Odorant-Degrading Enzymes (ODEs) are supposed to be involved in the signal inactivation step within the olfactory sensilla of insects by quickly removing odorant molecules from the vicinity of the olfactory receptors. Only three ODEs have been both identified at the molecular level and functionally characterized: two were specialized in the degradation of pheromone compounds and the last one was shown to degrade a plant odorant.
Previous work has shown that the antennae of the cotton leafworm Spodoptera littoralis, a worldwide pest of agricultural crops, express numerous candidate ODEs. We focused on an esterase overexpressed in males antennae, namely SlCXE7. We studied its expression patterns and tested its catalytic properties towards three odorants, i.e. the two female sex pheromone components and a green leaf volatile emitted by host plants.
SlCXE7 expression was concomitant during development with male responsiveness to odorants and during adult scotophase with the period of male most active sexual behaviour. Furthermore, SlCXE7 transcription could be induced by male exposure to the main pheromone component, suggesting a role of Pheromone-Degrading Enzyme. Interestingly, recombinant SlCXE7 was able to efficiently hydrolyze the pheromone compounds but also the plant volatile, with a higher affinity for the pheromone than for the plant compound. In male antennae, SlCXE7 expression was associated with both long and short sensilla, tuned to sex pheromones or plant odours, respectively. Our results thus suggested that a same ODE could have a dual function depending of it sensillar localisation. Within the pheromone-sensitive sensilla, SlCXE7 may play a role in pheromone signal termination and in reduction of odorant background noise, whereas it could be involved in plant odorant inactivation within the short sensilla.
Pheromone-degrading enzymes (PDEs) are supposed to be involved in the signal inactivation step within the olfactory sensilla of insects by quickly degrading pheromone molecules. Because esters are widespread insect pheromone components, PDEs belonging to the carboxylesterase (CCE) family have been the most studied. However, only two CCEs were both identified at the molecular level and functionally characterized as PDEs until recently. In the pest moth Spodoptera littoralis, we have identified an unsuspected diversity of antennal CCEs, with a total number of 30 genes. Two CCEs, enriched in antennae and belonging to distinct clades, were shown to present different substrate specificities toward pheromone and plant compounds. A same CCE was also shown to efficiently degrade both pheromone and plant components. Our results suggest that the structural evolution of antennal CCEs reflects their functional diversity and that a complex set of CCE-mediated reactions take place is the olfactory organs of moths.
lepidoptera; noctuidae; olfaction; carboxylesterase; odorant-degrading enzymes; pheromone
Insects respond to the spatial and temporal dynamics of a pheromone plume, which implies not only a strong response to 'odor on', but also to 'odor off'. This requires mechanisms geared toward a fast signal termination. Several mechanisms may contribute to signal termination, among which odorant-degrading enzymes. These enzymes putatively play a role in signal dynamics by a rapid inactivation of odorants in the vicinity of the sensory receptors, although direct in vivo experimental evidences are lacking. Here we verified the role of an extracellular carboxylesterase, esterase-6 (Est-6), in the sensory physiological and behavioral dynamics of Drosophila melanogaster response to its pheromone, cis-vaccenyl acetate (cVA). Est-6 was previously linked to post-mating effects in the reproductive system of females. As Est-6 is also known to hydrolyze cVA in vitro and is expressed in the main olfactory organ, the antenna, we tested here its role in olfaction as a putative odorant-degrading enzyme.
We first confirm that Est-6 is highly expressed in olfactory sensilla, including cVA-sensitive sensilla, and we show that expression is likely associated with non-neuronal cells. Our electrophysiological approaches show that the dynamics of olfactory receptor neuron (ORN) responses is strongly influenced by Est-6, as in Est-6° null mutants (lacking the Est-6 gene) cVA-sensitive ORN showed increased firing rate and prolonged activity in response to cVA. Est-6° mutant males had a lower threshold of behavioral response to cVA, as revealed by the analysis of two cVA-induced behaviors. In particular, mutant males exhibited a strong decrease of male-male courtship, in association with a delay in courtship initiation.
Our study presents evidence that Est-6 plays a role in the physiological and behavioral dynamics of sex pheromone response in Drosophila males and supports a role of Est-6 as an odorant-degrading enzyme (ODE) in male antennae. Our results also expand the role of Est-6 in Drosophila biology, from reproduction to olfaction, and highlight the role of ODEs in insect olfaction.
carboxylesterase; esterase 6; olfaction; pheromone; signal termination
Antennae-specific odorant-degrading enzymes (ODEs) are postulated to inactivate odorant molecules after they convey their signal. Different classes of insect ODEs are specific to esters, alcohols, and aldehydes – the major functional groups of female-produced, hydrophobic sex pheromones from moth species. Esterases that rapidly inactive acetate and other esters have been well-studied, but less is known about aldehyde oxidases (AOXs). Here we report cloning of an aldehyde oxidase, AtraAOX2, from the antennae of the navel orangeworm (NOW), Amyelois transitella, and the first activity characterization of a recombinant insect AOX. AtraAOX2 gene spans 3,813 bp and encodes a protein with 1,270 amino acid residues. AtraAOX2 cDNA was expressed in baculovirus-infected insect Sf21 cells as a ≈280 kDa homodimer with 140 kDa subunits. Recombinant AtraAOX2 degraded Z11Z13–16Ald and plant volatile aldehydes as substrates. However, as expected for aldehyde oxidases, recombinant AtraAOX2 did not show specificity for Z11Z13–16Ald, the main constituent of the sex pheromone, but showed high activity for plant volatile aldehydes. Our data suggest AtraAOX2 might be involved in degradation of a diversity of aldehydes including sex pheromones, plant-derived semiochemicals, and chemical cues for oviposition sites. Additionally, AtraAOX2 could protect the insect's olfactory system from xenobiotics, including pesticides that might reach the sensillar lymph surrounding the olfactory receptor neurons.
One of the challenges in insect chemical ecology is to understand how insect pheromones are synthesised, detected and degraded. Genome wide survey by comparative sequencing and gene specific expression profiling provide rich resources for this challenge. A. ipsilon is a destructive pest of many crops and further characterization of the genes involved in pheromone biosynthesis and transport could offer potential targets for disruption of their chemical communication and for crop protection.
Here we report 454 next-generation sequencing of the A. ipsilon pheromone gland transcriptome, identification and expression profiling of genes putatively involved in pheromone production, transport and degradation. A total of 23473 unigenes were obtained from the transcriptome analysis, 86% of which were A. ipsilon specific. 42 transcripts encoded enzymes putatively involved in pheromone biosynthesis, of which 15 were specifically, or mainly, expressed in the pheromone glands at 5 to 120-fold higher levels than in the body. Two transcripts encoding for a fatty acid synthase and a desaturase were highly abundant in the transcriptome and expressed more than 40-fold higher in the glands than in the body. The transcripts encoding for 2 acetyl-CoA carboxylases, 1 fatty acid synthase, 2 desaturases, 3 acyl-CoA reductases, 2 alcohol oxidases, 2 aldehyde reductases and 3 acetyltransferases were expressed at a significantly higher level in the pheromone glands than in the body. 17 esterase transcripts were not gland-specific and 7 of these were expressed highly in the antennae. Seven transcripts encoding odorant binding proteins (OBPs) and 8 encoding chemosensory proteins (CSPs) were identified. Two CSP transcripts (AipsCSP2, AipsCSP8) were highly abundant in the pheromone gland transcriptome and this was confirmed by qRT-PCR. One OBP (AipsOBP6) were pheromone gland-enriched and three OBPs (AipsOBP1, AipsOBP2 and AipsOBP4) were antennal-enriched. Based on these studies we proposed possible A. ipsilon biosynthesis pathways for major and minor sex pheromone components.
Our study identified genes potentially involved in sex pheromone biosynthesis and transport in A. ipsilon. The identified genes are likely to play essential roles in sex pheromone production, transport and degradation and could serve as targets to interfere with pheromone release. The identification of highly expressed CSPs and OBPs in the pheromone gland suggests that they may play a role in the binding, transport and release of sex pheromones during sex pheromone production in A. ipsilon and other Lepidoptera insects.
Nocturnal insects such as moths are ideal models to study the molecular bases of olfaction that they use, among examples, for the detection of mating partners and host plants. Knowing how an odour generates a neuronal signal in insect antennae is crucial for understanding the physiological bases of olfaction, and also could lead to the identification of original targets for the development of olfactory-based control strategies against herbivorous moth pests. Here, we describe an Expressed Sequence Tag (EST) project to characterize the antennal transcriptome of the noctuid pest model, Spodoptera littoralis, and to identify candidate genes involved in odour/pheromone detection.
By targeting cDNAs from male antennae, we biased gene discovery towards genes potentially involved in male olfaction, including pheromone reception. A total of 20760 ESTs were obtained from a normalized library and were assembled in 9033 unigenes. 6530 were annotated based on BLAST analyses and gene prediction software identified 6738 ORFs. The unigenes were compared to the Bombyx mori proteome and to ESTs derived from Lepidoptera transcriptome projects. We identified a large number of candidate genes involved in odour and pheromone detection and turnover, including 31 candidate chemosensory receptor genes, but also genes potentially involved in olfactory modulation.
Our project has generated a large collection of antennal transcripts from a Lepidoptera. The normalization process, allowing enrichment in low abundant genes, proved to be particularly relevant to identify chemosensory receptors in a species for which no genomic data are available. Our results also suggest that olfactory modulation can take place at the level of the antennae itself. These EST resources will be invaluable for exploring the mechanisms of olfaction and pheromone detection in S. littoralis, and for ultimately identifying original targets to fight against moth herbivorous pests.
Most animals rely on olfaction to find sexual partners, food or a habitat. The olfactory system faces the challenge of extracting meaningful information from a noisy odorous environment. In most moth species, males respond to sex pheromone emitted by females in an environment with abundant plant volatiles. Plant odours could either facilitate the localization of females (females calling on host plants), mask the female pheromone or they could be neutral without any effect on the pheromone. Here we studied how mixtures of a behaviourally-attractive floral odour, heptanal, and the sex pheromone are encoded at different levels of the olfactory pathway in males of the noctuid moth Agrotis ipsilon. In addition, we asked how interactions between the two odorants change as a function of the males' mating status. We investigated mixture detection in both the pheromone-specific and in the general odorant pathway. We used a) recordings from individual sensilla to study responses of olfactory receptor neurons, b) in vivo calcium imaging with a bath-applied dye to characterize the global input response in the primary olfactory centre, the antennal lobe and c) intracellular recordings of antennal lobe output neurons, projection neurons, in virgin and newly-mated males. Our results show that heptanal reduces pheromone sensitivity at the peripheral and central olfactory level independently of the mating status. Contrarily, heptanal-responding olfactory receptor neurons are not influenced by pheromone in a mixture, although some post-mating modulation occurs at the input of the sexually isomorphic ordinary glomeruli, where general odours are processed within the antennal lobe. The results are discussed in the context of mate localization.
Insect chemosensory proteins (CSPs) have been proposed to capture and transport hydrophobic chemicals from air to olfactory receptors in the lymph of antennal chemosensilla. They may represent a new class of soluble carrier protein involved in insect chemoreception. However, their specific functional roles in insect chemoreception have not been fully elucidated. In this study, we report for the first time three novel CSP genes (AlinCSP1-3) of the alfalfa plant bug Adelphocoris lineolatus (Goeze) by screening the antennal cDNA library. The qRT-PCR examinations of the transcript levels revealed that all three genes (AlinCSP1-3) are mainly expressed in the antennae. Interestingly, these CSP genes AlinCSP1-3 are also highly expressed in the 5th instar nymphs, suggesting a proposed function of these CSP proteins (AlinCSP1-3) in the olfactory reception and in maintaining particular life activities into the adult stage. Using bacterial expression system, the three CSP proteins were expressed and purified. For the first time we characterized the types of sensilla in the antennae of the plant bug using scanning electron microscopy (SEM). Immunocytochemistry analysis indicated that the CSP proteins were expressed in the pheromone-sensitive sensilla trichodea and general odorant-sensitive sensilla basiconica, providing further evidence of their involvement in chemoreception. The antennal activity of 55 host-related semiochemicals and sex pheromone compounds in the host location and mate selection behavior of A. lineolatus was investigated using electroantennogram (EAG), and the binding affinities of these chemicals to the three CSPs (AlinCSP1-3) were measured using fluorescent binding assays. The results showed several host-related semiochemicals, (Z)-3-hexen-1-ol, (E)-2-hexen-1-al and valeraldehyde, have a high binding affinity with AlinCSP1-3 and can elicit significant high EAG responses of A. lineolatus antennae. Our studies indicate the three antennae-biased CSPs may mediate host recognition in the alfalfa plant bug A. lineolatus.
In many insects, mate finding relies on female-released sex pheromones, which have to be deciphered by the male olfactory system within an odorous background of plant volatiles present in the environment of a calling female. With respect to pheromone-mediated mate localization, plant odorants may be neutral, favorable, or disturbing. Here we examined the impact of plant odorants on detection and coding of the major sex pheromone component, (Z)-11-hexadecenal (Z11-16:Ald) in the noctuid moth Heliothis virescens. By in vivo imaging the activity in the male antennal lobe (AL), we monitored the interference at the level of olfactory sensory neurons (OSN) to illuminate mixture interactions. The results show that stimulating the male antenna with Z11-16:Ald and distinct plant-related odorants simultaneously suppressed pheromone-evoked activity in the region of the macroglomerular complex (MGC), where Z11-16:Ald-specific OSNs terminate. Based on our previous findings that antennal detection of Z11-16:Ald involves an interplay of the pheromone binding protein (PBP) HvirPBP2 and the pheromone receptor (PR) HR13, we asked if the plant odorants may interfere with any of the elements involved in pheromone detection. Using a competitive fluorescence binding assay, we found that the plant odorants neither bind to HvirPBP2 nor affect the binding of Z11-16:Ald to the protein. However, imaging experiments analyzing a cell line that expressed the receptor HR13 revealed that plant odorants significantly inhibited the Z11-16:Ald-evoked calcium responses. Together the results indicate that plant odorants can interfere with the signaling process of the major sex pheromone component at the receptor level. Consequently, it can be assumed that plant odorants in the environment may reduce the firing activity of pheromone-specific OSNs in H. virescens and thus affect mate localization.
pheromone detection; antennal lobe; pheromone receptor; pheromone binding protein; olfaction
Carboxylesterase is a multifunctional superfamily and ubiquitous in all living organisms, including animals, plants, insects, and microbes. It plays important roles in xenobiotic detoxification, and pheromone degradation, neurogenesis and regulating development. Previous studies mainly used Dipteran Drosophila and mosquitoes as model organisms to investigate the roles of the insect COEs in insecticide resistance. However, genome-wide characterization of COEs in phytophagous insects and comparative analysis remain to be performed.
Based on the newly assembled genome sequence, 76 putative COEs were identified in Bombyx mori. Relative to other Dipteran and Hymenopteran insects, alpha-esterases were significantly expanded in the silkworm. Genomics analysis suggested that BmCOEs showed chromosome preferable distribution and 55% of which were tandem arranged. Sixty-one BmCOEs were transcribed based on cDNA/ESTs and microarray data. Generally, most of the COEs showed tissue specific expressions and expression level between male and female did not display obvious differences. Three main patterns could be classified, i.e. midgut-, head and integument-, and silk gland-specific expressions. Midgut is the first barrier of xenobiotics peroral toxicity, in which COEs may be involved in eliminating secondary metabolites of mulberry leaves and contaminants of insecticides in diet. For head and integument-class, most of the members were homologous to odorant-degrading enzyme (ODE) and antennal esterase. RT-PCR verified that the ODE-like esterases were also highly expressed in larvae antenna and maxilla, and thus they may play important roles in degradation of plant volatiles or other xenobiotics.
B. mori has the largest number of insect COE genes characterized to date. Comparative genomic analysis suggested that the gene expansion mainly occurred in silkworm alpha-esterases. Expression evidence indicated that the expanded genes were specifically expressed in midgut, integument and head, implying that these genes may have important roles in detoxifying secondary metabolites of mulberry leaves, contaminants in diet, and odorants. Our results provide some new insights into functions and evolutionary characteristics of COEs in phytophagous insects.
Insects rely on olfaction to locate food, mates, and suitable oviposition sites for successful completion of their life cycle. Agrilus planipennis Fairmaire (emerald ash borer) is a serious invasive insect pest that has killed tens of millions of North American ash (Fraxinus spp) trees and threatens the very existence of the genus Fraxinus. Adult A. planipennis are attracted to host volatiles and conspecifics; however, to date no molecular knowledge exists on olfaction in A. planipennis. Hence, we undertook an antennae-specific transcriptomic study to identify the repertoire of odor processing genes involved in A. planipennis olfaction.
Methodology and Principal Findings
We acquired 139,085 Roche/454 GS FLX transcriptomic reads that were assembled into 30,615 high quality expressed sequence tags (ESTs), including 3,249 isotigs and 27,366 non-isotigs (contigs and singletons). Intriguingly, the majority of the A. planipennis antennal transcripts (59.72%) did not show similarity with sequences deposited in the non-redundant database of GenBank, potentially representing novel genes. Functional annotation and KEGG analysis revealed pathways associated with signaling and detoxification. Several odor processing genes (9 odorant binding proteins, 2 odorant receptors, 1 sensory neuron membrane protein and 134 odorant/xenobiotic degradation enzymes, including cytochrome P450s, glutathione-S-transferases; esterases, etc.) putatively involved in olfaction processes were identified. Quantitative PCR of candidate genes in male and female A. planipennis in different developmental stages revealed developmental- and sex-biased expression patterns.
Conclusions and Significance
The antennal ESTs derived from A. planipennis constitute a rich molecular resource for the identification of genes potentially involved in the olfaction process of A. planipennis. These findings should help in understanding the processing of antennally-active compounds (e.g. 7-epi-sesquithujene) previously identified in this serious invasive pest.
Odorant binding proteins (OBPs) play important roles in insect olfactory processes. The Chinese pine caterpillar moth, Dendrolimus tabulaeformis (Lepidoptera, Lasiocampidae) is a serious economic pest in China, and the pheromones of this species have been identified to monitor their presence. However, the molecular mechanisms by which D. tabulaeformis perceive pheromones and host volatiles remain unknown. In this study, we identified and characterized three new OBPs, including one pheromone binding protein (PBP1) and two general odor binding proteins (GOBPs), from antennal cDNA of D. tabulaeformis. The deduced amino acid sequences of DtabPBP1, DtabGOBP1, and DtabGOBP2 revealed mature proteins of 140, 147, and 140 amino acids, respectively. Each has six cysteine residues in conserved positions relative to other known OBPs. Amino-acid alignments indicated that the two GOBPs are more conserved (DtabGOBP1 is 52.9–67.4 % identical to orthologs from other Lepidoptera, and DtabGOBP2 is 55.2–81.8 % identical) than the PBP (32.5–46.0 %). Real-time PCR indicated tissue- and sex-specific expression patterns of the three genes. DtabPBP1 was mainly expressed in the antennae of males, whereas female antennae had only 1.09 % the expression in male antennae. Both DtabGOBP1 and DtabGOBP2 were more highly expressed in antennae than in other tissues, while DtabGOBP1 was more abundant in male antennae and DtabGOBP2 in female antennae. In addition, the binding specificities of the three proteins were investigated, and all three OBPs exhibited high binding affinities for the pheromone component (5Z,7E)-5,7-dodecadien-1-yl propionate (Z5,E7-12:OPr). This suggests a role in binding pheromone for GOBPs, as well as PBP1, in D. tabulaeformis.
Electronic supplementary material
The online version of this article (doi:10.1007/s10886-014-0412-6) contains supplementary material, which is available to authorized users.
Pheromone binding proteins; General odorant binding proteins; Real-time PCR; Fluorescence competitive binding assay; Forest insect; Economic pest; Lepidoptera
Carboxyl/cholinesterases (CCEs) have pivotal roles in dietary detoxification, pheromone or hormone degradation and neurodevelopment. The recent completion of genome projects in various insect species has led to the identification of multiple CCEs with unknown functions. Here, we analyzed the phylogeny, expression and genomic distribution of 69 putative CCEs in the silkworm, Bombyx mori (Lepidoptera: Bombycidae).
A phylogenetic tree of CCEs in B. mori and other lepidopteran species was constructed. The expression pattern of each B. mori CCE was also investigated by a search of an expressed sequence tag (EST) database, and the relationship between phylogeny and expression was analyzed. A large number of B. mori CCEs were identified from a midgut EST library. CCEs expressed in the midgut formed a cluster in the phylogenetic tree that included not only B. mori genes but also those of other lepidopteran species. The silkworm, and possibly also other lepidopteran species, has a large number of CCEs, and this might be a consequence of the large cluster of midgut CCEs. Investigation of intron-exon organization in B. mori CCEs revealed that their positions and splicing site phases were strongly conserved. Several B. mori CCEs, including juvenile hormone esterase, not only showed clustering in the phylogenetic tree but were also closely located on silkworm chromosomes. We investigated the phylogeny and microsynteny of neuroligins in detail, among many CCEs. Interestingly, we found the evolution of this gene appeared not to be conserved between B. mori and other insect orders.
We analyzed 69 putative CCEs from B. mori. Comparison of these CCEs with other lepidopteran CCEs indicated that they had conserved expression and function in this insect order. The analyses showed that CCEs were unevenly distributed across the genome of B. mori and suggested that neuroligins may have a distinct evolutionary history from other insect order. It is possible that such an uneven genomic distribution and a unique neuroligin evolution are shared with other lepidopteran insects. Our genomic analysis has provided novel information on the CCEs of the silkworm, which will be of value to understanding the biology, physiology and evolution of insect CCEs.
The Mediterranean fruit fly (or medfly), Ceratitis capitata (Wiedemann; Diptera: Tephritidae), is a serious pest of agriculture worldwide, displaying a very wide larval host range with more than 250 different species of fruit and vegetables. Olfaction plays a key role in the invasive potential of this species. Unfortunately, the pheromone communication system of the medfly is complex and still not well established. In this study, we report the isolation of chemicals emitted by sexually mature individuals during the “calling” period and the electrophysiological responses that these compounds elicit on the antennae of male and female flies. Fifteen compounds with electrophysiological activity were isolated and identified in male emissions by gas chromatography coupled to electroantennography (GC–EAG). Within the group of 15 identified compounds, 11 elicited a response in antennae of both sexes, whilst 4 elicited a response only in female antennae. The binding affinity of these compounds, plus 4 additional compounds known to be behaviourally active from other studies, was measured using C. capitata OBP, CcapOBP83a-2. This OBP has a high homology to Drosophila melanogaster OBPs OS-E and OS-F, which are associated with trichoid sensilla and co-expressed with the well-studied Drosophila pheromone binding protein LUSH. The results provide evidence of involvement of CcapOBP83a-2 in the medfly's odorant perception and its wider specificity for (E,E)-α-farnesene, one of the five major compounds in medfly male pheromone emission. This represents the first step in the clarification of the C. capitata and pheromone reception pathway, and a starting point for further studies aimed towards the creation of new powerful attractants or repellents applicable in the actual control strategies.
•The pheromone blend produced by medfly males during the calling period is investigated using air entrainment and GC–EAG.•Out of 15 chemicals, 11 elicited antennal responses in both sexes and 4 elicited the response only in female antennae.•The first medfly odorant binding protein CcapOBP83a-2 has been expressed and purified (87).•CcapOBP83a-2 showed binding affinity for (E,E)-α-farnesene, one of the components of medfly pheromone.•The ability of CcapOBP83a-2 to bind other electrophysiologically active compounds shows its broad binding specificity.
Medfly; Ceratitis capitata; Olfaction; Odorant binding protein; Pheromone binding protein; Pheromone; Binding studies; Protein expression; Electroantennography; GC–EAG; Fluorescence displacement; OBP, Odorant binding protein; PBP, Pheromone binding protein; CSP, Chemosensory protein; OR, Odorant receptor; EAG, Electroantennography; GC–EAG, Gas chromatography coupled to electroantennography; SIT, Sterile Insect Technique
Most species of Parnara and Pelopidas (Hesperiidae) are important pests of rice. In this study, the antennal morphology, types of sensilla, and their distribution of four skipper butterflies, including Parnara guttata (Bremer & Grey), Pa. bada (Moore), Pelopidas mathias (Fabricius) and Pe. agna (Moore), were observed using a scanning electron microscope. Six distinct morphological types of sensilla were found on the antennae of all of these species: sensilla squamiformia, sensilla trichodea, sensilla chaetica, sensilla auricillica, sensilla coeloconica, and Böhm sensilla. The sensilla trichodea are the most abundant sensilla among the four skipper butterflies, and the sensilla auricillica are confirmed on the antennae of butterflies for the second time. In addition, the possible functions of these sensilla are discussed in the light of previously reported lepidopteran insects, which may provide useful information for further study of the function of these antennal sensilla and for related pests control by applying sex pheromones.
Lepidoptera; Hesperiidae; morphology; fine structure
In insects and other animals, intraspecific communication between individuals of the opposite sex is mediated in part by chemical signals called sex pheromones. In most moth species, male moths rely heavily on species-specific sex pheromones emitted by female moths to identify and orient towards an appropriate mating partner among a large number of sympatric insect species. The silkmoth, Bombyx mori, utilizes the simplest possible pheromone system, in which a single pheromone component, (E, Z)-10,12-hexadecadienol (bombykol), is sufficient to elicit full sexual behavior. We have previously shown that the sex pheromone receptor BmOR1 mediates specific detection of bombykol in the antennae of male silkmoths. However, it is unclear whether the sex pheromone receptor is the minimally sufficient determination factor that triggers initiation of orientation behavior towards a potential mate. Using transgenic silkmoths expressing the sex pheromone receptor PxOR1 of the diamondback moth Plutella xylostella in BmOR1-expressing neurons, we show that the selectivity of the sex pheromone receptor determines the chemical response specificity of sexual behavior in the silkmoth. Bombykol receptor neurons expressing PxOR1 responded to its specific ligand, (Z)-11-hexadecenal (Z11-16:Ald), in a dose-dependent manner. Male moths expressing PxOR1 exhibited typical pheromone orientation behavior and copulation attempts in response to Z11-16:Ald and to females of P. xylostella. Transformation of the bombykol receptor neurons had no effect on their projections in the antennal lobe. These results indicate that activation of bombykol receptor neurons alone is sufficient to trigger full sexual behavior. Thus, a single gene defines behavioral selectivity in sex pheromone communication in the silkmoth. Our findings show that a single molecular determinant can not only function as a modulator of behavior but also as an all-or-nothing initiator of a complex species-specific behavioral sequence.
Like many animal species, moths use chemical signals called sex pheromones to communicate with conspecific individuals of the opposite sex in the context of reproduction. Typically, male moths depend on sex pheromones emitted by conspecific females to identify and locate their mates. Therefore, the behavioral preference of male moths to conspecific pheromones is a critical factor for successful reproduction. Sex pheromone receptor proteins expressed in specialized antennal olfactory receptor neurons reportedly play a central role in sex pheromone discrimination. However, the causal relationship between sex pheromone receptor specificity and behavioral preference remains to be proven. We have addressed this question in a genetically tractable moth species, the silkmoth (Bombyx mori), because this species possesses the simplest possible pheromone system in which a single pheromone substance, bombykol, elicits full sexual behavior. Using transgenic silkmoths expressing a sex pheromone receptor from another moth species, we revealed that solely the chemical specificity of the odorant receptors in bombykol receptor neurons determines the behavioral preference in male silkmoths. Our results show that the initiation of a complex programmed sexual behavior can depend on the properties of a single pheromone receptor gene expressed in a population of olfactory receptor neurons.
Many animals rely on chemical cues to recognize and locate a resource, and they must extract the relevant information from a complex and changing odor environment. For example, in moths, finding a mate is mediated by a sex pheromone, which is detected in a rich environment of volatile plant compounds. Here, we investigated the effects of a volatile plant background on the walking response of male Spodoptera littoralis to the female pheromone. Males were stimulated by combining pheromone with one of three plant compounds, and their walking paths were recorded with a locomotion compensator and analyzed. We found that the addition of certain volatile plant compounds disturbed the orientation toward the sex pheromone. The effect on locomotion was correlated with the capacity of the plant compound to antagonize pheromone detection by olfactory receptor neurons, suggesting a masking effect of the background over the pheromone signal. Moths were more sensitive to changes in background compared to a constant background, suggesting that a background odor also acts as a distracting stimulus. Our experiments show that the effects of odorant background on insect responses to chemical signals are complex and cannot be explained by a single mechanism.
Insects possess antennae equipped with a large number of segments (flagellomeres) on which sensory organs (sensilla) are located. Hemimetabolous insects grow by molting until they reach adulthood. In these species, the sensory structures develop and mature during each stage of development; new flagellomeres are generated at each molt elongating the antennae, and new sensilla appear. The praying mantis (Tenodera aridifolia) is a hemimetabolous insect with 7 different instars before it reaches adulthood. Because their antennae are provided with an atypical sensillar distribution, we previously suggested that their antennae develop with a different mechanism to other hemimetaboulous insect species. In the present study, we measured the number, length and width of flagellomeres along the antennae in nymph and adult mantis Tenodera aridifolia. For this study, we developed a new and innovative methodology to reconstruct the antennal development based on the length of flagellomeres. We observed and confirmed that the antennae of mantises develop with the addition of new segments at two distinct sites. In addition, we constructed a complete database of the features of the flagellum for each stage of development. From our data, we found that sexual dimorphism appears from the 6 instar (larger number and wider flagellomeres in males) in accordance with the appearance of their genital apparatus. The antennal sexual dimorphism completes at adulthood with longer flagellomeres and the emergence of a huge number of grooved peg sensilla in males during the last molting, which suggests once again their function as sex-pheromone receptive sensilla.
An open question in olfactory coding is the extent of interglomerular connectivity: do olfactory glomeruli and their neurons regulate the odorant responses of neurons innervating other glomeruli? In the olfactory system of the moth Manduca sexta, the response properties of different types of antennal olfactory receptor cells are known. Likewise, a subset of antennal lobe glomeruli has been functionally characterized and the olfactory tuning of their innervating neurons identified. This provides a unique opportunity to determine functional interactions between glomeruli of known input, specifically, (1) glomeruli processing plant odors and (2) glomeruli activated by antennal stimulation with pheromone components of conspecific females. Several studies describe reciprocal inhibitory effects between different types of pheromone-responsive projection neurons suggesting lateral inhibitory interactions between pheromone component-selective glomerular neural circuits. Furthermore, antennal lobe projection neurons that respond to host plant volatiles and innervate single, ordinary glomeruli are inhibited during antennal stimulation with the female’s sex pheromone. The studies demonstrate the existence of lateral inhibitory effects in response to behaviorally significant odorant stimuli and irrespective of glomerular location in the antennal lobe. Inhibitory interactions are present within and between olfactory subsystems (pheromonal and non-pheromonal subsystems), potentially to enhance contrast and strengthen odorant discrimination.
Electrophysiology; Glomerulus; Neural coding; Olfaction; Pheromone
Below ground orientation in insects relies mainly on olfaction and taste. The economic impact of plant root feeding scarab beetle larvae gave rise to numerous phylogenetic and ecological studies. Detailed knowledge of the sensory capacities of these larvae is nevertheless lacking. Here, we present an atlas of the sensory organs on larval head appendages of Melolontha melolontha. Our ultrastructural and electrophysiological investigations allow annotation of functions to various sensory structures.
Three out of 17 ascertained sensillum types have olfactory, and 7 gustatory function. These sensillum types are unevenly distributed between antennae and palps. The most prominent chemosensory organs are antennal pore plates that in total are innervated by approximately one thousand olfactory sensory neurons grouped into functional units of three-to-four. In contrast, only two olfactory sensory neurons innervate one sensillum basiconicum on each of the palps. Gustatory sensilla chaetica dominate the apices of all head appendages, while only the palps bear thermo-/hygroreceptors. Electrophysiological responses to CO2, an attractant for many root feeders, are exclusively observed in the antennae. Out of 54 relevant volatile compounds, various alcohols, acids, amines, esters, aldehydes, ketones and monoterpenes elicit responses in antennae and palps. All head appendages are characterized by distinct olfactory response profiles that are even enantiomer specific for some compounds.
Chemosensory capacities in M. melolontha larvae are as highly developed as in many adult insects. We interpret the functional sensory units underneath the antennal pore plates as cryptic sensilla placodea and suggest that these perceive a broad range of secondary plant metabolites together with CO2. Responses to olfactory stimulation of the labial and maxillary palps indicate that typical contact chemo-sensilla have a dual gustatory and olfactory function.
Odorant binding proteins play a crucial role in transporting semiochemicals across the sensillum lymph to olfactory receptors within the insect antennal sensilla. In this study, the general odorant binding protein 2 gene was cloned from the antennae of Loxostege sticticalis, using reverse transcription PCR and rapid amplification of cDNA ends. Recombinant LstiGOBP2 was expressed in Escherichia coli and purified by Ni ion affinity chromatography. Real-time PCR assays indicated that LstiGOBP2 mRNA is expressed mainly in adult antennae, with expression levels differing with developmental age. Ligand-binding experiments using N-phenyl-naphthylamine (1-NPN) as a fluorescent probe demonstrated that the LstiGOBP2 protein has binding affinity to a broad range of odorants. Most importantly, trans-11-tetradecen-1-yl acetate, the pheromone component of Loxostege sticticalis, and trans-2-hexenal and cis-3-hexen-1-ol, the most abundant plant volatiles in essential oils extracted from host plants, had high binding affinities to LstiGOBP2 and elicited strong electrophysiological responses from the antennae of adults.
The navel orangeworm, Amyelois transitella Walker (Lepidoptera: Pyralidae), is the most serious insect pest of almonds and pistachios in California for which environmentally friendly alternative methods of control — like pheromone-based approaches — are highly desirable. Some constituents of the sex pheromone are unstable and could be replaced with parapheromones, which may be designed on the basis of molecular interaction of pheromones and pheromone-detecting olfactory proteins.
By analyzing extracts from olfactory and non-olfactory tissues, we identified putative olfactory proteins, obtained their N-terminal amino acid sequences by Edman degradation, and used degenerate primers to clone the corresponding cDNAs by SMART RACE. Additionally, we used degenerate primers based on conserved sequences of known proteins to fish out other candidate olfactory genes. We expressed the gene encoding a newly identified pheromone-binding protein, which was analyzed by circular dichroism, fluorescence, and nuclear magnetic resonance, and used in a binding assay to assess affinity to pheromone components.
We have cloned nine cDNAs encoding olfactory proteins from the navel orangeworm, including two pheromone-binding proteins, two general odorant-binding proteins, one chemosensory protein, one glutathione S-transferase, one antennal binding protein X, one sensory neuron membrane protein, and one odorant receptor. Of these, AtraPBP1 is highly enriched in male antennae. Fluorescence, CD and NMR studies suggest a dramatic pH-dependent conformational change, with high affinity to pheromone constituents at neutral pH and no binding at low pH.
Olfaction in animals is important for host localization, mating and reproduction in heterogeneous chemical environments. Studying the molecular basis of olfactory receptor neurons (ORNs) systems can elucidate the evolution of olfaction and associated behaviours. Odorant receptors (ORs) in insects have been identified, particularly in the holometabolous model Drosophila, and some of them have been functionally studied. However, ORs in the locust—a hemimetabolous model insect and the most important insect crop pest—have not yet been identified, hindering our understanding of locust olfaction. Here, we report for the first time four putative ORs in Locusta migratoria: LmigOR1, LmigOR2, LmigOR3 and LmigOR4.
These four putative OR genes encoded proteins with amino acids of 478, 436, 413 and 403 respectively. Sequence identity among them ranged from 19.7% to 35.4%. All ORs were tissue-specifically expressed in olfactory organs, without sex-biased characteristics. However, LmigOR1, LmigOR3 and LmigOR4 were only expressed in the antenna, while LmigOR2 could also be detected in mouthparts. In situ hybridization demonstrated that the LmigOR1antisense probe labelled olfactory receptor neurons (ORNs) in almost all segments of the antenna, but only a few segments housed ORNs expressing LmigOR2. The number of neurons labelled by LmigOR1 antisense probes in each antennal segment was much greater (>10 neurons/segment) than that labelled by LmigOR2 probes (generally 1–3 neurons/segment). Furthermore, some of the labelled neurons could be attributed to the basiconic sensilla, but LmigOR1 and LmigOR2 were expressed in different subtypes.
Our results strongly suggested that these newly discovered genes encode locust ORs and the differential expression patterns of LmigOR1 and LmigOR2 implied distinct functions. These results may offer insights into locust olfaction and contribute to the understanding of the evolution of insect chemoreception.
Odorant receptor; Locust; Odorant receptor neuron; Basiconic sensillum; Expression
Volatile odors are important sensory inputs that shape the behaviour of insects, including agricultural pests and disease vectors. Anopheles gambiae s.s. is a highly anthropophilic mosquito and is the major vector for human malaria in sub-Sahara Africa, while Anopheles quadriannulatus, largely due to its zoophilic behaviour, is considered a non-vector species in the same region. Careful studies of olfaction in these sibling species may lead to insights about the mechanisms that drive host preference behaviour. In the present study, the external anatomy of the antenna, the principle olfactory organ in the female mosquito of both species, was examined as an initial step toward more detailed comparisons.
Scanning electron and light microscopy were used to examine the antennae ultrastructures of adult female An. gambiae s.s. and An. quadriannulatus. Sensory structures, called sensilla, were categorized and counted; their distributions are reported here as well as densities calculated for each species.
Both An. gambiae s.s. and An. quadriannulatus bear five classes of sensilla on their antennae: chaetica (bristles), trichodea (hairs), basiconica (pegs), coeloconica (pitted pegs), and ampullacea (pegs in tubes). Female An. quadriannulatus antennae have approximately one-third more sensilla, and a proportionally larger surface area, than female An. gambiae s.s. antennae.
The same types of sensilla are found on the antennae of both species. While An. quadriannulatus has greater numbers of each sensilla type, sensilla densities are very similar for each species, suggesting that other factors may be more important to such olfactory-driven behaviours as host preference.
The three-dimensional structures of two odorant binding proteins (OBPs) and one chemosensory protein (CSP) from a polyphagous ectoparasitoid Scleroderma guani (Hymenoptera: Bethylidae) were resolved bioinformatically. The results show that both SguaOBP1 and OBP2 are classic OBPs, whereas SguaCSP1 belongs to non-classic CSPs which are considered as the “Plus-C” CSP in this report. The structural differences between the two OBPs and between OBP and CSP are thoroughly described, and the structural and functional significance of the divergent C-terminal regions (e.g., the prolonged C-terminal region in SguaOBP2 and the additional pair of cysteines in SguaCSP1) are discussed. The immunoblot analyses with antisera raised against recombinant SguaOBP1, OBP2, and CSP1, respectively, indicate that two SguaOBPs are specific to antennae, whereas SguaCSP1, which are more abundant than OBPs and detected in both male and female wasps, expresses ubiquitously across different tissues.
We also describe the ultrastructure of the antennal sensilla types in S. guani and compare them to 19 species of parasitic Hymenoptera. There are 11 types of sensilla in the flagellum and pedicel segments of antennae in both male and female wasps. Seven of them, including sensilla placodea (SP), long sensilla basiconica (LSB), sensilla coeloconica (SC), two types of double-walled wall pore sensilla (DWPS-I and DWPS-II), and two types of sensilla trichodea (ST-I and ST-II), are multiporous chemosensilla. The ultralsturctures of these sensilla are morphologically characterized. In comparison to monophagous specialists, the highly polyphagous generalist ectoparasitoids such as S. guani possess more diverse sensilla types which are likely related to their broad host ranges and complex life styles. Our immunocytochemistry study demonstrated that each of the seven sensilla immunoreacts with at least one antiserum against SguaOBP1, OBP2, and CSP1, respectively. Anti-OBP2 is specifically labeled in DWPS-II, whereas the anti-OBP1 shows a broad spectrum of immunoactivity toward four different sensilla (LSB, SP, ST-I and ST-II). On the other hand, anti-CSP1 is immunoactive toward SP, DWPS-I and SC. Interestingly, a cross co-localization pattern between SguaOBP1 and CSP1 is documented for the first time. Given that the numbers of OBPs and CSPs in many insect species greatly outnumber their antennal sensilla types, it is germane to suggest such phenomenon could be the rule rather than the exception.
Scleroderma guani; OBP; CSP; tertiary structure; sensilla; immunolocalization