Identification of candidate odorant degrading gene/enzyme systems in the antennal transcriptome of Drosophila melanogaster

Antennal transcriptome analysis reveals sensory receptors potentially associated with host detection in the livestock pest Lucilia cuprina

BACKGROUND

Lucilia cuprina (Wiedemann, 1830) (Diptera: Calliphoridae) is the main causative agent of flystrike of sheep in Australia and New Zealand. Female flies lay eggs in an open wound or natural orifice, and the developing larvae eat the host's tissues, a condition called myiasis. To improve our understanding of host-seeking behavior, we quantified gene expression in male and female antennae based on their behavior.

METHODS

A spatial olfactometer was used to evaluate the olfactory response of L. cuprina mated males and gravid females to fresh or rotting beef. Antennal RNA-Seq analysis was used to identify sensory receptors differentially expressed between groups.

RESULTS

Lucilia cuprina females were more attracted to rotten compared to fresh beef (> fivefold increase). However, males and some females did not respond to either type of beef. RNA-Seq analysis was performed on antennae dissected from attracted females, non-attracted females and males. Transcripts encoding sensory receptors from 11 gene families were identified above a threshold (≥ 5 transcript per million) including 49 ATP-binding cassette transporters (ABCs), two ammonium transporters (AMTs), 37 odorant receptors (ORs), 16 ionotropic receptors (IRs), 5 gustatory receptors (GRs), 22 odorant-binding proteins (OBPs), 9 CD36-sensory neuron membrane proteins (CD36/SNMPs), 4 chemosensory proteins (CSPs), 4 myeloid lipid-recognition (ML) and Niemann-Pick C2 disease proteins (ML/NPC2), 2 pickpocket receptors (PPKs) and 3 transient receptor potential channels (TRPs). Differential expression analyses identified sex-biased sensory receptors.

CONCLUSIONS

We identified sensory receptors that were differentially expressed between the antennae of both sexes and hence may be associated with host detection by female flies. The most promising for future investigations were as follows: an odorant receptor (LcupOR46) which is female-biased in L. cuprina and Cochliomyia hominivorax Coquerel, 1858; an ABC transporter (ABC G23.1) that was the sole sensory receptor upregulated in the antennae of females attracted to rotting beef compared to non-attracted females; a female-biased ammonia transporter (AMT_Rh50), which was previously associated with ammonium detection in Drosophila melanogaster Meigen, 1830. This is the first report suggesting a possible role for ABC transporters in L. cuprina olfaction and potentially in other insects.

Comparative transcriptomics reveals the conservation and divergence of reproductive genes across three sympatric Tomicus bark beetles

Three tree-killing bark beetles belonging to the genus Tomicus, Tomicus yunnanensis, Tomicus brevipilosus and Tomicus minor (Coleoptera; Curculionidae, Scolytinae), are serious wood-borers with larvae feeding on the phloem tissues of Pinus yunnanensis. The three Tomicus beetles, in some cases, coexist in a same habitat, providing a best system for exploring the conservation and divergence of reproductive genes. Here, we applied comparative transcriptomics and molecular biology approaches to characterize reproductive-related genes in three sympatric Tomicus species. Illumina sequencing of female and male reproductive systems and residual bodies generated a large number of clean reads, representing 185,920,232 sequences in T. yunnanensis, 169,153,404 in T. brevipilosus and 178,493,176 in T. minor that were assembled into 32,802, 56,912 and 33,670 unigenes, respectively. The majority of the genes had detectable expression in reproductive tissues (FPKM >1), particularly those genes in T. brevipilosus accounting for 76.61 % of the total genes. From the transcriptomes, totally 838 genes encoding 463 detoxification enzymes, 339 chemosensory membrane proteins and 36 ionotropic glutamate receptors (iGluRs) were identified, including 622 reproductive tissue-expressed genes. Of these, members of carboxylesterases (COEs), ionotropic receptors (IRs), sensory neuron membrane proteins (SNMPs) and iGluRs were highly conserved in gene numbers and sequence identities across three Tomicus species. Further, expression profiling analyses revealed a number of genes expressed in reproductive tissues and the diverse expression characteristics in these beetles. The results provide evidence for the conservation and differences of reproductive genes among three sympatric closely related beetles, helping understand their different reproductive strategies and the maximization of the reproductive success.

Antennal transcriptome analysis of Psyttalia incisi (silvestri) (Hymenoptera: Braconidae): identification and tissue expression profiling of candidate odorant-binding protein genes

BACKGROUND

Olfaction plays an important role in host-seeking by parasitoids, as they can sense chemical signals using sensitive chemosensory systems. Psyttalia incisi (Silvestri) (Hymenoptera: Braconidae) is the dominant parasitoid of Bactrocera dorsalis (Hendel) in fruit-producing regions of southern China. The olfactory behavior of P. incisi has been extensively studied; however, the chemosensory mechanisms of this species are not fully understood.

RESULTS

Bioinformatics analysis of 64,515 unigenes from the antennal transcriptome of both male and female adults P. incisi identified 87 candidate chemosensory genes. These included 13 odorant-binding proteins (OBPs), seven gustatory receptors (GRs), 55 odorant receptors (ORs), 10 ionotropic receptors (IRs), and two sensory neuron membrane proteins (SNMPs). Phylogenetic trees were constructed to predict evolutionary relationships between these chemosensory genes in hymenopterans. Moreover, the tissue expression profiles of 13 OBPs were analyzed by quantitative real-time PCR, revealing high expression of seven OBPs (1, 3, 6, 7, 8, 12, and 13) in the antennae.

CONCLUSION

This study represents the first identification of chemosensory genes and the determination of their expression patterns in different tissues of P. incisi. These results contribute to a better understanding of the function of the chemosensory system of this parasitoid species.

Molecular and functional characterization of an antenna-enriched glutathione S-transferase BminGSTd3 involved in undecanol degradation in the citrus fruit fly, Bactrocera minax (Enderlein) (Diptera Tephritidae)

Bactrocera minax is a disastrous pest of citrus crops in China. Numerous studies focused on the molecular mechanism of odorant perception of B. minax, but the molecular mechanism of odorant degradation remains unclear. Glutathione S-transferases (GSTs) are considered as a class of odorant-degrading enzymes involved in degrading odorant molecules in insects' olfactory system. Here, we identified a delta-class GST gene, BminGSTd3, from B. minax. It was predominantly expressed in adult's olfactory organ antennae. The bacterially expressed recombinant BminGSTd3 was able to catalyze the conjugation of glutathione (GSH) with 2, 4-dinitrochlorobenzene (CDNB). Spectrophotometric analysis showed that undecanol can inhibit catalytic activities of BminGSTd3. Metabolic assays exhibited that undecanol can be depleted by BminGSTd3. Undecanol is believed to be an important B. minax sex pheromone component. The other components of the pheromone remain unclear. To understand how BminGSTd3 specifically recognizes undecanol, a 3D model of BminGSTd3 was constructed by homology modeling. Molecular docking based on this model revealed that E64 and S65 are the key amino acids recognizing undecanol, and this was proven by site-directed mutagenesis and intrinsic fluorescence assays. We suggest that BminGSTd3 is an undecanol metabolizing GST in B.minax, and E64 and S65 may serve as the key binding sites.

Structure-activity analysis suggests an olfactory function for the unique antennal delta glutathione transferase of Apis mellifera

Glutathione transferases (GST) are detoxification enzymes that conjugate glutathione to a wide array of molecules. In the honey bee Apis mellifera, AmGSTD1 is the sole member of the delta class of GSTs, with expression in antennae. Here, we structurally and biochemically characterized AmGSTD1 to elucidate its function. We showed that AmGSTD1 can efficiently catalyse the glutathione conjugation of classical GST substrates. Additionally, AmGSTD1 exhibits binding properties with a range of odorant compounds. AmGSTD1 has a peculiar interface with a structural motif we propose to call 'sulfur sandwich'. This motif consists of a cysteine disulfide bridge sandwiched between the sulfur atoms of two methionine residues and is stabilized by CH…S hydrogen bonds and S…S sigma-hole interactions. Thermal stability studies confirmed that this motif is important for AmGSTD1 stability and, thus, could facilitate its functions in olfaction.

Investigating SnocCSP4 expression and key compound interactions with SnocOBP4 in Sirex noctilio Fabricius (Hymenoptera: Siricidae)

Sirex noctilio, a significant pest impacting Pinus sylvestris var. mongolica, presents control difficulties due to its wood-boring behavior, paucity of natural antagonists, and wide-ranging habitats. Our research aims to elucidate the functionality and operational mechanisms of chemosensory proteins 4 (SnocCSP4), providing strategic insights for pest management and fostering further exploration in CSPs. Techniques such as qPCR, fluorescence binding affinity assays, molecular docking, and dynamic simulations were utilized to investigate the tissue-specific distribution, ligand binding capacities, and mechanistic underpinnings of SnocCSP4. The findings revealed a high abundance of SnocCSP4 in male genitalia, significant sexual dimorphism in its expression, and high binding affinities to (-)-Globulol and 10-Oxodecanoic acid. Subsequent analysis identified hydrophobic cavities formed by non-polar amino acids (VAL, LEU, ILE, LYS) and the critical role of polar amino acids (ALA 46, GLU 45, THR 75) in maintaining system stability. These insights suggest the primary role of SnocCSP4 in binding or transporting these volatiles and indicate that modifying key amino acids could inform the design of more effective pest control measures.

Influences of Microbial Symbionts on Chemoreception of Their Insect Hosts

Chemical communication is widespread among insects and exploited to adjust their behavior, such as food and habitat seeking and preferences, recruitment, defense, and mate attraction. Recently, many studies have revealed that microbial symbionts could regulate host chemical communication by affecting the synthesis and perception of insect semiochemicals. In this paper, we review recent studies of the influence of microbial symbionts on insect chemoreception. Microbial symbionts may influence insect sensitivity to semiochemicals by regulating the synthesis of odorant-binding proteins or chemosensory proteins and olfactory or gustatory receptors and regulating host neurotransmission, thereby adjusting insect behavior. The manipulation of insect chemosensory behavior by microbial symbionts is conducive to their proliferation and dispersal and provides the impetus for insects to change their feeding habits and aggregation and dispersal behavior, which contributes to population differentiation in insects. Future research is necessary to reveal the material and information exchange between both partners to improve our comprehension of the evolution of chemoreception in insects. Manipulating insect chemoreception physiology by inoculating them with microbes could be utilized as a potential approach to managing insect populations.

Antennae-enriched expression of candidate odorant degrading enzyme genes in the turnip aphid, Lipaphis erysimi

Aphids heavily rely on their olfactory system for foraging behavior. Odorant-degrading enzymes (ODEs) are essential in preserving the olfactory acuity of aphids by removing redundant odorants in the antennae. Certain enzymes within this group stand out as being enriched and/or biased expressed in the antennae, such as carboxylesterases (CXEs), cytochrome P450 (CYPs), glutathione S-transferases (GSTs), and UDP-glycosyltransferases (UGTs). Here, we performed a comparative transcriptome analysis of antennae and body tissue to isolate the antennal ODE genes of turnip aphid Lipaphis erysimi. A dataset of one CXE, seven CYPs, two GSTs, and five UGTs enriched in the antennae was identified and subjected to sequence analysis. Furthermore, qRT-PCR analyses showed that 13 ODE genes (LeCXE6, LeCYP4c1, LeCYP6a2, LeCYP6a13, LeCYP6a14.2, LeCYP6k1, LeCYP18a1, LeGST1, LeUGT1-7, LeUGT2B7, LeUGT2B13, LeUGT2C1.1, and LeUGT2C1.2) were specifically or significantly elevated in antennal tissues. Among these antennae-enriched ODEs, LeCYP4c1, LeCYP6a2, LeCYP6a13, LeCYP6a14.2, LeCYP18a1, LeUGT2B7, and LeUGT2B13 were found to exhibit significantly higher expression levels in alate aphids compared to apterous and nymph aphids, suggesting their putative role in detecting new host plant location. The results presented in this study highlight the identification and expression of ODE genes in L. erysimi, paving the path to investigate their functional role in odorant degradation during the olfactory processes.

Structure of an antennally-expressed carboxylesterase suggests lepidopteran odorant degrading enzymes are broadly tuned

Insects rely on the detection of chemical cues present in the environment to guide their foraging and reproductive behaviour. As such, insects have evolved a sophisticated chemical processing system in their antennae comprised of several types of olfactory proteins. Of these proteins, odorant degrading enzymes are responsible for metabolising the chemical cues within the antennae, thereby maintaining olfactory system function. Members of the carboxyl/cholinesterase gene family are known to degrade odorant molecules with acetate-ester moieties that function as host recognition cues or sex pheromones, however, their specificity for these compounds remains unclear. Here, we evaluate expression levels of this gene family in the light-brown apple moth, Epiphyas postvittana, via RNAseq and identify putative odorant degrading enzymes. We then solve the apo-structure for EposCCE24 by X-ray crystallography to a resolution of 2.43 Å and infer substrate specificity based on structural characteristics of the enzyme's binding pocket. The specificity of EposCCE24 was validated by testing its ability to degrade biologically relevant and non-relevant sex pheromone components and plant volatiles using GC-MS. We found that EposCCE24 is neither capable of discriminating between linear acetate-ester odorant molecules of varying chain length, nor between molecules with varying double bond positions. EposCCE24 efficiently degraded both plant volatiles and sex pheromone components containing acetate-ester functional groups, confirming its role as a broadly-tuned odorant degrading enzyme in the moth olfactory organ.

Glutathione S-Transferase Highly Expressed in Holotrichia parallela Antennae Inactivates the Odorant Unsaturated Aldehyde Volatiles

Odorant-degrading enzymes in insects play a vital role in maintaining olfactory sensitivity. However, the role and molecular mechanism of glutathione S-transferases (GSTs) in odorant inactivation has been rarely studied. In the present study, 31 GSTs were identified from the antennal transcriptome of Holotrichia parallela. HpGSTd1 possesses the highest transcriptome expression level. Recombinant HpGSTd1 showed degradation activity toward various unsaturated aldehyde volatiles. Furthermore, the metabolite of cinnamaldehyde was identified by high-resolution mass spectrometry (HRMS). The molecular docking analysis and site-directed mutagenesis revealed the key residues of HpGSTd1 in degrading odorants. In addition, the unsaturated aldehyde volatiles elicited the behavioral and electrophysiological responses of H. parallela. Taken together, our findings suggest that HpGSTd1 may play an essential role in inactivating odorants in H. parallela, which provides new insights for identifying molecular targets and exploring effective olfactory regulators for this underground pest.

Current understandings of olfactory molecular events in the Asian corn borer, Ostrinia furnacalis (Lepidoptera: Crambidae)

The Asian corn borer Ostrinia furnacalis (Lepidoptera: Crambidae) is a serious corn pest with widespread distribution in East Asia. Its olfactory mechanism is a focus of scientific study, aiming to find good ways to control this pest. Molecular events are considered to be important in olfactory mechanism. Current understandings of olfactory molecular events in O. furnacalis, mainly involving sex pheromones and olfactory proteins, were summarized to provide a reference for further studies. O. furnacalis sex pheromone contains two components E-12-tetradecenyl acetate and Z-12-tetradecenyl acetate, which may be recognized and bound by the pheromone binding proteins OfurPBP3 and OfurPBP2, and then transported to the odorant receptors (ORs) OfurOR4 and OfurOR6 to activate them. The ORs OfurOR8, OfurOR7 and OfurOR5b mainly respond to the sex pheromone components of other Ostrinia species, E-11-tetradecenyl acetate, Z-11-tetradecenyl acetate and Z-9-tetradecenyl acetate. The OR OfurOR27 responds strongly to plant odorants nonanal, octanal and 1-octanol. Much work remains to be done to fully understand odorants with olfactory activity to O. furnacalis and the functions of its olfactory proteins. These studies will help to reveal olfactory mechanism in O. furnacalis, with the aim of regulating its behaviors to control this pest.

Dynamic Roles of Insect Carboxyl/Cholinesterases in Chemical Adaptation

Insects have evolved several intricate defense mechanisms to adapt to their chemical environment. Due to their versatile capabilities in hydrolytic biotransformation, insect carboxyl/cholinesterases (CCEs) play vital roles in the development of pesticide resistance, facilitating the adaptation of insects to their host plants, and manipulating insect behaviors through the olfaction system. CCEs confer insecticide resistance through the mechanisms of qualitative or quantitative changes of CCE-mediated enhanced metabolism or target-site insensitivity, and may contribute to the host plant adaptation. CCEs represent the first odorant-degrading enzymes (ODEs) discovered to degrade insect pheromones and plant odors and remain the most promising ODE candidates. Here, we summarize insect CCE classification, currently characterized insect CCE protein structure characteristics, and the dynamic roles of insect CCEs in chemical adaptation.

Role of Insect and Mammal Glutathione Transferases in Chemoperception

Glutathione transferases (GSTs) are ubiquitous key enzymes with different activities as transferases or isomerases. As key detoxifying enzymes, GSTs are expressed in the chemosensory organs. They fulfill an essential protective role because the chemosensory organs are located in the main entry paths of exogenous compounds within the body. In addition to this protective function, they modulate the perception process by metabolizing exogenous molecules, including tastants and odorants. Chemosensory detection involves the interaction of chemosensory molecules with receptors. GST contributes to signal termination by metabolizing these molecules. By reducing the concentration of chemosensory molecules before receptor binding, GST modulates receptor activation and, therefore, the perception of these molecules. The balance of chemoperception by GSTs has been shown in insects as well as in mammals, although their chemosensory systems are not evolutionarily connected. This review will provide knowledge supporting the involvement of GSTs in chemoperception, describing their localization in these systems as well as their enzymatic capacity toward odorants, sapid molecules, and pheromones in insects and mammals. Their different roles in chemosensory organs will be discussed in light of the evolutionary advantage of the coupling of the detoxification system and chemosensory system through GSTs.

Sensing of a spore surface protein by a Drosophila chemosensory protein induces behavioral defense against fungal parasitic infections

In addition to innate immunity in a physiological context, insects have evolved behavioral defenses against parasite attacks. Here, we report that Drosophila can sense the CFEM (common in fungal extracellular membrane) protein Mcdc9, which acts as a negative virulence factor of the entomopathogenic fungus Metarhizium robertsii. The individual deletions of 18 CFEM genes in Metarhizium followed by fly infection identified three null mutants that could kill the flies more quickly than the wild-type strain, among which Mcdc9 can coat fungal spores and interact with the fly chemosensory protein CheA75a. The deletion of Mcdc9 in the fungus or the knockdown of CheA75a in flies had a similar effect, in which a greater number of fungal spores were left on flies than on the respective controls after topical infection. Thus, similar to the accelerated death of the wild-type flies treated with ΔMcdc9, the CheA75a^RNAi flies succumbed more quickly than the control insects topically challenged with the wild-type strain. The CheA75a gene is highly transcribed in fly legs and wings, and positive electrophysiological responses were evidenced in tarsal sensilla after stimulation with the Mcdc9 protein. The results imply that this CFEM protein could be sensed as a contact elicitor inducing the hygienic behavior of flies against fungal parasitic infection, which reveals a previously unsuspected mechanism of fungus-insect interactions.

Antennal transcriptomic analysis of carboxylesterases and glutathione S-transferases associated with odorant degradation in the tea gray geometrid, Ectropis grisescens (Lepidoptera, Geometridae)

Introduction: Carboxylesterases (CXEs) and glutathione S-transferases (GSTs) can terminate olfactory signals during chemosensation by rapid degradation of odorants in the vicinity of receptors. The tea grey geometrid, Ectropis grisescens (Lepidoptera, Geometridae), one of the most devastating insect herbivores of tea plants in China, relies heavily on plant volatiles to locate the host plants as well as the oviposition sites. However, CXEs and GSTs involved in signal termination and odorant clearance in E. grisescens remains unknown. Methods: In this study, identification and spatial expression profiles of CXEs and GSTs in this major tea pest were investigated by transcriptomics and qRT-PCR, respectively. Results: As a result, we identified 28 CXEs and 16 GSTs from female and male antennal transcriptomes. Phylogenetic analyses clustered these candidates into several clades, among which antennal CXEs, mitochondrial and cytosolic CXEs, and delta group GSTs contained genes commonly associated with odorants degradation. Spatial expression profiles showed that most CXEs (26) were expressed in antennae. In comparison, putative GSTs exhibited a diverse expression pattern across different tissues, with one GST expressed specifically in the male antennae. Disscussion: These combined results suggest that 12 CXEs (EgriCXE1, 2, 4, 6, 8, 18, 20-22, 24, 26, and 29) and 5 GSTs (EgriGST1 and EgriGST delta group) provide a major source of candidate genes for odorants degradation in E. grisescens.

Genetic mechanisms modulating behaviour through plastic chemosensory responses in insects

The ability to transition between different behavioural stages is a widespread phenomenon across the animal kingdom. Such behavioural adaptations are often linked to changes in the sensitivity of those neurons that sense chemical cues associated with the respective behaviours. To identify the genetic mechanisms that regulate neuronal sensitivity, and by that behaviour, typically *omics approaches, such as RNA- and protein-sequencing, are applied to sensory organs of individuals displaying differences in behaviour. In this review, we discuss these genetic mechanisms and how they impact neuronal sensitivity, summarize the correlative and functional evidence for their role in regulating behaviour and discuss future directions. As such, this review can help interpret *omics data by providing a comprehensive list of already identified genes and mechanisms that impact behaviour through changes in neuronal sensitivity.

Proteomic Characterization of Drosophila melanogaster Proboscis

Drosophila melanogaster flies use their proboscis to taste and distinguish edible compounds from toxic compounds. With their proboscis, flies can detect sex pheromones at a close distance or by contact. Most of the known proteins associated with probosci's detection belong to gustatory receptor families. To extend our knowledge of the proboscis-taste proteins involved in chemo-detection, we used a proteomic approach to identify soluble proteins from Drosophila females and males. This investigation, performed with hundreds of dissected proboscises, was initiated by the chromatographic separation of tryptic peptides, followed by tandem mass spectrometry, allowing for femtomole detection sensitivity. We found 586 proteins, including enzymes, that are involved in intermediary metabolism and proteins dedicated to various functions, such as nucleic acid metabolism, ion transport, immunity, digestion, and organ development. Among 60 proteins potentially involved in chemosensory detection, we identified two odorant-binding proteins (OBPs), i.e., OBP56d (which showed much higher expression in females than in males) and OBP19d. Because OBP56d was also reported to be more highly expressed in the antennae of females, this protein can be involved in the detection of both volatile and contact male pheromone(s). Our proteomic study paves the way to better understand the complex role of Drosophila proboscis in the chemical detection of food and pheromonal compounds.

Transcriptomic Identification and Expression Profile Analysis of Odorant-Degrading Enzymes from the Asian Corn Borer Moth, Ostrinia furnacalis

The Asian corn borer moth Ostrinia furnacalis is an important lepidopteran pest of maize in Asia. Odorant-degrading enzymes (ODEs), including carboxylesterases (CCEs), glutathione S-transferases (GSTs), cytochrome P450s (CYPs), UDP-glycosyltransferases (UGTs), and aldehyde oxidases (AOXs), are responsible for rapid inactivation of odorant signals in the insect antennae. In this study, we performed a transcriptome assembly for the antennae of O. furnacalis to identify putative ODE genes. Transcriptome sequencing revealed 35,056 unigenes, and 21,012 (59.94%) of these were annotated by searching against the reference sequences in the NCBI non-redundant (NR) protein database. For functional classification, these unigenes were subjected to Gene Ontology (GO), Eukaryotic Orthologous Groups (KOG), and Kyoto Encyclopedia of Genes and Genomes (KEGG) annotations. We identified 79 genes encoding putative ODEs: 19 CCEs, 17 GSTs, 24 CYPs, 13 UGTs, and 6 AOXs. BLASTX best hit results indicated that these genes shared quite high amino acid identities with their respective orthologs from other lepidopteran species. Reverse transcription-quantitative PCR showed that OfurCCE2, OfurCCE5, and OfurCCE18 were enriched in male antennae, while OfurCCE7 and OfurCCE10 were enriched in female antennae. OfurCCE14 and OfurCCE15 were expressed at near-equal amounts in the antennae of both sexes. Our findings establish a solid foundation for future studies aimed at understanding the olfactory functions of these genes in O. furnacalis.

Genome-wide and expression-profiling analyses of the cytochrome P450 genes in Tenebrionidea

Cytochrome P450 monooxygenases (CYPs) are present in almost all areas of the tree of life. As one of the largest and most diverse superfamilies of multifunctional enzymes, they play important roles in the metabolism of xenobiotics and biosynthesis of endogenous compounds, shaping the success of insects. In this study, the CYPome (an omics term for all the CYP genes in a genome) diversification was examined in the four Tenebrionidea species through genome-wide analysis. A total of 483 CYP genes were identified, of which 103, 157, 122, and 101 were respectively deciphered from the genomes of Tebebrio molitor, Asbolus verucosus, Hycleus cichorii and Hycleus phaleratus. These CYPs were classified into four major clans (mitochondrial, CYP2, CYP3, and CYP4), and clans CYP3 and CYP4 are most diverse. Phylogenetic analysis showed that most CYPs of these Tenebrionidea beetles from each clan had a very close 1:1 orthology to each other, suggesting that they originate closely and have evolutionally conserved function. Expression analysis at different developmental stages and in various tissues showed the life stage-, gut-, salivary gland-, fat body-, Malpighian tubule-, antennae-, ovary- and testis-specific expression patterns of T. molitor CYP genes, implying their various potential roles in development, detoxification, immune response, digestion, olfaction, and reproduction. Our studies provide a platform to understand the evolution of Tenebrionidea CYP gene superfamily, and a basis for further functional investigation of the T. molitor CYPs involved in various biological processes.

Pheromone sensing in Drosophila requires support cell-expressed Osiris 8

Background

The nose of most animals comprises multiple sensory subsystems, which are defined by the expression of different olfactory receptor families. Drosophila melanogaster antennae contain two morphologically and functionally distinct subsystems that express odorant receptors (Ors) or ionotropic receptors (Irs). Although these receptors have been thoroughly characterized in this species, the subsystem-specific expression and roles of other genes are much less well-understood.

Results

Here we generate subsystem-specific transcriptomic datasets to identify hundreds of genes, encoding diverse protein classes, that are selectively enriched in either Or or Ir subsystems. Using single-cell antennal transcriptomic data and RNA in situ hybridization, we find that most neuronal genes—other than sensory receptor genes—are broadly expressed within the subsystems. By contrast, we identify many non-neuronal genes that exhibit highly selective expression, revealing substantial molecular heterogeneity in the non-neuronal cellular components of the olfactory subsystems. We characterize one Or subsystem-specific non-neuronal molecule, Osiris 8 (Osi8), a conserved member of a large, insect-specific family of transmembrane proteins. Osi8 is expressed in the membranes of tormogen support cells of pheromone-sensing trichoid sensilla. Loss of Osi8 does not have obvious impact on trichoid sensillar development or basal neuronal activity, but abolishes high sensitivity responses to pheromone ligands.

Conclusions

This work identifies a new protein required for insect pheromone detection, emphasizes the importance of support cells in neuronal sensory functions, and provides a resource for future characterization of other olfactory subsystem-specific genes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01425-w.

Identification and characterization of candidate detoxification genes in Pharsalia antennata Gahan (Coleoptera: Cerambycidae)

The wood-boring beetles, including the majority of Cerambycidae, have developed the ability to metabolize a variety of toxic compounds derived from host plants and the surrounding environment. However, detoxification mechanisms underlying the evolutionary adaptation of a cerambycid beetle Pharsalia antennata to hosts and habitats are largely unexplored. Here, we characterized three key gene families in relation to detoxification (cytochrome P450 monooxygenases: P450s, carboxylesterases: COEs and glutathione-S-transferases: GSTs), by combinations of transcriptomics, gene identification, phylogenetics and expression profiles. Illumina sequencing generated 668,701,566 filtered reads in 12 tissues of P. antennata, summing to 100.28 gigabases data. From the transcriptome, 215 genes encoding 106 P450s, 77 COEs and 32 GSTs were identified, of which 107 relatives were differentially expressed genes. Of the identified 215 genes, a number of relatives showed the orthology to those in Anoplophora glabripennis, revealing 1:1 relationships in 94 phylogenetic clades. In the trees, P. antennata detoxification genes mainly clustered into one or two subfamilies, including 64 P450s in the CYP3 clan, 33 COEs in clade A, and 20 GSTs in Delta and Epsilon subclasses. Combining transcriptomic data and PCR approaches, the numbers of detoxification genes expressed in abdomens, antennae and legs were 188, 148 and 141, respectively. Notably, some genes exhibited significantly sex-biased levels in antennae or legs of both sexes. The findings provide valuable reference resources for further exploring xenobiotics metabolism and odorant detection in P. antennata.

Antennae-abundant expression of candidate cytochrome P450 genes associated with odorant degradation in the asian citrus psyllid, Diaphorina citri

The Asian citrus psyllid, Diaphorina citri, is a notorious pest that is an efficient vector for Candidatus Liberibacter asiaticus (CLas), the causal agent of citrus huanglongbing (HLB). The olfactory system of insects is crucial for foraging and mating behavior. Antennae-abundant odorant degrading enzymes (ODEs), including cytochrome P450 (CYPs), are important in degrading redundant odorant molecules to recover the insect olfactory. In this study, to isolate the antennal CYP genes of D. citri, we generated four transcriptomes from female/male antennae and body through deep sequencing of RNA libraries. Seven DcCYP genes preferentially expressed in antennae were first identified by comparing the antennal and body transcriptomes. Phylogenetic analysis grouped four DcCYPs (DcCYP6a13, DcCYP6j1, DcCYP6k1, and DcCYP6a2) into the CYP3 class, whereas DcCYP4d2, DcCYP4c62, and DcCYP4d8 were clustered in the CYP4 clade. qRT-PCR analyses across developmental stages and tissues showed they were antennae-abundant in both genders and constantly expressed from the first instar nymph to the adult. The results presented here highlight the isolation and expression of CYP genes in D. citri antennae, providing valuable insights into their putative role in odorant degradation.

Identification and expression patterns of candidate carboxylesterases in Carposina sasakii Matsumura (Lepidoptera: Carposinidae), an important pest of fruit trees

Carposina sasakii Matsumura (Lepidoptera: Carposinidae) is an important pest of fruit trees in a large area of Asia. The adults mainly depend on olfaction to communicate with the environment, but the olfactory mechanism has not been well known. Odorant degrading enzymes (ODEs) are important olfactory proteins, which inactivate and degrade odorants to free odorant receptors for maintaining olfactory sensitivity. Carboxylesterases (CXEs) are considered to be a major group of moth ODEs. In this study, four candidate CXEs (CsasCXE1 ~ CsasCXE4) were identified by using head transcriptomic data from C. sasakii adult females and males. Sequence alignment showed conserved amino acid residues and their variations in C. sasakii CXEs. Phylogenetic analysis indicated the CXEs with the variations cluster well, and each C. sasakii CXE clusters in a clade with some of the other lepidopteran CXEs, with a high enough bootstrap value. Gene expression analysis revealed that CsasCXE2 and CsasCXE3 have similar tissue and sex expression patterns in C. sasakii adults. The two CXEs have relatively high expression levels in the heads and are expressed more abundantly in the female heads than male heads. CsasCXE1 and CsasCXE4 are expressed at higher levels in the male heads than female heads, but not dominantly expressed in the heads among the different tissues. Whether these CXEs function as ODEs remains to be further researched. This study laid the foundation for exploring functions of C. sasakii CXEs.

Expression Patterns of Drosophila Melanogaster Glutathione Transferases

Glutathione transferases (GSTs) are ubiquitous enzymes that catalyze the conjugation of glutathione to various molecules. Among the 42 GSTs identified in Drosophila melanogaster, Delta and Epsilon are the largest classes, with 25 members. The Delta and Epsilon classes are involved in different functions, such as insecticide resistance and ecdysone biosynthesis. The insect GST number variability is due mainly to these classes. Thus, they are generally considered supports during the evolution for the adaptability of the insect species. To explore the link between Delta and Epsilon GST and their evolution, we analyzed the sequences using bioinformatic tools. Subgroups appear within the Delta and Epsilon GSTs with different levels of diversification. The diversification also appears in the sequences showing differences in the active site. Additionally, amino acids essential for structural stability or dimerization appear conserved in all GSTs. Quantitative real-time polymerase chain reaction (qRT-PCR) analysis revealed that the transcripts corresponding to these two classes are heterogeneously expressed within D. melanogaster. Some GSTs, such as GSTD1, are highly expressed in all tissues, suggesting their general function in detoxification. Conversely, some others, such as GSTD11 or GSTE4, are specifically expressed at a high level specifically in antennae, suggesting a potential role in olfaction.

Alternative transcript splicing regulates UDP-glucosyltransferase-catalyzed detoxification of DIMBOA in the fall armyworm (Spodoptera frugiperda)

Herbivorous insects often possess the ability to detoxify chemical defenses from their host plants. The fall armyworm (Spodoptera frugiperda), which feeds principally on maize, detoxifies the maize benzoxazinoid 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) by stereoselective re-glucosylation using a UDP-glucosyltransferase, SfUGT33F28. SfUGT33F28 activity is induced by feeding on a DIMBOA-containing diet, but how this induction is regulated is unknown. In the present work, we describe the alternative splicing of the SfUGT33F28 transcript. Variant transcripts are differentially expressed in response to DIMBOA, and this transcriptional response is mediated by an insect aryl hydrocarbon receptor. These variants have large deletions leading to the production of truncated proteins that have no intrinsic UGT activity with DIMBOA but interact with the full-length enzyme to raise or lower its activity. Therefore, the formation of SfUGT33F28 splice variants induces DIMBOA-conjugating UGT activity when DIMBOA is present in the insect diet and represses activity in the absence of this plant defense compound.

Systematic analysis of olfactory protein-protein interactions network of fruitfly, Drosophila melanogaster

Olfaction is one of the physiological traits of insect behavior. Insects have evolved a sophisticated olfactory system and use a combined coding strategy to process general odor. Drosophila melanogaster is a powerful model to reveal the molecular and cellular mechanisms of odor detection. Identifying new olfactory targets through complex interactions will contribute to a better understanding of the functions, interactions, and signaling pathways of olfactory proteins. However, the mechanism of D. melanogaster olfaction is still unclear, and more olfactory proteins are required to be discovered. In this study, we tried to explore essential proteins in the olfactory system of D. melanogaster and conduct protein-protein interactions (PPIs) analysis. We constructed the PPIs network of the olfactory system of D. melanogaster, consisting of 863 proteins and 18,959 interactions. Various methods were used to perform functional enrichment analysis, topological analysis and cluster analysis. Our results confirmed that Class B scavenger receptors (SR-Bs), glutathione S-transferases (GSTs), and UDP-glycosyltransferases (UGTs) play an essential role in olfaction of D. melanogaster. The proteins obtained in this study can be used for subsequent functional identification in D. melanogaster olfactory.

UGT440A1 Is Associated With Motility, Reproduction, and Pathogenicity of the Plant-Parasitic Nematode Bursaphelenchus xylophilus

Pine wilt disease (PWD) caused by Bursaphelenchus xylophilus is considered a major threat to pine forests worldwide. Uridine diphosphate (UDP)-glycosyltransferases (UGTs) catalyze the conjugation of small lipophilic compounds with sugars and play crucial roles in the detoxification and homeostatic processes in all living organisms. We investigated the molecular characteristics and biological functions of the gene UGT440A1 that encodes UGTs in B. xylophilus. The in situ hybridization results indicated that UGT440A1 is expressed in all developmental stages of B. xylophilus, particularly in the head, intestine, and hypodermis of the second-stage of juveniles (J2), third-stage of juveniles (J3) and fourth-stage of juveniles (J4) females and in almost the whole body of J4 males and adults. Recombinant UGT440A1 was observed mainly in the inclusion bodies, and the enzyme activity assay revealed that UGT440A1 could catalyze the glycosylation reaction of two types of flavonols (kaempferol and quercetin). RNA interference (RNAi) of UGT440A1 suppressed motility, feeding, and reproduction of B. xylophilus. Furthermore, UGT440A1 knockdown caused a delay in the development of PWD symptoms in the pine seedlings inoculated with the nematodes. These results suggest that UGT440A1 is involved in the pathogenic process of B. xylophilus and the information may facilitate a better understanding of the molecular mechanism of PWD.

Identification and Characterization of UDP-Glycosyltransferase Genes in a Cerambycid Beetle, Pharsalia antennata Gahan, 1894 (Coleoptera: Cerambycidae)

The cerambycid beetle, Pharsalia antennata Gahan, 1894 (Coleoptera: Cerambycidae), is a wood-boring pest that spends most of its life cycle in the trunks or under the bark of trees. These distinctive biological characteristics make it likely that this beetle will encounter a number of plant defensive compounds, coupled with a broad range of host plants, possibly resulting in the overexpression or expansion of uridine diphosphate (UDP)-glycosyltransferase (UGT) genes. Here, we identified and characterized the UGT gene family in P. antennata through transcriptome data, sequence and phylogenetic analyses, and PCR and homology modeling approaches. In total, 59 transcripts encoding UGTs were identified, 34 of which harbored full-length sequences and shared high conservation with the UGTs of Anoplophora glabripennis. Of the 34 PantUGTs, only 31.78% amino acid identity was observed on average, but catalytic and sugar binding residues were highly conserved. Phylogenetic analyses revealed four Cerambycidae-specific clades, including 30 members from P. antennata. Combining the transcriptome and PCR data showed that PantUGTs had a wide tissue expression, and the majority of the genes were presented mainly in antennae or abdomens, suggesting their putative roles in olfaction and detoxification. This study provides, for the first time, information on the molecular and genetic basis of P. antennata, greatly enhancing our knowledge of the detoxification-related UGT gene family.

Neuroligin 3 from common cutworm enhances the GABA-induced current of recombinant SlRDL1 channel

BACKGROUND

Neuroligin (NLG) protein is a nerve cell adhesion molecule and plays a key role in the precision apposition of presynaptic domains on inhibitory and excitatory synapses. Existing studies mainly focused on the function of NLG3 against the excitatory channel. However, the interaction between insect NLG3 and ionotropic GABA receptor, which is the main inhibitory channel, remains unclear. In this study, the Nlg3 of common cutworm (CCW), Spodoptera litura Fabricius, one important agricultural Lepidopteron, is selected to explore its function in the inhibitory channel.

RESULTS

The SlNlg3 was obtained and the SlNLG3 contains the characteristic features including transmembrane domain, PDZ-binding motif and type-B carboxylesterases signature 2 motif. The SlNlg3 messenger RNA (mRNA) was most abundant in midgut, and exhibited multiple expression patterns in different developmental stages and tissues or body parts. Compared with the single injection of SlRDL1, the median effective concentration value of GABA in activating currents was smaller in Xenopus laevis oocytes co-injected with SlRDL1 and SlNlg3. In addition, SlNlg3 could enhance the GABA-induced current of homomeric SlRDL1 channel from -391.86 ± 15.41 to -2152.51 ± 30.09 nA. DsSlNlg3 depressed the expression level of SlNlg3 mRNA more than 64.29% at 6 h. After exposure to median lethal dose of fluralaner, the mortality of CCW injected with dsSlNlg3 was significantly decreased by 13.34% and 30.00% at 24 and 48 h, respectively, compared to injection of dsEGFP.

CONCLUSION

NLG3 should have physiological function on ionotropic GABA receptor in vitro, which provided a favorable foundation for further research on the physiological function of Nlg gene in Lepidopteron. © 2021 Society of Chemical Industry.

Identification and Expression Profile of Chemosensory Receptor Genes in Aromia bungii (Faldermann) Antennal Transcriptome

The red-necked longicorn beetle, Aromia bungii (Faldermann) (Coleoptera: Cerambycidae), is a major destructive, wood-boring pest, which is widespread throughout the world. The sex pheromone of A. bungii was reported earlier; however, the chemosensory mechanism of the beetle remains almost unknown. In this study, 45 AbunORs, 6 AbunGRs and 2 AbunIRs were identified among 42,197 unigenes derived from the antennal transcriptome bioinformatic analysis of A. bungii adults. The sequence of putative Orco (AbunOR25) found in this study is highly conserved with the known Orcos from other Coleoptera species, and these Orco genes might be potentially used as target genes for the future development of novel and effective control strategies. Tissue expression analysis showed that 29 AbunOR genes were highly expressed in antennae, especially in the antennae of females, which was consistent with the idea that females might express more pheromone receptors for sensing pheromones, especially the sex pheromones produced by males. AbunOR5, 29, 31 and 37 were clustered with the pheromone receptors of the cerambycid Megacyllene caryae, suggesting that they might be putative pheromone receptors of A. bungii. All six AbunGRs were highly expressed in the mouthparts, indicating that these GRs may be involved in the taste perception process. Both AbunIRs were shown to be female-mouthparts-biased, suggesting that they might also be related to the tasting processes. Our study provides some basic information towards a deeper understanding of the chemosensing mechanism of A. bungii at a molecular level.

Functional Interaction Between Drosophila Olfactory Sensory Neurons and Their Support Cells

Insects detect volatile chemicals using antennae, which house a vast variety of olfactory sensory neurons (OSNs) that innervate hair-like structures called sensilla where odor detection takes place. In addition to OSNs, the antenna also hosts various support cell types. These include the triad of trichogen, tormogen, and thecogen support cells that lie adjacent to their respective OSNs. The arrangement of OSN supporting cells occurs stereotypically for all sensilla and is widely conserved in evolution. While insect chemosensory neurons have received considerable attention, little is known about the functional significance of the cells that support them. For instance, it remains unknown whether support cells play an active role in odor detection, or only passively contribute to homeostasis, e.g., by maintaining sensillum lymph composition. To investigate the functional interaction between OSNs and support cells, we used optical and electrophysiological approaches in Drosophila. First, we characterized the distribution of various supporting cells using genetic markers. By means of an ex vivo antennal preparation and genetically-encoded Ca²⁺ and K⁺ indicators, we then studied the activation of these auxiliary cells during odor presentation in adult flies. We observed acute responses and distinct differences in Ca²⁺ and K⁺ fluxes between support cell types. Finally, we observed alterations in OSN responses upon thecogen cell ablation in mature adults. Upon inducible ablation of thecogen cells, we notice a gain in mechanical responsiveness to mechanical stimulations during single-sensillum recording, but a lack of change to the neuronal resting activity. Taken together, these results demonstrate that support cells play a more active and responsive role during odor processing than previously thought. Our observations thus reveal that support cells functionally interact with OSNs and may be important for the extraordinary ability of insect olfactory systems to dynamically and sensitively discriminate between odors in the turbulent sensory landscape of insect flight.

Identification and characterization of CYPs induced in the Drosophila antenna by exposure to a plant odorant

Members of the cytochrome p450 (CYP) enzyme family are abundantly expressed in insect olfactory tissues, where they are thought to act as Odorant Degrading Enzymes (ODEs). However, their contribution to olfactory signaling in vivo is poorly understood. This is due in part to the challenge of identifying which of the dozens of antennal-expressed CYPs might inactivate a given odorant. Here, we tested a high-throughput deorphanization strategy in Drosophila to identify CYPs that are transcriptionally induced by exposure to odorants. We discovered three CYPs selectively upregulated by geranyl acetate using transcriptional profiling. Although these CYPs are broadly expressed in the antenna in non-neuronal cells, electrophysiological recordings from CYP mutants did not reveal any changes in olfactory neuron responses to this odorant. Neurons were desensitized by pre-exposing flies to the odorant, but this effect was similar in CYP mutants. Together, our data suggest that the induction of a CYP gene by an odorant does not necessarily indicate a role for that CYP in neuronal responses to that odorant. We go on to show that some CYPs have highly restricted expression patterns in the antenna, and suggest that such CYPs may be useful candidates for further studies on olfactory CYP function.

Identification of Candidate Carboxylesterases Associated With Odorant Degradation in Holotrichia parallela Antennae Based on Transcriptome Analysis

Insects rely on their olfactory systems in antennae to recognize sex pheromones and plant volatiles in surrounding environments. Some carboxylesterases (CXEs) are odorant-degrading enzymes (ODEs), degrading odorant signals to protect the olfactory neurons against continuous excitation. However, there is no report about CXEs in Holotrichia parallela, one of the most major agricultural underground pests in China. In the present study, 20 candidate CXEs were identified based on transcriptome analysis of female and male antennae. Sequence alignments and phylogenetic analysis were performed to investigate the characterization of these candidate CXEs. The expression profiles of CXEs were compared by RT-qPCR analysis between olfactory and non-olfactory tissues of both genders. HparCXE4, 11, 16, 17, 18, 19, and 20 were antenna-biased expressed genes, suggesting their possible roles as ODEs. HparCXE6, 10, 11, 13, and 16 showed significantly higher expression profiles in male antennae, whereas HparCXE18 was expressed more in female antennae. This study highlighted candidate CXE genes linked to odorant degradation in antennae, and provided a useful resource for further work on the H. parallela olfactory mechanism and selection of target genes for integrative control of H. parallela.

Identification and Characterization of an Antennae-Specific Glutathione S-Transferase From the Indian Meal Moth

Insect glutathione-S-transferases (GSTs) play essential roles in metabolizing endogenous and exogenous compounds. GSTs that are uniquely expressed in antennae are assumed to function as scavengers of pheromones and host volatiles in the odorant detection system. Based on this assumption, antennae-specific GSTs have been identified and functionally characterized in increasing number of insect species. In the present study, 17 putative GSTs were identified from the antennal transcriptomic dataset of the Indian meal moth, Plodia interpunctella, a severe stored-grain pest worldwide. Among the GSTs, only PiGSTd1 is antennae-specific according to both Fragments Per Kilobase Million (FPKM) and quantitative real-time PCR (qRT-PCR) analysis. Sequence analysis revealed that PiGSTd1 has a similar identity as many delta GSTs from other moths. Enzyme kinetic assays using 1-chloro-2,4-dinitrobenzene (CDNB) as substrates showed that the recombinant PiGSTd1 gave a K_m of 0.2292 ± 0.01805 mM and a V_max of 14.02 ± 0.2545 μmol·mg⁻¹·min⁻¹ under the optimal catalytic conditions (35°C and pH = 7.5). Further analysis revealed that the recombinant PiGSTd1 could efficiently degrade the sex pheromone component Z9-12:Ac (75.63 ± 5.52%), as well as aldehyde volatiles, including hexanal (89.10 ± 2.21%), heptanal (63.19 ± 5.36%), (E)-2-octenal (73.58 ± 3.92%), (E)-2-nonenal (75.81 ± 1.90%), and (E)-2-decenal (61.13 ± 5.24%). Taken together, our findings suggest that PiGSTd1 may play essential roles in degrading and inactivating a variety of odorants, especially sex pheromones and host volatiles of P. interpunctella.

Mating increases Drosophila melanogaster females' choosiness by reducing olfactory sensitivity to a male pheromone

Females that are highly selective when choosing a mate run the risk of remaining unmated or delaying commencing reproduction. Therefore, low female choosiness would be beneficial when males are rare but it would be maladaptive if males become more frequent. How can females resolve this issue? Polyandry would allow mating-status-dependent choosiness, with virgin females selecting their first mate with little selectivity and becoming choosier thereafter. This plasticity in choosiness would ensure timely acquisition of sperm and enable females to increase offspring quality during later mating. Here, we show that Drosophila melanogaster females display such mating-status-dependent choosiness by becoming more selective once mated and identify the underlying neurohormonal mechanism. Mating releases juvenile hormone, which desensitizes Or47b olfactory neurons to a pheromone produced by males, resulting in increased preference for pheromone-rich males. Besides providing a mechanism to a long-standing evolutionary prediction, these data suggest that intersexual selection in D. melanogaster, and possibly in all polyandrous, sperm-storing species, is mainly the domain of mated females since virgin females are less selective. Juvenile hormone influences behaviour by changing cue responsiveness across insects; the neurohormonal modulation of olfactory neurons uncovered in D. melanogaster provides an explicit mechanism for how this hormone modulates behavioural plasticity.

Knockdown of UGT352A5 decreases the thiamethoxam resistance in Bemisia tabaci (Hemiptera: Gennadius)

Uridine diphosphate (UDP)-glycosyltransferases (UGTs), which are major phase II detoxification enzymes, have been implicated in the glycosylation of lipophilic endobiotics and xenobiotics and thus potentially lead to the evolution of insecticide resistance. In this study, we identified and cloned two putative UGT genes from transcriptome data which are named UGT352A4 and UGT352A5. As demonstrated by qRT-PCR, two UGT genes were over-expressed in the thiamethoxam-resistant (THQR) strain relative to the susceptible (THQS) strain. Moreover, the induction experiment revealed that the expression of the UGT352A5 gene was significantly increased following exposure to thiamethoxam in the THQR strain. Furthermore, the expression of both UGT352A4 and UGT352A5 was downregulated after RNA interference, whereas only the silencing of UGT352A5 resulted in a noticeable increase in the mortality of THQR adults. Our results represent the first line of evidence showing that UGT352A5 might be responsible for conferring thiamethoxam resistance in B. tabaci. The results will be shed new insights for obtaining a better understanding of the role of UGTs in the evolution of insecticide resistance and developing new insect resistance management tactics within the sustainable integrated pest management framework.

Comparative analysis of mite genomes reveals positive selection for diet adaptation

Diet is a powerful evolutionary force for species adaptation and diversification. Acari is one of the most abundant clades of Arachnida, exhibiting diverse dietary types, while the underlying genetic adaptive mechanisms are not fully understood. Based on comparative analyses of 15 Acari genomes, we found genetic bases for three specialized diets. Herbivores experienced stronger selection pressure than other groups; the olfactory genes and gene families involving metabolizing toxins showed strong adaptive signals. Genes and gene families related to anticoagulation, detoxification, and haemoglobin digestion were found to be under strong selection pressure or significantly expanded in the blood-feeding species. Lipid metabolism genes have a faster evolutionary rate and been subjected to greater selection pressures in fat-feeding species; one positively selected site in the fatty-acid amide hydrolases 2 gene was identified. Our research provides a new perspective for the evolution of Acari and offers potential target loci for novel pesticide development.

Interactions Between Odorants and Glutathione Transferases in the Human Olfactory Cleft

Xenobiotic metabolizing enzymes and other proteins, including odorant-binding proteins located in the nasal epithelium and mucus, participate in a series of processes modulating the concentration of odorants in the environment of olfactory receptors (ORs) and finely impact odor perception. These enzymes and transporters are thought to participate in odorant degradation or transport. Odorant biotransformation results in 1) changes in the odorant quantity up to their clearance and the termination of signaling and 2) the formation of new odorant stimuli (metabolites). Enzymes, such as cytochrome P450 and glutathione transferases (GSTs), have been proposed to participate in odorant clearance in insects and mammals as odorant metabolizing enzymes. This study aims to explore the function of GSTs in human olfaction. Using immunohistochemical methods, GSTs were found to be localized in human tissues surrounding the olfactory epithelium. Then, the activity of 2 members of the GST family toward odorants was measured using heterologously expressed enzymes. The interactions/reactions with odorants were further characterized using a combination of enzymatic techniques. Furthermore, the structure of the complex between human GSTA1 and the glutathione conjugate of an odorant was determined by X-ray crystallography. Our results strongly suggest the role of human GSTs in the modulation of odorant availability to ORs in the peripheral olfactory process.

The UDP-Glycosyltransferase Family in Drosophila melanogaster: Nomenclature Update, Gene Expression and Phylogenetic Analysis

UDP-glycosyltransferases (UGTs) are important conjugation enzymes found in all kingdoms of life, catalyzing a sugar conjugation with small lipophilic compounds and playing a crucial role in detoxification and homeostasis. The UGT gene family is defined by a signature motif in the C-terminal domain where the uridine diphosphate (UDP)-sugar donor binds. UGTs have been identified in a number of insect genomes over the last decade and much progress has been achieved in characterizing their expression patterns and molecular functions. Here, we present an update of the complete repertoire of UGT genes in Drosophila melanogaster and provide a brief overview of the latest research in this model insect. A total of 35 UGT genes are found in the D. melanogaster genome, localized to chromosomes 2 and 3 with a high degree of gene duplications on the chromosome arm 3R. All D. melanogaster UGT genes have now been named in FlyBase according to the unified UGT nomenclature guidelines. A phylogenetic analysis of UGT genes shows lineage-specific gene duplications. Analysis of anatomical and induced gene expression patterns demonstrate that some UGT genes are differentially expressed in various tissues or after environmental treatments. Extended searches of UGT orthologs from 18 additional Drosophila species reveal a diversity of UGT gene numbers and composition. The roles of Drosophila UGTs identified to date are briefly reviewed, and include xenobiotic metabolism, nicotine resistance, olfaction, cold tolerance, sclerotization, pigmentation, and immunity. Together, the updated genomic information and research overview provided herein will aid further research in this developing field.

Putative carboxylesterase gene identification and their expression patterns in Hyphantria cunea (Drury)

The olfactory system of insects is important for behavioral activities as it recognizes internal and external volatile stimuli in the environment. Insect odorant degrading enzymes (ODEs), including antennal-specific carboxylesterases (CXEs), are known to degrade redundant odorant molecules or to hydrolyze important olfactory sex pheromone components and plant volatiles. Compared to many well-studied Type-I sex pheromone-producing lepidopteran species, the molecular mechanisms of the olfactory system of Type-II sex pheromone-producing Hyphantria cunea (Drury) remain poorly understood. In the current study, we first identified a total of ten CXE genes based on our previous H. unea antennal transcriptomic data. We constructed a phylogenetic tree to evaluate the relationship of HcunCXEs with other insects’ CXEs, and used quantitative PCR to investigate the gene expression of H. cunea CXEs (HcunCXEs). Our results indicate that HcunCXEs are highly expressed in antennae, legs and wings, suggesting a potential function in degrading sex pheromone components, host plant volatiles, and other xenobiotics. This study not only provides a theoretical basis for subsequent olfactory mechanism studies on H. cunea, but also offers some new insights into functions and evolutionary characteristics of CXEs in lepidopteran insects. From a practical point of view, these HcunCXEs might represent meaningful targets for developing behavioral interference control strategies against H. cunea.

Olfactory coding in honeybees

With less than a million neurons, the western honeybee Apis mellifera is capable of complex olfactory behaviors and provides an ideal model for investigating the neurophysiology of the olfactory circuit and the basis of olfactory perception and learning. Here, we review the most fundamental aspects of honeybee's olfaction: first, we discuss which odorants dominate its environment, and how bees use them to communicate and regulate colony homeostasis; then, we describe the neuroanatomy and the neurophysiology of the olfactory circuit; finally, we explore the cellular and molecular mechanisms leading to olfactory memory formation. The vastity of histological, neurophysiological, and behavioral data collected during the last century, together with new technological advancements, including genetic tools, confirm the honeybee as an attractive research model for understanding olfactory coding and learning.

Molecular Mechanism of the UDP-Glucuronosyltransferase 2B20-like Gene (AccUGT2B20-like) in Pesticide Resistance of Apis cerana cerana

UDP-glucuronosyltransferases (UGTs), being multifunctional detoxification enzymes, play a major role in the process of resistance to various pesticides in insects. However, the mechanism underlying the molecular regulation of pesticide resistance remains unclear, especially in Apis cerana cerana. In this study, all of the UGTs in Apis cerana cerana (AccUGT) have been identified through the multiple alignment and phylogenetic analysis. Expression of AccUGT genes under different pesticides, and antioxidant genes after silencing of AccUGT2B20-like, were detected by qRT-PCR. The resistance of overexpressed AccUGT2B20-like to oxidative stress was investigated by an Escherichia coli overexpression system. Also, antioxidant-related enzyme activity was detected after silencing of the AccUGT2B20-like gene. Expression pattern analysis showed that almost all UGT genes were upregulated under different pesticide treatments. This result indicated that AccUGTs participate in the detoxification process of pesticides. AccUGT2B20-like was the major gene because it was more highly induced than the others. Overexpression of AccUGT2B20-like in E. coli could effectively improve oxidative stress resistance. Specifically, silencing the AccUGT2B20-like gene increased oxidative stress by repressing the expression of oxidation-related genes, decreasing antioxidant-related enzyme activity, and increasing malondialdehyde concentration. Taken together, our results indicate that AccUGTs are involved in pesticide resistance, among which, AccUGT2B20-like contributes to the detoxification of pesticides by eliminating oxidative stress in Apis cerana cerana. This study explains the molecular basis for the resistance of bees to pesticides and provides an important safeguard for maintaining ecological balance.

Chemosensory-Related Gene Family Members of the Horn Fly, Haematobia irritans irritans (Diptera: Muscidae), Identified by Transcriptome Analysis

Horn flies are one of the most significant economic pests of cattle in the United States and worldwide. Chemical control methods have been routinely utilized to reduce populations of this pest, but the steady development of insecticide resistance has prompted evaluation of alternative control strategies. Behavior modifying compounds from natural products have shown some success in impacting horn fly populations, and a more thorough understanding of the horn fly chemosensory system would enable improvements in the development of species-specific compounds. Using an RNA-seq approach, we assembled a transcriptome representing genes expressed in adult female and male horn fly head appendages (antennae, maxillary palps, and proboscides) and adult fly bodies from which heads were removed. Differential gene expression analysis identified chemosensory gene family members that were enriched in head appendage tissues compared with headless bodies. Candidate members included 43 odorant binding proteins (OBP) and 5 chemosensory binding proteins (CSP), as well as 44 odorant receptors (OR), 27 gustatory receptors (GR), and 34 ionotropic receptors (IR). Sex-biased expression of these genes was not observed. These findings provide a resource to enable future studies targeting horn fly chemosensation as part of an integrated strategy to control this blood-feeding pest.

Construction and analysis of the protein-protein interaction network for the olfactory system of the silkworm Bombyx mori

Olfaction plays an essential role in feeding and information exchange in insects. Previous studies on the olfaction of silkworms have provided a wealth of information about genes and proteins, yet, most studies have only focused on a single gene or protein related to the insect's olfaction. The aim of the current study is to determine key proteins in the olfactory system of the silkworm, and further understand protein-protein interactions (PPIs) in the olfactory system of Lepidoptera. To achieve this goal, we integrated Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG), and network analyses. Furthermore, we selected 585 olfactory-related proteins and constructed a (PPI) network for the olfactory system of the silkworm. Network analysis led to the identification of several key proteins, including GSTz1, LOC733095, BGIBMGA002169-TA, BGIBMGA010939-TA, GSTs2, GSTd2, Or-2, and BGIBMGA013255-TA. A comprehensive evaluation of the proteins showed that glutathione S-transferases (GSTs) had the highest ranking. GSTs also had the highest enrichment levels in GO and KEGG. In conclusion, our analysis showed that key nodes in the biological network had a significant impact on the network, and the key proteins identified via network analysis could serve as new research targets to determine their functions in olfaction.

Molecular mechanisms of olfactory detection in insects: beyond receptors

Insects thrive in diverse ecological niches in large part because of their highly sophisticated olfactory systems. Over the last two decades, a major focus in the study of insect olfaction has been on the role of olfactory receptors in mediating neuronal responses to environmental chemicals. In vivo, these receptors operate in specialized structures, called sensilla, which comprise neurons and non-neuronal support cells, extracellular lymph fluid and a precisely shaped cuticle. While sensilla are inherent to odour sensing in insects, we are only just beginning to understand their construction and function. Here, we review recent work that illuminates how odour-evoked neuronal activity is impacted by sensillar morphology, lymph fluid biochemistry, accessory signalling molecules in neurons and the physiological crosstalk between sensillar cells. These advances reveal multi-layered molecular and cellular mechanisms that determine the selectivity, sensitivity and dynamic modulation of odour-evoked responses in insects.

Sex pheromones and olfactory proteins in Antheraea moths: A. pernyi and A. polyphemus (Lepidoptera: Saturniidae)

Olfaction is essential for regulating the physiological and behavioral actions of insects with specific recognition of various odors. Antheraea moths (Lepidoptera: Saturniidae) possess relatively large bodies and antennae so that they are good subjects for exploring molecular aspects of insect olfaction. Current knowledge of the molecular aspects of Antheraea olfaction is focused on the Chinese tussah silkmoth A. pernyi Guérin-Méneville and another species A. polyphemus (Cramer) in their pheromones, odorant-binding proteins (OBPs), odorant receptors (ORs), odorant receptor coreceptors (ORCOs), sensory neuron membrane proteins (SNMPs), and odorant-degrading enzymes (ODEs). The first insect OBP, SNMP, and ODE were identified from A. polyphemus. This review summarizes the principal findings associated with the olfactory physiology and its molecular components in the two Antheraea species. Three types of olfactory neurons may have specific ORs for three respective sex-pheromone components, with the functional sensitivity and specificity mediated by three respective OBPs. SNMPs and ODEs are likely to play important roles in sex-pheromone detection, inactivation, and degradation. Identification and functional analysis of the olfactory molecules remain to be further performed in A. pernyi, A. polyphemus, and other Antheraea species.

Antennal Proteome of the Solenopsis invicta (Hymenoptera: Formicidae): Caste Differences in Olfactory Receptors and Chemosensory Support Proteins

Little is known about the expression pattern of odorant and pheromone transporters, receptors, and deactivation enzymes in the antennae of ants carrying out different tasks. In order to begin filling in this information gap, we compared the proteomes of the antennae of workers and males of the red fire ant, Solenopsis invicta Buren (Hymenoptera: Formicidae). Male ants do not perform any colony work, and their only activity is to leave the nest on a mating flight. Previous studies showed that male ants express fewer types of odorant receptors than workers. Thus, we expected to find large differences between male and worker antennae for expression of receptors, transporters, and deactivators of signaling chemicals. We found that the abundance of receptors was consistent with the expected caste-specific signaling complexity, but the numbers of different antenna-specific transporters and deactivating enzymes in males and workers were similar. It is possible that some of these proteins have antenna-specific functions that are unrelated to chemosensory reception. Alternatively, the similar complexity could be a vestige of ant progenitors that had more behaviorally active males. As the reduced behavior of male ants evolved, the selection process may have favored a complex repertoire of transporters and deactivating enzymes alongside a limited repertoire of odorant receptors.

Sex-biased gene expression in antennae of Drosophila suzukii

Drosophila suzukii differs from other members of the genus Drosophila in its host preference and oviposition behavior. The flies are attracted to ripening fruits, and females have a serrated ovipositor enabling eggs to be laid inside the fruit. In addition to its huge economic impact, its unique chemoecological, morphological, and physiological characteristics have garnered considerable research interests. In this study, we analyzed D. suzukii antennal transcriptomes to identify sex-biased genes by comparison of differential gene expressions between male antennae (MA) and female antennae (FA). Among 13,583 total genes of the fly genome, 11,787 genes were expressed in either MA or FA. There are only 132 genes (9 in MA, 7 in FA, and 116 in both, FPKM >1) were expressed in antennae exclusively, and 2,570 genes (9 in MA, 0 in FA, and 2,561 in both) were enriched in antennae containing 185 and 113 sex-biased genes in MA and FA, respectively. Interestingly, many immune-related genes were highly expressed in MA, whereas several chemosensory genes were at high rank in FA. We identified 27 sex-biased chemosensory genes including odorant and gustatory receptors, odorant-binding proteins, chemosensory proteins, ionotropic receptors, and cytochrome P450s, and validated the gene expressions using quantitative real-time PCR. The highly expressed sex-biased genes in antennae are likely involved in the fly specific mating, host-finding behaviors, or sex-specific functions. The molecular results demonstrated here will facilitate to find the unique chemoreception of D. suzukii, as well as on the development of new management strategies for this pest.

Behavioral and Transcriptional Response to Selection for Olfactory Behavior in Drosophila

The detection, discrimination, and behavioral responses to chemical cues in the environment can have marked effects on organismal survival and reproduction, eliciting attractive or aversive behavior. To gain insight into mechanisms mediating this hedonic valence, we applied thirty generations of divergent artificial selection for Drosophila melanogaster olfactory behavior. We independently selected for positive and negative behavioral responses to two ecologically relevant chemical compounds: 2,3-butanedione and cyclohexanone. We also tested the correlated responses to selection by testing behavioral responses to other odorants and life history traits. Measurements of behavioral responses of the selected lines and unselected controls to additional odorants showed that the mechanisms underlying responses to these odorants are, in some cases, differentially affected by selection regime and generalization of the response to other odorants was only detected in the 2,3-butanedione selection lines. Food consumption and lifespan varied with selection regime and, at times, sex. An analysis of gene expression of both selection regimes identified multiple differentially expressed genes. New genes and genes previously identified in mediating olfactory behavior were identified. In particular, we found functional enrichment of several gene ontology terms, including cell-cell adhesion and sulfur compound metabolic process, the latter including genes belonging to the glutathione S-transferase family. These findings highlight a potential role for glutathione S-transferases in the evolution of hedonic valence to ecologically relevant volatile compounds and set the stage for a detailed investigation into mechanisms by which these genes mediate attraction and aversion.