Odorant receptors for floral- and plant-derived volatiles in the yellow fever mosquito, Aedes aegypti (Diptera: Culicidae)

Adult mosquitoes require regular sugar meals, usually floral nectar, to survive and flourish in natural habitats. Both males and females locate potential sugar sources using sensory proteins called odorant receptors activated by plant volatiles that facilitate orientation toward flowers or honeydew. The Yellow Fever mosquito, Aedes aegypti (Linnaeus, 1762), possesses a large repertoire of odorant receptors, many of which are likely to support floral odor detection and nectar-seeking. In this study, we have employed a heterologous expression system and the two-electrode voltage clamping technique to identify environmentally relevant chemical compounds that activate specific odorant receptors. Importantly, we have uncovered ligand-receptor pairings for a suite of Aedes aegypti odorant receptors likely to mediate appetitive or aversive behavioral responses, thus shaping a critical aspect of the life history of a medically important mosquito. Moreover, the high degree of conservation of these receptors in other disease-transmitting species suggests common mechanisms of floral odor detection. This knowledge can be used to further investigate mosquito foraging behavior to either enhance existing, or develop novel, control strategies, especially those that incorporate mosquito bait-and-kill or attractive toxic sugar bait technologies.

The evolution of borneol repellency in culicine mosquitoes

Insects have developed remarkable adaptations to effectively interact with plant secondary metabolites and utilize them as cues to identify suitable hosts. Consequently, humans have used aromatic plants for centuries to repel mosquitoes. The repellent effects of plant volatile compounds are mediated through olfactory structures present in the antennae, and maxillary palps of mosquitoes. Mosquito maxillary palps contain capitate-peg sensilla, which house three olfactory sensory neurons, of which two are mainly tuned to either carbon dioxide or octenol - two animal host odorants. However, the third neuron, which expresses the OR49 receptor, has remained without a known ecologically-relevant odorant since its initial discovery. In this study, we used odorant mixtures and terpenoid-rich Cannabis essential oils to investigate the activation of OR49. Our results demonstrate that two monoterpenoids, borneol and camphor, selectively activate OR49, and OR9-expressing neurons, as well as the MD3 glomerulus in the antennal lobe. We confirm that borneol repels female mosquitoes, and knocking out the gene encoding the OR49 receptor suppresses the response of the corresponding olfactory sensory neuron. Importantly, this molecular mechanism of action is conserved across culicine mosquito species, underscoring its significance in their olfactory systems.

Carboxylic acids that drive mosquito attraction to humans activate ionotropic receptors

The mosquito, Aedes aegypti, is highly anthropophilic and transmits debilitating arboviruses within human populations and between humans and non-human primates. Female mosquitoes are attracted to sources of blood by responding to odor plumes that are emitted by their preferred hosts. Acidic volatile compounds, including carboxylic acids, represent particularly salient odors driving this attraction. Importantly, carboxylic acids are major constituents of human sweat and volatiles generated by skin microbes. As such, they are likely to impact human host preference, a dominant factor in disease transmission cycles. A more complete understanding of mosquito host attraction will necessitate the elucidation of molecular mechanisms of volatile odor detection that function in peripheral sensory neurons. Recent studies have shown that members of the variant ionotropic glutamate receptor gene family are necessary for physiological and behavioral responses to acidic volatiles in Aedes. In this study, we have identified a subfamily of variant ionotropic receptors that share sequence homology across several important vector species and are likely to be activated by carboxylic acids. Moreover, we demonstrate that selected members of this subfamily are activated by short-chain carboxylic acids in a heterologous cell expression system. Our results are consistent with the hypothesis that members of this receptor class underlie acidic volatile sensitivity in vector mosquitoes and provide a frame of reference for future development of novel mosquito attractant and repellent technologies.

PrimedSherlock: a tool for rapid design of highly specific CRISPR-Cas12 crRNAs

Background

CRISPR-Cas based diagnostic assays provide a portable solution which bridges the benefits of qRT-PCR and serological assays in terms of portability, specificity and ease of use. CRISPR-Cas assays are rapidly fieldable, specific and have been rigorously validated against a number of targets, including HIV and vector-borne pathogens. Recently, CRISPR-Cas12 and CRISPR-Cas13 diagnostic assays have been granted FDA approval for the detection of SARS-CoV-2. A critical step in utilizing this technology requires the design of highly-specific and efficient CRISPR RNAs (crRNAs) and isothermal primers. This process involves intensive manual curation and stringent parameters for design in order to minimize off-target detection while also preserving detection across divergent strains. As such, a single, streamlined bioinformatics platform for rapidly designing crRNAs for use with the CRISPR-Cas12 platform is needed. Here we offer PrimedSherlock, an automated, computer guided process for selecting highly-specific crRNAs and primers for targets of interest.

Results

Utilizing PrimedSherlock and publicly available databases, crRNAs were designed against a selection of Flavivirus genomes, including West Nile, Zika and all four serotypes of Dengue. Using outputs from PrimedSherlock in concert with both wildtype A.s Cas12a and Alt-R Cas12a Ultra nucleases, we demonstrated sensitive detection of nucleic acids of each respective arbovirus in in-vitro fluorescence assays. Moreover, primer and crRNA combinations facilitated the detection of their intended targets with minimal off-target background noise.

Conclusions

PrimedSherlock is a novel crRNA design tool, specific for CRISPR-Cas12 diagnostic platforms. It allows for the rapid identification of highly conserved crRNA targets from user-provided primer pairs or PrimedRPA output files. Initial testing of crRNAs against arboviruses of medical importance demonstrated a robust ability to distinguish multiple strains by exploiting polymorphisms within otherwise highly conserved genomic regions. As a freely-accessible software package, PrimedSherlock could significantly increase the efficiency of CRISPR-Cas12 diagnostics. Conceptually, the portability of detection kits could also be enhanced when coupled with isothermal amplification technologies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12859-022-04968-5.

Aedes japonicus japonicus in Nebraska and Texas

Aedes japonicus japonicus continues to spread westward and in this study, its presence is documented in 8 counties in Nebraska and in Bowie County, TX. In 1998, Ae. japonicus was collected in Connecticut, New Jersey, and New York for the 1st records of this species in North America. Except for Louisiana, it has been reported from all states that border or are east of the Mississippi River. In Canada, it has been reported in Ontario and all eastern provinces. In the Pacific Northwest, it has been reported in Washington, Oregon, and British Columbia, and in the midwestern states that do not border the Mississippi River, Kansas, Oklahoma, and South Dakota are the only states to have reported its presence in peer-reviewed journals.

Identification and functional characterization of olfactory indolergic receptors in Musca domestica

In mosquitoes, indolic compounds are detected by a group of olfactory indolergic Odorant Receptors (indolORs). The ancient origin of indole and 3-methylindole as chemical signals suggest that they may be detected by insects outside the Culicidae clade. To test this hypothesis, we have identified potential indolOR genes in brachyceran flies based on sequence homology. Because of the crucial roles of indolic compounds in oviposition and foraging, we have focused our attention on the housefly Musca domestica. Using a heterologous expression system, we have identified indolOR transcript expression in the female antennae, and have characterized MdomOR30a and MdomOR49b as 3-methylindole and indole receptors, respectively. We have identified a set of 92 putative indolOR genes encoded in the genomes of Culicoidea, Psychodidae and brachycera, described their phylogenetic relationships, and exon/intron structures. Further characterization of indolORs will impact our understanding of insect chemical ecology and will provide targets for the development of novel odor-based tools that can be integrated into existing vector surveillance and control programs.

Ammonium transporter AcAmt mutagenesis uncovers reproductive and physiological defects without impacting olfactory responses to ammonia in the malaria vector mosquito Anopheles coluzzii

Anopheline mosquitoes are the sole vectors of malaria and rely on olfactory cues for host seeking in which ammonia derived from human sweat plays an essential role. To investigate the function of the Anopheles coluzzii ammonium transporter (AcAmt) in the mosquito olfactory system, we generated an AcAmt null mutant line using CRISPR/Cas9. AcAmt^-/- mutants displayed a series of novel phenotypes compared with wild-type mosquitoes including significantly lower insemination rates during mating and increased mortality during eclosion. Furthermore, AcAmt^-/- males showed significantly lower sugar consumption while AcAmt^-/- females and pupae displayed significantly higher ammonia levels than their wild-type counterparts. Surprisingly, in contrast to previous studies in Drosophila that revealed that the mutation of the ammonium transporter (DmAmt) induces a dramatic reduction of ammonia responses in antennal coeloconic sensilla, no significant differences were observed across a range of peripheral sensory neuron responses to ammonia and other odorants between wild-type and AcAmt^-/- females. These data support the existence in mosquitoes of novel compensatory ammonia-sensing mechanisms that are likely to have evolved as a result of the importance of ammonia in host-seeking and other behaviors.

Distribution of Culex coronator in the USA

In 1920, Culex coronator was reported from San Benito, Texas, and later in Arizona, New Mexico, and Oklahoma. In 2005, this species was reported to be spreading across the southeastern USA. Now reported in 14 states, it has been found as far north as northern Oklahoma; Memphis, TN; and Suffolk, VA. The public health significance of Cx. coronator is not firmly established, even though it has been implicated as a potential vector of several arboviral diseases. This study aims to document additional Cx. coronator county-level records, to provide information about its continued expansion across the southern USA, and to provide a short research update into its vector potential. Data acquired through multistate collaborations and author collections resulted in 146 new county records from Alabama, Arkansas, Florida, Georgia, Louisiana, Mississippi, North Carolina, Oklahoma, South Carolina, and Texas. No new county records were presented for Arizona, New Mexico, Tennessee, or Virginia, which had previously reported this species. With these new data, this species has been documented in 386 counties in 14 states of the continental USA.

Carboxylic acid responses by a conserved odorant receptor in culicine vector mosquitoes

Many mosquito behaviours that are critical for survival and reproduction depend upon timely responses to chemical cues. Of interest are the effects of volatile organic compounds like carboxylic acids (CAs) that are released by potential blood meal hosts. Short chain CAs are among the primary attractants for host-seeking females and influence host selection in vector species. Although the behavioural relevance of CA's has been established, less is known about the molecular receptive events that evoke responses to specific compounds, with the Ir family of chemoreceptors being broadly implicated in their detection. In this study, we demonstrate that Or orthologs from two vector species, Aedes aegypti (L.) and Aedes albopictus (Skuse), are selectively activated by straight chain carboxylic acids and that these responses are attenuated by the commercial insect repellant N,N-Diethyl-meta-toluamide. Our results suggest that multiple chemoreceptors, representing diverse families, are able to mediate molecular responses to CAs and may therefore underlie important behaviours that directly impact disease-transmission cycles.

Feeding Habits of Vector Mosquitoes in Harris County, TX, 2018

Mosquito-borne pathogens contribute significantly to the global burden of infectious diseases and are a continuing public health concern in the United States. Blood feeding by vector mosquitoes is a critical step in the transmission of human pathogens. Continuous surveillance of mosquito feeding patterns, especially in major population centers, is necessary for sustainable, effective control strategies. To better understand female feeding habits in Harris County, TX, we trapped mosquitoes from various locations, distributed among urban and semi-urban environments. Bloodmeal hosts were determined using a cytochrome C oxidase I DNA barcoding strategy. We identified a diverse array of vertebrate hosts with a high degree of avian host utilization, most surprisingly from anthropophilic species like Aedes aegypti (L.). We also detected sequences from two different vertebrate hosts in about half of specimens examined, suggesting that multiple bloodmeals had been acquired in the same feeding cycle by a sizable fraction of females in both urban and semi-urban locations. The high proportion of feeding on domestic chickens may indicate that a significant number of homeowners are rearing chickens within close proximity to study trap sites. As non-amplifying hosts, chickens may have a diluting effect on West Nile virus, as well as a zooprophylactic effect in their immediate vicinities. Ultimately, spatial and temporal host utilization patterns add insight into potential disease transmission dynamics, thereby informing vector control strategies in Harris County and other metropolitan areas.

Heterogeneous expression of the ammonium transporter AgAmt in chemosensory appendages of the malaria vector, Anopheles gambiae

Ammonia is one of the principal kairomones originating from human and other animal emanations and in that context, plays an essential role in the host-seeking behaviors of the malaria vector mosquito Anopheles gambiae. Nevertheless, despite its importance in directing host-seeking, the mechanisms underlying ammonia detection in the mosquito olfactory system remains largely unknown. In addition to ongoing efforts to identify and characterize the molecular receptors that underlie ammonia sensitivity, previous studies have revealed a prominent role for ammonium transporters (Amt) in modulating antennal and behavioral responses in Drosophila melanogaster and An. gambiae. In the former, localization of DmAmt in antennal sensilla to auxiliary cells surrounding the ammonia sensory neurons led to the hypothesis that its role was to clear excess ammonium ions in the sensillar lymph. In the latter, RT-PCR and heterologous expression have been used to examine the expression and functional characteristics of the An. gambiae ammonium transporter, AgAmt. We now employ advanced transgenic tools to comprehensively examine AgAmt spatial localization across the peripheral chemosensory appendages in larvae and adult female An. gambiae. In the larval antennae, AgAmt appears localized in both neuronal and auxiliary cells. In contrast to D. melanogaster, in the adult antennae, AgAmt-derived signals are observed in both non-neuronal auxiliary cells and in sensory neurons in ammonia-responsive basiconic and coeloconic sensilla. In the maxillary palps, labella, and tarsi, AgAmt appears restricted to sensory neurons. We have also characterized the responses to ammonia of adult antennal coeloconic sensilla and maxillary palp capitate pegs revealing a correlation between sensillar AgAmt expression and ammonia sensitivity. Taken together, these data suggest that AgAmt may play heterogeneous roles in the adult and larval chemosensory apparatus and potentially broad utility as a supra-receptor target in mosquito control.

Identification of Mosquito Bloodmeals Collected in Diverse Habitats in Malaysian Borneo Using COI Barcoding

Land cover and land use change (LCLUC) acts as a catalyst for spillover of arthropod-borne pathogens into novel hosts by shifting host and vector diversity, abundance, and distribution, ultimately reshaping host–vector interactions. Identification of bloodmeals from wild-caught mosquitoes provides insight into host utilization of particular species in particular land cover types, and hence their potential role in pathogen maintenance and spillover. Here, we collected 134 blood-engorged mosquitoes comprising 10 taxa across 9 land cover types in Sarawak, Malaysian Borneo, a region experiencing intense LCLUC and concomitant spillover of arthropod-borne pathogens. Host sources of blood were successfully identified for 116 (87%) mosquitoes using cytochrome oxidase subunit I (COI) barcoding. A diverse range of hosts were identified, including reptiles, amphibians, birds, and mammals. Sixteen engorged Aedes albopictus, a major vector of dengue virus, were collected from seven land cover types and found to feed exclusively on humans (73%) and boar (27%). Culex tritaeniohynchus (n = 2), Cx. gelidus (n = 3), and Cx. quiquefasciatus (n = 3), vectors of Japanese encephalitis virus, fed on humans and pigs in the rural built-up land cover, creating potential transmission networks between these species. Our data support the use of COI barcoding to characterize mosquito–host networks in a biodiversity hotspot.

An odorant receptor from Anopheles gambiae that demonstrates enantioselectivity to the plant volatile, linalool

Insects express chemical receptors within sensory neurons that are activated by specific cues in the environment, thereby influencing the acquisition of critical resources. A significant gap in our current understanding of insect chemical ecology is defining the molecular mechanisms that underlie sensitivity to plant-emitted volatiles. Linalool is a commonly-occurring monoterpene that has various effects on insect behavior, either acting as an attractant or a repellent, and existing in nature as one of two possible stereoisomers, (R)-(-)-linalool and (S)-(+)-linalool. In this study, we have used a cell-based functional assay to identify linalool and structurally-related compounds as ligands of Odorant receptor 29, a labellum-expressed receptor in the malaria vector mosquito, Anopheles gambiae (AgamOr29). While (R)-(-)-linalool activates AgamOr29, a mixture of the (R) and (S) stereoisomers activates the receptor with higher potency, implying enantiomeric selectivity. Orthologs of Or29 are present in the genomes of Anophelines within the Cellia subgenus. The conservation of this receptor across Anopheline lineages suggests that this ecologically important compound might serve as an attraction cue for nectar-seeking mosquitoes. Moreover, the characterization of a mosquito terpene receptor could serve as a foundation for future ligand-receptor studies of plant volatiles and for the discovery of compounds that can be integrated into push-pull vector control strategies.

Variant Ionotropic Receptors in the Malaria Vector Mosquito Anopheles gambiae Tuned to Amines and Carboxylic Acids

The principal Afrotropical human malaria vector mosquito, Anopheles gambiae, remains a significant threat to global health. A critical component in the transmission of malaria is the ability of An. gambiae females to detect and respond to human-derived chemical kairomones in their search for blood meal hosts. The basis for host odor responses resides in olfactory receptor neurons (ORNs) that express chemoreceptors encoded by large gene families, including the odorant receptors (ORs) and the variant ionotropic receptors (IRs). While ORs have been the focus of extensive investigation, functional IR complexes and the chemical compounds that activate them have not been identified in An. gambiae. Here we report the transcriptional profiles and functional characterization of three An. gambiae IR (AgIr) complexes that specifically respond to amines or carboxylic acids - two classes of semiochemicals that have been implicated in mediating host-seeking by adult females but are not known to activate An. gambiae ORs (AgOrs). Our results suggest that AgIrs play critical roles in the detection and behavioral responses to important classes of host odors that are underrepresented in the AgOr chemical space.

Disease vectors in the era of next generation sequencing

Almost 20 % of all infectious human diseases are vector borne and, together, are responsible for over one million deaths per annum. Over the past decade, the decreasing costs of massively parallel sequencing technologies have facilitated the agnostic interrogation of insect vector genomes, giving medical entomologists access to an ever-expanding volume of high-quality genomic and transcriptomic data. In this review, we highlight how genomics resources have provided new insights into the physiology, behavior, and evolution of human disease vectors within the context of the global health landscape.

Antennal-expressed ammonium transporters in the malaria vector mosquito Anopheles gambiae

The principal Afrotropical malaria vector mosquito, Anopheles gambiae remains a significant threat to human health. In this anthropophagic species, females detect and respond to a range of human-derived volatile kairomones such as ammonia, lactic acid, and other carboxylic acids in their quest for blood meals. While the molecular underpinnings of mosquito olfaction and host seeking are becoming better understood, many questions remain unanswered. In this study, we have identified and characterized two candidate ammonium transporter genes, AgAmt and AgRh50 that are expressed in the mosquito antenna and may contribute to physiological and behavioral responses to ammonia, which is an important host kairomone for vector mosquitoes. AgAmt transcripts are highly enhanced in female antennae while a splice variant of AgRh50 appears to be antennal-specific. Functional expression of AgAmt in Xenopus laevis oocytes facilitates inward currents in response to both ammonium and methylammonium, while AgRh50 is able to partially complement a yeast ammonium transporter mutant strain, validating their conserved roles as ammonium transporters. We present evidence to suggest that both AgAmt and AgRh50 are in vivo ammonium transporters that are important for ammonia sensitivity in An. gambiae antennae, either by clearing ammonia from the sensillar lymph or by facilitating sensory neuron responses to environmental exposure. Accordingly, AgAmt and AgRh50 represent new and potentially important targets for the development of novel vector control strategies.

A blood-free protein meal supporting oogenesis in the Asian tiger mosquito, Aedes albopictus (Skuse)

Female mosquitoes require blood meals to complete oogenesis, or egg development. Current methods of maintaining laboratory colonies of mosquitoes generally rely on the use of whole blood to feed females. Blood feeding protocols require special handling techniques, impart numerous potential health hazards, involve significant costs, and are widely variable in terms of their success rates. In this study, a simple protein formulation was provided to Aedes albopictus using a membrane feeding system. Under the experimental conditions tested, females readily accepted the blood-free meal and produced eggs in greater numbers than cohort females that were fed with whole human blood. Moreover, fertility was comparable between treatments and survivorship of hatched larvae was equal among feedings. This implies that a readily available blood-free meal could be utilized in the laboratory rearing of this species. The elimination of blood handling, reduced cost, and consistency of blood-free meals would potentially be advantageous to mosquito rearing facilities generally, and in terms of scale, to mass rearing facilities specifically.

Review: Improving our knowledge of male mosquito biology in relation to genetic control programmes

The enormous burden placed on populations worldwide by mosquito-borne diseases, most notably malaria and dengue, is currently being tackled by the use of insecticides sprayed in residences or applied to bednets, and in the case of dengue vectors through reduction of larval breeding sites or larviciding with insecticides thereof. However, these methods are under threat from, amongst other issues, the development of insecticide resistance and the practical difficulty of maintaining long-term community-wide efforts. The sterile insect technique (SIT), whose success hinges on having a good understanding of the biology and behaviour of the male mosquito, is an additional weapon in the limited arsenal against mosquito vectors. The successful production and release of sterile males, which is the mechanism of population suppression by SIT, relies on the release of mass-reared sterile males able to confer sterility in the target population by mating with wild females. A five year Joint FAO/IAEA Coordinated Research Project brought together researchers from around the world to investigate the pre-mating conditions of male mosquitoes (physiology and behaviour, resource acquisition and allocation, and dispersal), the mosquito mating systems and the contribution of molecular or chemical approaches to the understanding of male mosquito mating behaviour. A summary of the existing knowledge and the main novel findings of this group is reviewed here, and further presented in the reviews and research articles that form this Acta Tropica special issue.

The roles of kairomones, synomones and pheromones in the chemically-mediated behaviour of male mosquitoes

Despite decades of intensive study of the chemical ecology of female mosquitoes, relatively little is known about the chemical ecology of males. This short review summarizes the current state of knowledge of the chemicals that mediate male mosquito behaviour. Various trophic interactions including insect-plant, insect-host, and insect-insect responses are emphasized. The relevance of the chemical ecology of male mosquitoes in the context of vector control programmes is discussed.

Odorant receptor-mediated sperm activation in disease vector mosquitoes

Insects, such as the malaria vector mosquito, Anopheles gambiae, depend upon chemoreceptors to respond to volatiles emitted from a range of environmental sources, most notably blood meal hosts and oviposition sites. A subset of peripheral signaling pathways involved in these insect chemosensory-dependent behaviors requires the activity of heteromeric odorant receptor (OR) ion channel complexes and ligands for numerous A. gambiae ORs (AgOrs) have been identified. Although AgOrs are expressed in nonhead appendages, studies characterizing potential AgOr function in nonolfactory tissues have not been conducted. In the present study, we explore the possibility that AgOrs mediate responses of spermatozoa to endogenous signaling molecules in A. gambiae. In addition to finding AgOr transcript expression in testes, we show that the OR coreceptor, AgOrco, is localized to the flagella of A. gambiae spermatozoa where Orco-specific agonists, antagonists, and other odorant ligands robustly activate flagella beating in an Orco-dependent process. We also demonstrate Orco expression and Orco-mediated activation of spermatozoa in the yellow fever mosquito, Aedes aegypti. Moreover, we find Orco localization in testes across distinct insect taxa and posit that OR-mediated responses in spermatozoa may represent a general characteristic of insect reproduction and an example of convergent evolution.

Blood meal-induced changes to antennal transcriptome profiles reveal shifts in odor sensitivities in Anopheles gambiae

Olfactory-driven behaviors are central to the lifecycle of the malaria vector mosquito Anopheles gambiae and are initiated by peripheral signaling in the antenna and other olfactory tissues. To continue gaining insight into the relationship between gene expression and olfaction, we have performed cohort comparisons of antennal transcript abundances at five time points after a blood meal, a key event in both reproduction and disease transmission cycles. We found that more than 5,000 transcripts displayed significant abundance differences, many of which were correlated by cluster analysis. Within the chemosensory gene families, we observed a general reduction in the level of chemosensory gene transcripts, although a subset of odorant receptors (AgOrs) was modestly enhanced in post-blood-fed samples. Integration of AgOr transcript abundance data with previously characterized AgOr excitatory odorant response profiles revealed potential changes in antennal odorant receptivity that coincided with the shift from host-seeking to oviposition behaviors in blood-fed female mosquitoes. Behavioral testing of ovipositing females to odorants highlighted by this synthetic analysis identified two unique, unitary oviposition cues for An. gambiae, 2-propylphenol and 4-methylcyclohexanol. We posit that modest, yet cumulative, alterations of AgOr transcript levels modulate peripheral odor coding resulting in biologically relevant behavioral effects. Moreover, these results demonstrate that highly quantitative, RNAseq transcript abundance data can be successfully integrated with functional data to generate testable hypotheses.

Feeding frequency and survival of Anopheles gambiae in a rice-growing area in Ghana

Mortality rates, determined by dissection, of predominantly M form female Anopheles gambiae (Diptera: Culicidae) were estimated. Mosquitoes were collected in tent traps and light traps in an irrigation project village in Ghana in June and July 2010, when much of the area was flooded. Both M and S form larvae were collected from rice fields (74 of 80 specimens were M form). Adults were collected in equal proportions from the two traps (90 of 107 specimens from the light trap and 106 of 116 specimens from the tent trap were M form). During the study, collection numbers rose from 105 to 972 per night. A total of 1787 of the 15 431 An. gambiae collected were dissected. Of these, 953 (53%) were found to have taken their first bloodmeal, either as virgins or following mating. The age profiles of mosquitoes collected alive and dead, respectively, were similar. Eighteen of 2933 (0.61 ± 0.49%) specimens were found to be positive for sporozoites in an enzyme-linked immunosorbent assay (ELISA). Lagged cross correlations among the different age groups implied that the mosquitoes fed on days 2 and 4 following emergence prior to oviposition and every 2.65 ± 0.17 days thereafter. The best model to describe the observed population patterns implied a daily mortality of 84%. The results are discussed in relation to possible mosquito control measures for the village.

Novel high-throughput screens of Anopheles gambiae odorant receptors reveal candidate behaviour-modifying chemicals for mosquitoes

Despite many decades of multilateral global efforts, a significant portion of the world population continues to be plagued with one or more mosquito-vectored diseases. These include malaria and filariasis as well as numerous arboviral-associated illnesses including Dengue and Yellow fevers. The dynamics of disease transmission by mosquitoes is complex, and involves both vector competence and vectorial capacity. One area of intensive effort is the study of chemosensory-driven behaviours in the malaria vector mosquito Anopheles gambiae Giles, the modulation of which are likely to provide opportunities for disease reduction. In this context recent studies have characterized a large divergent family of An. gambiae odorant receptors (AgORs) that play critical roles in olfactory signal transduction. This work has facilitated high-throughput, cell-based calcium mobilization screens of AgOR-expressing HEK cells that have identified a large number of conventional AgOR ligands, as well as the first non-conventional Orco (olfactory receptor co-receptor) family agonist. As such, ligand-mediated modulation serves as a proof-of-concept demonstration that AgORs represent viable targets for high-throughput screening and for the eventual development of behaviour-modifying olfactory compounds. Such attractants or repellents could foster malaria reduction programmes.

Studies on the behaviour of peridomestic and endophagic M form Anopheles gambiae from a rice growing area of Ghana

The 'paddy paradox', the occurrence of large populations of vectors but low amounts of malaria transmission where irrigated rice is grown, was investigated in a village in Ghana where M form Anopheles gambiae are common. Peridomestic and indoor host-seeking mosquitoes were collected in tent traps and light traps over 21 consecutive nights at the start of the rainy season in June 2009 when the population increased exponentially from less than 100 per night to over 1000. Infection rates in the overall mosquito population were 0.3% and in the estimated parous population were 1.9%. Numbers of An. gambiae in the tent trap peaked between midnight and 02:40 am. The majority of insects were taking their first blood meal, as virgins or shortly after mating. More than expected were collected in the light trap during a rainstorm at the start of the rains but overall numbers were not affected. Fewer than expected were collected after a subsequent storm. Recruitment to the adult population decreased over the following days. It is hypothesised that the 'paddy paradox' is due to young pre-gravid insects dispersing more widely than gravid ones, not necessarily to low survival in the mosquito.

Transcriptome profiling of chemosensory appendages in the malaria vector Anopheles gambiae reveals tissue- and sex-specific signatures of odor coding

Background

Chemosensory signal transduction guides the behavior of many insects, including Anopheles gambiae, the major vector for human malaria in sub-Saharan Africa. To better understand the molecular basis of mosquito chemosensation we have used whole transcriptome RNA sequencing (RNA-seq) to compare transcript expression profiles between the two major chemosensory tissues, the antennae and maxillary palps, of adult female and male An. gambiae.

Results

We compared chemosensory tissue transcriptomes to whole body transcriptomes of each sex to identify chemosensory enhanced genes. In the six data sets analyzed, we detected expression of nearly all known chemosensory genes and found them to be highly enriched in both olfactory tissues of males and females. While the maxillary palps of both sexes demonstrated strict chemosensory gene expression overlap, we observed acute differences in sensory specialization between male and female antennae. The relatively high expression levels of chemosensory genes in the female antennae reveal its role as an organ predominately assigned to chemosensation. Remarkably, the expression of these genes was highly conserved in the male antennae, but at much lower relative levels. Alternatively, consistent with a role in mating, the male antennae displayed significant enhancement of genes involved in audition, while the female enhancement of these genes was observed, but to a lesser degree.

Conclusions

These findings suggest that the chemoreceptive spectrum, as defined by gene expression profiles, is largely similar in female and male An. gambiae. However, assuming sensory receptor expression levels are correlated with sensitivity in each case, we posit that male and female antennae are perceptive to the same stimuli, but possess inverse receptive prioritizations and sensitivities. Here we have demonstrated the use of RNA-seq to characterize the sensory specializations of an important disease vector and grounded future studies investigating chemosensory processes.

Conservation of indole responsive odorant receptors in mosquitoes reveals an ancient olfactory trait

Aedes aegypti and Anopheles gambiae are among the best-characterized mosquito species within the Culicinae and Anophelinae mosquito clades which diverged ∼150 million years ago. Despite this evolutionary distance, the olfactory systems of these mosquitoes exhibit similar morphological and physiological adaptations. Paradoxically, mosquito odorant receptors, which lie at the heart of chemosensory signal transduction pathways, belong to a large and highly divergent gene family. We have used 2 heterologous expression systems to investigate the functional characteristics of a highly conserved subset of Ors between Ae. aegypti and An. gambiae to investigate whether protein homology correlates with odorant-induced activation. We find that these receptors share similar odorant response profiles and that indole, a common and ecologically relevant olfactory cue, elicits strong responses from these homologous receptors. The identification of other highly conserved members of this Or clade from mosquito species of varying phylogenetic relatedness supports a model in which high sensitivity to indole represents an ancient ecological adaptation that has been preserved as a result of its life cycle importance. These results provide an understanding of how similarities and disparities among homologous OR proteins relate to olfactory function, which can lead to greater insights into the design of successful strategies for the control of mosquito-borne diseases.

Distinct olfactory signaling mechanisms in the malaria vector mosquito Anopheles gambiae

Anopheles gambiae is the principal Afrotropical vector for human malaria, in which olfaction mediates a wide range of both adult and larval behaviors. Indeed, mosquitoes depend on the ability to respond to chemical cues for feeding, host preference, and mate location/selection. Building upon previous work that has characterized a large family of An. gambiae odorant receptors (AgORs), we now use behavioral analyses and gene silencing to examine directly the role of AgORs, as well as a newly identified family of candidate chemosensory genes, the An. gambiae variant ionotropic receptors (AgIRs), in the larval olfactory system. Our results validate previous studies that directly implicate specific AgORs in behavioral responses to DEET as well as other odorants and reveal the existence of at least two distinct olfactory signaling pathways that are active in An. gambiae. One system depends directly on AgORs; the other is AgOR-independent and requires the expression and activity of AgIRs. In addition to clarifying the mechanistic basis for olfaction in this system, these advances may ultimately enhance the development of vector control strategies, targeting olfactory pathways in mosquitoes to reduce the catastrophic effects of malaria and other mosquito-borne diseases.

Anopheles gambiae TRPA1 is a heat-activated channel expressed in thermosensitive sensilla of female antennae

Heat sensitivity is a sensory modality that plays a critical role in close-range host-seeking behaviors of adult female Anopheles gambiae, the principal Afrotropical vector for human malaria. An essential step in this activity is the ability to discriminate and respond to increases in environmental temperature gradients through the process of peripheral thermoreception. Here, we report on the characterization of the anopheline homolog of the transient receptor potential (TRP) A1/ANKTM1 channel that is consistent with its role as a heat-sensor in host-seeking adult female mosquitoes. We identify a set of distal antennal sensory structures that specifically respond to temperature gradients and express AgTRPA1. Functional characterization of AgTRPA1 in Xenopus oocytes supports its role in the molecular transduction of temperature gradients in An. gambiae, providing a basis for targeting mosquito heat responses as a means toward reducing malaria transmission.

Molecular characterization of the Aedes aegypti odorant receptor gene family

The olfactory-driven blood-feeding behaviour of female Aedes aegypti mosquitoes is the primary transmission mechanism by which the arboviruses causing dengue and yellow fevers affect over 40 million individuals worldwide. Bioinformatics analysis has been used to identify 131 putative odourant receptors from the A. aegypti genome that are likely to function in chemosensory perception in this mosquito. Comparison with the Anopheles gambiae olfactory subgenome demonstrates significant divergence of the odourant receptors that reflects a high degree of evolutionary activity potentially resulting from their critical roles during the mosquito life cycle. Expression analyses in the larval and adult olfactory chemosensory organs reveal that the ratio of odourant receptors to antennal glomeruli is not necessarily one to one in mosquitoes.

Odor coding in the maxillary palp of the malaria vector mosquito Anopheles gambiae

BACKGROUND

Many species of mosquitoes, including the major malaria vector Anopheles gambiae, utilize carbon dioxide (CO(2)) and 1-octen-3-ol as olfactory cues in host-seeking behaviors that underlie their vectorial capacity. However, the molecular and cellular basis of such olfactory responses remains largely unknown.

RESULTS

Here, we use molecular and physiological approaches coupled with systematic functional analyses to define the complete olfactory sensory map of the An. gambiae maxillary palp, an olfactory appendage that mediates the detection of these compounds. In doing so, we identify three olfactory receptor neurons (ORNs) that are organized in stereotyped triads within the maxillary-palp capitate-peg-sensillum population. One ORN is CO(2)-responsive and characterized by the coexpression of three receptors that confer CO(2) responses, whereas the other ORNs express characteristic odorant receptors (AgORs) that are responsible for their in vivo olfactory responses.

CONCLUSIONS

Our results describe a complete and highly concordant map of both the molecular and cellular olfactory components on the maxillary palp of the adult female An. gambiae mosquito. These results also facilitate the understanding of how An. gambiae mosquitoes sense olfactory cues that might be exploited to compromise their ability to transmit malaria.

Antennal sensilla of two female anopheline sibling species with differing host ranges

BACKGROUND

Volatile odors are important sensory inputs that shape the behaviour of insects, including agricultural pests and disease vectors. Anopheles gambiae s.s. is a highly anthropophilic mosquito and is the major vector for human malaria in sub-Sahara Africa, while Anopheles quadriannulatus, largely due to its zoophilic behaviour, is considered a non-vector species in the same region. Careful studies of olfaction in these sibling species may lead to insights about the mechanisms that drive host preference behaviour. In the present study, the external anatomy of the antenna, the principle olfactory organ in the female mosquito of both species, was examined as an initial step toward more detailed comparisons.

METHODS

Scanning electron and light microscopy were used to examine the antennae ultrastructures of adult female An. gambiae s.s. and An. quadriannulatus. Sensory structures, called sensilla, were categorized and counted; their distributions are reported here as well as densities calculated for each species.

RESULTS

Both An. gambiae s.s. and An. quadriannulatus bear five classes of sensilla on their antennae: chaetica (bristles), trichodea (hairs), basiconica (pegs), coeloconica (pitted pegs), and ampullacea (pegs in tubes). Female An. quadriannulatus antennae have approximately one-third more sensilla, and a proportionally larger surface area, than female An. gambiae s.s. antennae.

CONCLUSION

The same types of sensilla are found on the antennae of both species. While An. quadriannulatus has greater numbers of each sensilla type, sensilla densities are very similar for each species, suggesting that other factors may be more important to such olfactory-driven behaviours as host preference.

Identification of a Chemosensory Receptor from the Yellow Fever Mosquito, Aedes aegypti, that is Highly Conserved and Expressed in Olfactory and Gustatory Organs

Aedes aegypti is a highly anthropophilic mosquito responsible for the transmission of dengue and yellow fever around the world. Like other mosquitoes, the biting and host preference behaviors of this disease vector are largely influenced by its sense of smell, which is presumably facilitated by G protein-coupled receptor signaling cascades. Here, we report the identification and characterization of AaOr7, the first candidate odorant receptor from Ae. aegypti. This receptor displays extremely high primary amino acid conservation with AgOr7 another candidate odorant receptor identified in the Aftrotropical malaria vector, Anopheles gambiae as well as several previously identified candidate odorant receptors in Drosophila melanogaster and other insects. Its transcript is expressed in adult chemosensory tissues and during several stages of Ae. aegypti development. Within the adult olfactory system, AaOr7 protein is found specifically within most antennal and maxillary palp sensilla, as well as in a subset of proboscis sensilla. These results are consistent with a role for AaOr7 in olfaction and gustation supporting the hypothesis that AaOr7 and its orthologs may be of general importance to chemosensory processes throughout the lifetime of an insect.

A highly conserved candidate chemoreceptor expressed in both olfactory and gustatory tissues in the malaria vector Anopheles gambiae

Anopheles gambiae is a highly anthropophilic mosquito responsible for the majority of malaria transmission in Africa. The biting and host preference behavior of this disease vector is largely influenced by its sense of smell, which is presumably facilitated by G protein-coupled receptor signaling [Takken, W. & Knols, B. (1999) Annu. Rev. Entomol. 44, 131-157]. Because of the importance of host preference to the mosquitoes' ability to transmit disease, we have initiated studies intended to elucidate the molecular mechanisms underlying olfaction in An. gambiae. In the course of these studies, we have identified a number of genes potentially involved in signal transduction, including a family of candidate odorant receptors. One of these receptors, encoded by GPRor7 (hereafter referred to as AgOr7), is remarkably similar to an odorant receptor that is expressed broadly in olfactory tissues and has been identified in Drosophila melanogaster and other insects [Krieger, J., Klink, O., Mohl, C., Raming, K. & Breer, H. (2003) J. Comp. Physiol. A 189, 519-526; Vosshall, L. B., Amrein, H., Morozov, P. S., Rzhetsky, A. & Axel, R. (1999) Cell 96, 725-736]. We have observed AgOr7 expression in olfactory and gustatory tissues in adult An. gambiae and during several stages of the mosquitoes' development. Within the female adult peripheral chemosensory system, antiserum against the AgOR7 polypeptide labels most sensilla of the antenna and maxillary palp as well as a subset of proboscis sensilla. Furthermore, AgOR7 antiserum labeling is observed within the larval antenna and maxillary palpus. These results are consistent with a role for AgOr7 in both olfaction and gustation in An. gambiae and raise the possibility that AgOr7 orthologs may also be of general importance to both modalities of chemosensation in other insects.

Molecular characterization of arrestin family members in the malaria vector mosquito, Anopheles gambiae

Olfaction influences many insect behaviours including mate seeking and host selection. The molecular machinery underlying insect olfactory systems is a G protein-coupled receptor pathway that, in addition to activation, requires adaptation for olfactory sensitivity and discrimination. We have previously identified ARR1 (henceforth AgARR1), a sensory arrestin from the malaria vector mosquito Anopheles gambiae that has been postulated to modulate olfactory adaptation. This report describes three additional arrestin family members including ARR2 (henceforth AgARR2), which is similar to previously characterized insect sensory arrestins and is expressed at significantly higher levels in the antennae of male vs. female A. gambiae mosquitoes. This finding is consistent with the hypothesis that AgARR2 may be important for the regulation of olfactory-driven behaviours particular to male mosquitoes.

G protein-coupled receptors in Anopheles gambiae

We used bioinformatic approaches to identify a total of 276 G protein-coupled receptors (GPCRs) from the Anopheles gambiae genome. These include GPCRs that are likely to play roles in pathways affecting almost every aspect of the mosquito's life cycle. Seventy-nine candidate odorant receptors were characterized for tissue expression and, along with 76 putative gustatory receptors, for their molecular evolution relative to Drosophila melanogaster. Examples of lineage-specific gene expansions were observed as well as a single instance of unusually high sequence conservation.

A cluster of candidate odorant receptors from the malaria vector mosquito, Anopheles gambiae

Olfaction is critical to the host preference selection behavior of many disease-transmitting insects, including the mosquito Anopheles gambiae sensu stricto (hereafter A. gambiae), one of the major vectors for human malaria. In order to more fully understand the molecular biology of olfaction in this insect, we have previously identified several members member of a family of candidate odorant receptor proteins from A. gambiae (AgORs). Here we report the cloning and characterization of an additional AgOR gene, denoted as AgOr5, which shows significant similarity to putative odorant receptors in A. gambiae and Drosophila melanogaster and which is selectively expressed in olfactory organs. AgOr5 is tightly clustered within the A. gambiae genome to two other highly homologous candidate odorant receptors, suggesting that these genes are derived from a common ancestor. Analysis of the developmental expression within members of this AgOR gene cluster reveals considerable variation between these AgORs as compared to candidate odorant receptors from D. melanogaster.

Visual arrestins in olfactory pathways of Drosophila and the malaria vector mosquito Anopheles gambiae

Arrestins are important components for desensitization of G protein-coupled receptor cascades that mediate neurotransmission as well as olfactory and visual sensory reception. We have isolated AgArr1, an arrestin-encoding cDNA from the malaria vector mosquito, Anopheles gambiae, where olfaction is critical for vectorial capacity. Analysis of AgArr1 expression revealed an overlap between chemosensory and photoreceptor neurons. Furthermore, an examination of previously identified arrestins from Drosophila melanogaster exposed similar bimodal expression, and Drosophila arrestin mutants demonstrate impaired electrophysiological responses to olfactory stimuli. Thus, we show that arrestins in Drosophila are required for normal olfactory physiology in addition to their previously described role in visual signaling. These findings suggest that individual arrestins function in both olfactory and visual pathways in Dipteran insects; these genes may prove useful in the design of control strategies that target olfactory-dependent behaviors of insect disease vectors.

Candidate odorant receptors from the malaria vector mosquito Anopheles gambiae and evidence of down-regulation in response to blood feeding

Olfaction plays a major role in host preference and blood feeding, integral behaviors for disease transmission by the malaria vector mosquito Anopheles gambiae sensu stricto (henceforth A. gambiae). We have identified four genes encoding candidate odorant receptors from A. gambiae that are selectively expressed in olfactory organs, contain approximately seven transmembrane domains, and show significant similarity to several putative odorant receptors in Drosophila melanogaster. Furthermore, one of the putative A. gambiae odorant receptors exhibits female-specific antennal expression and is down-regulated 12 h after blood feeding, a period during which substantial reduction in olfactory responses to human odorants has been observed. Taken together, these data suggest these genes encode a family of odorant receptors in A. gambiae, whose further study may aid in the design of novel antimalarial programs.

Isolation and characterization of the Xanthine dehydrogenase gene of the Mediterranean fruit fly, Ceratitis capitata

Xanthine dehydrogenase (XDH) is a member of the molybdenum hydroxylase family of enzymes catalyzing the oxidation of hypoxanthine and xanthine to uric acid. The enzyme is also required for the production of one of the major Drosophila eye pigments, drosopterin. The XDH gene has been isolated in many species representing a broad cross section of the major groups of living organisms, including the cDNA encoding XDH from the Mediterranean fruit fly Ceratitis capitata (CcXDH) described here. CcXDH is closely related to other insect XDHs and is able to rescue the phenotype of the Drosophila melanogaster XDH mutant, rosy, in germline transformation experiments. A previously identified medfly mutant, termed rosy, whose phenotype is suggestive of a disruption in XDH function, has been examined for possible mutations in the XDH gene. However, we find no direct evidence that a mutation in the CcXDH gene or that a reduction in the CcXDH enzyme activity is present in rosy medflies. Conclusive studies of the nature of the medfly rosy mutant will require rescue by germline transformation of mutant medflies.

Genomic organization and characterization of the white locus of the Mediterranean fruitfly, Ceratitis capitata

An approximately 14-kb region of genomic DNA encoding the wild-type white eye (w+) color gene from the medfly, Ceratitis capitata has been cloned and characterized at the molecular level. Comparison of the intron-exon organization of this locus among several dipteran insects reveals distinct organizational patterns that are consistent with the phylogenetic relationships of these flies and the dendrogram of the predicted primary amino acid sequence of the white loci. An examination of w+ expression during medfly development has been carried out, displaying overall similarity to corresponding studies for white gene homologues in Drosophila melanogaster and other insects. Interestingly, we have detected two phenotypically neutral allelic forms of the locus that have arisen as the result of an apparently novel insertion or deletion event located in the large first intron of the medfly white locus. Cloning and sequencing of two mutant white alleles, w1 and w2, from the we,wp and M245 strains, respectively, indicate that the mutant conditions in these strains are the result of independent events--a frameshift mutation in exon 6 for w1 and a deletion including a large part of exon 2 in the case of w2.

Auxin and ethylene promote root hair elongation in Arabidopsis

Genetic and physiological studies implicate the phytohormones auxin and ethylene in root hair development. To learn more about the role of these compounds, we have examined the root hair phenotype of a number of auxin- and ethylene-related mutants. In a previous study, Masucci and Schiefelbein (1996) showed that neither the auxin response mutations aux1 and axr1 nor the ethylene response mutations etr1 and ein2 have a significant effect on root hair initiation. In this study, we found that mutants deficient in either auxin or ethylene response have a pronounced effect on root hair length. Treatment of wild-type, axr1 and etr1 seedlings with the synthetic auxin, 2,4-D, or the ethylene precursor ACC, led to the development of longer root hairs than untreated seedlings. Furthermore, axr1 seedlings grown in the presence of ACC produce ectopic root hairs and an unusual pattern of long root hairs followed by regions that completely lack root hairs. These studies indicate that both auxin and ethylene are required for normal root hair elongation.