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

Genome-Wide Identification, Evolution, and Female-Biased Expression Analysis of Odorant Receptors in Tuta absoluta (Lepidoptera: Gelechiidae)

The tomato leafminer, Tuta absoluta (Lepidoptera: Gelechiidae), is a highly destructive invasive pest targeting Solanaceae crops. Its olfactory system plays a crucial role in host location, mate finding, and other behavioral activities. However, there is a notable gap in the literature regarding the characterization of its chemosensory genes. In this study, we conducted a genome-wide identification of 58 odorant receptors (ORs) of T. absoluta. The identified ORs exhibit coding sequence (CDS) lengths ranging from 1062 bp to 1419 bp, encoding proteins of 354 to 473 amino acids. Gene structure analysis showed that the majority of these ORs consist of five, seven, eight, or nine exons, collectively representing 67% of the total ORs identified. Through chromosomal mapping, we identified several tandemly duplicate genes, including TabsOR12a, TabsOR12b, TabsOR12c, TabsOR21a, TabsOR21b, TabsOR34a, TabsOR34b, TabsOR34c, TabsOR62a, and TabsOR62b. The phylogenetic analysis indicated that six TabsORs were clustered within the lepidopteran sex pheromone receptor clade, while an expansion clade containing ten TabsORs resulted from tandem duplication events. Additionally, five TabsORs were classified into a specific OR clade in T. absoluta. Furthermore, through RNA-Seq and RT-qPCR analyses, we identified five TabsORs (TabsOR21a, TabsOR26a, TabsOR34a, TabsOR34c, and TabsOR36) exhibiting female-antennae-biased expression. Our study provides a valuable foundation to further investigations into the molecular and ecological functions of TabsORs, particularly in relation to oviposition behavior. These findings provide foundational data for the future exploration of the functions of female-biased expression OR genes in T. absoluta, thereby facilitating the further development of eco-friendly attract-and-kill techniques for the prevention and control of T. absoluta.

Identification of a receptor for the sex pheromone of the vine mealybug, Planococcus ficus

The vine mealybug, Planococcus ficus, is a significant pest of vineyards in all major grape growing regions of the world. This pest causes significant aesthetic damage to berry clusters through its feeding behavior and secretion of "honeydew", which leads to significant decreases in crop marketability. More importantly, the vine mealybug is a vector of several grapevine viruses which are the causal agent of grapevine leafroll disease, one of the most destructive and economically devastating diseases of the grape industry worldwide. As there is no cure for grapevine leafroll disease, the only control measures available to reduce its spread are to remove infected vines whilst simultaneously controlling mealybug populations. Using transcriptomic libraries prepared from male and female mealybugs and a draft genome, we identified and evaluated expression levels of members of the odorant receptor gene family. Interestingly, of the 50 odorant receptors identified from these P. ficus genetic resources, only 23 were found to be expressed in females, suggesting this flightless life stage has a decreased reliance on the olfactory system. In contrast, 46 odorant receptors were found to be expressed in the alate male life stage. Heterologous expression of eight of these receptors, along with the obligate co-receptor, Orco, in HEK293 cells allowed for the identification of two receptors that respond to lavandulyl senecioate, the sole constituent of the sex pheromone used by this species. Interestingly, one of these receptors, PficOR8, also responded to the sex pheromone used by the Japanese mealybug, Planococcus kraunhiae. The data presented here represent the first report of odorant receptor gene family expression levels, as well as the identification of the first sex pheromone receptor, in soft-scale insects. The identification of a receptor for the vine mealybug sex pheromone will allow for the development of novel, species-specific pest control tools and monitoring devices.

High-throughput ligand profile characterization in novel cell lines expressing seven heterologous insect olfactory receptors for the detection of volatile plant biomarkers

Agriculturally important crop plants emit a multitude of volatile organic compounds (VOCs), which are excellent indicators of their health status and their interactions with pathogens and pests. In this study, we have developed a novel cellular olfactory panel for detecting fungal pathogen-related VOCs we had identified in the field, as well as during controlled inoculations of several crop plants. The olfactory panel consists of seven stable HEK293 cell lines each expressing a functional Drosophila olfactory receptor as a biosensing element along with GCaMP6, a fluorescent calcium indicator protein. An automated 384-well microplate reader was used to characterize the olfactory receptor cell lines for their sensitivity to reference VOCs. Subsequently, we profiled a set of 66 VOCs on all cell lines, covering a concentration range from 1 to 100 μM. Results showed that 49 VOCs (74.2%) elicited a response in at least one olfactory receptor cell line. Some VOCs activated the cell lines even at nanomolar (ppb) concentrations. The interaction profiles obtained here will support the development of biosensors for agricultural applications. Additionally, the olfactory receptor proteins can be purified from these cell lines with sufficient yields for further processing, such as structure determination or integration with sensor devices.

Odorant receptor orthologues in conifer-feeding beetles display conserved responses to ecologically relevant odours

Insects are able to detect a plethora of olfactory cues using a divergent family of odorant receptors (ORs). Despite the divergent nature of this family, related species frequently express several evolutionarily conserved OR orthologues. In the largest order of insects, Coleoptera, it remains unknown whether OR orthologues have conserved or divergent functions in different species. Using HEK293 cells, we addressed this question through functional characterization of two groups of OR orthologues in three species of the Curculionidae (weevil) family, the conifer-feeding bark beetles Ips typographus L. ("Ityp") and Dendroctonus ponderosae Hopkins ("Dpon") (Scolytinae), and the pine weevil Hylobius abietis L. ("Habi"; Molytinae). The ORs of H. abietis were annotated from antennal transcriptomes. The results show highly conserved response specificities, with one group of orthologues (HabiOR3/DponOR8/ItypOR6) responding exclusively to 2-phenylethanol (2-PE), and the other group (HabiOR4/DponOR9/ItypOR5) responding to angiosperm green leaf volatiles (GLVs). Both groups of orthologues belong to the coleopteran OR subfamily 2B, and share a common ancestor with OR5 in the cerambycid Megacyllene caryae, also tuned to 2-PE, suggesting a shared evolutionary history of 2-PE receptors across two beetle superfamilies. The detected compounds are ecologically relevant for conifer-feeding curculionids, and are probably linked to fitness, with GLVs being used to avoid angiosperm nonhost plants, and 2-PE being important for intraspecific communication and/or playing a putative role in beetle-microbe symbioses. To our knowledge, this study is the first to reveal evolutionary conservation of OR functions across several beetle species and hence sheds new light on the functional evolution of insect ORs.

Expression Profiling and Functional Analysis of Candidate Odorant Receptors in Galeruca daurica

Simple Summary

Odorant receptors (ORs) play an important role in the olfactory system in insects. However, there is no functional research on the odorant receptors of Galeruca daurica. In this study, 21 OR genes were identified from the transcriptome database of G. daurica adults. Most GdauORs were mainly expressed in antennae, and the expression levels of GdauORs in adults were affected by age. When GdauOR4, GdauOR15, and GdauORco were silenced by RNAi, the electrophysiological responses to host plant volatiles were significantly decreased.

Abstract

Galeruca daurica (Joannis) is an oligophagous pest in the grasslands of Inner Mongolia, China, which feed mainly on Allium spp. Odorant receptors (ORs) play an important role in the olfactory system in insects, and function together with olfactory co-receptor (ORco). In this study, 21 OR genes were identified from the transcriptome database of G. daurica adults, and named GdauOR1-20 and GdauORco. The expression profiles were examined by RT-qPCR and RNA interference (RNAi) and electroantennogram (EAG) experiments were conducted to further identify the olfactory functions of GdauOR4, GdauOR11, GdauOR15, and GdauORco. It was found that 15 GdauORs (OR1, OR3-6, OR8, OR11-13, OR15, OR17-20, and ORco) were mainly expressed in antennae, and the expression levels of GdauORs in adults were affected by age. When GdauOR4, GdauOR15, and GdauORco were silenced by RNAi, the electrophysiological responses to host plant volatiles were significantly decreased in G. daurica. This study lays a necessary foundation for clarifying the mechanism on finding host plants in G. daurica.

Sex pheromone receptors of the light brown apple moth, Epiphyas postvittana, support a second major pheromone receptor clade within the Lepidoptera

Sex pheromones facilitate species-specific sex communication within the Lepidoptera. They are detected by specialised pheromone receptors (PRs), most of which to date fall into a single monophyletic receptor lineage (frequently referred to as "the PR clade") within the odorant receptor (OR) family. Here we investigated PRs of the invasive horticultural pest, Epiphyas postvittana, commonly known as the light brown apple moth. Ten candidate PRs were selected, based on their male-biased expression in antennae or their relationship to the PR clade, for functional assessment in both HEK293 cells and Xenopus oocytes. Of these, six ORs responded to compounds that include components of the E. postvittana ('Epos') sex pheromone blend or compounds that antagonise sex pheromone attraction. In phylogenies, four of the characterised receptors (EposOR1, 6, 7 and 45) fall within the PR clade and two other male-biased receptors (EposOR30 and 34) group together well outside the PR clade. This new clade of pheromone receptors includes the receptor for (E)-11-tetradecenyl acetate (EposOR30), which is the main component of the sex pheromone blend for this species. Interestingly, receptors of the two clades do not segregate by preference for compounds associated with behavioural response (agonist or antagonist), isomer type (E or Z) or functional group (alcohol or acetate), with examples of each scattered across both clades. Phylogenetic comparison with PRs from other species supports the existence of a second major clade of lepidopteran ORs including, EposOR30 and 34, that has been co-opted into sex pheromone detection in the Lepidoptera. This second clade of sex pheromone receptors has an origin that likely predates the split between the major lepidopteran families.

Codon Optimization of Insect Odorant Receptor Genes May Increase Their Stable Expression for Functional Characterization in HEK293 Cells

Insect odorant receptor (OR) genes are routinely expressed in Human Embryonic Kidney (HEK) 293 cells for functional characterization ("de-orphanization") using transient or stable expression. However, progress in this research field has been hampered because some insect ORs are not functional in this system, which may be due to insufficient protein levels. We investigated whether codon optimization of insect OR sequences for expression in human cells could facilitate their functional characterization in HEK293 cells with stable and inducible expression. We tested the olfactory receptor co-receptor (Orco) proteins from the bark beetles Ips typographus ("Ityp") and Dendroctonus ponderosae ("Dpon"), and six ItypORs previously characterized in Xenopus laevis oocytes and/or HEK cells. Western blot analysis indicated that codon optimization yielded increased cellular protein levels for seven of the eight receptors. Our experimental assays demonstrated that codon optimization enabled functional characterization of two ORs (ItypOR25 and ItypOR29) which are unresponsive when expressed from wildtype (non-codon optimized) genes. Similar to previous Xenopus oocyte recordings, ItypOR25 responded primarily to the host/conifer monoterpene (+)-3-carene. ItypOR29 responded primarily to (+)-isopinochamphone and similar ketones produced by fungal symbionts and trees. Codon optimization also resulted in significantly increased responses in ItypOR49 to its pheromone ligand (R)-(-)-ipsdienol, and improved responses to the Orco agonist VUAA1 in ItypOrco. However, codon optimization did not result in functional expression of DponOrco, ItypOR23, ItypOR27, and ItypOR28 despite higher protein levels as indicated by Western blots. We conclude that codon optimization may enable or improve the functional characterization of insect ORs in HEK cells, although this method is not sufficient for all ORs that are not functionally expressed from wildtype genes.

Identification and expression profiling of chemosensory membrane protein genes in Achelura yunnanensis (Lepidoptera: Zygaenidae)

During the past decade, antennal transcriptome sequencing has been applied to at least 50 species from 16 families of the Lepidoptera order of insects, emphasizing the identification and characterization of chemosensory-related genes. However, little is known about the chemosensory genes in the Zygaenidae family of Lepidoptera. Herein, we report the transmembrane protein gene repertoires involved in chemoreception from Achelura yunnanensis (Lepidoptera: Zygaenidae) through transcriptome sequencing, bioinformatics, phylogenetics and polymerase chain reaction (PCR) approaches. Transcriptome analysis led to the generation of 555.47 million clean reads and accumulation of 83.30 gigabases of data. From this transcriptome, 132 transcripts encoding 69 odorant receptors (ORs), 33 gustatory receptors (GRs), 26 ionotropic receptors (IRs), and four sensory neuron membrane proteins (SNMPs) were identified, 69 of which were full-length sequences. Notably, the number of SNMPs in A. yunnanensis was the largest set in Lepidoptera to date. Phylogenetic analysis combined with sequence homology highlighted several conserved groups of chemoreceptors, including pheromone receptors (a so-called pheromone receptor (PR) clade: AyunOR50 and novel PR members: AyunOR39 and OR40), a phenylacetaldehyde-sensing OR (AyunOR28), carbon dioxide receptors (AyunGR1-3), and antennal IRs (13 A-IRs). In addition, a Zygaenidae-specific OR expansion was observed, including 15 A. yunnanensis members. Expression profiles revealed 99 detectable chemosensory genes in the antennae and 20 in the reproductive tissues, some of which displayed a sex-biased expression. This study identifies potential olfactory molecular candidates for sensing sex pheromones, phenylacetaldehyde or other odorants, and provides preliminary evidence for the putative reproductive function of chemosensory membrane protein genes in A. yunnanensis.

Putative ligand binding sites of two functionally characterized bark beetle odorant receptors

BACKGROUND

Bark beetles are major pests of conifer forests, and their behavior is primarily mediated via olfaction. Targeting the odorant receptors (ORs) may thus provide avenues towards improved pest control. Such an approach requires information on the function of ORs and their interactions with ligands, which is also essential for understanding the functional evolution of these receptors. Hence, we aimed to identify a high-quality complement of ORs from the destructive spruce bark beetle Ips typographus (Coleoptera, Curculionidae, Scolytinae) and analyze their antennal expression and phylogenetic relationships with ORs from other beetles. Using 68 biologically relevant test compounds, we next aimed to functionally characterize ecologically important ORs, using two systems for heterologous expression. Our final aim was to gain insight into the ligand-OR interaction of the functionally characterized ORs, using a combination of computational and experimental methods.

RESULTS

We annotated 73 ORs from an antennal transcriptome of I. typographus and report the functional characterization of two ORs (ItypOR46 and ItypOR49), which are responsive to single enantiomers of the common bark beetle pheromone compounds ipsenol and ipsdienol, respectively. Their responses and antennal expression correlate with the specificities, localizations, and/or abundances of olfactory sensory neurons detecting these enantiomers. We use homology modeling and molecular docking to predict their binding sites. Our models reveal a likely binding cleft lined with residues that previously have been shown to affect the responses of insect ORs. Within this cleft, the active ligands are predicted to specifically interact with residues Tyr84 and Thr205 in ItypOR46. The suggested importance of these residues in the activation by ipsenol is experimentally supported through site-directed mutagenesis and functional testing, and hydrogen bonding appears key in pheromone binding.

CONCLUSIONS

The emerging insight into ligand binding in the two characterized ItypORs has a general importance for our understanding of the molecular and functional evolution of the insect OR gene family. Due to the ecological importance of the characterized receptors and widespread use of ipsenol and ipsdienol in bark beetle chemical communication, these ORs should be evaluated for their potential use in pest control and biosensors to detect bark beetle infestations.

Putative ligand binding sites of two functionally characterized bark beetle odorant receptors

BACKGROUND

Bark beetles are major pests of conifer forests, and their behavior is primarily mediated via olfaction. Targeting the odorant receptors (ORs) may thus provide avenues towards improved pest control. Such an approach requires information on the function of ORs and their interactions with ligands, which is also essential for understanding the functional evolution of these receptors. Hence, we aimed to identify a high-quality complement of ORs from the destructive spruce bark beetle Ips typographus (Coleoptera, Curculionidae, Scolytinae) and analyze their antennal expression and phylogenetic relationships with ORs from other beetles. Using 68 biologically relevant test compounds, we next aimed to functionally characterize ecologically important ORs, using two systems for heterologous expression. Our final aim was to gain insight into the ligand-OR interaction of the functionally characterized ORs, using a combination of computational and experimental methods.

RESULTS

We annotated 73 ORs from an antennal transcriptome of I. typographus and report the functional characterization of two ORs (ItypOR46 and ItypOR49), which are responsive to single enantiomers of the common bark beetle pheromone compounds ipsenol and ipsdienol, respectively. Their responses and antennal expression correlate with the specificities, localizations, and/or abundances of olfactory sensory neurons detecting these enantiomers. We use homology modeling and molecular docking to predict their binding sites. Our models reveal a likely binding cleft lined with residues that previously have been shown to affect the responses of insect ORs. Within this cleft, the active ligands are predicted to specifically interact with residues Tyr84 and Thr205 in ItypOR46. The suggested importance of these residues in the activation by ipsenol is experimentally supported through site-directed mutagenesis and functional testing, and hydrogen bonding appears key in pheromone binding.

CONCLUSIONS

The emerging insight into ligand binding in the two characterized ItypORs has a general importance for our understanding of the molecular and functional evolution of the insect OR gene family. Due to the ecological importance of the characterized receptors and widespread use of ipsenol and ipsdienol in bark beetle chemical communication, these ORs should be evaluated for their potential use in pest control and biosensors to detect bark beetle infestations.

The role of SNMPs in insect olfaction

The sense of smell enables insects to recognize olfactory signals crucial for survival and reproduction. In insects, odorant detection highly depends on the interplay of distinct proteins expressed by specialized olfactory sensory neurons (OSNs) and associated support cells which are housed together in chemosensory units, named sensilla, mainly located on the antenna. Besides odorant-binding proteins (OBPs) and olfactory receptors, so-called sensory neuron membrane proteins (SNMPs) are indicated to play a critical role in the detection of certain odorants. SNMPs are insect-specific membrane proteins initially identified in pheromone-sensitive OSNs of Lepidoptera and are indispensable for a proper detection of pheromones. In the last decades, genome and transcriptome analyses have revealed a wide distribution of SNMP-encoding genes in holometabolous and hemimetabolous insects, with a given species expressing multiple subtypes in distinct cells of the olfactory system. Besides SNMPs having a neuronal expression in subpopulations of OSNs, certain SNMP types were found expressed in OSN-associated support cells suggesting different decisive roles of SNMPs in the peripheral olfactory system. In this review, we will report the state of knowledge of neuronal and non-neuronal members of the SNMP family and discuss their possible functions in insect olfaction.

Molecular signatures of sexual communication in the phlebotomine sand flies

Phlebotomine sand flies employ an elaborate system of pheromone communication wherein males produce pheromones that attract other males to leks (thus acting as an aggregation pheromone) and females to the lekking males (sex pheromone). In addition, the type of pheromone produced varies among populations. Despite the numerous studies on sand fly chemical communication, little is known of their chemosensory genome. Chemoreceptors interact with chemicals in an organism’s environment to elicit essential behaviors such as the identification of suitable mates and food sources. Thus, they play important roles during adaptation and speciation. Major chemoreceptor gene families, odorant receptors (ORs), gustatory receptors (GRs) and ionotropic receptors (IRs) together detect and discriminate the chemical landscape. Here, we annotated the chemoreceptor repertoire in the genomes of Lutzomyia longipalpis and Phlebotomus papatasi, major phlebotomine vectors in the New World and Old World, respectively. Comparison with other sequenced Diptera revealed a large and unique expansion where over 80% of the ~140 ORs belong to a single, taxonomically restricted clade. We next conducted a comprehensive analysis of the chemoreceptors in 63 L. longipalpis individuals from four different locations in Brazil representing allopatric and sympatric populations and three sex-aggregation pheromone types (chemotypes). Population structure based on single nucleotide polymorphisms (SNPs) and gene copy number in the chemoreceptors corresponded with their putative chemotypes, and corroborate previous studies that identified multiple populations. Our work provides genomic insights into the underlying behavioral evolution of sexual communication in the L. longipalpis species complex in Brazil, and highlights the importance of accounting for the ongoing speciation in central and South American Lutzomyia that could have important implications for vectorial capacity.

Phlebotomine sand flies are the primary vectors of Leishmania parasites, the causative agents of cutaneous and visceral leishmaniasis. Due to the lack of vaccines, control of leishmaniasis relies upon reducing human exposure to sand flies. Sand flies produce sex-aggregation pheromones that elicit robust olfactory behaviors, and the molecular targets for pheromone detection remain unknown. We identified chemoreceptors in the genomes of L. longipalpis and P. papatasi, and used these gene models to explore chemoreceptor evolution in 63 L. longipalpis individuals representing different pheromone types. These analyses identified genomic loci underlying chemosensory behavior in sand flies. This paves the way for understanding the sand fly species diversity at the molecular level, and functional characterization of these candidate genes will isolate and identify chemostimuli that can directly be tested as potential attractants for odor-baited traps.

The general odorant receptor GmolOR9 from Grapholita molesta (Lepidoptera: Tortricidae) is mainly tuned to eight host-plant volatiles

Grapholita molesta is one of the most destructive fruit pests distributed worldwide. Odorant receptors (ORs) located on the dendritic membrane of chemosensory neurons are deemed to be key molecules for sensing exogenous chemical signals. In this study, GmolOR9, a general OR from G. molesta, was functionally characterized. Quantitative real-time polymerase chain reaction revealed that GmolOR9 was more highly expressed in adults than in other stages, including eggs, larvae, and pupae. GmolOR9 expression was highly significantly more in the antennae of females than in those of males, and the highest level occurred in the antennae of 3-day-old female adults. GmolOR9 was broadly tuned to eight of 47 odorant components tested, including (Z)-3-hexenyl acetate, butyl propionate, ethyl hexanoate, ethyl heptanoate, 1-hexanol, (Z)-3-hexenol, 2-ethyl-1-hexanol, and linalool, by in vitro heterologous expression. Furthermore, electroantennogram responses indicated that the effects of dsOR9-injected females to (Z)-3-hexenyl acetate dramatically decreased. These results suggested that GmolOR9 might be involved in detecting host-plant volatiles. Moreover, (Z)-3-hexenyl acetate might serve as a potential attractant for the biological control of G. molesta.

Identification and expression profiles of candidate chemosensory receptors in Histia rhodope (Lepidoptera: Zygaenidae)

Insect olfaction and vision play important roles in survival and reproduction. Diurnal butterflies mainly rely on visual cues whereas nocturnal moths rely on olfactory signals to locate external resources. Histia rhodope Cramer (Lepidoptera: Zygaenidae) is an important pest of the landscape tree Bischofia polycarpa in China and other Southeast Asian regions. As a diurnal moth, H. rhodope represents a suitable model for studying the evolutionary shift from olfactory to visual communication. However, only a few chemosensory soluble proteins have been characterized and information on H. rhodope chemoreceptor genes is currently lacking. In this study, we identified 45 odorant receptors (ORs), nine ionotropic receptors (IRs), eight gustatory receptors (GRs) and two sensory neuron membrane proteins (SNMPs) from our previously acquired H. rhodope antennal transcriptomic data. The number of chemoreceptors of H. rhodope was less compared with that found in many nocturnal moths. Some specific chemoreceptors such as OR co-receptor (ORco), ionotropic receptors co-receptor, CO₂ receptors, sugar receptors and bitter receptors were predicted by phylogenetic analysis. Notably, two candidate pheromone receptors (PRs) were identified within a novel PR lineage. qRT-PCR results showed that almost all tested genes (22/24) were predominantly expressed in antennae, indicating that they may be important in olfactory function. Among these antennae-enriched genes, six ORs, five IRs and two GRs displayed female-biased expression, while two ORs displayed male-biased expression. Additionally, HrhoIR75q.2 and HrhoGR67 were more highly expressed in heads and legs. This study enriches the olfactory gene inventory of H. rhodope and provides the foundation for further research of the chemoreception mechanism in diurnal moths.

Evolution of sex pheromone receptors in Dendrolimus punctatus Walker (lepidoptera: Lasiocampidae) is divergent from other moth species

Dendrolimus punctatus Walker (Lepidoptera: Lasiocampidae) is a pine caterpillar moth distributed in most areas of southern China and is an economically important pest of pine, due to its defoliation activity. Understanding fundamental sex pheromone perception mechanisms in D. punctatus may provide effective and sustainable options for novel control strategies. However, the identification and function of pheromone receptors, key genes that receipt the pheromone of this pest, are both unclear now. Previous researches suggested several candidate pheromone receptors whose expression levels were male antennae bias in D. punctatus. In this study, we cloned six candidate pheromone receptors (DpunOR 20/45/46/51/54/58) and Orco from D. punctatus. Phylogenetic tree analysis showed that lepidopteran PRs tend to be conserved and clustered together; however, D. punctatus candidate PRs were located in a distinct clade. Motif analysis of PRs showed clear sequences differences between Dendrolimus spp. and other tested moth species. To illustrate the ligand response properties of the candidate PRs of D. punctatus, each of the six genes was expressed with an Orco gene in Xenopus oocytes and using two-electrode voltage-clamp recordings. Finally, we successfully identified two sex pheromone receptors (PR45 and PR46). Our study, which identified a novel lineage of PRs tuned to Type I pheromones in Lepidoptera, provides evidence for the new evolution origin of sex pheromone communication in moths, and lays a foundation for the development of novel control strategies of D. punctatus.

Identification and Expression Analysis of Chemosensory Receptor Genes in Bradysia odoriphaga (Diptera: Sciaridae)

The chive midge, Bradysia odoriphaga, is a major insect pest affecting Chinese chive production in China. Its adult life stage is nonfeeding and has a short life span. Hence, the perception of chemical stimuli is important for its adult behavior and reproductive success. To better understand its chemosensory process at the molecular level, chemosensory receptor genes were identified based on transcriptomes of B. odoriphaga. In total, 101 chemosensory genes were identified from the antenna and body transcriptomes, including 71 odorant receptors (ORs), 18 ionotropic receptors (IRs), 5 gustatory receptors (GRs), and 7 sensory neuron membrane proteins (SNMPs). Phylogenetic analysis indicated that most of these genes have homologs among other Dipteran insects. A transcript abundance comparison based on FPKM values was conducted to analyze the sex- and tissue-specific expression profiles of these chemosensory genes. Moreover, quantitative real-time PCR of OR transcripts was performed on different tissues (female antennae, male antennae, heads, and legs) to verify the transcriptional expression levels of ORs in the transcriptomes. This analysis suggested that 44 ORs showed significantly higher expression in the female antennae, while 16 OR transcripts were most highly expressed in the male antennae and may play significant roles in sex pheromone detection. In addition, some IRs and GRs might be involved in CO2 and sugar detection and temperature sensing. In the present study, 101 chemosensory genes were identified, and their putative functions were predicted. This work could provide a basis to facilitate functional clarification of these chemosensory genes at the molecular level.

Functional characterization of odorant receptors from the moth Eriocrania semipurpurella: A comparison of results in the Xenopus oocyte and HEK cell systems

The Xenopus oocyte and the Human Embryonic Kidney (HEK) 293 cell expression systems are frequently used for functional characterization (deorphanization) of insect odorant receptors (ORs). However, the inherent characteristics of these heterologous systems differ in several aspects, which raises the question of whether the two systems provide comparable results, and how well the results correspond to the responses obtained from olfactory sensory neurons in vivo. Five candidate pheromone receptors were previously identified in the primitive moth Eriocrania semipurpurella (Esem) and their responses were characterized in HEK cells. We re-examined the responses of these five EsemORs in Xenopus oocytes. We showed that in both systems, EsemOR1 specifically responded to the plant volatile β-caryophyllene. EsemOR3 responded stronger to the pheromone component (S,Z)-6-nonen-2-ol than to its enantiomer (R,Z)-6-nonen-2-ol, the second pheromone component. However, EsemOR3 also responded secondarily to the plant volatile β-caryophyllene in the oocyte system, but not in the HEK cell system. EsemOR4 was unresponsive in the HEK cells, but responded primarily to (R,Z)-6-nonen-2-ol followed by (S,Z)-6-nonen-2-ol in the oocytes, representing a discovery of a new pheromone receptor in this species. EsemOR5 was broadly tuned in both systems, but the rank order among the most active pheromone compounds and antagonists was different. EsemOR6 showed no response to any compound in either system. We compared the results obtained in the two different heterologous systems with the activity previously recorded in vivo, and performed in situ hybridization to localize the expression of these OR genes in the antennae. In spite of similar results overall, differences in OR responses between heterologous expression systems suggest that conclusions about the function of individual ORs may differ depending on the system used for deorphanization.

The diversity and evolution of odorant receptors in beetles (Coleoptera)

The insect odorant receptors (ORs) are amongst the largest gene families in insect genomes and the primary means by which insects recognize volatile compounds. The evolution of ORs is thus instrumental in explaining the chemical ecology of insects and as a model of evolutionary biology. However, although ORs have been described from numerous insect species, their analysis within and amongst the insect orders has been hindered by a combination of limited genomic information and a tendency of the OR family toward rapid divergence, gain, and loss. We addressed these issues in the insect order Coleoptera through a targeted genomic annotation effort that included 1181 ORs from one species of the sister order Strepsiptera and 10 species representing the four coleopteran suborders. The numbers of ORs in each species varied from hundreds to fewer than 10, but coleopteran ORs could nevertheless be represented within a scheme of nine monophyletic subfamilies. We observed many radiations and losses of genes amongst OR subfamilies, and the diversity of ORs appeared to parallel the host breadth of the study species. However, some small lineages of ORs persisted amongst many coleopteran families, suggesting receptors of key function that underlie the olfactory ecology of beetles.

Genomic content of chemosensory genes correlates with host range in wood-boring beetles (Dendroctonus ponderosae, Agrilus planipennis, and Anoplophora glabripennis)

Background

Olfaction and gustation underlie behaviors that are crucial for insect fitness, such as host and mate selection. The detection of semiochemicals is mediated via proteins from large and rapidly evolving chemosensory gene families; however, the links between a species’ ecology and the diversification of these genes remain poorly understood. Hence, we annotated the chemosensory genes from genomes of select wood-boring coleopterans, and compared the gene repertoires from stenophagous species with those from polyphagous species.

Results

We annotated 86 odorant receptors (ORs), 60 gustatory receptors (GRs), 57 ionotropic receptors (IRs), 4 sensory neuron membrane proteins (SNMPs), 36 odorant binding proteins (OBPs), and 11 chemosensory proteins (CSPs) in the mountain pine beetle (Dendroctonus ponderosae), and 47 ORs, 30 GRs, 31 IRs, 4 SNMPs, 12 OBPs, and 14 CSPs in the emerald ash borer (Agrilus planipennis). Four SNMPs and 17 CSPs were annotated in the polyphagous wood-borer Anoplophora glabripennis. The gene repertoires in the stenophagous D. ponderosae and A. planipennis are reduced compared with those in the polyphagous A. glabripennis and T. castaneum, which is largely manifested through small gene lineage expansions and entire lineage losses. Alternative splicing of GR genes was limited in D. ponderosae and apparently absent in A. planipennis, which also seems to have lost one carbon dioxide receptor (GR1). A. planipennis has two SNMPs, which are related to SNMP3 in T. castaneum. D. ponderosae has two alternatively spliced OBP genes, a novel OBP “tetramer”, and as many as eleven IR75 members. Simple orthology was generally rare in beetles; however, we found one clade with orthologues of putative bitter-taste GRs (named the “GR215 clade”), and conservation of IR60a from Drosophila melanogaster.

Conclusions

Our genome annotations represent important quantitative and qualitative improvements of the original datasets derived from transcriptomes of D. ponderosae and A. planipennis, facilitating evolutionary analysis of chemosensory genes in the Coleoptera where only a few genomes were previously annotated. Our analysis suggests a correlation between chemosensory gene content and host specificity in beetles. Future studies should include additional species to consolidate this correlation, and functionally characterize identified proteins as an important step towards improved control of these pests.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-6054-x) contains supplementary material, which is available to authorized users.

SRD-1 in AWA neurons is the receptor for female volatile sex pheromones in C. elegans males

Pheromones are critical cues for attracting mating partners for successful reproduction. Sexually mature Caenorhabditis remanei virgin females and self-sperm-depleted Caenorhabditis elegans hermaphrodites produce volatile sex pheromones to attract adult males of both species from afar. The chemoresponsive receptor in males has remained unknown. Here, we show that the male chemotactic behavior requires amphid sensory neurons (AWA neurons) and the G-protein-coupled receptor SRD-1. SRD-1 expression in AWA neurons is sexually dimorphic, with the levels being high in males but undetectable in hermaphrodites. Notably, srd-1 mutant males lack the chemotactic response and pheromone-induced excitation of AWA neurons, both of which can be restored in males and hermaphrodites by AWA-specific srd-1 expression, and ectopic expression of srd-1 in AWB neurons in srd-1 mutants results in a repulsive behavioral response in both sexes. Furthermore, we show that the C-terminal region of SRD-1 confers species-specific differences in the ability to perceive sex pheromones between C. elegans and C. remanei These findings offer an excellent model for dissecting how a single G-protein-coupled receptor expressed in a dimorphic neural system contributes to sex-specific behaviors in animals.

Antennal Transcriptome Analysis of the Chemosensory Gene Families From Trichoptera and Basal Lepidoptera

The chemosensory gene families of insects encode proteins that are crucial for host location, mate finding, oviposition, and avoidance behaviors. The insect peripheral chemosensory system comprises odorant receptors (ORs), gustatory receptors (GRs), ionotropic receptors (IRs), odorant binding proteins (OBPs), chemosensory proteins (CSPs), and sensory neuron membrane proteins (SNMPs). These protein families have been identified from a large number of insect species, however, they still remain unidentified from several taxa that could provide important clues to their evolution. These taxa include older lepidopteran lineages and the sister order of Lepidoptera, Trichoptera (caddisflies). Studies of these insects should improve evolutionary analyses of insect chemoreception, and in particular shed light on the origin of certain lepidopteran protein subfamilies. These include the pheromone receptors (PRs) in the "PR clade", the pheromone binding proteins (PBPs), general odorant binding proteins (GOBPs), and certain presumably Lepidoptera-specific IR subfamilies. Hence, we analyzed antennal transcriptomes from Rhyacophila nubila (Trichoptera), Eriocrania semipurpurella, and Lampronia capitella (representing two old lepidopteran lineages). We report 37 ORs, 17 IRs, 9 GRs, 30 OBPs, 7 CSPs, and 2 SNMPs in R. nubila; 37 ORs, 17 IRs, 3 GRs, 23 OBPs, 14 CSPs, and 2 SNMPs in E. semipurpurella; and 53 ORs, 20 IRs, 5 GRs, 29 OBPs, 17 CSPs, and 3 SNMPs in L. capitella. We identified IR members of the "Lepidoptera-specific" subfamilies IR1 and IR87a also in R. nubila, demonstrating that these IRs also occur in Trichoptera. Members of the GOBP subfamily were only found in the two lepidopterans. ORs grouping within the PR clade, as well as PBPs, were only found in L. capitella, a species that in contrast to R. nubila and E. semipurpurella uses a so-called Type I pheromone similar to the pheromones of most species of the derived Lepidoptera (Ditrysia). Thus, in addition to providing increased coverage for evolutionary analyses of chemoreception in insects, our findings suggest that certain subfamilies of chemosensory genes have evolved in parallel with the transition of sex pheromone types in Lepidoptera. In addition, other chemoreceptor subfamilies show a broader taxonomic occurrence than hitherto acknowledged.

Functional characterization of odorant receptors from Lampronia capitella suggests a non-ditrysian origin of the lepidopteran pheromone receptor clade

The odorant receptors (ORs) of insects are crucial for host and mate recognition. In moths (Lepidoptera), specialized ORs are involved in male detection of the sex pheromone produced by females. Most moth sex pheromones are C₁₀-C₁₈ acetates, alcohols, and aldehydes (Type I pheromones), and most pheromone receptors (PRs) characterized to date are from higher Lepidoptera (Ditrysia), responding to these types of compounds. With few exceptions, functionally characterized PRs fall into what has been called the "PR-clade", which also contains receptors that have yet to be characterized. While it has been suggested that moth PRs have evolved from plant odor-detecting ORs, it is not known when receptors for Type I pheromones arose. This is largely due to a lack of functionally characterized PRs from non-ditrysian Lepidoptera. The currant shoot borer moth, Lampronia capitella (Prodoxidae), belongs to a non-ditrysian lineage, and uses Type I pheromone compounds. We identified 53 ORs from antennal transcriptomes of this species, and analyzed their phylogenetic relationships with known lepidopteran ORs. Using a HEK293 cell-based assay, we showed that three of the LcapORs with male-biased expression (based on FPKM values) respond to Type I pheromone compounds. Two of them responded to pheromone components of L. capitella and one to a structurally related compound. These PRs are the first from a non-ditrysian moth species reported to respond to Type I compounds. They belong to two of the more early-diverging subfamilies of the PR-clade for which a role in pheromone detection had not previously been demonstrated. Hence, our definition of the monophyletic lepidopteran PR-clade includes these receptors from a non-ditrysian species, based on functional support.

Enormous expansion of the chemosensory gene repertoire in the omnivorous German cockroach Blattella germanica

The acquisition of genome sequences from a wide range of insects and other arthropods has revealed a broad positive correlation between the complexity of their chemical ecology and the size of their chemosensory gene repertoire. The German cockroach Blattella germanica is an extreme omnivore and has the largest chemosensory gene repertoire known for an arthropod, exceeding even the highly polyphagous spider mite Tetranychus urticae. While the Odorant Receptor family is not particularly large, with 123 genes potentially encoding 134 receptors (105 intact), the Gustatory Receptor family is greatly expanded to 431 genes potentially encoding 545 receptors (483 intact), the largest known for insects and second only to the spider mite. The Ionotropic Receptor family of olfactory and gustatory receptors is vastly expanded to at least 897 genes (604 intact), the largest size known in arthropods, far surpassing the 150 known from the dampwood termite Zootermopsis nevadensis. Commensurately, the Odorant Binding Protein family is expanded to the largest known for insects at 109 genes (all intact). Comparison with the far more specialized, but phylogenetically related termite, within the Dictyoptera, reveals considerable gene losses from the termite, and massive species-specific gene expansions in the cockroach. The cockroach has lost function of 11%-41% of these three chemoreceptor gene families to pseudogenization, and most of these are young events, implying rapid turnover of genes along with these major expansions, presumably in response to changes in its chemical ecology.

Expressional and functional interactions of two Apis cerana cerana olfactory receptors

Apis cerana cerana relies on its sensitive olfactory system to perform foraging activities in the surrounding environment. Olfactory receptors (ORs) are a primary requirement for odorant recognition and coding. However, the molecular recognition of volatile compounds with ORs in A. cerana cerana is still not clear. Hence, in the present study, we achieved transient transfection and cell surface expression of A. cerana cerana ORs (AcerOr1 and AcerOr2; AcerOr2 is orthologous to the co-receptor) in Spodoptera frugiperda (Sf9) cells. AcerOr2 narrowly responded to N-(4-ethylphenyl)-2-((4-ethyl-5-(3-pyridinyl)-4H-1,2,4-triazol-3-yl) thio) acetamide (VUAA1), whereas AcerOr1 was sensitive to eugenol, lauric acid, ocimene, 1-nonanol, linolenic acid, hexyl acetate, undecanoic acid, 1-octyl alcohol, and nerol. Of the compounds tested, AcerOr1 showed the highest sensitivity to these odorants with EC₅₀ values of 10⁻⁷ and 10⁻⁸ M, and AcerOr2 recognized VUAA1 with higher sensitivity [EC₅₀ = (6.621 ± 0.26) × 10⁻⁸]. These results indicate that AcerOr2 is an essential gene for olfactory signaling, and AcerOr1 is a broadly tuned receptor. We discovered ligands that were useful for probing receptor activity during odor stimulation and validated three of them by electroantennography. The response increased with concentration of the odorant. The present study provides insight into the mechanism of olfactory discrimination in A. cerana cerana.

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 characterization and evolution of a chemosensory receptor gene family in three notorious rice planthoppers, Nilaparvata lugens, Sogatella furcifera and Laodelphax striatellus, based on genome and transcriptome analyses

BACKGROUND

The white-backed planthopper (WBPH) Sogatella furcifera, the brown planthopper (BPH) Nilaparvata lugens, and the small brown planthopper (SBPH) Laodelphax striatellus (Hemiptera: Delphacidae) are rice pests that damage rice plants by sap-sucking and by transmitting viruses. Host-seeking behavior involves chemosensory receptor genes that include odorant receptors (ORs), ionotropic receptors (IRs) and gustatory receptors (GRs).

RESULTS

We used genome and transcriptome data to identify 141 ORs, 28 GRs and 25 IRs in BPH; 135 ORs, 18 GRs and 16 IRs in WBPH; and 37 ORs, 14 GRs and 6 IRs in SBPH. A phylogenetic analysis identified several specific OR clades of rice planthoppers, the results indicating that these OR members might be used to respond to specific host volatiles. OR co-receptor (Orco) is the most conserved and essential OR gene among these species and RNA interference (RNAi) can decrease their mRNA expression level to <50%. RNAi knockdown rice planthoppers were anosmia and were unable to seek or locate rice plants in behavioral tests.

CONCLUSION

The results demonstrate the importance of the planthopper Orco genes in locating rice plants. This information may aid in the development of RNAi-based transgenic rice and other pest management technologies. © 2018 Society of Chemical Industry.

Endogenous insensitivity to the Orco agonist VUAA1 reveals novel olfactory receptor complex properties in the specialist fly Mayetiola destructor

Insect olfactory receptors are routinely expressed in heterologous systems for functional characterisation. It was recently discovered that the essential olfactory receptor co-receptor (Orco) of the Hessian fly, Mayetiola destructor (Mdes), does not respond to the agonist VUAA1, which activates Orco in all other insects analysed to date. Here, using a mutagenesis-based approach we identified three residues in MdesOrco, located in different transmembrane helices as supported by 3D modelling, that confer sensitivity to VUAA1. Reciprocal mutations in Drosophila melanogaster (Dmel) and the noctuid moth Agrotis segetum (Aseg) Orcos diminish sensitivity of these proteins to VUAA1. Additionally, mutating these residues in DmelOrco and AsegOrco compromised odourant receptor (OR) dependent ligand-induced Orco activation. In contrast, both wild-type and VUAA1-sensitive MdesOrco were capable of forming functional receptor complexes when coupled to ORs from all three species, suggesting unique complex properties in M. destructor, and that not all olfactory receptor complexes are “created” equal.