Expression of a pheromone receptor in ovipositor sensilla of the female moth (Heliothis virescens)

Sensing volatiles throughout the body: geographic- and tissue-specific olfactory receptor expression in the fig wasp

An essential adaptive strategy in insects is the evolution of olfactory receptors (ORs) to recognize important volatile environmental chemical cues. Our model species, Ceratosolen fusciceps, a specialist wasp pollinator of Ficus racemosa, likely possesses an OR repertoire that allows it to distinguish fig-specific volatiles in highly variable environments. Using a newly assembled genome-guided transcriptome, we annotated 63 ORs in the species and reconstructed the phylogeny of Ceratosolen ORs in conjunction with other hymenopteran species. Expression analysis showed that though ORs were mainly expressed in the female antennae, 20% were also expressed in nonantennal tissues such as the head, thorax, abdomen, legs, wings, and ovipositor. Specific upregulated expression was observed in OR30C in the head and OR60C in the wings. We identified OR expression from all major body parts of female C. fusciceps, suggesting novel roles of ORs throughout the body. Further examination of the OR expression of C. fusciceps in widely separated geographical locations, that is, South (urban) and Northeast (rural) India, revealed distinct OR expression levels in different locations. This discrepancy likely parallels the observed variation in fig volatiles between these regions and provides new insights into the evolution of insect ORs and their expression across geographical locations and tissues.

Functional characterization of sex pheromone receptors PflaOR29 and PflaOR44 involved in the chemoreception of a diurnal moth, Phauda flammans (Walker) (Lepidoptera: Phaudidae)

Recognition of sex pheromones released by heterosexual moths via sex pheromone receptors is key for establishing mating connections in moths. The day-flying moth Phauda flammans is an oligophagous pest in southern cities of China and Southeast Asian countries. Our previous study reported that male P. flammans can be attracted to two sex pheromone compounds [Z-9-hexadecenal and (Z, Z, Z)-9,12,15-octadecadienal] released by females in the field; however, the mechanism of olfactory recognition is not clear. In this study, two sex pheromone receptor genes (PflaOR29 and PflaOR44) were cloned. Among the different tissues, both PflaOR29 and PflaOR44 were highly expressed in the antennae of mated male adults. At different developmental stages, the expression levels of PflaOR29 and PflaOR44 were significantly greater in mated male adults than other stages. The fluorescence signals of PflaOR29 and PflaOR44 were mostly distributed on the dorsal side of the antennae, with a large number of trichoid sensilla. The results of the gene function of PflaOR29 and PflaOR44 based on a Drosophila empty neuron heterologous expression system indicated that PflaOR29 strongly responded to (Z, Z, Z)-9,12,15-octadecadienal but not to Z-9-hexadecenal, whereas PflaOR44 did not respond to the two sex pheromones. Our findings clarify the sex pheromone receptor gene corresponding to (Z, Z, Z)-9,12,15-octadecatrienal. These results provide essential information for analyzing the mechanism of sexual communication in diurnal moths and for identifying target genes for the development of efficient attractants.

The Evolutionary Importance of Intraspecific Variation in Sexual Communication Across Sensory Modalities

The evolution of sexual communication is critically important in the diversity of arthropods, which are declining at a fast pace worldwide. Their environments are rapidly changing, with increasing chemical, acoustic, and light pollution. To predict how arthropod species will respond to changing climates, habitats, and communities, we need to understand how sexual communication systems can evolve. In the past decades, intraspecific variation in sexual signals and responses across different modalities has been identified, but never in a comparative way. In this review, we identify and compare the level and extent of intraspecific variation in sexual signals and responses across three different modalities, chemical, acoustic, and visual, focusing mostly on insects. By comparing causes and possible consequences of intraspecific variation in sexual communication among these modalities, we identify shared and unique patterns, as well as knowledge needed to predict the evolution of sexual communication systems in arthropods in a changing world.

Chemosensory Receptor Expression in the Abdomen Tip of the Female Codling Moth, Cydia pomonella L. (Lepidoptera: Tortricidae)

In insects, the chemical senses influence most vital behaviors, including mate seeking and egg laying; these sensory modalities are predominantly governed by odorant receptors (ORs), ionotropic receptors (IRs), and gustatory receptors (GRs). The codling moth, Cydia pomonella, is a global pest of apple, pear, and walnut, and semiochemically based management strategies limit the economic impacts of this species. The previous report of expression of a candidate pheromone-responsive OR in female codling moth ovipositor and pheromone glands raises further questions about the chemosensory capacity of these organs. With an RNA-sequencing approach, we examined chemoreceptors' expression in the female codling moth abdomen tip, sampling tissues from mated and unmated females and pupae. We report 37 ORs, 22 GRs, and 18 IRs expressed in our transcriptome showing overlap with receptors expressed in adult antennae as well as non-antennal candidate receptors. A quantitative PCR approach was also taken to assess the effect of mating on OR expression in adult female moths, revealing a few genes to be upregulated or downregulating after mating. These results provide a better understanding of the chemosensory role of codling moth female abdomen tip organs in female-specific behaviors. Future research will determine the function of specific receptors to augment current semiochemical-based strategies for codling moth management.

Sex- and tissue-specific expression of chemosensory receptor genes in a hawkmoth

For the nocturnal hawkmoth Manduca sexta, olfactory and gustatory cues are essential for finding partners, food, and oviposition sites. Three chemosensory receptor families, odorant receptors (ORs), ionotropic receptors (IRs), and gustatory receptors (GRs) are involved in the detection of these stimuli. While many chemosensory receptor genes have been identified, knowledge of their expression profile in potentially chemoreceptive organs is incomplete. Here, we studied the expression patterns of chemosensory receptors in different tissues including the antennae, labial palps, proboscis, legs, wings and ovipositor. We compared the receptors’ expression in female and male moths both before and after mating by using the NanoString platform. This tool allowed us to measure expression levels of chemosensory receptor genes in a single reaction using probes designed against 71 OR, 29 IR and 49 GR transcripts. In all tissues investigated, we detected expression of genes from all three receptor families. The highest number of receptors was detected in the antennae (92), followed by the ovipositor (59), while the least number was detected in the hindlegs (21). The highest number of OR genes were expressed in the antennae (63), of which 24 were specific to this main olfactory organ. The highest number of IRs were also expressed in the antennae (16), followed by the ovipositor (15). Likewise, antennae and ovipositor expressed the highest number of GRs (13 and 14). Expression of the OR co-receptor MsexORCo, presumably a prerequisite for OR function, was found in the antennae, labial palps, forelegs and ovipositor. IR co-receptors MsexIR25a and MsexIR76b were expressed across all tested tissues, while expression of the IR co-receptor MsexIR8a was restricted to antennae and ovipositor. Comparing the levels of all 149 transcripts across the nine tested tissues allowed us to identify sex-biased gene expression in the antennae and the legs, two appendages that are also morphologically different between the sexes. However, none of the chemosensory receptors was differentially expressed based on the moths’ mating state. The observed gene expression patterns form a strong base for the functional characterization of chemosensory receptors and the understanding of olfaction and gustation at the molecular level in M. sexta.

Identification and Characterization of Chemosensory Receptors in the Pheromone Gland-Ovipositor of Spodoptera frugiperda (J. E. Smith)

Simple Summary

Chemical cues are generally thought to be primarily detected by the cephalic organ antennae, maxillary palps, and proboscises in insects. Although several recent studies have reported the chemosensory roles of ovipositors in some moth species, the expression of chemosensory receptors and their functions in the ovipositor remain largely unknown. Here, we systematically analyzed the pheromone gland-ovipositor (PG-OV) transcriptome of the fall armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae). A total of 26 candidate chemosensory receptor genes were revealed, including 12 odorant receptors (ORs), 4 gustatory receptors (GRs), and 10 ionotropic receptors (IRs). Specific genes including pheromone receptors, ORco, CO₂ receptors, sugar receptors, and IR co-receptors were identified. Tissue expression profiling demonstrated that the annotated receptor genes were mainly expressed in the antennae (for ORs and IRs) or proboscis (for GRs), but two ORs, two GRs, and two IRs were also highly enriched in the PG-OV, with expression levels only slightly lower or even similar to those in the antennae/proboscis. This report provides the first large-scale description of chemosensory receptors in the PG-OV of S. frugiperda. It may inspire researchers to investigate how chemosensory receptors function in the ovipositor of S. frugiperda, as well as in the ovipositors of other moths.

Abstract

Chemoreception by moth ovipositors has long been suggested, but underlying molecular mechanisms are mostly unknown. To reveal such chemosensory systems in the current study, we sequenced and assembled the pheromone gland-ovipositor (PG-OV) transcriptome of females of the fall armyworm, Spodoptera frugiperda, a pest of many crops. We annotated a total of 26 candidate chemosensory receptor genes, including 12 odorant receptors (ORs), 4 gustatory receptors (GRs), and 10 ionotropic receptors (IRs). The relatedness of these chemosensory receptors with those from other insect species was predicted by phylogenetic analyses, and specific genes, including pheromone receptors, ORco, CO₂ receptors, sugar receptors, and IR co-receptors, were reported. Although real-time quantitative-PCR analyses of annotated genes revealed that OR and IR genes were mainly expressed in S. frugiperda antennae, two ORs and two IRs expressed in antennae were also highly expressed in the PG-OV. Similarly, GR genes were mainly expressed in the proboscis, but two were also highly expressed in the PG-OV. Our study provides the first large-scale description of chemosensory receptors in the PG-OV of S. frugiperda and provides a foundation for exploring the chemoreception mechanisms of PG-OV in S. frugiperda and in other moth species.

Description of Chemosensory Genes in Unexplored Tissues of the Moth Spodoptera littoralis

Illumina-based transcriptome sequencing of chemosensory organs has become a standard in deciphering the molecular bases of chemical senses in insects, especially in non-model species. A plethora of antennal transcriptomes is now available in the literature, describing large sets of chemosensory receptors and binding proteins in a diversity of species. However, little is still known on other organs such as mouthparts, legs and ovipositors, which are also known to carry chemosensory sensilla. This is the case of the noctuid Spodoptera littoralis, which has been established as a model insect species in molecular chemical ecology thanks to the description of many—but not all—chemosensory genes. To fulfill this gap, we present here an unprecedented transcriptomic survey of chemosensory tissues in this species. RNAseq from male and female proboscis, labial palps, legs and female ovipositors allowed us to annotate 115 putative chemosensory gene transcripts, including 30 novel genes in this species. Especially, we doubled the number of candidate gustatory receptor transcripts described in this species. We also evidenced ectopic expression of many chemosensory genes. Remarkably, one third of the odorant receptors were found to be expressed in the proboscis. With a total of 196 non-overlapping chemosensory genes annotated, the S. littoralis repertoire is one of the most complete in Lepidoptera. We further evaluated the expression of transcripts between males and females, pinpointing sex-specific transcripts. We identified five female-specific transcripts, including one odorant receptor, one gustatory receptor, one ionotropic receptor and one odorant-binding protein, and one male-specific gustatory receptor. Such sex-biased expression suggests that these transcripts participate in sex-specific behaviors, such as host choice for oviposition in females and/or mating partner recognition in both sexes.

Research Progress on Oviposition-Related Genes in Insects

Oviposition-related genes have remained a consistent focus of insect molecular biology. Previous research has gradually clarified our mechanistic understanding of oviposition-related genes, including those related to oviposition-gland-related genes, oogenesis-related genes, oviposition-site-selection-related genes, and genes related to ovulation and hatching. Moreover, some of this research has revealed how the expression of single oviposition-related genes affects the expression of related genes, and more importantly, how individual node genes function to link the expression of upstream and downstream genes. However, the research to date is not sufficient to completely explain the overall interactions among the genes of the insect oviposition system. Through a literature review of a large number of studies, this review provides references for future research on oviposition-related genes in insects and the use of RNAi or CRISPR/Cas9 technology to verify the functions of oviposition-related genes and to prevent and control harmful insects.

Host plant odours and their recognition by the odourant-binding proteins of Diaphorina citri Kuwayama (Hemiptera: Psyllidae)

BACKGROUND

The Asian citrus psyllid (ACP), Diaphorina citri, is one of the major pests in citrus production because it transmits huanglongbing, a devastating disease of citrus plants. Odourant-binding proteins (OBPs) play an important role in the olfactory perception of insects. Revealing the function of DcitOBPs is beneficial to the development of new ACP management strategies.

RESULTS

An analysis of the components of volatiles from the new shoots of six host plant species showed that β-caryophyllene was the characteristic volatile compound in flushing shoots and the most abundant volatile compound in three of the six tested ACP host plant species. The tissue expression profiles of nine known DcitOBPs were analyzed based on a transcriptome database, and DcitOBP3 and DcitOBP6 exhibited high expression in the antennae of both sexes and the ovipositor of females. The binding ability of two recombinant proteins with eight ligands was studied through competitive binding analysis; the results showed that DcitOBP6 exhibited stronger binding to β-caryophyllene. Behavioural trials indicated that sexually mature female adults of D. citri were significantly attracted to β-caryophyllene at concentrations of 0.1 μL mL^-1 and 10 μL mL^-1 . RNAi analysis in female D. citri showed that the reduction of DcitOBP6 transcript abundance led to a decrease in antennae EAG activity and behavioural responses to β-caryophyllene.

CONCLUSION

The results demonstrate that DcitOBP6 is involved in the perception of an important host plant volatile, β-caryophyllene, in the ACP, and provide a theoretical foundation for behavioural interference in ACP management.

A moth odorant receptor highly expressed in the ovipositor is involved in detecting host-plant volatiles

Antennae are often considered to be the nostrils of insects. Here, we sequenced the transcriptome of the pheromone gland-ovipositor complex of Helicoverpa assulta and discovered that an odorant receptor (OR) gene, HassOR31, had much higher expression in the ovipositor than in antennae or other tissues. To determine whether the ovipositor was involved in odorant detection, we co-expressed HassOR31 and its co-receptor, HassORco, in a Xenopus oocyte model system, and demonstrated that the OR was responsive to 12 plant odorants, especially Z-3-hexenyl butyrate. These odorants elicited electrophysiological responses of some sensilla in the ovipositor, and HassOR31 and HassORco were co-expressed within ovipositor sensilla. Two oviposition preference experiments showed that female moths lacking antennae still preferentially selected oviposition sites containing plant volatiles. We suggest that the expression of HassOR31 in the ovipositor of H. assulta helps females to determine precise egg-laying sites in host plants.

When most insects reproduce they lay eggs that hatch into juveniles known as larvae. To provide good sources of food for the larvae, the adult insects have to carefully select where to lay the eggs. Host plants produce specific sets of chemicals known as odorants that the adult insects are able to smell using proteins called odorant receptors.

It is generally thought that odorant receptors in the antennae on the head are responsible for guiding adult insects to good egg-laying sites. However, recent studies have reported that odorant receptors are also present in the egg-laying organs of several different species of moth. It remains unclear what role these odorant receptors may play in egg-laying.

The oriental tobacco budworm (Helicoverpa assulta) is considered a serious pest in agriculture. The adult moths lay their eggs on a narrow range of plants in the nightshade family including tobacco and hot pepper. Li et al. have now investigated the odorant receptors of H. assulta and found that one gene for an odorant receptor called HassOR31 was expressed much more in the egg-laying organs of the moths than in the antennae. Further experiments showed that this receptor was tuned to respond to 12 odorants that also stimulated responses in the egg-laying organ of H. assulta. Together these findings suggest that this odorant receptor in the egg-laying organ helps the moths find suitable host plants to lay their eggs on.

The work of Li et al. may help us understand how H. assulta evolved to lay its eggs on specific members of the nightshade family and lead to new methods of controlling this pest. An insect’s sense of smell guides many other behaviors including finding food, mates and avoiding enemies. Therefore, these findings may inspire researchers to investigate whether odorant receptors in the antennae or other organs guide these behaviors.

Chemosensory Gene Families in the Oligophagous Pear Pest Cacopsylla chinensis (Hemiptera: Psyllidae)

Chemosensory systems play an important role in insect behavior, and some key associated genes have potential as novel targets for pest control. Cacopsylla chinensis is an oligophagous pest and has become one of the main pests of pear trees, but little is known about the molecular-level means by which it locates its hosts. In this study, we assembled the head transcriptome of C. chinensis using Illumina sequencing, and 63,052 Unigenes were identified. A total of 36 candidate chemosensory genes were identified, including five different families: 12 odorant binding proteins (OBPs), 11 chemosensory proteins (CSPs), 7 odorant receptors (ORs), 4 ionotropic receptors (IRs), and 2 gustatory receptors (GRs). The number of chemosensory gene families is consistent with that found in other Hemipteran species, indicating that our approach successfully obtained the chemosensory genes of C. chinensis. The tissue expression of all genes using quantitative real-time PCR (qRT-PCR) found that some genes displayed male head, female head, or nymph-biased specific/expression. Our results enrich the gene inventory of C. chinensis and provide valuable resources for the analysis of the functions of some key genes. This will help in developing molecular targets for disrupting feeding behavior in C. chinensis.

Insect Pheromone Receptors - Key Elements in Sensing Intraspecific Chemical Signals

Pheromones are chemicals that serve intraspecific communication. In animals, the ability to detect and discriminate pheromones in a complex chemical environment substantially contributes to the survival of the species. Insects widely use pheromones to attract mating partners, to alarm conspecifics or to mark paths to rich food sources. The various functional roles of pheromones for insects are reflected by the chemical diversity of pheromonal compounds. The precise detection of the relevant intraspecific signals is accomplished by specialized chemosensory neurons housed in hair-like sensilla located on the surface of body appendages. Current data indicate that the extraordinary sensitivity and selectivity of the pheromone-responsive neurons (PRNs) is largely based on specific pheromone receptors (PRs) residing in their ciliary membrane. Besides these key elements, proper ligand-induced responses of PR-expressing neurons appear to generally require a putative co-receptor, the so-called "sensory neuron membrane protein 1" (SNMP1). Regarding the PR-mediated chemo-electrical signal transduction processes in insect PRNs, ionotropic as well as metabotropic mechanisms may be involved. In this review, we summarize and discuss current knowledge on the peripheral detection of pheromones in the olfactory system of insects with a focus on PRs and their specific role in the recognition and transduction of volatile intraspecific chemical signals.

Pheromone Binding Protein EhipPBP1 Is Highly Enriched in the Male Antennae of the Seabuckthorn Carpenterworm and Is Binding to Sex Pheromone Components

The seabuckthorn carpenterworm moth Eogystia hippophaecolus is a major threat to seabuckthorn plantations, causing considerable ecological and economic losses in China. Transcriptomic analysis of E. hippophaecolus previously identified 137 olfactory proteins, including three pheromone-binding proteins (PBPs). We investigated the function of E. hippophaecolus PBP1 by studying its mRNA and protein expression profiles and its binding ability with different compounds. The highest levels of expression were in the antennae, particularly in males, with much lower levels of expression in the legs and external genitals. Recombinant PBP1 showed strong binding to sex-pheromone components, suggesting that antennal EhipPBP1 is involved in binding sex-pheromone components during pheromone communication.

Characterization of Odorant Receptors from a Non-ditrysian Moth, Eriocrania semipurpurella Sheds Light on the Origin of Sex Pheromone Receptors in Lepidoptera

Pheromone receptors (PRs) are essential in moths to detect sex pheromones for mate finding. However, it remains unknown from which ancestral proteins these specialized receptors arose. The oldest lineages of moths, so-called non-ditrysian moths, use short-chain pheromone components, secondary alcohols, or ketones, so called Type 0 pheromones that are similar to many common plant volatiles. It is, therefore, possible that receptors for these ancestral pheromones evolved from receptors detecting plant volatiles. Hence, we identified the odorant receptors (ORs) from a non-ditrysian moth, Eriocrania semipurpurella (Eriocraniidae, Lepidoptera), and performed functional characterization of ORs using HEK293 cells. We report the first receptors that respond to Type 0 pheromone compounds; EsemOR3 displayed highest sensitivity toward (2S, 6Z)-6-nonen-2-ol, whereas EsemOR5 was most sensitive to the behavioral antagonist (Z)-6-nonen-2-one. These receptors also respond to plant volatiles of similar chemical structures, but with lower sensitivity. Phylogenetically, EsemOR3 and EsemOR5 group with a plant volatile-responding receptor from the tortricid moth Epiphyas postvittana (EposOR3), which together reside outside the previously defined lepidopteran PR clade that contains the PRs from more derived lepidopteran families. In addition, one receptor (EsemOR1) that falls at the base of the lepidopteran PR clade, responded specifically to β-caryophyllene and not to any other additional plant or pheromone compounds. Our results suggest that PRs for Type 0 pheromones have evolved from ORs that detect structurally-related plant volatiles. They are unrelated to PRs detecting pheromones in more derived Lepidoptera, which, in turn, also independently may have evolved a novel function from ORs detecting plant volatiles.

Molecular and functional characterization of a candidate sex pheromone receptor OR1 in Spodoptera litura

Olfaction is primarily mediated by highly specified olfactory receptors (ORs). Here, we cloned and identified an olfactory receptor, named SlituOR1 (Genbank no. JN835269), from Spodoptera litura and found evidence that it is a candidate pheromone receptor. It exhibited male-biased expression in the antennae, where it was localized at the base of sensilla trichoidea, the antennal sensilla mainly responsive to pheromones in moths. Conserved orthologues of this receptor, found among known pheromone receptors within the Lepidoptera, and SlituOR1 were placed among a clade of candidate pheromone receptors in a phylogeny tree of insect OR gene sequences. SlituOR1 showed differential expression in S. litura populations attracted to traps baited with different ratios of the two sex pheromone components (9Z,11E)-tetradecadienyl acetate (Z9E11-14:OAc) and (9Z,12E)-tetradecadienyl acetate (Z9E12-14:OAc). Knocking down of SlituOR1 by RNA interference reduced the electroantennogram (EAG) response to Z9E11-14:OAc, and this result is consistent with the field trapping experiment. We infer that variation in transcription levels of olfactory receptors may modulate sex pheromone perception in male moths and could provide some of the flexibility required to maintain the functionality of communication with females when a population is adapting to a new niche and reproductive isolation becomes an advantage.

Ultrastructure of sensilla of antennae and ovipositor of Sitotroga cerealella (Lepidoptera: Gelechiidae), and location of female sex pheromone gland

The Angoumois grain moth, Sitotroga cerealella, is a serious pest of stored grains worldwide. Presently, the best effective control against the moth is to disrupt the sexual communication between sexes. Sexual communication in moths includes two processes in which females produce and release pheromones from the sex pheromone gland and males detect and respond to them with a relatively sophisticated olfactory system in their antennae. To better understand these processes, we studied the ultrastructure of antennal and ovipositor sensilla of S. cerealella and determined the location of the female sex pheromone gland. Seven types of antennal sensilla were identified on both sexes: sensilla trichodea, sensilla chaetica, sensilla coeloconica, sensilla styloconica, sensilla auricillica, sensilla squamiformia and Bӧhm bristles. Of these sensilla, the sensilla trichodea were significantly more abundant on male antennae than on those of females, suggesting that these sensilla may detect the sex pheromones. On the ovipositor, only sensilla chaetica of various lengths were found. The sexual gland was an eversible sac of glandular epithelium that was situated dorsally in the intersegmental membrane between the 8^th and 9^th abdominal segments. These results will lead to a better understanding of mate finding with sex pheromones for this worldwide pest species.

A Sex Pheromone Receptor in the Hessian Fly Mayetiola destructor (Diptera, Cecidomyiidae)

The Hessian fly, Mayetiola destructor Say (Diptera, Cecidomyiidae), is a pest of wheat and belongs to a group of gall-inducing herbivores. This species has a unique life history and several ecological features that differentiate it from other Diptera such as Drosophila melanogaster and blood-feeding mosquitoes. These features include a short, non-feeding adult life stage (1-2 days) and the use of a long-range sex pheromone produced and released by adult females. Sex pheromones are detected by members of the odorant receptor (OR) family within the Lepidoptera, but no receptors for similar long-range sex pheromones have been characterized from the Diptera. Previously, 122 OR genes have been annotated from the Hessian fly genome, with many of them showing sex-biased expression in the antennae. Here we have expressed, in HEK293 cells, five MdesORs that display male-biased expression in antennae, and we have identified MdesOR115 as a Hessian fly sex pheromone receptor. MdesOR115 responds primarily to the sex pheromone component (2S,8E,10E)-8,10-tridecadien-2-yl acetate, and secondarily to the corresponding Z,E-isomer. Certain sensory neuron membrane proteins (i.e., SNMP1) are important for responses of pheromone receptors in flies and moths. The Hessian fly genome is unusual in that it encodes six SNMP1 paralogs, of which five are expressed in antennae. We co-expressed each of the five antennal SNMP1 paralogs together with each of the five candidate sex pheromone receptors from the Hessian fly and found that they do not influence the response of MdesOR115, nor do they confer responsiveness in any of the non-responsive ORs to any of the sex pheromone components identified to date in the Hessian fly. Using Western blots, we detected protein expression of MdesOrco, all MdesSNMPs, and all MdesORs except for MdesOR113, potentially explaining the lack of response from this OR. In conclusion, we report the first functional characterization of an OR from the Cecidomyiidae, extending the role of ORs as long-range sex pheromone detectors from the Lepidoptera into the Diptera.

Differential expression of the chemosensory transcriptome in two populations of the stemborer Sesamia nonagrioides

Among the proposed mechanisms of local adaptation to different ecological environments, transcriptional changes may play an important role. In this study, we investigated whether such variability occurred within the chemosensory organs of a herbivorous insect, for which chemosensation guides most of its host preferences. A European and an African population of the noctuid Sesamia nonagrioides that display significant differences in their ecological preferences were collected on Zea mays and Typha domingensis, respectively. RNAseq were used between the two populations for digital expression profiling of chemosensory organs from larval antennae and palps. Preliminary data on adult female antennae and ovipositors were also collected. We found 6,550 differentially expressed transcripts in larval antennae and palps. Gene ontology enrichment analyses suggested that transcriptional activity was overrepresented in the French population and that virus and defense activities were overrepresented in the Kenyan population. In addition, we found differential expression of a variety of cytochrome P450s, which may be linked to the different host-plant diets. Looking at olfactory genes, we observed differential expression of numerous candidate odorant-binding proteins, chemosensory proteins, and one olfactory receptor, suggesting that differences in olfactory sensitivity participate in insect adaptation.

Genes involved in sex pheromone biosynthesis of Ephestia cautella, an important food storage pest, are determined by transcriptome sequencing

BACKGROUND

Insects use pheromones, chemical signals that underlie all animal behaviors, for communication and for attracting mates. Synthetic pheromones are widely used in pest control strategies because they are environmentally safe. The production of insect pheromones in transgenic plants, which could be more economical and effective in producing isomerically pure compounds, has recently been successfully demonstrated. This research requires information regarding the pheromone biosynthetic pathways and the characterization of pheromone biosynthetic enzymes (PBEs). We used Illumina sequencing to characterize the pheromone gland (PG) transcriptome of the Pyralid moth, Ephestia cautella, a destructive storage pest, to reveal putative candidate genes involved in pheromone biosynthesis, release, transport and degradation.

RESULTS

We isolated the E. cautella pheromone compound as (Z,E)-9,12-tetradecadienyl acetate, and the major pheromone precursors 16:acyl, 14:acyl, E14-16:acyl, E12-14:acyl and Z9,E12-14:acyl. Based on the abundance of precursors, two possible pheromone biosynthetic pathways are proposed. Both pathways initiate from C16:acyl-CoA, with one involving ∆14 and ∆9 desaturation to generate Z9,E12-14:acyl, and the other involving the chain shortening of C16:acyl-CoA to C14:acyl-CoA, followed by ∆12 and ∆9 desaturation to generate Z9,E12-14:acyl-CoA. Then, a final reduction and acetylation generates Z9,E12-14:OAc. Illumina sequencing yielded 83,792 transcripts, and we obtained a PG transcriptome of ~49.5 Mb. A total of 191 PBE transcripts, which included pheromone biosynthesis activating neuropeptides, fatty acid transport proteins, acetyl-CoA carboxylases, fatty acid synthases, desaturases, β-oxidation enzymes, fatty acyl-CoA reductases (FARs) and fatty acetyltransferases (FATs), were selected from the dataset. A comparison of the E. cautella transcriptome data with three other Lepidoptera PG datasets revealed that 45% of the sequences were shared. Phylogenetic trees were constructed for desaturases, FARs and FATs, and transcripts that clustered with the ∆14, ∆12 and ∆9 desaturases, PG-specific FARs and potential candidate FATs, respectively, were identified. Transcripts encoding putative pheromone degrading enzymes, and candidate pheromone carrier and receptor proteins expressed in the E. cautella PG, were also identified.

CONCLUSIONS

Our study provides important background information on the enzymes involved in pheromone biosynthesis. This information will be useful for the in vitro production of E. cautella sex pheromones and may provide potential targets for disrupting the pheromone-based communication system of E. cautella to prevent infestations.

Large number of putative chemoreception and pheromone biosynthesis genes revealed by analyzing transcriptome from ovipositor-pheromone glands of Chilo suppressalis

The chemoreception role of moth ovipositor has long been suggested, but its molecular mechanism is mostly unknown. By transcriptomic analysis of the female ovipositor-pheromone glands (OV-PG) of Chilo suppressalis, we obtained 31 putative chemoreception genes (9 OBPs, 10 CSPs, 2 ORs, 1 SNMP, 8 CXEs and 1 AOX), in addition to 32 genes related to sex pheromone biosynthesis (1 FAS, 6 Dess, 10 FARs, 2 ACOs, 1 ACC, 4 FATPs, 3 ACBPs and 5 ELOs). Tissue expression profiles further revealed that CsupCSP2 and CsupCSP10 were OV-PG biased, while most chemoreception genes were highly and preferably expressed in antennae. This suggests that OV-PG employs mostly the same chemoreception proteins as in antennae, although the physiological roles of these proteins might be different in OV-PG. Of the 32 pheromone biosynthesis related genes, CsupDes4, CsupDes5 and CsupFAR2 are strongly OV-PG biased, and clustered with functionally validated genes from other moths, strongly indicating their involvement in specific step of the pheromone biosynthesis. Our study for the first time identified a large number of putative chemoreception genes, and provided an important basis for exploring the chemoreception mechanisms of OV-PG in C. suppressalis, as well as other moth species.

Candidate chemosensory genes in the Stemborer Sesamia nonagrioides

The stemborer Sesamia nonagrioides is an important pest of maize in the Mediterranean Basin. Like other moths, this noctuid uses its chemosensory system to efficiently interact with its environment. However, very little is known on the molecular mechanisms that underlie chemosensation in this species. Here, we used next-generation sequencing (454 and Illumina) on different tissues from adult and larvae, including chemosensory organs and female ovipositors, to describe the chemosensory transcriptome of S. nonagrioides and identify key molecular components of the pheromone production and detection systems. We identified a total of 68 candidate chemosensory genes in this species, including 31 candidate binding-proteins and 23 chemosensory receptors. In particular, we retrieved the three co-receptors Orco, IR25a and IR8a necessary for chemosensory receptor functioning. Focusing on the pheromonal communication system, we identified a new pheromone-binding protein in this species, four candidate pheromone receptors and 12 carboxylesterases as candidate acetate degrading enzymes. In addition, we identified enzymes putatively involved in S. nonagrioides pheromone biosynthesis, including a ∆11-desaturase and different acetyltransferases and reductases. RNAseq analyses and RT-PCR were combined to profile gene expression in different tissues. This study constitutes the first large scale description of chemosensory genes in S. nonagrioides.

Expression patterns and binding properties of three pheromone binding proteins in the diamondback moth, Plutella xyllotella

Pheromone binding proteins (PBPs) play a key role in transporting hydrophobic sex pheromone components emitted by con-specific female across aqueous sensillar lymph to the surface of olfactory receptor neurons. A number of PBPs have been cloned, however, details of their function are still largely unknown. Here three pheromone binding protein genes in the diamondback moth, Plutella xyllotella were cloned. The three PxylPBP genes are not only expressed in chemosensory tissues but also expressed in female reproductive organs and male legs. To better understand the functions of PxylPBPs in the initial steps of pheromone recognition, three PxylPBPs were expressed in Escherichia coli and the ligand-binding specificities of purified recombinant PBPs were investigated. Fluorescence binding assays indicate that three PxylPBPs not only robustly bound all four sex pheromone components but also significantly bound pheromone analogs with at least one double bond, while weakly bound tested plant volatiles. Although pheromone analogs bound PBPs, they could not elicit the moth's electrophysiological response. These experiments provide evidence that PxylPBPs have limited selectivity of pheromone components and analogs and some downstream components such as odor receptors might be involved in selectivity and specificity of pheromone perception in P. xyllotella.

Candidate chemosensory ionotropic receptors in a Lepidoptera

A new family of candidate chemosensory ionotropic receptors (IRs) related to ionotropic glutamate receptors (iGluRs) was recently discovered in Drosophila melanogaster. Through Blast analyses of an expressed sequenced tag library prepared from male antennae of the noctuid moth Spodoptera littoralis, we identified 12 unigenes encoding proteins related to D. melanogaster and Bombyx mori IRs. Their full length sequences were obtained and the analyses of their expression patterns suggest that they were exclusively expressed or clearly enriched in chemosensory organs. The deduced protein sequences were more similar to B. mori and D. melanogaster IRs than to iGluRs and showed considerable variations in the predicted ligand-binding domains; none have the three glutamate-interacting residues found in iGluRs, suggesting different binding specificities. Our data suggest that we identified members of the insect IR chemosensory receptor family in S. littoralis and we report here the first demonstration of IR expression in Lepidoptera.

Functional characterization of pheromone receptors in the tobacco budworm Heliothis virescens

Functional analyses of candidate Heliothis virescens pheromone odorant receptors (HvORs) were conducted using heterologous expression in Xenopus oocytes. HvOR6 was found to be highly tuned to Z9-14:Ald, while HvOR13, HvOR14 and HvOR16 showed specificity for Z11-16:Ald, Z11-16:OAc and Z11-16:OH, respectively. HvOR15, which had been considered a candidate receptor for Z9-14:Ald did not respond to any of the pheromone compounds tested, nor to 50 other general odorants. Thus, while HvOR15 is specifically expressed in H. virescens male antennae, its role in pheromone reception remains unknown. Based on our results and previous research we can now assign pheromone receptors in H. virescens males to each of the critical H. virescens agonistic pheromone compounds and two antagonistic compounds produced by heterospecific females.

Differential expression of odorant receptor genes involved in the sexual isolation of two Heliothis moths

Moth sexual communication systems are highly diverse, but the mechanisms underlying their evolutionary diversification remain unclear. Recently, genes coding for odorant receptors (ORs) OR6, OR14, OR15 and OR16 have been genetically associated with species-specific male response to female pheromone blends in Heliothis virescens (Hv) and Heliothis subflexa (Hs). Quantitative real-time PCR analysis indicates that expression of HvOR6, HsOR6, HvOR14, HsOR14, HvOR15 and HsOR15 is male biased, which supports the hypothesis that they have a role in mediating female sex pheromone detection. The genes HvOR14, HvOR15 and HvOR16 are expressed at higher levels than their corresponding orthologues HsOR14, HsOR15 and HsOR16 in male antennae, while HvOR6 and HsOR6 transcripts are equally abundant in male antennae. The lack of higher expression of any of the receptor genes in H. subflexa antennae suggests that interspecific sequence differences, rather than gene regulation differences, underly the species-specific male response to pheromone components.

Transcriptome analysis of the sex pheromone gland of the noctuid moth Heliothis virescens

BACKGROUND

The chemical components of sex pheromones have been determined for more than a thousand moth species, but so far only a handful of genes encoding enzymes responsible for the biosynthesis of these compounds have been identified. For understanding the evolution of moth sexual communication, it is essential to know which genes are involved in the production of specific pheromone components and what controls the variation in their relative frequencies in the pheromone blend. We used a transcriptomic approach to characterize the pheromone gland of the Noctuid moth Heliothis virescens, an important agricultural pest, in order to obtain substantial general sequence information and to identify a range of candidate genes involved in the pheromone biosynthetic pathway.

RESULTS

To facilitate identifying sets of genes involved in a broad range of processes and to capture rare transcripts, we developed our majority of ESTs from a normalized cDNA library of Heliothis virescens pheromone glands (PG). Combining these with a non-normalized library yielded a total of 17,233 ESTs, which assembled into 2,082 contigs and 6,228 singletons. Using BLAST searches of the NR and Swissprot databases we were able to identify a large number of putative unique gene elements (unigenes), which we compared to those derived from previous transcriptomic surveys of the larval stage of Heliothis virescens. The distribution of unigenes among GO Biological Process functional groups shows an overall similarity between PG and larval transcriptomes, but with distinct enrichment of specific pathways in the PG. In addition, we identified a large number of candidate genes in the pheromone biosynthetic pathways.

CONCLUSION

These data constitute one of the first large-scale EST-projects for Noctuidae, a much-needed resource for exploring these pest species. Our analysis shows a surprisingly complex transcriptome and we identified a large number of potential pheromone biosynthetic pathway and immune-related genes that can be applied to population and systematic studies of Heliothis virescens and other Noctuidae.