of 27 OBPs, 9 CSPs, 3 NPC2s, 155 ORs, 49 GRs, 23 IRs and 8 SNMPs genes, were identified based on the transcriptomes and manual annotation. Phylogenetic trees of the chemosensory genes were constructed to investigate the orthologs between B. dioryctriae and other insect species. Additionally, twenty-eight chemosensory genes showed female antennae- and ovipositor-biased expression, which was validated by RT-qPCR. These findings not only built a molecular basis for further research on the processes of chemosensory perception in B. dioryctriae, but also enriched the identification of chemosensory genes from various tissues of Eulophidae wasps.

Spatial Expression Analysis of Odorant Binding Proteins in Both Sexes of the Aphid Parasitoid Aphidius gifuensis and Their Ligand Binding Properties

In China, Aphidius gifuensis is one of the most common endoparasitoids of the green peach aphid Myzus persicae and grain aphid Sitobion miscanthi in the field. Insect odorant-binding proteins (OBPs) play vital roles in odor perception during feeding, host searching, mating and oviposition. In addition, some OBPs are involved in other physiological processes such as gustation and reproduction. In the present study, a comparative antennal transcriptomic analysis was applied between male and female A. gifuensis. The spatial expression patterns among antennae, heads, thoraxes, abdomens and legs of OBPs in both sexes were further profiled. Fifteen AgifOBPs were predicted, and 14 of them were identified by gene cloning, including 12 classic OBPs and 2 min-C OBPs. As expected, all OBPs were mainly expressed at high levels in antennae, heads or legs which are sensory organs and tissues. Finally, ligand binding properties of 2 OBPs (AgifOBP7 and AgifOBP9) were further evaluated. Female leg specifically expressed AgifOBP9 displays a broad and high binding property to aphid alarm pheromones, plant green volatiles and aphid sex pheromones (Ki < 10 μΜ). However, female leg specifically expressed AgifOBP7 displays poor affinity for all tested ligands except CAU-II-11 ((E)-3,7-dimethylocta-2,6-dien-1-yl-2-hydroxy-3-methoxybenzoate), a reported (E)-β-farnesene (EBF) analog with an exceptionally high binding affinity (Ki = 1.07 ± 0.08 μΜ). In summary, we reported the spatial expression pattern of the OBP repertoire in A. gifuensis, and further studied the binding properties of OBP7 and OBP9, which are mainly expressed in female legs, laying the foundation for the dissection of the contribution of OBPs to chemosensation in A. gifuensis.

Identification of chemosensory genes by antennal transcriptome analysis and expression profiles of odorant-binding proteins in parasitoid wasp Aulacocentrum confusum

The endoparasitoid wasp, Aulacocentrum confusum (Hymenoptera: Braconidae), is a preponderant natural enemy of the larvae of Glyphodes pyloalis Walker (Lepidoptera: Pyralidae), which is a destructive pest of mulberry trees. We first constructed the antennal transcriptome database of A. confusum. In total, we obtained 48,262,304 clean reads from the dataset and assembled 24,324 unigenes. A total of 12,690 (52.17%) unigenes indicated significant similarity (E-value < 10^-5) compared to known protein sequences of other species from the NCBI non-redundant protein database. Gene ontology (GO) and cluster of orthologous groups (COG) analyses were used to determine the functional categories of these genes. A total of 84 putative chemosensory genes were identified from the antennal transcriptome of A. confusum, including 11 putative odorant-binding protein (OBP) genes, six chemosensory protein (CSP) genes, 44 olfactory receptor (OR) genes (including one olfactory co-receptor, Orco), 19 ionotropic receptor (IR) genes, and four sensory neuron membrane protein (SNMP) genes. Results of qPCR assays indicated that among of 11 AconOBPs, nine AconOBP genes were significantly expressed in the antennae of A. confusum adults. AconOBP8 was significantly expressed in the abdomen and AconOBP10 was highly expressed in the thorax. These findings can build a basis for further study on the processes of chemosensory perception in A. confusum at the molecular level.

Comparative Transcriptome Analysis Reveals Sex-Based Differences during the Development of the Adult Parasitic Wasp Cotesia vestalis (Hymenoptera: Braconidae)

The endoparasitic wasp Cotesia vestalis is an important biological agent for controlling the population of Plutella xylostella, a major pest of cruciferous crops worldwide. Though the genome of C. vestalis has recently been reported, molecular mechanisms associated with sexual development have not been comprehensively studied. Here, we combined PacBio Iso-Seq and Illumina RNA-Seq to perform genome-wide profiling of pharate adult and adult development of male and female C. vestalis. Taking advantage of Iso-Seq full-length reads, we identified 14,466 novel transcripts as well as 8770 lncRNAs, with many lncRNAs showing a sex- and stage-specific expression pattern. The differentially expressed gene (DEG) analyses showed 2125 stage-specific and 326 sex-specific expressed genes. We also found that 4819 genes showed 11,856 alternative splicing events through combining the Iso-Seq and RNA-Seq data. The results of comparative analyses showed that most genes were alternatively spliced across developmental stages, and alternative splicing (AS) events were more prevalent in females than in males. Furthermore, we identified six sex-determining genes in this parasitic wasp and verified their sex-specific alternative splicing profiles. Specifically, the characterization of feminizer and doublesex splicing between male and female implies a conserved regulation mechanism of sexual development in parasitic wasps.

Sensory gene identification in the transcriptome of the ectoparasitoid Quadrastichus mendeli

Sensory genes play a key role in the host location of parasitoids. To date, the sensory genes that regulate parasitoids to locate gall-inducing insects have not been uncovered. An obligate ectoparasitoid, Quadrastichus mendeli Kim & La Salle (Hymenoptera: Eulophidae: Tetrastichinae), is one of the most important parasitoids of Leptocybe invasa, which is a global gall-making pest in eucalyptus plantations. Interestingly, Q. mendeli can precisely locate the larva of L. invasa, which induces tumor-like growth on the eucalyptus leaves and stems. Therefore, Q. mendeli-L. invasa provides an ideal system to study the way that parasitoids use sensory genes in gall-making pests. In this study, we present the transcriptome of Q. mendeli using high-throughput sequencing. In total, 31,820 transcripts were obtained and assembled into 26,925 unigenes in Q. mendeli. Then, the major sensory genes were identified, and phylogenetic analyses were performed with these genes from Q. mendeli and other model insect species. Three chemosensory proteins (CSPs), 10 gustatory receptors (GRs), 21 ionotropic receptors (IRs), 58 odorant binding proteins (OBPs), 30 odorant receptors (ORs) and 2 sensory neuron membrane proteins (SNMPs) were identified in Q. mendeli by bioinformatics analysis. Our report is the first to obtain abundant biological information on the transcriptome of Q. mendeli that provided valuable information regarding the molecular basis of Q. mendeli perception, and it may help to understand the host location of parasitoids of gall-making pests.