Functional Analysis of the Chemosensory Protein GmolCSP8 From the Oriental Fruit Moth, Grapholita molesta (Busck) (Lepidoptera: Tortricidae)

Fine structure of the compound eyes of the crepuscular moth Grapholita molesta (Busck 1916) (Lepidoptera: Tortricidae)

Morphological organization, ultrastructure and adaptational changes under different light intensities (10000, 100, 1, and 0.01 mW/m2) of the compound eye of the oriental fruit moth Grapholita molesta (Busck 1916) were investigated. Its superposition type of eyes consist of approximately 1072 ommatidia in males and 1029 ommatidia in females with ommatidial diameters of around 15 μm. Each ommatidium features a laminated corneal lens densely covered by corneal nipples of 256 nm in height. Crystalline cones are formed by four cone cells, proximally tapering to form a narrow crystalline tract with a diameter of 1.5 μm. Eight retinula cells, two primary and six secondary pigment cells per ommatidium are present. The 62.3 μm long rhabdom is divided into a thin 1.8 μm wide distal and a 5.2 μm wide proximal region. Distally the fused rhabdom consists of the rhabdomeres of seven retinula cells (R1-R7) and connects with the crystalline cone. In the proximal rhabdom region, the pigment-containing retinula cell R8 occupies a position in centre of the rhabdom while R1-R7 cells have taken peripheral positions. At this level each ommatidial group of retinula cells is surrounded by a tracheal tapetum. In response to changes from bright-light to dim-light adaptations, the pigment granules in the secondary pigment cells and retinula cells migrate distally, with a decrease in the length of crystalline tract.

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.

Characterization of CrufCSP1 and Its Potential Involvement in Host Location by Cotesia ruficrus (Hymenoptera: Braconidae), an Indigenous Parasitoid of Spodoptera frugiperda (Lepidoptera: Noctuidae) in China

Chemosensory proteins (CSPs) are a class of soluble proteins that facilitate the recognition of chemical signals in insects. While CSP genes have been identified in many insect species, studies investigating their function remain limited. Cotesia ruficrus (Hymenoptera: Braconidae) holds promise as an indigenous biological control agent for managing the invasive pest Spodoptera frugiperda (Lepidoptera: Noctuidae) in China. This study aimed to shed light on the gene expression, ligand binding, and molecular docking of CrufCSP1 in C. ruficrus. A RT-qPCR analysis revealed that the expression of CrufCSP1 was higher in the wings, with male adults exhibiting significantly higher relative expression levels than other developmental stages. A fluorescence competitive binding analysis further demonstrated that CrufCSP1 has a high binding ability with several host-related volatiles, with trans-2-hexenal, octanal, and benzaldehyde showing the strongest affinity to CrufCSP1. A molecular docking analysis indicated that specific amino acid residues (Phe24, Asp25, Thr53, and Lys81) of CrufCSP1 can bind to these specific ligands. Together, these findings suggest that CrufCSP1 may play a crucial role in the process of C. ruficrus locating hosts. This knowledge can contribute to the development of more efficient and eco-friendly strategies for protecting crops and managing pests.

Chemosensory protein regulates the behavioural response of Frankliniella intonsa and Frankliniella occidentalis to tomato zonate spot virus-Infected pepper (Capsicum annuum)

Many herbivorous insects rely on plant volatiles to locate their host plants. Vector-borne viral infections induce changes in plant volatiles, which render infected plants more attractive to insect vectors. However, the detailed mechanisms underlying the olfactory responses of insect vectors induced by the volatiles produced by virus-infected plants are poorly understood. Here, we show that volatiles emitted by pepper (Capsicum annuum) plants infected with tomato zonate spot virus (TZSV), particularly the volatile cis-3-hexenal, which is recognized by chemosensory protein 1 of the thrips Frankliniella intonsa (FintCSP1), are more attractive to F. intonsa than the volatiles emitted by non-infected pepper plants. FintCSP1 is highly abundant in the antenna of F. intonsa. Silencing of FintCSP1 significantly decreased electroantennogram responses of F. intonsa antennae to cis-3-hexenal and impaired thrips' responses to TZSV-infected pepper plants and cis-3-hexenal, as assessed using a Y-tube olfactometer. Three-dimensional model predictions indicated that FintCSP1 consists of seven α-helixes and two disulfide bridges. Molecular docking analysis suggested that cis-3-hexenal is positioned deep inside the binding pocket of FintCSP1 and binds to residues of the protein. We combined site-directed mutagenesis and fluorescence binding assays and identified three hydrophilic residues, Lys26, Thr28, and Glu67, of FintCSP1 as being critical for cis-3-hexenal binding. Furthermore, CSP of F. occidentalis (FoccCSP) is also a key olfactory protein involved in modulating the behaviour of F. occidentalis to TZSV-infected pepper. This study revealed the specific binding characteristics of CSPs to cis-3-hexenal and confirmed the general hypothesis that virus infections induce changes in host volatiles, which can be recognized by the olfactory proteins of the insect vector to enhance vector attraction and this may facilitate viral spread and transmission.

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.

Sublethal Effects of Abamectin on the Development, Reproduction, Detoxification Enzyme Activity, and Related Gene Expression of the Oriental Fruit Moth (Lepidoptera: Tortricidae)

Grapholita molesta is one of the most important fruit pests worldwide. Abamectin is a biological pesticide frequently used to control fruit borers like G. molesta in part owing to its translaminar properties. In this study, we characterized the toxicity of abamectin to G. molesta larvae using the diet incorporation method. The sublethal effects of abamectin on the development, reproduction, detoxification enzyme activity, and related gene expression of G. molesta were assessed. The results showed that the LC20 and LC50 values of the insecticide against G. molesta 72 h post-treatment were 1.17 mg L-1 and 5.85 mg L-1, whereas the LC20 and LC50 values 96 h post-treatment were 0.34 mg L-1 and 3.63 mg L-1. When compared to the control, sublethal concentrations of abamectin 1) significantly increased the mortality of the larvae, prepupae, and pupae of G. molesta, 2) prolonged the duration of 3rd to 5th instar larva, prepupal and pupal periods, 3) shortened the longevity of adults, and 4) reduced female fecundity. The enzymatic activity of glutathione S-transferase (GST) varied significantly after exposure to sublethal concentrations of abamectin, but the cytochrome P450 monooxygenases and carboxylesterase activity were not significantly affected. Thirteen of the 25 GST genes were significantly upregulated under different sublethal concentrations of abamectin. The combined findings increase our understanding of the effects of abamectin on G. molesta and the potential role of GSTs in the metabolic interactions of abamectin in this pest, and have applications for more rational and effective use of abamectin to control G. molesta.

Involvement of chemosensory proteins in host plant searching in the bird cherry-oat aphid

Chemosensory systems are considered to play an important role in host plant selection in herbivorous insects. However, few studies have focused on chemosensory proteins (CSPs) for aphid host-location mechanisms. The roles of CSPs in searching for different Poaceae species (wheat, barley, triticale, maize and sorghum) were tested in Rhopalosiphum padi, an important cereal pest. The olfactometer assays showed that R. padi responds to plant odors. Seven R. padi CSP genes were identified. Influence of aphid morph, tissue and starvation state on expression patterns of CSPs was evaluated. Expression levels of CSP1, CSP4, CSP5 and CSP6 in winged aphids were significantly higher than those in wingless ones. Transcription levels of four genes (CSP1, CSP4, CSP5 and CSP6) were relatively higher in the head with antennae, and the four genes tended to be upregulated following starvation. Silencing of three CSPs (CSP4, CSP5 and CSP6) altered aphid host-location behavior in response to the five different host plants tested. Three volatile compounds of host plants (octanal, [E]-2-hexenol and linalool) have significant attraction to winged R. padi according to the four-arm olfactometer tests. Molecular docking predicted hydrogen bonding sites which played key roles in the binding of CSP4, CSP5 and CSP6 with volatile compounds. Knockdown of CSP4 or CSP5 significantly decreased the staying time of R. padi in the arms with octanal. However, knockdown of CSP6 could not affect the response of R. padi to octanal. These results bring evidence for the involvement of three CSPs in R. padi host-location behavior.

Involvement of Chemosensory Protein BodoCSP1 in Perception of Host Plant Volatiles in Bradysia odoriphaga

Chemosensory proteins (CSPs) can bind and transport odorant molecules and play important roles in insect chemoreception. In this study, we focused on the roles of a chemosensory protein (BodoCSP1) in perception of host plant volatiles in Bradysia odoriphaga. The expression of BodoCSP1 was significantly higher in adults than in larvae and pupae, without a significant difference between male and female adults. Recombinant protein BodoCSP1 exhibited relatively high binding affinities to 9 out of 10 tested ligands (K_i < 10 μM). Behavioral assays revealed that adults of B. odoriphaga showed a significant preference for five compounds. The predicted three-dimensional (3D) structure of BodoCSP1 has the typical six α-helices that form the hydrophobic ligand-binding pocket. Molecular docking and site-directed mutagenesis combined with ligand-binding assays indicated that Val48 and Thr66 may be the key binding site in BodoCSP1 for host plant volatiles. RNAi results indicated that dsBodoCSP1-treated adults showed significant reductions in response to diallyl disulfide, dipropyl disulfide, and allyl methyl disulfide. These results indicated that BodoCSP1 plays essential functions in the perception of host plant volatiles in B. odoriphaga.

Functional characteristics of a novel odorant binding protein in the legume pod borer, Maruca vitrata

Insect olfaction system plays a key role in the foraging food, pollination, mating, oviposition, reproduction and other insect physiological behavior. Odorant binding protein are widely found in the various olfactory sensilla of different insect antennae and involved in chemical signals discrimination from natural environment. In this study, a novel OBP gene, MvitOBP3 is identified from the legume pod borer, Maruca vitrata, which it mainly harms important legume vegetables including cowpea, soybean and lablab bean. Real-time PCR results demonstrated that MvitOBP3 gene was abundantly expressed in the antennal tissue of M. vitrata, while low levels were distributed in the head, thorax, abdomen, leg and wing of adult moths. The recombinant OBP3 protein was purified using the prokaryotic expression and affinity chromatography system. Fluorescence competitive binding experiments indicated that that MvitOBP3 protein exhibited greater binding affinities with host-plant flower volatiles including Butanoic acid butyl ester, Limonene, 1H-indol-4-ol and 2-methyl-3-phenylpropanal, highlighting they may have attractant activities for the oviposition of female moths on the legume vegetables. Moreover, protein homology modeling and molecular docking analysis revealed that there are six amino acid sites of MvitOBP3 involved in the binding of the host-plant volatiles. These findings will further promote to understand the key role of odorant binding protein during host perception and oviposition of M. vitrata moths, which improve the efficiency of semiochemical-based prevention and monitoring for this pest in the legume vegetables field.

Detecting Host-Plant Volatiles with Odorant Receptors from Grapholita molesta (Busck) (Lepidoptera: Tortricidae)

Grapholita molesta is a global pest of stone and pome fruits. The sensitive olfactory system plays a crucial role in regulating key behavioral activities of insects and G. molesta relies heavily on general odorant receptors (ORs) to detect host-plant volatiles. In this study, three general OR genes from G. molesta (GmolOR12, GmolOR20, and GmolOR21) were identified. Quantitative polymerase chain reaction revealed that GmolORs expression was considerably higher in adults and adult antennae than in any other life stages and body parts, respectively. Moreover, the expression of GmolORs was significantly higher in the antennae of females than in those of males, with a peak in the antennae of 3-days-old adult females. GmolOR20 and GmolOR21 displayed no responses to any of the odorant compounds tested in the Xenopus oocyte system. GmolOR12 was tuned mainly to 5 of the 47 odorant components tested (including decanol, heptanal, octanal, nonanal, and decanal), and the response to aldehydes among the 5 components was the highest. Additionally, they all elicited female and male antennae electroantennogram responses, and the aldehydes elicited the highest response among the 5 components. These results suggested that GmolOR12 in the G. molesta olfactory system plays an important role in sensing aldehydes and that GmolOR12 is involved in sensing host-plant volatiles. These findings provide insight into the possibility of using host-plant volatiles for the control of G. molesta.