Identification and cloning of a female antenna-specific odorant-binding protein in the mosquito Culex quinquefasciatus

Behavioural response of Culex pipiens biotype molestus to oviposition pheromone

Insect behaviour relies on an olfactory sensory system that controls a range of activities, from food choice and mating to oviposition, where pheromones play a central role. In Culex mosquitoes, egg-laying is accompanied by the release of mosquito oviposition pheromone (MOP), which has been shown to affect the oviposition behaviour of conspecifics. Here, we investigated for the first time the effect of MOP on the oviposition rate of Culex pipiens biotype molestus, examining separately males and females, before and after mating and oviposition. Our results demonstrate that MOP is more likely to act as an oviposition stimulant rather than an attractant, since more gravid females laid eggs in its presence, while the number of male or female mosquitoes (virgin or mated) captured on pheromone-treated pots was similar to those treated with control water.

Social environment affects sensory gene expression in ant larvae

Social insects depend on communication to regulate social behaviour. This also applies to their larvae, which are commonly exposed to social interactions and can react to social stimulation. However, how social insect larvae sense their environment is not known. Using RNAseq, we characterized expression of sensory-related genes in larvae of the ant Formica fusca, upon exposure to two social environments: isolation without contact to other individuals, and stimulation via the presence of other developing individuals. Expression of key sensory-related genes was higher following social stimulation, and larvae expressed many of the same sensory-related genes as adult ants and larvae of other insects, including genes belonging to the major insect chemosensory gene families. Our study provides first insights into the molecular changes associated with social information perception in social insect larvae.

Screening of olfactory genes related to blood-feeding behaviors in Culex pipiens quinquefasciatus and Culex pipiens molestus by transcriptome analysis

Background

Culex pipiens quinquefasciatus Say (Cx. quinquefasciatus) and Culex pipiens form molestus Forskal (Cx. molestus) in the Culex pipiens complex group show considerable differences in host seeking, blood feeding, mating behavior and in vector competence. Blood-feeding mosquito behaviors are closely related to their olfactory gene expression and olfactory gene repertoire composition. Comparing olfactory genes between these two subspecies with significantly different blood-feeding behaviors can support further research on the molecular mechanism of the Culex pipiens complex olfactory sensory system, providing a new approach for determining candidate attractant or repellent compounds.

Methods

Non-blood-feeding (NBF) and post-blood-feeding (PBF) olfactory system transcriptomes of the two subspecies were sequenced, and the biological functions of their differentially expressed genes were described by bioinformatics analysis. A quantitative polymerase chain reaction (qPCR) was applied to validate the RNA-seq data. The roles of particular olfactory receptors in Cx. quinquefasciatus blood-feeding behaviors were evaluated by RNAi.

Results

Five, 7, 24, and 3 Cx. quinquefasciatus-specific OBPs, Cx. molestus-specific OBPs, Cx. quinquefasciatus-specific ORs and Cx. molestus-specific ORs were identified, respectively. The majority of selected ORs were consistent with the predicted transcriptome sequencing results after qRT-PCR validation. OR5 was expressed only in Cx. quinquefasciatus, and OR65 was the only gene upregulated after blood feeding in Cx. molestus. The blood-feeding rates of the OR5 and OR78 dsRNA groups were significantly lower (4.3%±3.1% and 13.3%±11.5%) than those of the enhanced green fluorescence protein (EGFP) group (64.5%±8.7%).

Conclusion

Most OBPs and ORs were expressed in both subspecies but showed divergence in expression level. OR5 and OR65 might be species-specific expressed genes that regulate the olfactory behaviors of Cx. quinquefasciatus and Cx. molestus, respectively. The RNA interference of OR5 and OR78 could inhibit the blood-feeding behavior of Cx. quinquefasciatus, providing new targets for screening effective repellent compounds to control mosquito-borne diseases effectively and efficiently.

The transcriptomic gene expression of the olfactory tissues of Cx. quinquefasciatusthe and Cx. molestus differ significantly. The majority of ORs and OBPs are expressed in both subspecies but are clearly differentiated in expression level. OR5 and OR65 may be species-specific olfactory genes expressed in Cx. quinquefasciatus and Cx. molestus, respectively. After the microinjection of OR5-dsRNA and OR78-dsRNA into female Cx. quinquefasciatus adults, the blood-feeding rate was significantly lower than that of the control group, suggesting that OR5 and OR78 are associated with the blood-feeding behavior of Cx. quinquefasciatus.

Pleiotropic Odorant-Binding Proteins Promote Aedes aegypti Reproduction and Flavivirus Transmission

Insect odorant-binding proteins (OBPs) are small soluble proteins that have been assigned roles in olfaction, but their other potential functions have not been extensively explored. Using CRISPR/Cas9-mediated disruption of Aedes aegyptiObp10 and Obp22, we demonstrate the pleiotropic contribution of these proteins to multiple processes that are essential for vectorial capacity. Mutant mosquitoes have impaired host-seeking and oviposition behavior, reproduction, and arbovirus transmission. Here, we show that Obp22 is linked to the male-determining sex locus (M) on chromosome 1 and is involved in male reproduction, likely by mediating the development of spermatozoa. Although OBP10 and OBP22 are not involved in flavivirus replication, abolition of these proteins significantly reduces transmission of dengue and Zika viruses through a mechanism affecting secretion of viral particles into the saliva. These results extend our current understanding of the role of insect OBPs in insect reproduction and transmission of human pathogens, making them essential determinants of vectorial capacity.

Transcript Assembly and Quantification by RNA-Seq Reveals Significant Differences in Gene Expression and Genetic Variants in Mosquitoes of the Culex pipiens (Diptera: Culicidae) Complex

The taxonomy of Culex pipiens complex of mosquitoes is still debated, but in North America it is generally regarded to include Culex pipiens pipiens, Culex pipiens molestus, and Culex quinquefasciatus (or Culex pipiens quinquefasciatus). Although these mosquitoes have very similar morphometry, they each have unique life strategies specifically adapted to their ecological niche. Differences include the capability for overwintering diapause, bloodmeal preference, mating behaviors, and reliance on blood meals to produce eggs. Here, we used RNA-seq transcriptome analysis to investigate the differential gene expression and nucleotide polymorphisms that may link to the divergent traits specifically between Cx. pipiens pipiens and Cx. pipiens molestus.

Olfactory systems across mosquito species

There are 3559 species of mosquitoes in the world (Harbach 2018) but, so far, only a handful of them have been a focus of olfactory neuroscience and neurobiology research. Here we discuss mosquito olfactory anatomy and function and connect these to mosquito ecology. We highlight the least well-known and thus most interesting aspects of mosquito olfactory systems and discuss promising future directions. We hope this review will encourage the insect neuroscience community to work more broadly across mosquito species instead of focusing narrowly on the main disease vectors.

Citronellal perception and transmission by Anopheles gambiae s.s. (Diptera: Culicidae) females

Anopheles gambiae s.s. is a key vector of Plasmodium parasites. Repellents, which may be a promising alternative to pesticides used to control malaria mosquitoes. Although citronellal is a known mosquito repellent, its repellency characteristics are largely unknown. Determining the specific odorant-binding proteins (OBPs) and odorant receptors (ORs) that detect and transfer the citronellal molecule in A. gambiae s.s. will help to define the mode of action of this compound. In this research, we assessed the repellent activity of citronellal in A. gambiae s.s. using a Y-tube olfactory meter, screened candidate citronellal-binding OBPs and ORs using reverse molecular docking, clarified the binding properties of predicted proteins for citronellal using fluorescence competition binding assay. Results showed that citronellal had a dosage effect on repelling A. gambiae s.s.. The 50% repellent rate was determined to be 4.02 nmol. Results of simulated molecular docking showed that the only proteins that bound tightly with citronellal were AgamOBP4 and AgamORC7. Fluorescence competitive binding assays confirmed the simulations. This research determined that citronellal was captured by AgamOBP4 and transmitted to AgamORC7 in A. gambiae s.s.. Our study will be beneficial in the further understanding the repellent mechanism of citronellal against A. gambiae s.s..

Molecular and functional characterization of three odorant-binding proteins from the wheat blossom midge, Sitodiplosis mosellana

Sitodiplosis mosellana, a periodic but devastating wheat pest, relies on wheat spike volatiles as a cue in selecting hosts for oviposition. Insect odorant-binding proteins (OBPs) are thought to play essential roles in filtering, binding and transporting hydrophobic odorant molecules to specific receptors. To date, the molecular mechanisms underlying S. mosellana olfaction are poorly understood. Here, three S. mosellana antenna-specific OBP genes, SmosOBP11, 16 and 21, were cloned and bacterially expressed. Binding properties of the recombinant proteins to 28 volatiles emitted from wheat spikes were investigated using fluorescence competitive binding assays. Sequence analysis suggested that these SmosOBPs belong to the Classic OBP subfamily. Ligand-binding analysis showed that all three SmosOBPs preferentially bound alcohol, ester and ketone compounds, and SmosOBP11 and 16 also selectively bound terpenoid compounds. In particular, the three SmosOBPs had high binding affinities (K_i < 20 μmol/L) to 3-hexanol and cis-3-hexenylacetate that elicited strong electroantennogram (EAG) response from female antennae. In addition, SmosOBP11 displayed significantly higher binding (K_i < 8 μmol/L) than SmosOBP16 and 21 to 1-octen-3-ol, D-panthenol, α-pinene and heptyl acetate which elicited significant EAG response, suggesting that SmosOBP11 plays a major role in recognition and transportation of these volatiles. These findings have provided important insight into the molecular mechanism by which S. mosellana specifically recognizes plant volatiles for host selection, and have facilitated identification of effective volatile attractants that are potentially useful for pest monitoring and trapping.

Identification and expression profiles of novel odorant binding proteins and functional analysis of OBP99a in Bactrocera dorsalis

Odorant-binding proteins (OBPs) in insects are essential for mating and oviposition host selection. How these OBPs respond to different hosts at the mRNA level and their effects on behavior remain poorly characterized. The oriental fruit fly Bactrocera dorsalis is a highly invasive agricultural pest with an extremely broad host range and high fecundity. Based on our previously constructed B. dorsalis transcriptome, six OBPs that were differentially expressed during three different physiological adult stages were identified. A phylogenetic tree was constructed to illustrate the relationships of these six OBPs with OBP sequences from three other dipteran species (Drosophila melanogaster, Anopheles gambiae, and Ceratitis capitata). The spatiotemporal expression profiles of the six OBPs were analyzed using quantitative real-time PCR. Our results revealed that OBP19c, OBP44a, OBP99a, and OBP99d were abundantly expressed from the prepupa stage to the adult stage, and most of the OBPs were mainly expressed in the head, wings, and antennae. The expression levels of these OBPs were upregulated when female flies were exposed to their preferred hosts. Silencing OBP99a resulted fewer eggs being laid compared with the control group when the females were exposed to their preferred host, that is, banana, whereas more eggs were laid when a non-preferred host, that is, tomato, was used. Furthermore, silencing OBP99a led to sexually dimorphic mating behavior. dsOBP99a-injected males dramatically reduced courtship, whereas enhanced courtship was observed in the treated females. These data indicate that OBPs may participate in different biological processes of B. dorsalis. Our study will provide insight into the molecular mechanism of chemoreception and help develop ecologically friendly pest-control strategies.

Genome-wide identification and characterization of odorant-binding protein (OBP) genes in the malaria vector Anopheles sinensis (Diptera: Culicidae)

Anopheles sinensis is a major malaria vector. Insect odorant-binding proteins (OBPs) may function in the reception of odorants in the olfactory system. The classification and characterization of the An. sinensis OBP genes have not been systematically studied. In this study, 64 putative OBP genes were identified at the whole-genome level of An. sinensis based on the comparison between OBP conserved motifs, PBP_GOBP, and phylogenetic analysis with An. gambiae OBPs. The characterization of An. sinensis OBPs, including the motif's conservation, gene structure, genomic organization and classification, were investigated. A new gene, AsOBP73, belonging to the Plus-C subfamily, was identified with the support of transcript and conservative motifs. These An. sinensis OBP genes were classified into three subfamilies with 37, 15 and 12 genes in the subfamily Classic, Atypical and Plus-C, respectively. The genomic organization of An. sinensis OBPs suggests a clustered distribution across nine different scaffolds. Eight genes (OBP23-28, OBP63-64) might originate from a single gene through a series of historic duplication events at least before divergence of Anopheles, Culex and Aedes. The microsynteny analyses indicate a very high synteny between An. sinensis and An. gambiae OBPs. OBP70 and OBP71 earlier classified under Plus-C in An. gambiae are recognized as belonging to the group Obp59a of the Classic subfamily, and OBP69 earlier classified under Plus-C has been moved to the Atypical subfamily in this study. The study established a basic information frame for further study of the OBP genes in insects as well as in An. sinensis.

The Anopheles stephensi odorant binding protein 1 (AsteObp1) gene: a new molecular marker for biological forms diagnosis

Anopheles (Cellia) stephensi Liston 1901 is known as an Asian malaria vector. Three biological forms, namely "mysorensis", "intermediate", and "type" have been earlier reported in this species. Nevertheless, the present morphological and molecular information is insufficient to diagnose these forms. During this investigation, An. stephensi biological forms were morphologically identified and sequenced for odorant-binding protein 1 (Obp1) gene. Also, intron I sequences were used to construct phylogenetic trees. Despite nucleotide sequence variation in exon of AsteObp1, nearly 100% identity was observed at the amino acid level among the three biological forms. In order to overcome difficulties in using egg morphology characters, intron I sequences of An. stephensi Obp1 opens new molecular way to the identification of the main Asian malaria vector biological forms. However, multidisciplinary studies are needed to establish the taxonomic status of An. stephensi.

Selectivity of odorant-binding proteins from the southern house mosquito tested against physiologically relevant ligands

As opposed to humans, insects rely heavily on an acute olfactory system for survival and reproduction. Two major types of olfactory proteins, namely, odorant-binding proteins (OBPs) and odorant receptors (ORs), may contribute to the selectivity and sensitivity of the insects' olfactory system. Here, we aimed at addressing the question whether OBPs highly enriched in the antennae of the southern house mosquito, Culex quinquefasciatus, contribute at least in part to the selective reception of physiologically relevant compounds. Using a fluorescence reporter and a panel of 34 compounds, including oviposition attractants, human-derived attractants, and repellents, we measured binding affinities of CquiOBP1, CquiOBP2, and CquiOBP5. Based on dissociation constants, we surmised that CquiOBP2 is a carrier for the oviposition attractant skatole, whereas CquiOBP1 and CquiOBP5 might transport the oviposition pheromone MOP, a human-derived attractant nonanal, and the insect repellent picardin. Binding of these three ligands to CquiOBP1 was further analyzed by examining the influence of pH on apparent affinity as well as by docking these three ligands into CquiOBP1. Our findings suggest that CquiOBP1 might discriminate MOP from nonanal/picaridin on the basis of the midpoint transition of a pH-dependence conformational change, and that MOP is better accommodated in the binding cavity than the other two ligands. These findings, along with previous experimental evidence suggesting that CquiOBP1 does not detect nonanal in vivo, suggest that OBP selectivity may not be clearly manifested in their dissociation constants.

Venom of the parasitoid wasp Pteromalus puparum contains an odorant binding protein

Odorant binding proteins (OBPs) are crucial for insects to detect food, mates, predators, or other purposes. They are mostly located on antennae and other olfactory sensilla. In this study, we identified an OBP from the venom of Pteromalus puparum, designated as PpOBP. The cDNA of PpOBP is 517 bp in length, encoding 132 amino acids. Phylogenetic analysis revealed that PpOBP was clustered with OBP68 and OBP67 of Nasonia vitripennis. PpOBP was highly expressed in the venom apparatus at the transcriptional and translational levels. PpOBP was located in all parts of venom apparatus including venom gland, venom reservoir, and Dufour's gland. During 0-6 days post adult eclosion, the PpOBP mRNA level peaked at 2 days in the venom apparatus, whereas the protein remained at a high level. In the venom apparatus, the PpOBP mRNA was significantly upregulated following feeding with honey and parasitization. We propose that PpOBP is involved in parasitoid-host interactions.

Binding of a fluorescence reporter and a ligand to an odorant-binding protein of the yellow fever mosquito, Aedes aegypti

Odorant-binding proteins (OBPs), also named pheromone-binding proteins when the odorant is a pheromone, are essential for insect olfaction. They solubilize odorants that reach the port of entry of the olfactory system, the pore tubules in antennae and other olfactory appendages. Then, OBPs transport these hydrophobic compounds through an aqueous sensillar lymph to receptors embedded on dendritic membranes of olfactory receptor neurons. Structures of OBPs from mosquito species have shed new light on the mechanism of transport, although there is considerable debate on how they deliver odorant to receptors. An OBP from the southern house mosquito, Culex quinquefasciatus, binds the hydrophobic moiety of a mosquito oviposition pheromone (MOP) on the edge of its binding cavity. Likewise, it has been demonstrated that the orthologous protein from the malaria mosquito binds the insect repellent DEET on a similar edge of its binding pocket. A high school research project was aimed at testing whether the orthologous protein from the yellow fever mosquito, AaegOBP1, binds DEET and other insect repellents, and MOP was used as a positive control. Binding assays using the fluorescence reporter N-phenyl-1-naphtylamine (NPN) were inconclusive. However, titration of NPN fluorescence emission in AaegOBP1 solution with MOP led to unexpected and intriguing results. Quenching was observed in the initial phase of titration, but addition of higher doses of MOP led to a stepwise increase in fluorescence emission coupled with a blue shift, which can be explained at least in part by formation of MOP micelles to house stray NPN molecules.

Binding of a fluorescence reporter and a ligand to an odorant-binding protein of the yellow fever mosquito, Aedes aegypti

Odorant-binding proteins (OBPs), also named pheromone-binding proteins when the odorant is a pheromone, are essential for insect olfaction. They solubilize odorants that reach the port of entry of the olfactory system, the pore tubules in antennae and other olfactory appendages. Then, OBPs transport these hydrophobic compounds through an aqueous sensillar lymph to receptors embedded on dendritic membranes of olfactory receptor neurons. Structures of OBPs from mosquito species have shed new light on the mechanism of transport, although there is considerable debate on how they deliver odorant to receptors. An OBP from the southern house mosquito, Culex quinquefasciatus, binds the hydrophobic moiety of a mosquito oviposition pheromone (MOP) on the edge of its binding cavity. Likewise, it has been demonstrated that the orthologous protein from the malaria mosquito binds the insect repellent DEET on a similar edge of its binding pocket. A high school research project was aimed at testing whether the orthologous protein from the yellow fever mosquito, AaegOBP1, binds DEET and other insect repellents, and MOP was used as a positive control. Binding assays using the fluorescence reporter N-phenyl-1-naphtylamine (NPN) were inconclusive. However, titration of NPN fluorescence emission in AaegOBP1 solution with MOP led to unexpected and intriguing results. Quenching was observed in the initial phase of titration, but addition of higher doses of MOP led to a stepwise increase in fluorescence emission coupled with a blue shift, which can be explained at least in part by formation of MOP micelles to house stray NPN molecules.

Morphology and physiology of the olfactory system of blood-feeding insects

Several blood-feeding (hematophagous) insects are vectors of a number of diseases including dengue, Chagas disease and leishmaniasis which persistently affect public health throughout Latin America. The vectors of those diseases include mosquitoes, triatomine bugs and sandflies. As vector control is an efficient way to prevent these illnesses it is important to understand the sensory biology of those harmful insects. We study the physiology of the olfactory system of those insects and apply that knowledge on the development of methods to manipulate their behavior. Here we review some of the latest information on insect olfaction with emphasis on hematophagous insects. The insect olfactory sensory neurons are housed inside hair-like organs called sensilla which are mainly distributed on the antenna and mouthparts. The identity of many of the odor compounds that those neurons detect are already known in hematophagous insects. They include several constituents of host (vertebrate) odor, sex, aggregation and alarm pheromones, and compounds related to egg-deposition behavior. Recent work has contributed significant knowledge on how odor information is processed in the insect first odor-processing center in the brain, the antennal lobe. The quality, quantity, and temporal features of the odor stimuli are encoded by the neural networks of the antennal lobe. Information regarding odor mixtures is also encoded. While natural mixtures evoke strong responses, synthetic mixtures that deviate from their natural counterparts in terms of key constituents or proportions of those constituents evoke weaker responses. The processing of olfactory information is largely unexplored in hematophagous insects. However, many aspects of their olfactory behavior are known. As in other insects, responses to relevant single odor compounds are weak while natural mixtures evoke strong responses. Future challenges include studying how information about odor mixtures is processed in their brain. This could help develop highly attractive synthetic odor blends to lure them into traps.

Construction and analysis of cDNA libraries from the antennae of Batocera horsfieldi and expression pattern of putative odorant binding proteins

A high-quality cDNA library was constructed from female and male antenna of the longhorned beetle, Batocera horsfieldi (Hope) (Coleoptera: Cerambycidae), a serious pest of Populus (Salicales: Salicaceae). The titer was approximately 2.37 × 106 pfu/mL, and this complies with the test requirement. From the libraries, 692 clones were selected randomly, sequenced, and further analyzed, and the recombinational efficiency reached 93.85%. By alignment and cluster analysis, we identified four odorant binding proteins, two pheromone-binding proteins (have the characteristic six conserved cysteine residues), four Minus-C odorant binding proteins (lost two conserved cysteines), and three chemosensory proteins. In this study, we describe the identification and characterization of four new cDNAs that encode Minus-C odorant binding proteins (Minus-C OBPs) from B. horsfieldi antennal cDNA libraries. Our investigation focused on the expression pattern of the Minus-C OBP genes in various tissues in both sexes at different developmental stages, using reverse transcription PCR (RT-PCR) and realtime PCR (qPCR) strategies. Minus-C OBP1, 2, and 3 were expressed in all tested tissues, with the exception of the head (without antenna, labial palps, and maxillary palps). Minus-C OBP4 was expressed in the antenna, legs, and abdomen, but not in the labial palps, maxillary palps, or head. The qPCR results revealed MinusC OBPs were expressed in the antenna throughout the adult life, and that the transcript levels of these genes depended on the sex, age, and mating status of adults.

Molecular and functional characterization of odorant-binding protein genes in an invasive vector mosquito, Aedes albopictus

Aedes albopictus is a major vector of dengue and Chikungunya viruses. Olfaction plays a vital role in guiding mosquito behaviors and contributes to their ability to transmit pathogens. Odorant-binding proteins (OBPs) are abundant in insect olfactory tissues and involved in the first step of odorant reception. While comprehensive descriptions are available of OBPs from Aedes aegypti, Culex quinquefasciatus and Anopheles gambiae, only a few genes from Ae. albopictus have been reported. In this study, twenty-one putative AalbOBP genes were cloned using their homologues in Ae. aegypti to query an Ae. albopictus partial genome sequence. Two antenna-specific OBPs, AalbOBP37 and AalbOBP39, display a remarkable similarity in their overall folding and binding pockets, according to molecular modeling. Binding affinity assays indicated that AalbOBP37 and AalbOBP39 had overlapping ligand affinities and are affected in different pH condition. Electroantennagrams (EAG) and behavioral tests show that these two genes were involved in olfactory reception. An improved understanding of the Ae. albopictus OBPs is expected to contribute to the development of more efficient and environmentally-friendly mosquito control strategies.

Odorant reception in insects: roles of receptors, binding proteins, and degrading enzymes

Our knowledge of the molecular basis of odorant reception in insects has grown exponentially over the past decade. Odorant receptors (ORs) from moths, fruit flies, mosquitoes, and the honey bees have been deorphanized, odorant-degrading enzymes (ODEs) have been isolated, and the functions of odorant-binding proteins (OBPs) have been unveiled. OBPs contribute to the sensitivity of the olfactory system by transporting odorants through the sensillar lymph, but there are competing hypotheses on how they act at the end of the journey. A few ODEs that have been demonstrated to degrade odorants rapidly may act in signal inactivation alone or in combination with other molecular traps. Although ORs in Drosophila melanogaster respond to multiple odorants and seem to work in combinatorial code involving both periphery and antennal lobes, reception of sex pheromones by moth ORs suggests that their labeled lines rely heavily on selectivity at the periphery.

Insect olfaction from model systems to disease control

Great progress has been made in the field of insect olfaction in recent years. Receptors, neurons, and circuits have been defined in considerable detail, and the mechanisms by which they detect, encode, and process sensory stimuli are being unraveled. We provide a guide to recent progress in the field, with special attention to advances made in the genetic model organism Drosophila. We highlight key questions that merit additional investigation. We then present our view of how recent advances may be applied to the control of disease-carrying insects such as mosquitoes, which transmit disease to hundreds of millions of people each year. We suggest how progress in defining the basic mechanisms of insect olfaction may lead to means of disrupting host-seeking and other olfactory behaviors, thereby reducing the transmission of deadly diseases.

Characterization of olfactory genes in the antennae of the Southern house mosquito, Culex quinquefasciatus

Odorant reception in insects is mediated by different families of olfactory proteins. Here we focus on the characterization of odorant-binding proteins (OBPs), "plus-C" odorant-binding proteins ("plus-C" OBPs), chemosensory proteins (CSPs) and sensory neuron membrane proteins (SNMPs) families from the Southern house mosquito, Culex quinquefasciatus, a vector of pathogens implicated in multiple human diseases. Using bioinformatics and molecular approaches, we have identified a diversity of genes in the genome of Culex quinquefasciatus and examined their expression profiles by RT-PCR and real-time quantitative PCR. Based on their high transcript enrichment in female antennae compared to non-olfactory tissues, we have identified twelve OBPs, two "plus-C" OBPs and two SNMPs that likely play important roles in odorant reception. Transcripts of two genes were clearly enriched in female antennae compared to male antennae, whereas other genes displayed relatively equivalent transcript levels in antennae of both sexes. Additionally, eight genes were found to be transcribed at very high levels in female antennae compared to CquiOR7, suggesting they might encode highly abundant olfactory proteins. Comparative analysis across different mosquito species revealed that olfactory genes of Culex quinquefasciatus are related to putative orthologs in other species, indicating that they might perform similar functions. Understanding how mosquitoes are able to detect ecologically relevant odorant cues might help designing better control strategies. We have identified olfactory genes from different families which are likely important in Culex quinquefasciatus behaviors, thus paving the way towards a better understanding of the diversity of proteins involved in the reception of semiochemicals in this species.

A review of chemosensation and related behavior in aquatic insects

Insects that are secondarily adapted to aquatic environments are able to sense odors from a diverse array of sources. The antenna of these insects, as in all insects, is the main chemosensory structure and its input to the brain allows for integration of sensory information that ultimately ends in behavioral responses. Only a fraction of the aquatic insect orders have been studied with respect to their sensory biology and most of the work has centered either on the description of the different types of sensilla, or on the behavior of the insect as a whole. In this paper, the literature is exhaustively reviewed and ways in which antennal morphology, brain structure, and associated behavior can advance better understanding of the neurobiology involved in processing of chemosensory information are discussed. Moreover, the importance of studying such group of insects is stated, and at the same time it is shown that many interesting questions regarding olfactory processing can be addressed by looking into the changes that aquatic insects undergo when leaving their aquatic environment.

Crystal and solution structures of an odorant-binding protein from the southern house mosquito complexed with an oviposition pheromone

Culex mosquitoes introduce the pathogens responsible for filariasis, West Nile virus, St. Louis encephalitis, and other diseases into humans. Currently, traps baited with oviposition semiochemicals play an important role in detection efforts and could provide an environmentally friendly approach to controlling their populations. The odorant binding proteins (OBPs) in the female's antenna play a crucial, if yet imperfectly understood, role in sensing oviposition cues. Here, we report the X-ray crystallography and NMR 3D structures of OBP1 for Culex quinquefasciatus (CquiOBP1) bound to an oviposition pheromone (5R,6S)-6-acetoxy-5-hexadecanolide (MOP). In both studies, CquiOBP1 had the same overall six-helix structure seen in other insect OBPs, but a detailed analysis revealed an important previously undescribed feature. There are two models for OBP-mediated signal transduction: (i) direct release of the pheromone from an internal binding pocket in a pH-dependent fashion and (ii) detection of a pheromone-induced conformational change in the OBP·pheromone complex. Although CquiOBP1 binds MOP in a pH-dependent fashion, it lacks the C terminus required for the pH-dependent release model. This study shows that CquiOBP binds MOP in an unprecedented fashion using both a small central cavity for the lactone head group and a long hydrophobic channel for its tail.

Odorant-binding proteins of the malaria mosquito Anopheles funestus sensu stricto

BACKGROUND

The mosquito Anopheles funestus is one of the major malaria vector species in sub-Saharan Africa. Olfaction is essential in guiding mosquito behaviors. Odorant-binding proteins (OBPs) are highly expressed in insect olfactory tissues and involved in the first step of odorant reception. An improved understanding of the function of malaria mosquito OBPs may contribute to identifying new attractants/repellents and assist in the development of more efficient and environmentally friendly mosquito controlling strategies.

METHODOLOGY

In this study, a large screening of over 50 ecologically significant odorant compounds led to the identification of 12 ligands that elicit significant electroantennographic (EAG) responses from An. funestus female antennae. To compare the absolute efficiency/potency of these chemicals, corrections were made for differences in volatility by determining the exact amount in a stimulus puff. Fourteen AfunOBP genes were cloned and their expression patterns were analyzed. AfunOBP1, 3, 7, 20 and 66 showed olfactory tissue specificity by reverse transcriptase PCR (RT-PCR). Quantitative real-time PCR (qRT-PCR) analysis showed that among olfactory-specific OBPs, AfunOBP1 and 3 are the most enriched OBPs in female antennae. Binding assay experiments showed that at pH 7, AfunOBP1 significantly binds to 2-undecanone, nonyl acetate, octyl acetate and 1-octen-3-ol but AfunOBP3, which shares 68% identify with AfunOBP1 at amino acid level, showed nearly no binding activity to the selected 12 EAG-active odorant compounds.

CONCLUSION

This work presents for the first time a study on the odorants and OBPs of the malaria vector mosquito An. funestus, which may provide insight into the An. funestus olfactory research, assist in a comparative study between major malaria mosquitoes An. gambiae and An. funestus olfactory system, and help developing new mosquito control strategies to reduce malaria transmission.

The Anopheles gambiae odorant binding protein 1 (AgamOBP1) mediates indole recognition in the antennae of female mosquitoes

Haematophagous insects are frequently carriers of parasitic diseases, including malaria. The mosquito Anopheles gambiae is the major vector of malaria in sub-Saharan Africa and is thus responsible for thousands of deaths daily. Although the role of olfaction in A. gambiae host detection has been demonstrated, little is known about the combinations of ligands and odorant binding proteins (OBPs) that can produce specific odor-related responses in vivo. We identified a ligand, indole, for an A. gambiae odorant binding protein, AgamOBP1, modeled the interaction in silico and confirmed the interaction using biochemical assays. RNAi-mediated gene silencing coupled with electrophysiological analyses confirmed that AgamOBP1 binds indole in A. gambiae and that the antennal receptor cells do not respond to indole in the absence of AgamOBP1. This case represents the first documented instance of a specific A. gambiae OBP-ligand pairing combination, demonstrates the significance of OBPs in odor recognition, and can be expanded to the identification of other ligands for OBPs of Anopheles and other medically important insects.

Knockdown of a mosquito odorant-binding protein involved in the sensitive detection of oviposition attractants

Odorant-binding proteins (OBPs) were discovered almost three decades ago, but there is still considerable debate regarding their role(s) in insect olfaction, particularly due to our inability to knockdown OBPs and demonstrate their direct phenotypic effects. By using RNA interference (RNAi), we reduced transcription of a major OBP gene, CquiOBP1, in the antennae of the Southern house mosquito, Culex quinquefasciatus. Previously, we had demonstrated that the mosquito oviposition pheromone (MOP) binds to CquiOBP1, which is expressed in MOP-sensitive sensilla. Antennae of RNAi-treated mosquitoes showed significantly lower electrophysiological responses to known mosquito oviposition attractants than the antennae of water-injected, control mosquitoes. While electroantennogram (EAG) responses to MOP, skatole, and indole were reduced in the knockdowns, there was no significant difference in the EAG responses from RNAi-treated and water-injected mosquito antennae to nonanal at all doses tested. These data suggest that CquiOBP1 is involved in the reception of some oviposition attractants, and that high levels of OBPs expression are essential for the sensitivity of the insect's olfactory system.

Identification and characterization of odorant-binding protein 1 gene from the Asian malaria mosquito, Anopheles stephensi

Insect odorant-binding proteins (OBPs) are small, water-soluble molecules that are thought to transport the hydrophobic odorants to their receptors in the chemosensory neurones. Here we report the identification and molecular characterization of the Anopheles stephensi odorant-binding protein 1 gene (AsteObp1), an Obp1 gene in An. stephensi, a major malaria vector in Asia. We show that AsteObp1 and Anopheles gambiae Obp1 (AgamObp1) are orthologues. These two genes share similar coding sequences and conserved noncoding sequences (CNSs) that may be involved in their regulation. Transcript of AsteObp1 was observed in larvae and reached a relatively high level in late pupae. Quantitative real-time PCR on female adult chemosensory tissues showed approximately 900-fold higher expression of AsteObp1 in antennae than in maxillary palp and proboscis. The amount of AsteObp1 in female legs was approximately 15-fold lower than that of maxillary palp and proboscis. The level of AsteObp1 transcript was seven and 85-fold higher in females than in males in the antennae, and maxillary palp and proboscis, respectively. Moreover, the AsteObp1 level was reduced by approximately 20-fold in maxillary palp and proboscis 24 h after a bloodmeal. Our results indicate that AsteObp1 is likely to function in the female olfactory response and may also be involved in blood-feeding behaviour.

Chemical ecology of animal and human pathogen vectors in a changing global climate

Infectious diseases affecting livestock and human health that involve vector-borne pathogens are a global problem, unrestricted by borders or boundaries, which may be exacerbated by changing global climate. Thus, the availability of effective tools for control of pathogen vectors is of the utmost importance. The aim of this article is to review, selectively, current knowledge of the chemical ecology of pathogen vectors that affect livestock and human health in the developed and developing world, based on key note lectures presented in a symposium on "The Chemical Ecology of Disease Vectors" at the 25th Annual ISCE meeting in Neuchatel, Switzerland. The focus is on the deployment of semiochemicals for monitoring and control strategies, and discusses briefly future directions that such research should proceed along, bearing in mind the environmental challenges associated with climate change that we will face during the 21st century.

Acute olfactory response of Culex mosquitoes to a human- and bird-derived attractant

West Nile virus, which is transmitted by Culex mosquitoes while feeding on birds and humans, has emerged as the dominant vector borne disease in North America. We have identified natural compounds from humans and birds, which are detected with extreme sensitivity by olfactory receptor neurons (ORNs) on the antennae of Culex pipiens quinquefasciatus (Cx. quinquefasciatus). One of these semiochemicals, nonanal, dominates the odorant spectrum of pigeons, chickens, and humans from various ethnic backgrounds. We determined the specificity and sensitivity of all ORN types housed in different sensilla types on Cx. quinquefasciatus antennae. Here, we present a comprehensive map of all antennal ORNs coding natural ligands and their dose-response functions. Nonanal is detected by a large array of sensilla and is by far the most potent stimulus; thus, supporting the assumption that Cx. quinquefasciatus can smell humans and birds. Nonanal and CO(2) synergize, thus, leading to significantly higher catches of Culex mosquitoes in traps baited with binary than in those with individual lures.

Molecular characterization and expression pattern of two general odorant binding proteins from the diamondback moth, Plutella xylostella

In the Lepidoptera, odorant signals are thought to be mediated by general odorant binding proteins (GOBPs) in the sensillar lymph surrounding the olfactory receptors. We describe the identification and characterization of two new cDNAs encoding GOBPs from the antennae of the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), a species for which no GOBPs have been identified to date. We focused our investigation on this olfactory protein family by using reverse transcription-polymerase chain reaction strategies. The deduced amino acid sequences of PxylGOBP1 and PxylGOBP2 revealed open reading frames of 168 and 163 amino acids, respectively, with six cysteine residues in conserved positions relative to other known GOBPs. The alignment of the mature PxylGOBPs with other Lepidoptera GOBPs showed high sequence identity (70-80%) with other full-length sequences from GenBank. Sequence identity between PxylGOBP1and PxylGOBP2 was only 50%, suggesting that the two proteins belong to different classes of lepidopteran GOBPs. The expression patterns of the two PxylGOBP genes, with respect to tissue distribution and sex, were further investigated by reverse transcription-polymerase chain reaction (RT-PCR) and real-time PCR. Although the two GOBP genes were expressed only in the antennae of both sexes, reflecting the antennal specificity of GOBPs, the transcription levels of these genes depended on the sex, the age, the mating status, and the genes.

Genome analysis and expression patterns of odorant-binding proteins from the Southern House mosquito Culex pipiens quinquefasciatus

Olfactory-based behaviors in mosquitoes are mediated by odorant-binding proteins (OBPs). They form a multigenic family involved in the peripheral events in insect olfaction, specifically the transport of odorants to membrane-bound odorant receptors. OBPs contribute to the remarkable sensitivity of the insect's olfactory system and may be involved in the selective transport of odorants.We have employed a combination of bioinformatics and molecular approaches to identify and characterize members of the "classic" OBP family in the Southern House mosquito Culex pipiens quinquefasciatus ( = Cx. quinquefasciatus), a vector of pathogens causing several human diseases. By taking advantage of the recently released genome sequences, we have identified fifty-three putative Cx. quinquefasciatus OBP genes by Blast searches. As a first step towards their molecular characterization, expression patterns by RT-PCR revealed thirteen genes that were detected exclusively and abundantly in chemosensory tissues. No clear differences were observed in the transcripts levels of olfactory-specific OBPs between antennae of both sexes using semi-quantitative RT-PCR. Phylogenetic and comparative analysis revealed orthologous of Cx. quinquefasciatus OBPs in Anopheles gambiae and Aedes aegypti. The identification of fifty-three putative OBP genes in Cx. quinquefasciatus highlights the diversity of this family. Tissue-specificity study suggests the existence of different functional classes within the mosquito OBP family. Most genes were detected in chemosensory as well as non chemosensory tissues indicating that they might be encapsulins, but not necessarily olfactory proteins. On the other hand, thirteen "true" OBP genes were detected exclusively in olfactory tissues and might be involved specifically in the detection of "key" semiochemicals. Interestingly, in Cx. quinquefasciatus olfactory-specific OBPs belong exclusively to four distinct phylogenetic groups which are particularly well conserved among three mosquito species.

Reverse and conventional chemical ecology approaches for the development of oviposition attractants for Culex mosquitoes

Synthetic mosquito oviposition attractants are sorely needed for surveillance and control programs for Culex species, which are major vectors of pathogens causing various human diseases, including filariasis, encephalitis, and West Nile encephalomyelitis. We employed novel and conventional chemical ecology approaches to identify potential attractants, which were demonstrated in field tests to be effective for monitoring populations of Cx. p. quinquefasciatus in human dwellings. Immunohistochemistry studies showed that an odorant-binding protein from this species, CquiOBP1, is expressed in trichoid sensilla on the antennae, including short, sharp-tipped trichoid sensilla type, which house an olfactory receptor neuron sensitive to a previously identified mosquito oviposition pheromone (MOP), 6-acetoxy-5-hexadecanolide. CquiOBP1 exists in monomeric and dimeric forms. Monomeric CquiOBP1 bound MOP in a pH-dependent manner, with a change in secondary structure apparently related to the loss of binding at low pH. The pheromone antipode showed higher affinity than the natural stereoisomer. By using both CquiOBP1 as a molecular target in binding assays and gas chromatography-electroantennographic detection (GC-EAD), we identified nonanal, trimethylamine (TMA), and skatole as test compounds. Extensive field evaluations in Recife, Brazil, a region with high populations of Cx. p. quinquefasciatus, showed that a combination of TMA (0.9 µg/l) and nonanal (0.15 ng/µl) is equivalent in attraction to the currently used infusion-based lure, and superior in that the offensive smell of infusions was eliminated in the newly developed synthetic mixture.

Semiochemicals for biting fly control: their identification and exploitation

Insects that feed on the blood of vertebrates are difficult to control, and many previous efforts have been unsuccessful. This is becoming an ever increasing issue, not only in developing countries, but also in developed countries, as exemplified by the recent spread of West Nile virus by mosquitoes across the USA and recent dengue outbreaks in Singapore and Australia. Investigating the ways in which biting insects interact with each other, their environment and their hosts is providing valuable knowledge that will lead to the development of improved control technologies. For instance, recent advances in chemical ecology research have led to the identification of new semiochemicals that show great potential as control agents against biting insects. Exciting new chemical ecology tools and control technologies for the future are discussed.

Maxillary Palps Are Broad Spectrum Odorant Detectors in Culex quinquefasciatus

A single type of olfactory sensilla on maxillary palps in many species of mosquitoes houses a very sensitive olfactory receptor neuron (ORN) for carbon dioxide reception. We performed extensive single sensillum recordings from this peg sensillum in Culex quinquefasciatus and have characterized the response threshold and kinetics for CO(2) reception, with a detection threshold less than the CO(2) concentration in the atmosphere. This ORN responded in a tonic mode to lower concentrations of CO(2), whereas higher concentrations generated a phasic-tonic mode of action potential firing. Sensillum potentials accurately represented the response magnitude and kinetics of carbon dioxide-elicited excitatory responses. Stimulation of these ORNs with human breath, a complex mixture of mosquito kairomones and up to 4.5% CO(2), elicited excitatory responses that were reliably detected by CO(2)-sensitive ORNs. Another ORN housed in these sensilla responded to 1-octen-3-ol and to various plant-derived compounds, particularly floral and green leaf volatiles. This ORN showed remarkable sensitivity to the natural enantiomer, (R)-(-)-1-octen-3-ol, rivaling pheromone-detecting ORNs in moths. Maximum neuronal response was elicited with a 10 ng dose. A biological, ecological role of maxillary palps in detection of plant- and nectar-related sources is proposed.

A detailed larval salivary gland polytene chromosome photomap for Culex quinquefasciatus (Diptera: Culicidae) from Johannesburg, South Africa

The suitability of polytene chromosomes for population genetic and in situ DNA physical mapping was investigated in Johannesburg, South Africa, Culex quinquefasciatus Say and Culex pipiens L.. The most defined and consistent polytene spreads were obtained from salivary glands of early fourth instars from Cx. quinquefasciatus. A photomap is provided for a Cx. quinquefasciatus colony originating from Johannesburg. Variations in banding patterns, chromosome aberrations, and paracentric inversions within the colony and comparisons with previous published descriptions are discussed. Chromosomes of sufficient quality were obtained such that a DNA coding for an odorant binding protein could be mapped by colorimetric in situ hybridization.

Molecular biology of insect olfaction: recent progress and conceptual models

Insects have an enormous impact on global public health as disease vectors and as agricultural enablers as well as pests and olfaction is an important sensory input to their behavior. As such it is of great value to understand the interplay of the molecular components of the olfactory system which, in addition to fostering a better understanding of insect neurobiology, may ultimately aid in devising novel intervention strategies to reduce disease transmission or crop damage. Since the first discovery of odorant receptors in vertebrates over a decade ago, much of our view on how the insect olfactory system might work has been derived from observations made in vertebrates and other invertebrates, such as lobsters or nematodes. Together with the advantages of a wide range of genetic tools, the identification of the first insect odorant receptors in Drosophila melanogaster in 1999 paved the way for rapid progress in unraveling the question of how olfactory signal transduction and processing occurs in the fruitfly. This review intends to summarize much of this progress and to point out some areas where advances can be expected in the near future.

Antennal expressed genes of the yellow fever mosquito (Aedes aegypti L.); characterization of odorant-binding protein 10 and takeout

A small cDNA library was constructed from antennae of 100 adult male Aedes aegypti yellow fever mosquitoes. Sequencing of 80 clones identified 49 unique gene products, including a member of the Odorant Binding Protein family (Aaeg-OBP10), a homologue of Takeout (Aaeg-TO), and transposable elements of the LINE, SINE and MITE classes. Aaeg-OBP10 encodes a 140 amino acid protein including a predicted 25 amino acid signal peptide. Aaeg-OBP10 expression was adult male enriched, increased with adult age, and greatest in antennae and wings but also present in maxillary palps, proboscis and leg. Aaeg-OBP10 is a likely orthologue of Agam-OBP10 of the malaria mosquito Anopheles gambiae and shares significant similarity with members of the OBP56 gene cluster of Drosophila melanogaster. These OBP genes may represent a unified class of OBPs with unique roles in chemodetection; the expression pattern of Aaeg-OBP10 suggests it may play a role in adult male chemosensory behavior. Aaeg-TO encodes a 248 amino acid protein including a predicted 22 amino acid signal peptide. Aaeg-TO is homologous with the circadian/feeding regulated D. melanogaster Takeout protein (Dmel-TO) and a subclass of Juvenile Hormone Binding Proteins (JHBP) characterized by Moling from Manduca sexta; both Dmel-TO and Moling are sensitive to feeding, suggesting Aaeg-TO might regulate the antennal response to food, host or pheromonal odors in a JH sensitive manner. Aaeg-TO was used to identify 25 D. melanogaster and 13 A. gambiae homologues by Blast analysis suggesting these may comprise a relatively large class of protein involved in the hormonal regulation of behavior.

Intriguing olfactory proteins from the yellow fever mosquito, Aedes aegypti

Four antennae-specific proteins (AaegOBP1, AaegOBP2, AaegOBP3, and AaegASP1) were isolated from the yellow fever mosquito, Aedes aegypti and their full-length cDNAs were cloned. RT-PCR indicated that they are expressed in female and, to a lesser extent, in male antennae, but not in control tissues (legs). AaegOBP1 and AaegOBP3 showed significant similarity to previously identified mosquito odorant-binding proteins (OBPs) in cysteine spacing pattern and sequence. Two of the isolated proteins have a total of eight cysteine residues. The similarity of the spacing pattern of the cysteine residues and amino acid sequence to those of previously identified olfactory proteins suggests that one of the cysteine-rich proteins (AaegOBP2) is an OBP. The other (AaegASP1) did not belong to any group of known OBPs. Structural analyses indicate that six of the cysteine residues in AaegOBP2 are linked in a similar pattern to the previously known cysteine pairing in OBPs, i.e., Cys-24-Cys-55, Cys-51-Cys-104, Cys-95-Cys-113. The additional disulfide bridge, Cys-38-Cys-125, knits the extended C-terminal segment of the protein to a predicted alpha2-helix. As indicated by circular dichroism (CD) spectra, the extra rigidity seems to prevent the predicted formation of a C-terminal alpha-helix at low pH.

Genomics spawns novel approaches to mosquito control

In spite of advances in medicine and public health, malaria and other mosquito-borne diseases are on the rise worldwide. Although vaccines, genetically modified mosquitoes and safer insecticides are under development, herein we examine a promising new approach to malaria control through better repellents. Current repellents, usually based on DEET, inhibit host finding by impeding insect olfaction, but have significant drawbacks. We discuss how comparative genomics, using data from the Anopheles genome project, allows the rapid identification of members of three protein classes critical to insect olfaction: odorant-binding proteins, G-protein-coupled receptors, and odorant-degrading enzymes. A rational design approach similar to that used by the pharmaceutical industry for drug development can then be applied to the development of products that interfere with mosquito olfaction. Such products have the potential to provide more complete, safer and longer lasting protection than conventional repellents, preventing disease transmission by interrupting the parasite life cycle.

Odorant binding protein homologues of the malaria mosquito Anopheles gambiae; possible orthologues of the OS-E and OS-F OBPs OF Drosophila melanogaster

Twenty-nine Anopheles gambiae candidate Odorant Binding Proteins (OBPs) were characterized for similarity to OBPs of Drosophila melanogaster and other insects. Twenty-five of these sequences were identified by BLAST searching the A. gambiae genome database. Several A. gambiae sequences were significantly similar to the D. melanogaster OBPs OS-E/OS-F, LUSH and PBPRP2/PBPRP5. Exon boundary comparisons suggests that two A. gambiae genes are orthologues of OS-E and OS-F, justifying the names AgamOS-E (EAA0 1090, AF437886) and AgamOS-F (EAA14641, AF437884). If these are orthologues. then the gene duplication establishing the OS-E and OS-F lineages predated the divergence of mosquitoes and flies. The identification of orthologous OBPs and other chemosensory genes between D. melanogaster and A. gambiae should accelerate the transfer of physiological and behavioral information between these two species.