Characterization of candidate odorant-binding proteins and chemosensory proteins in the tea geometrid Ectropis obliqua Prout (Lepidoptera: Geometridae)

Evolutionary genomics of three agricultural pest moths reveals rapid evolution of host adaptation and immune-related genes

BACKGROUND

Understanding the genotype of pest species provides an important baseline for designing integrated pest management (IPM) strategies. Recently developed long-read sequence technologies make it possible to compare genomic features of nonmodel pest species to disclose the evolutionary path underlying the pest species profiles. Here we sequenced and assembled genomes for 3 agricultural pest gelechiid moths: Phthorimaea absoluta (tomato leafminer), Keiferia lycopersicella (tomato pinworm), and Scrobipalpa atriplicella (goosefoot groundling moth). We also compared genomes of tomato leafminer and tomato pinworm with published genomes of Phthorimaea operculella and Pectinophora gossypiella to investigate the gene family evolution related to the pest species profiles.

RESULTS

We found that the 3 solanaceous feeding species, P. absoluta, K. lycopersicella, and P. operculella, are clustered together. Gene family evolution analyses with the 4 species show clear gene family expansions on host plant-associated genes for the 3 solanaceous feeding species. These genes are involved in host compound sensing (e.g., gustatory receptors), detoxification (e.g., ABC transporter C family, cytochrome P450, glucose-methanol-choline oxidoreductase, insect cuticle proteins, and UDP-glucuronosyl), and digestion (e.g., serine proteases and peptidase family S1). A gene ontology enrichment analysis of rapid evolving genes also suggests enriched functions in host sensing and immunity.

CONCLUSIONS

Our results of family evolution analyses indicate that host plant adaptation and pathogen defense could be important drivers in species diversification among gelechiid moths.

Functional characterization of chemosensory proteins in response to artificial light treatment in Thalassodes immissaria

BACKGROUND

Chemosensory proteins (CSPs) play a vital role in the response to environmental stimuli in insects. However, the involvement of insect CSPs in the stress response to night-time environmental changes has not been examined.

RESULTS

In the current study, four TiCSP genes were first cloned from Thalassodes immissaria by transcriptome and RACE-PCR techniques. TiCSPs had typical characteristics of insect CSPs, including a highly conserved four-cysteine motif and olfactory-specific protein D (OS-D) or OS-D superfamily domains. TiCSP1-4 were clustered classified within different clades in a phylogenetic analysis and were differentially expressed at all developmental stages. Under night-time artificial light stress, the expression levels of TiCSP1 in males were significantly decreased at 24 h, and those of TiCSP2 were decreased in both adult sexes at 48 h. In a molecular docking analysis, TiCSPs showed relatively higher binding affinities with sex pheromone components than with host plant volatile molecules.

CONCLUSION

Taking the reduced expression levels of TiCSPs and binding affinities into account, TiCSP1 and TiCSP2 are involved in the stress response processes of T. immissaria under light treatment. Our study supplies basic data for the evaluation of the effects of light interference control technology - an emerging physical control measure on nontarget pests of lychee orchards. © 2023 Society of Chemical Industry.

The Potential of Parapanteles hyposidrae and Protapanteles immunis (Hymenoptera: Braconidae) as Biocontrol Agents for the Tea Grey Geometrid Ectropis grisescens (Lepidoptera)

The tea grey geometrid Ectropis grisescens has long been a significant insect pest of tea plants in China. Two parasitoids, Parapanteles hyposidrae and Protapanteles immunis (Hymenoptera: Braconidae: Microgastrinae), are the most important parasitoids in the larval stage of E. grisescens. Yet, the potential of these two parasitoids for controlling the tea grey geometrid is not known. Here, we studied the parasitism performance of these two parasitoid species on different host densities under different temperatures as well as the interference effect of parasitoid density. The results showed that both parasitoid species, Pa. hyposidrae and Pr. immunis, exhibited a Type II functional response towards the tea grey geometrid E. grisescens at four tested temperatures. With increasing the density of E. grisescens larvae, the number of parasitized larvae increased until a maximum was reached. The highest number of hosts parasitized by Pa. hyposidrae or Pr. immunis reached 14.5 or 14.75 hosts d^-1 at 22 °C, respectively. The estimated values of instantaneous searching efficiency (a) and handling time (h) for Pa. hyposidrae or Pr. immunis were 1.420 or 3.621 and 0.04 or 0.053 at 22 °C, respectively. Pr. immunis performed better than Pa. hyposidrae under higher temperatures. The parasitism rate by a single female parasitoid decreased with increasing parasitoid density at different temperatures, resulting in a reduction of searching efficiency. The findings of this study showed that Pr.immunis could be a better effective biocontrol agent than Pa. hyposidrae against the tea grey geometrid.

Identification of Olfactory Genes From the Greater Wax Moth by Antennal Transcriptome Analysis

The olfactory system is used by insects to find hosts, mates, and oviposition sites. Insects have different types of olfactory proteins, including odorant-binding proteins (OBPs), chemosensory proteins (CSPs), odorant receptors (ORs), ionotropic receptors (IRs), and sensory neuron membrane proteins (SNMPs) to perceive chemical cues from the environment. The greater wax moth, Galleria mellonella, is an important lepidopteran pest of apiculture. However, the molecular mechanism underlying odorant perception in this species is unclear. In this study, we performed transcriptome sequencing of G. mellonella antennae to identify genes involved in olfaction. A total of 42,544 unigenes were obtained by assembling the transcriptome. Functional classification of these unigenes was determined by searching against the Gene Ontology (GO), eukaryotic orthologous groups (KOG), and the Kyoto Encyclopedia of Genes and Genomes (KEGG) databases. We identified a total of 102 olfactory-related genes: 21 OBPs, 18 CSPs, 43 ORs, 18 IRs, and 2 SNMPs. Results from BLASTX best hit and phylogenetic analyses showed that most of the genes had a close relationship with orthologs from other Lepidoptera species. A large number of OBPs and CSPs were tandemly arrayed in the genomic scaffolds and formed gene clusters. Reverse transcription-quantitative PCR results showed that GmelOBP19 and GmelOR47 are mainly expressed in male antennae. This work provides a transcriptome resource for olfactory genes in G. mellonella, and the findings pave the way for studying the function of these genes.

Identification of chemosensory genes from the antennal transcriptome of Semiothisa cinerearia

Olfaction plays vital roles in the survival and reproduction of insects. The completion of olfactory recognition requires the participation of various complex protein families. However, little is known about the olfactory-related proteins in Semiothisa cinerearia Bremer et Grey, an important pest of Chinese scholar tree. In this study, we sequenced the antennal transcriptome of S. cinerearia and identified 125 olfactory-related genes, including 25 odorant-binding proteins (OBPs), 15 chemosensory proteins (CSPs), two sensory neuron membrane proteins (SNMPs), 52 odorant receptors (ORs), eight gustatory receptors (GRs) and 23 ionotropic receptors (IRs). BLASTX best hit results and phylogenetic analyses indicated that these genes were most identical to their respective orthologs from Ectropis obliqua. Further quantitative real-time PCR (qRT-PCR) analysis revealed that three ScinOBPs and three ScinORs were highly expressed in male antennae, while seven ScinOBPs and twelve ScinORs were female-specifically expressed. Our study will be useful for the elucidation of olfactory mechanisms in S. cinerearia.

Functional characterization of a binding protein for Type-II sex pheromones in the tea geometrid moth Ectropis obliqua Prout

The tea geometrid moth Ectropis obliqua Prout is one of the most serious moth pests in tea plants, and its sex pheromones have been identified as typical Type-II polyunsaturated hydrocarbons and epoxide derivatives. Therefore, the E. obliqua male olfactory system provides a good model to study the molecular basis of Type-II sex pheromone recognition as well as functional gene evolution towards structurally different types of moth sex pheromones. In this study, we identified the full-length sequence of a pheromone-binding protein, EoblPBP2 and revealed that it clustered together with the lepidopteran PBP2 subfamily, which binds Type I acetate pheromones. These findings suggest that the EoblPBP2 sequence and physiological function are conserved, although E. obliqua evolved Type II hydrocarbon and epoxide sex pheromones structurally different from Type I acetates. To examine this hypothesis, we studied the expression patterns and in vitro functions of EoblPBP2 in detail. Quantitative real-time PCR experiments showed that EoblPBP2 was predominantly expressed in male E. obliqua antennae. Fluorescence in situ hybridization further demonstrated that the EoblPBP2 gene was abundantly expressed in the pheromone-sensitive sensilla trichodea Str-I in male E. obliqua. The physiological function of recombinant EoblPBP2 was then examined using a competitive binding assay. The results showed that EoblPBP2 had high affinities for three E. obliqua Type II sex pheromone components and Type I acetate pheromones in comparison to some plant volatiles. These results indicate that PBP2 is involved in the detection of Type II pheromones in E. obliqua and it still retains high binding affinities to acetate pheromones and some green leaf ester volatiles.

Transcriptomic Analysis Reveals Insect Hormone Biosynthesis Pathway Involved in Desynchronized Development Phenomenon in Hybridized Sibling Species of Tea Geometrids (Ectropis grisescens and Ectropis obliqua)

Ectropis grisescens and Ectropis obliqua are sibling species of tea-chewing pests. An investigation of the distribution of tea geometrids was implemented for enhancing controlling efficiency. E. grisescens is distributed across a wider range of tea-producing areas than Ectropis obliqua in China with sympatric distribution found in some areas. In order to explore reproductive isolation mechanisms in co-occurrence areas, hybridization experiments were carried out. Results showed they can mate but produce infertile hybrids. During experiments, the desynchronized development phenomenon was found in the hybridized generation of sibling tea geometrids. Furthermore, transcriptome analysis of those individuals of fast-growing and slow-growing morphs revealed that the insect hormone biosynthesis pathway was enriched in two unsynchronized development groups of hybrid offspring. More importantly, some genes regulating the synthesis of moulting hormone showed significantly up-regulated expression in fast-growing groups. Above all, metabolism of the juvenile hormone and synthesis of the ecdysone pathway were found to be crucially involved in the desynchronized development phenomenon. This research finding contributes to a better understanding of the mechanisms of insect development and reproductive isolation of two sibling species.

The sensilla trichodea-biased EoblPBP1 binds sex pheromones and green leaf volatiles in Ectropis obliqua Prout, a geometrid moth pest that uses Type-II sex pheromones

Pheromone-binding proteins (PBPs) are considered to play critical roles in sex pheromone detection. Lepidopteran moths can be divided into two taxa, those that use Type-I sex pheromones, such as C10-C18 unsaturated aldehydes, alcohols and acetates, and those that use Type-II pheromones, which are C17-C23 polyunsaturated hydrocarbons and their epoxide derivatives. To date, nearly all the characterized PBPs have been reported in moths with Type-I sex pheromones, and the physiological functions of PBPs in moths that use Type-II sex pheromones remains unclear. In the present study we functionally examine EoblPBP1 in Ectropis obliqua Prout, an important geometrid moth pest that uses Type-II sex pheromones. The phylogenetic analysis of the sequence indicated that EoblPBP1 clustered together with ScerPBP1, a geometrid PBP for detecting Type-II sex pheromones. Scanning electron microscopy showed that E. obliqua moths of both sexes mainly had six types of antennal sensilla, including two types of sensilla trichodea, Str-I and Str-II, sensilla basiconica (Sba), sensilla styloconica (Sst), sensilla chaetica (Sch) and sensilla auricillica (Sau). Of these, Str-I was confirmed to be male moth-specific and had five different subtypes. Fluorescence in situ hybridization revealed that EoblPBP1 was primarily expressed at the base of Str-I. A comparative binding assay showed that recombinant EoblPBP1 bound three sex pheromone components of E. obliqua, demonstrating its involvement in the detection of Type-II sex pheromones. Besides, EoblPBP1 also highly bound unsaturated acetates pheromones and the green leaf volatiles. These results indicate that PBP1 is associated with detecting Type-II sex pheromones in E. obliqua but cannot differentiate Type-II sex pheromones from Type-I sex pheromones or green leaf volatiles. Our findings provide a foundation for further study on molecular basis of Type-II sex pheromone recognition in lepidopteran moths.

Expression patterns and colocalization of two sensory neurone membrane proteins in Ectropis obliqua Prout, a geometrid moth pest that uses Type-II sex pheromones

Sensory neurone membrane proteins (SNMPs) function as essential cofactors for insect sex pheromone detection. In this study, we report two SNMPs in Ectropis obliqua Prout, a serious geometrid pest that produces typical Type-II sex pheromones. Sequence alignments and phylogenetic analyses showed that EoblSNMP1 and EoblSNMP2 belong to two distinct SNMP subfamilies. Quantitative real-time PCR suggested that EoblSNMP1 was male antennae-biased, whereas EoblSNMP2 was highly expressed on male antennae but was also expressed on female antennae and other chemosensory tissues. Additionally, EoblSNMP1 and EoblSNMP2 differed in their developmental expression profiles. In situ hybridization revealed that EoblSNMP1 was sensilla trichodea I specific, whereas EoblSNMP2 was expressed in sensilla trichodea I and the sensilla basiconica; furthermore, EoblSNMP1 and EoblSNMP2 were co-expressed in sensilla trichodea I but in different cells. This study suggests that EoblSNMP1 is functionally distinct from EoblSNMP2 in E. obliqua; EoblSNMP1 may specifically contribute to the recognition of sex pheromones, whereas EoblSNMP2 exhibits multiple olfactory roles. Our findings comprehensively reveal the expression patterns of SNMPs in a lepidopteran species that uses Type-II sex pheromones, providing new insights into the functional evolution of SNMPs from lepidopteran moths with Type-I sex pheromones to those with Type-II sex pheromones.

Comparison of male antennal morphology and sensilla physiology for sex pheromone olfactory sensing between sibling moth species: Ectropis grisescens and Ectropis obliqua (Geometridae)

Ectropis grisescens and Ectropis obliqua (Lepidoptera: Geometridae) are sibling pest species that co-occur on tea plants. The sex pheromone components of both species contain (Z,Z,Z)-3,6,9-octadecatriene and (Z,Z)-3,9-cis-6,7-epoxy-octadecadiene. E. obliqua has (Z,Z)-3,9-cis-6,7-epoxy-nonadecadiene as an additional sex pheromone component, which ensures reproductive segregation between the two species. To ascertain the detection mechanism of olfactory organs for sex pheromone components of E. grisescens and E. obliqua, we applied scanning electron microscopy and single sensillum recording to compare antennal morphology and sensillum physiology in the two species. There was no apparent morphological difference between the antennae of the two species. Both species responded similarly to all three sex pheromone components, including, E. obliqua specific component. The distribution patterns of antennal sensilla trichodea differed between the two species. Sex pheromone olfactory sensing in these sibling species appears to be determined by the density of different types of olfactory sensing neurons. Dose-dependent responses of sensilla trichodea type 1 to (Z,Z)-3,9-cis-6,7-epoxy-octadecadiene, the most abundant component, showed an "all or none" pattern and the other two components showed sigmoidal dose-response curves with a half threshold of 10^-4 (dilution equal to the concentration of 10 μg/μl). These results suggest that the major sex pheromone component functions as an on-off controller while secondary components function as modulators during olfactory transmission to the primary olfactory center.

Candidate olfactory genes identified in Heortia vitessoides (Lepidoptera: Crambidae) by antennal transcriptome analysis

Heortia vitessoides Moore is the most severe defoliating pest of Aquilaria sinensis (Lour.) Gilg (Thymelaeaceae) forests. Olfaction in insects is essential for host identification, mating, and oviposition, in which olfactory proteins, including odorant-binding proteins (OBPs), chemosensory proteins (CSPs), olfactory receptors (ORs), ionotropic receptors (IRs), and sensory neuron membrane proteins (SNMPs), are responsible for chemical signaling. Here, we determined the transcriptomes of male and female adult antennae of H. vitessoides. We assembled 52,383 unigenes and annotated their putative gene functions based on the gene ontology (GO), eukaryotic ortholog groups (KOG), and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases. Overall, 61 olfactory-related transcripts, including nine OBPs, 10 CSPs, 28 ORs, 12 IRs, and two SNMPs, were identified. Expression patterns of OBPs and CSPs in the female antennae, male antennae, and legs were performed using reverse transcription quantitative PCR (RT-qPCR). The results revealed that HvitOBP1, HvitOBP6, and HvitGOBP1 were enriched in the female antennae, while HvitOBP2, HvitOBP3, HvitOBP5, HvitGOBP2, and HvitPBP1 were enriched in the male antennae. HvitOBP4 was expressed at nearly the same level in the antennae of both males and females. Four CSPs (HvitCSP3, HvitCSP5, HvitCSP7, and HvitCSP10) and two CSPs (HvitCSP1 and HvitCSP4) were expressed at higher levels in the female and male antennae, respectively. HvitCSP6 was expressed at higher levels both in the female antennae and legs. Three CSP genes (HvitCSP2, HvitCSP8, and HvitCSP9) were expressed at higher levels in the legs. These results provide a basis for further studies on the molecular olfactory mechanisms of H. vitessoides.

Alimentary Tract Transcriptome Analysis of the Tea Geometrid, Ectropis oblique (Lepidoptera: Geometridae)

Ectropis oblique Prout (Lepidoptera: Geometridae) is one of the main pests that damages the tea crop in Southeast Asia. To understand the molecular mechanisms of its feeding biology, transcriptomes of the alimentary tract (AT) and of the body minus the AT of E. oblique were successfully sequenced and analyzed in this study. A total of 36,950 unigenes from de novo sequences were assembled. After analysis using six annotation databases (e.g., Gene Ontology, Kyoto Encyclopedia of Genes and Genome, and NCBI nr), a series of putative genes were found for this insect species that were related to digestion, detoxification, the immune system, and Bacillus thuringiensis (Bt) receptors. From this series of genes, 21 were randomly selected to verify the relative expression levels of transcripts using quantitative real-time polymerase chain reaction. These results will provide an invaluable genomic resource for future studies on the molecular mechanisms of E. oblique, which will be useful in developing biological control strategies for this pest.

Functional Characteristics, Electrophysiological and Antennal Immunolocalization of General Odorant-Binding Protein 2 in Tea Geometrid, Ectropis obliqua

As one of the main lepidopteran pests in Chinese tea plantations, Ectropisobliqua Warren (tea geometrids) can severely decrease yields of tea products. The olfactory system of the adult tea geometrid plays a significant role in seeking behaviors, influencing their search for food, mating partners, and even spawning grounds. In this study, a general odorant-binding protein (OBP) gene, EoblGOBP2, was identified in the antennae of E. obliqua using reverse transcription quantification PCR (RT-qPCR). Results showed that EoblGOBP2 was more highly expressed in the antennae of males than in females relative to other tissues. The recombinant EoblGOBP2 protein was prepared in Escherichia coli and then purified through affinity chromatography. Ligand-binding assays showed that EoblGOBP2 had a strong binding affinity for some carbonyl-containing tea leaf volatiles (e.g., (E)-2-hexenal, methyl salicylate, and acetophenone). Electrophysiological tests confirmed that the male moths were more sensitive to these candidate tea plant volatiles than the female moths. Immunolocalization results indicated that EoblGOBP2 was regionally confined to the sensilla trichoid type-II in the male antennae. These results indicate that EoblGOBP2 may be primarily involved in the olfactory activity of male E. obliqua moths, influencing their ability to sense tea leaf volatiles. This study provides a new perspective of insect GOBPs and implies that olfactory function can be used to prevent and control the tea geometrid.

Identification and Expression Patterns of Putative Diversified Carboxylesterases in the Tea Geometrid Ectropis obliqua Prout

Carboxylesterases (CXEs) belong to a family of metabolic enzymes. Some CXEs act as odorant-degrading enzymes (ODEs), which are reportedly highly expressed in insect olfactory organs and participate in the rapid deactivation of ester pheromone components and plant volatiles. The tea geometrid Ectropis obliqua Prout produces sex pheromones consisting of non-ester functional compounds but relies heavily on acetic ester plant volatiles to search for host plants and locate oviposition sites. However, studies characterizing putative candidate ODEs in this important tea plant pest are still relatively scarce. In the present study, we identified 35 candidate EoblCXE genes from E. obliqua chemosensory organs based on previously obtained transcriptomic data. The deduced amino acid sequences possessed the typical characteristics of the insect CXE family, including oxyanion hole residues, the Ser-Glu-His catalytic triad, and the Ser active included in the conserved pentapeptide characteristic of esterases, Gly-X-Ser-X-Gly. Phylogenetic analyses revealed that the EoblCXEs were diverse, belonging to several different insect esterase clades. Tissue- and sex-related expression patterns were studied via reverse-transcription and quantitative real-time polymerase chain reaction analyses (RT- and qRT-PCR). The results showed that 35 EoblCXE genes presented a diversified expression profile; among these, 12 EoblCXEs appeared to be antenna-biased, two EoblCXEs were non-chemosensory organ-biased, 12 EoblCXEs were ubiquitous, and nine EoblCXEs showed heterogeneous expression levels among different tissues. Intriguingly, two EoblCXE genes, EoblCXE7 and EoblCXE13, were not only strongly localized to antennal sensilla tuned to odorants, such as the sensilla trichodea (Str I and II) and sensilla basiconica (Sba), but were also expressed in the putative gustatory sensilla styloconica (Sst), indicating that these two CXEs might play multiple physiological roles in the E. obliqua chemosensory processing system. This study provides the first elucidation of CXEs in the chemosensory system of a geometrid moth species and will enable a more comprehensive understanding of the functions of insect CXEs across lepidopteran species.

Identification and Characterization of Odorant Binding Proteins in the Forelegs of Adelphocoris lineolatus (Goeze)

The chemosensory system is essential for insects to detect exogenous compounds, and odorant binding proteins (OBPs) play crucial roles in odorant binding and transduction. In the alfalfa plant bug Adelphocoris lineolatus, an important pest of multiple crops, our understanding of the physiological roles of antenna-biased OBPs has increased dramatically, whereas OBPs related to gustation have remained mostly unexplored. In this study, we employed RNA sequencing and RACE PCR methods to identify putative OBPs from the adult forelegs of both sexes. Eight candidate OBPs were identified, and three OBPs (AlinOBP15, 16, and 17) were novel. Full-length sequence alignment and phylogenetic analyses suggested that these three candidate OBPs had characteristics typical of the insect OBP family. AlinOBP16 and 17 displayed six highly conserved cysteines, placing them in the classic OBP subfamily, whereas AlinOBP15 resembled AlinOBP14 and clustered with the Plus-C clade. Quantitative real-time PCR (qRT-PCR) revealed distinct and significant tissue- and sex-biased expression patterns. AlinOBP15 was highly expressed in female heads, and AlinOBP16 and 17 were strongly expressed in female antennae. In particular, AlinOBP11, the most abundant OBP gene in our foreleg transcriptome dataset, was predominately expressed in adult legs. Furthermore, four types of sensilla hairs were observed on the forelegs of adult A. lineolatus, including sensilla trichodea, setae, and two types of sensilla chaetica (Sch1 and Sch2). Anti-AlinOBP11 antiserum strongly labeled the outer sensillum lymph of Sch2, implying that it has important gustatory functions in A. lineolatus. Our current findings provide evidence that OBPs can be functionally expressed in the tarsal gustatory sensilla of hemipteran mirid species, broadening our understanding of OBP chemosensory function in insects and facilitating the discovery of new functional targets for the regulation of insect host-searching behaviors.