Efficient ΦC31 integrase-mediated site-specific germline transformation of Anopheles gambiae

HM, Antoshechkin I, Marshall JM, Akbari OS. Eliminating malaria vectors with precision-guided sterile males

Controlling the principal African malaria vector, the mosquito Anopheles gambiae, is considered essential to curtail malaria transmission. However, existing vector control technologies rely on insecticides, which are becoming increasingly ineffective. Sterile insect technique (SIT) is a powerful suppression approach that has successfully eradicated a number of insect pests, yet the A. gambiae toolkit lacks the requisite technologies for its implementation. SIT relies on iterative mass releases of nonbiting, nondriving, sterile males which seek out and mate with monandrous wild females. Once mated, females are permanently sterilized due to mating-induced refractoriness, which results in population suppression of the subsequent generation. However, sterilization by traditional methods renders males unfit, making the creation of precise genetic sterilization methods imperative. Here, we introduce a vector control technology termed precision-guided sterile insect technique (pgSIT), in A. gambiae for inducible, programmed male sterilization and female elimination for wide-scale use in SIT campaigns. Using a binary CRISPR strategy, we cross separate engineered Cas9 and gRNA strains to disrupt male-fertility and female-essential genes, yielding >99.5% male sterility and >99.9% female lethality in hybrid progeny. We demonstrate that these genetically sterilized males have good longevity, are able to induce sustained population suppression in cage trials, and are predicted to eliminate wild A. gambiae populations using mathematical models, making them ideal candidates for release. This work provides a valuable addition to the malaria genetic biocontrol toolkit, enabling scalable SIT-like confinable, species-specific, and safe suppression in the species.

Targeting mosquito X-chromosomes reveals complex transmission dynamics of sex ratio distorting gene drives

Engineered sex ratio distorters (SRDs) have been proposed as a powerful component of genetic control strategies designed to suppress harmful insect pests. Two types of CRISPR-based SRD mechanisms have been proposed: X-shredding, which eliminates X-bearing sperm, and X-poisoning, which eliminates females inheriting disrupted X-chromosomes. These differences can have a profound impact on the population dynamics of SRDs when linked to the Y-chromosome: an X-shredder is invasive, constituting a classical meiotic Y-drive, whereas X-poisoning is self-limiting, unable to invade but also insulated from selection. Here, we establish X-poisoning strains in the malaria vector Anopheles gambiae targeting three X-linked genes during spermatogenesis, resulting in male bias. We find that sex distortion is primarily driven by a loss of X-bearing sperm, with limited evidence for postzygotic lethality of female progeny. By leveraging a Drosophila melanogaster model, we show unambiguously that engineered SRD traits can operate differently in these two insects. Unlike X-shredding, X-poisoning could theoretically operate at early stages of spermatogenesis. We therefore explore premeiotic Cas9 expression to target the mosquito X-chromosome. We find that, by pre-empting the onset of meiotic sex chromosome inactivation, this approach may enable the development of Y-linked SRDs if mutagenesis of spermatogenesis-essential genes is functionally balanced.

Selecting Transgenic Mosquito Larvae with Puromycin

The selection of mosquito transgenic larvae using a nonfluorescent approach can be advantageous to reserve fluorophores for downstream applications, such as immunostaining or for the study of promoter activity by cloning a fluorescence reporter gene under the control of that promoter. We previously reported that puromycin selection is efficient in transgenic Anopheles gambiae or Anopheles coluzzii larvae expressing an OpIE2-pac selection marker. A concentration of puromycin of >10 µg/mL is lethal for Anopheles larvae, unless they carry the resistance gene, conferring them resistance to puromycin concentrations of 25-80 µg/mL. A drawback of this fully dominant selection marker is that, unlike with fluorescence markers, homozygous transgenics cannot be distinguished from heterozygotes. Here, we outline the procedure for selecting puromycin-resistant transgenic Anopheles larvae.

Sorting Mosquito Larvae with a COPAS Machine

Transgenic mosquitoes are widely used in mosquito research. To distinguish transgenic individuals from wild types, genes for fluorescent proteins are the most commonly used genetic markers in transgenic constructs, offering all the advantages of visual selection. Although manual selection under a fluorescence binocular microscope is perfect for the selection of first-generation transgenics, managing established fluorescent lines can be facilitated by complex object parametric analyzer and sorter (COPAS) sorting, which we describe in this protocol. COPAS sorting allows researchers to purify large mosquito larval populations containing only homozygous transgenic individuals, only heterozygotes, or a mix of homozygous, wild types, and heterozygotes in desired proportions. Sorting large populations of a single sex is also possible. Finally, especially when several transgenes of different fluorescence colors are inserted in the same docking site (a recombination site previously inserted in the mosquito genome, which can be used to insert new transgenes into the same locus), they can be maintained together in a single mosquito population to save insectarium space and labor. COPAS sorting can then be used to extract the desired genotype when needed and to readjust transgene frequencies every few generations in case drift is observed.

Screening Mosquito Larvae Under a Fluorescence Binocular Microscope

Transgenic mosquitoes are used in many aspects of mosquito research, and convenient selection markers are crucial to identifying transgenic individuals even among large numbers of wild types. Visual markers, in the form of fluorescent proteins expressed in larval and adult mosquito tissues, are the most commonly used. This requires observing large numbers of mosquitoes under the fluorescence microscope and recognizing positive individuals expressing the fluorescent genetic marker. As research models, mosquito larvae possess the following two advantages over many other insects, greatly facilitating fluorescence screening: (1) Being aquatic, they can be isolated in a drop of clear water, an ideal medium for live observations under the binocular microscope; and (2) synchronous hatching from many eggs is easily obtained, so that large populations of larvae can be screened in batches of several tens of thousands at a time. Screening at the neonate stage, when larvae are ∼1-mm-long, allows the concentration of hundreds of larvae in a drop of water that fits in the observation field of the microscope at medium magnification. Thus, many individuals can be screened rapidly. We strongly recommend screening larvae at the neonate stage and before any feeding for two reasons, as follows: (1) Food particles can be strongly autofluorescent, thereby dramatically increasing the fluorescence background noise around larvae, and (2) tissue autofluorescence increases during development, notably in the digestive tract, significantly decreasing the specific signal-to-noise ratio. In this protocol, we guide the experimenter step-by-step for a fast and efficient medium-throughput manual screening for fluorescent larvae.

CRISPR-mediated germline mutagenesis for genetic sterilization of Anopheles gambiae males

Rapid spread of insecticide resistance among anopheline mosquitoes threatens malaria elimination efforts, necessitating development of alternative vector control technologies. Sterile insect technique (SIT) has been successfully implemented in multiple insect pests to suppress field populations by the release of large numbers of sterile males, yet it has proven difficult to adapt to Anopheles vectors. Here we outline adaptation of a CRISPR-based genetic sterilization system to selectively ablate male sperm cells in the malaria mosquito Anopheles gambiae. We achieve robust mosaic biallelic mutagenesis of zero population growth (zpg, a gene essential for differentiation of germ cells) in F1 individuals after intercrossing a germline-expressing Cas9 transgenic line to a line expressing zpg-targeting gRNAs. Approximately 95% of mutagenized males display complete genetic sterilization, and cause similarly high levels of infertility in their female mates. Using a fluorescence reporter that allows detection of the germline leads to a 100% accurate selection of spermless males, improving the system. These males cause a striking reduction in mosquito population size when released at field-like frequencies in competition cages against wild type males. These findings demonstrate that such a genetic system could be adopted for SIT against important malaria vectors.

Aedes aegypti CCEae3A carboxylase expression confers carbamate, organophosphate and limited pyrethroid resistance in a model transgenic mosquito

Insecticide resistance is a serious threat to our ability to control mosquito vectors which transmit pathogens including malaria parasites and arboviruses. Understanding the underlying mechanisms is an essential first step in tackling the challenges presented by resistance. This study aimed to functionally characterise the carboxylesterase, CCEae3A, the elevated expression of which has been implicated in temephos resistance in Aedes aegypti and Aedes albopictus larvae. Using our GAL4/UAS expression system, already established in insecticide-sensitive Anopheles gambiae mosquitoes, we produced transgenic An. gambiae mosquitoes that express an Ae. aegypti CCEae3A ubiquitously. This new transgenic line permits examination of CCEae3A expression in a background in which there is not a clear orthologue in Vectorbase and allows comparison with existing An. gambiae GAL4-UAS lines. Insecticide resistance profiling of these transgenic An. gambiae larvae indicated significant increases in resistance ratio for three organophosphate insecticides, temephos (6), chloropyriphos (6.6) and fenthion (3.2) when compared to the parental strain. Cross resistance to adulticides from three major insecticide classes: organophosphates (malathion, fenitrothion and pirimiphos methyl), carbamates (bendiocarb and propoxur) and pyrethroid (alpha-cypermethrin) was also detected. Resistance to certain organophosphates and carbamates validates conclusions drawn from previous expression and phenotypic data. However, detection of resistance to pirimiphos methyl and alphacypermethrin has not previously been formally associated with CCEae3A, despite occurring in Ae. aegypti strains where this gene was upregulated. Our findings highlight the importance of characterising individual resistance mechanisms, thereby ensuring accurate information is used to guide future vector control strategies.

HM, Antoshechkin I, Marshall JM, Akbari OS. Eliminating Malaria Vectors with Precision Guided Sterile Males

Controlling the principal African malaria vector, the mosquito Anopheles gambiae, is considered essential to curtail malaria transmission. However existing vector control technologies rely on insecticides, which are becoming increasingly ineffective. Sterile insect technique (SIT) is a powerful suppression approach that has successfully eradicated a number of insect pests, yet the A. gambiae toolkit lacks the requisite technologies for its implementation. SIT relies on iterative mass-releases of non-biting, non-driving, sterile males which seek out and mate with monandrous wild females. Once mated, females are permanently sterilized due to mating-induced refractoriness, which results in population suppression of the subsequent generation. However, sterilization by traditional methods renders males unfit, making the creation of precise genetic sterilization methods imperative. Here we develop precision guided Sterile Insect Technique (pgSIT) in the mosquito A. gambiae for inducible, programmed male-sterilization and female-elimination for wide scale use in SIT campaigns. Using a binary CRISPR strategy, we cross separate engineered Cas9 and gRNA strains to disrupt male-fertility and female-essential genes, yielding >99.5% male-sterility and >99.9% female-lethality in hybrid progeny. We demonstrate that these genetically sterilized males have good longevity, are able to induce population suppression in cage trials, and are predicted to eliminate wild A. gambiae populations using mathematical models, making them ideal candidates for release. This work provides a valuable addition to the malaria genetic biocontrol toolkit, for the first time enabling scalable SIT-like confinable suppression in the species.

CRISPR-mediated germline mutagenesis for genetic sterilization of Anopheles gambiae males

Rapid spread of insecticide resistance among anopheline mosquitoes threatens malaria elimination efforts, necessitating development of alternative vector control technologies. Sterile Insect Technique (SIT) has been successfully implemented in multiple insect pests to suppress field populations by the release of large numbers of sterile males, yet it has proven difficult to adapt to Anopheles vectors. Here we outline adaptation of a CRISPR-based genetic sterilization system to selectively ablate male sperm cells in the malaria mosquito Anopheles gambiae. We achieve robust mosaic biallelic mutagenesis of zero population growth (zpg, a gene essential for differentiation of germ cells) in F1 individuals after intercrossing a germline-expressing Cas9 transgenic line to a line expressing zpg-targeting gRNAs. Approximately 95% of mutagenized males display complete genetic sterilization, and cause similarly high levels of infertility in their female mates. Using a fluorescence reporter that allows detection of the germline leads to a 100% accurate selection of spermless males, improving the system. These males cause a striking reduction in mosquito population size when released at field-like frequencies in competition cages against wild type males. These findings demonstrate that such a genetic system could be adopted for SIT against important malaria vectors.

Neurogenetic identification of mosquito sensory neurons

Anopheles mosquitoes, as vectors for the malaria parasite, are a global threat to human health. To find and bite a human, they utilize neurons within their sensory appendages. However, the identity and quantification of sensory appendage neurons are lacking. Here we use a neurogenetic approach to label all neurons in Anopheles coluzzii mosquitoes. We utilize the homology assisted CRISPR knock-in (HACK) approach to generate a T2A-QF2^w knock-in of the synaptic gene bruchpilot. We use a membrane-targeted GFP reporter to visualize the neurons in the brain and to quantify neurons in all major chemosensory appendages (antenna, maxillary palp, labella, tarsi, and ovipositor). By comparing labeling of brp>GFP and Orco>GFP mosquitoes, we predict the extent of neurons expressing ionotropic receptors (IRs) or other chemosensory receptors. This work introduces a valuable genetic tool for the functional analysis of Anopheles mosquito neurobiology and initiates characterization of the sensory neurons that guide mosquito behavior.

A toolbox of engineered mosquito lines to study salivary gland biology and malaria transmission

Mosquito saliva is a vehicle for the transmission of vector borne pathogens such as Plasmodium parasites and different arboviruses. Despite the key role of the salivary glands in the process of disease transmission, knowledge of host-pathogen interactions taking place within this organ is very limited. To improve the experimental tractability of the salivary glands, we have generated fluorescent reporter lines in the African malaria mosquito Anopheles coluzzii using the salivary gland-specific promoters of the anopheline antiplatelet protein (AAPP), the triple functional domain protein (TRIO) and saglin (SAG) coding genes. Promoter activity was specifically observed in the distal-lateral lobes or in the median lobe of the salivary glands. Besides a comparison of the expression patterns of the selected promoters, the fluorescent probes allowed us to evaluate the inducibility of the selected promoters upon blood feeding and to measure intracellular redox changes. We also combined the aapp-DsRed fluorescent reporter line with a pigmentation-deficient yellow(-) mosquito mutant to assess the feasibility of in vivo microscopy of parasitized salivary glands. This combination allowed locating the salivary gland through the cuticle and imaging of individual sporozoites in vivo, which facilitates live imaging studies of salivary gland colonization by Plasmodium sporozoites.

Discrete roles of Ir76b ionotropic coreceptor impact olfaction, blood feeding, and mating in the malaria vector mosquito Anopheles coluzzii

Anopheline mosquitoes rely on their highly sensitive chemosensory apparatus to detect diverse chemical stimuli that drive the host-seeking and blood-feeding behaviors required to vector pathogens for malaria and other diseases. This process incorporates a variety of chemosensory receptors and transduction pathways. We used advanced in vivo gene-editing and -labeling approaches to localize and functionally characterize the ionotropic coreceptor AcIr76b in the malaria mosquito Anopheles coluzzii, where it impacts both olfactory and gustatory systems. AcIr76b has a broad expression pattern in female adult antennal grooved pegs, coeloconic sensilla, and T1 and T2 sensilla on the labellum, stylets, and tarsi, as well as the larval sensory peg. AcIr76b is colocalized with the Orco odorant receptor (OR) coreceptor in a subset of cells across the female antennae and labella. In contrast to Orco and Ir8a, chemosensory coreceptors that appear essential for the activity of their respective sets of chemosensory neurons in mosquitoes, AcIr76b−/− mutants maintain wild-type peripheral responses to volatile amines on the adult palps, labellum, and larval sensory cone. Interestingly, AcIr76b−/− mutants display significantly increased responses to amines in antennal grooved peg sensilla, while coeloconic sensilla reveal significant deficits in responses to several acids and amines. Behaviorally, AcIr76b mutants manifest significantly female-specific insemination deficits, and although AcIr76b−/− mutant females can locate, alight on, and probe artificial blood hosts, they are incapable of blood feeding successfully. Taken together, our findings reveal a multidimensional functionality of Ir76b in anopheline olfactory and gustatory pathways that directly impacts the vectorial capacity of these mosquitoes.

Multiplexed drug-based selection and counterselection genetic manipulations in Drosophila

The power of Drosophila melanogaster as a model system relies on tractable germline genetic manipulations. Despite Drosophila's expansive genetics toolbox, such manipulations are still accomplished one change at a time and depend predominantly on phenotypic screening. We describe a drug-based genetic platform consisting of four selection and two counterselection markers, eliminating the need to screen for modified progeny. These markers work reliably individually or in combination to produce specific genetic outcomes. We demonstrate three example applications of multiplexed drug-based genetics by generating (1) transgenic animals, expressing both components of binary overexpression systems in a single transgenesis step; (2) dual selectable and counterselectable balancer chromosomes; and (3) selectable, fluorescently tagged P[acman] bacterial artificial chromosome (BAC) strains. We perform immunoprecipitation followed by proteomic analysis on one tagged BAC line, demonstrating our platform's applicability to biological discovery. Lastly, we provide a plasmid library resource to facilitate custom transgene design and technology transfer to other model systems.

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.

Improved transient silencing of gene expression in the mosquito female Aedes aegypti

Gene silencing using RNA interference (RNAi) has become a widely used genetic technique to study gene function in many organisms. In insects, this technique is often applied through the delivery of dsRNA. In the adult female Aedes aegypti, a main vector of human-infecting arboviruses, efficiency of gene silencing following dsRNA injection varies greatly according to targeted genes. Difficult knockdowns using dsRNA can thus hamper gene function analysis. Here, by analysing silencing of three different genes in female Ae. aegypti (p400, ago2 and E75), we show that gene silencing can indeed be dsRNA sequence dependent but different efficiencies do not correlate with dsRNA length. Our findings suggest that silencing is likely also gene dependent, probably due to gene-specific tissue expression and/or feedback mechanisms. We demonstrate that use of high doses of dsRNA can improve knockdown efficiency, and injection of a transfection reagent along with dsRNA reduces the variability in efficiency between replicates. Finally, we show that gene silencing cannot be achieved using siRNA injection in Ae. aegypti adult females. Overall, this work should help future gene function analyses using RNAi in adult females Ae. aegypti, leading toward a better understanding of physiological and infectious processes.

Gene editing reveals obligate and modulatory components of the CO2 receptor complex in the malaria vector mosquito, Anopheles coluzzii

The sensitivity to volatile carbon dioxide (CO₂) produced by humans and other animals is a critical component in the host preference behaviors of the malaria vector mosquito Anopheles coluzzii. The molecular receptors responsible for the ability to sense CO₂ are encoded by three putative gustatory receptor (Gr) genes (Gr22,23,24) which are expressed in a distinctive array of sensory neurons housed in maxillary palp capitate peg sensilla of An. coluzzii. Despite the identification of these components and subsequent studies, there is a paucity of understanding regarding the respective roles of these three GRs in the mosquito's CO₂ transduction process. To address this, we have used CRISPR/Cas9-based gene editing technique combined with in vivo electrophysiological recordings to directly examine the role of Gr22,23,24 in detecting CO₂ in An. coluzzii. These studies reveal that both Gr23 and Gr24 are absolutely required to maintain in vivo CO₂ sensitivity while, in contrast, Gr22 knock out mutants are still able to respond to CO₂ stimuli albeit with significantly weaker sensitivity. Our data supports a model in which Gr22 plays a modulatory role to enhance the functionality of Gr23/24 complexes that are responsible for CO₂ sensitivity of mosquitoes.

Gene editing reveals obligate and modulatory components of the CO2 receptor complex in the malaria vector mosquito, Anopheles coluzzii

The sensitivity to volatile carbon dioxide (CO₂) produced by humans and other animals is a critical component in the host preference behaviors of the malaria vector mosquito Anopheles coluzzii. The molecular receptors responsible for the ability to sense CO₂ are encoded by three putative gustatory receptor (Gr) genes (Gr22,23,24) which are expressed in a distinctive array of sensory neurons housed in maxillary palp capitate peg sensilla of An. coluzzii. Despite the identification of these components and subsequent studies, there is a paucity of understanding regarding the respective roles of these three GRs in the mosquito's CO₂ transduction process. To address this, we have used CRISPR/Cas9-based gene editing technique combined with in vivo electrophysiological recordings to directly examine the role of Gr22,23,24 in detecting CO₂ in An. coluzzii. These studies reveal that both Gr23 and Gr24 are absolutely required to maintain in vivo CO₂ sensitivity while, in contrast, Gr22 knock out mutants are still able to respond to CO₂ stimuli albeit with significantly weaker sensitivity. Our data supports a model in which Gr22 plays a modulatory role to enhance the functionality of Gr23/24 complexes that are responsible for CO₂ sensitivity of mosquitoes.

Mutagenesis of the orco odorant receptor co-receptor impairs olfactory function in the malaria vector Anopheles coluzzii

Mosquitoes rely heavily on their olfactory systems for host seeking, selection of oviposition sites, and avoiding predators and other environmental dangers. Of these behaviors, the preferential selection of a human blood-meal host drives the vectorial capacity of anthropophilic female Anopheles coluzzii mosquitoes. Olfactory receptor neurons (ORNs) are dispersed across several appendages on the head and express an obligate odorant receptor co-receptor (Orco) coupled with a "tuning" odorant receptor (OR) to form heteromeric, odor-gated ion channels in the membrane of these neurons. To examine the mechanistic and functional contributions of Orco/OR complexes to the chemosensory processes of An. coluzzii, we utilized CRISPR/Cas9 gene editing to create a line of homozygous, Orco-knockout, mutant mosquitoes. As expected, orco-/- ORNs across both adult and larval stages of An. coluzzii display significantly lower background activity and lack nearly all odor-evoked responses. In addition, blood-meal-seeking, adult female, orco-/- mutant mosquitoes exhibit severely reduced attraction to human- and non-human-derived odors while gravid females are significantly less responsive to established oviposition attractants. These results reinforce observations in other insects that Orco is crucial in maintaining the activity of ORNs. In that light, it significantly influences a range of olfactory-driven behaviors central to the anthropophilic host preference that is critical to the vectorial capacity of An. coluzzii as a primary vector for human malaria.

Isolation and transcriptomic analysis of Anopheles gambiae oenocytes enables the delineation of hydrocarbon biosynthesis

The surface of insects is coated in cuticular hydrocarbons (CHCs); variations in the composition of this layer affect a range of traits including adaptation to arid environments and defence against pathogens and toxins. In the African malaria vector, Anopheles gambiae quantitative and qualitative variance in CHC composition have been associated with speciation, ecological habitat and insecticide resistance. Understanding how these modifications arise will inform us of how mosquitoes are responding to climate change and vector control interventions. CHCs are synthesised in sub-epidermal cells called oenocytes that are very difficult to isolate from surrounding tissues. Here we utilise a transgenic line with fluorescent oenocytes to purify these cells for the first time. Comparative transcriptomics revealed the enrichment of biological processes related to long chain fatty acyl-CoA biosynthesis and elongation of mono-, poly-unsaturated and saturated fatty acids and enabled us to delineate, and partially validate, the hydrocarbon biosynthetic pathway in An. gambiae.

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.

Hemocyte-targeted gene expression in the female malaria mosquito using the hemolectin promoter from Drosophila

Hemocytes, the immune cells in mosquitoes, participate in immune defenses against pathogens including malaria parasites. Mosquito hemocytes can also be infected by arthropod-borne viruses but the pro- or anti-viral nature of this interaction is unknown. Although there has been progress on hemocyte characterization during pathogen infection in mosquitoes, the specific contribution of hemocytes to immune responses and the hemocyte-specific functions of immune genes and pathways remain unresolved due to the lack of genetic tools to manipulate gene expression in these cells specifically. Here, we used the Gal4-UAS system to characterize the activity of the Drosophila hemocyte-specific hemolectin promoter in the adults of Anopheles gambiae, the malaria mosquito. We established an hml-Gal4 driver line that we further crossed to a fluorescent UAS responder line, and examined the expression pattern in the adult progeny driven by the hml promoter. We show that the hml regulatory region drives hemocyte-specific transgene expression in a subset of hemocytes, and that transgene expression is triggered after a blood meal. The hml promoter drives transgene expression in differentiating prohemocytes as well as in differentiated granulocytes. Analysis of different immune markers in hemocytes in which the hml promoter drives transgene expression revealed that this regulatory region could be used to study phagocytosis as well as melanization. Finally, the hml promoter drives transgene expression in hemocytes in which o'nyong-nyong virus replicates. Altogether, the Drosophila hml promoter constitutes a good tool to drive transgene expression in hemocyte only and to analyze the function of these cells and the genes they express during pathogen infection in Anopheles gambiae.

Methods for the generation of heritable germline mutations in the disease vector Culex quinquefasciatus using clustered regularly interspaced short palindrome repeats-associated protein 9

Culex quinquefasciatus is a vector of many diseases that adversely impact human and animal health; however, compared to other mosquito vectors limited genome engineering technologies have been characterized for this vector. Clustered regularly interspaced short palindrome repeats-associated protein 9 (CRISPR-Cas9) based technologies are a powerful tool for genome engineering and functional genetics and consequently have transformed genetic studies in many organisms. Our objective was to improve upon the limited technologies available for genome editing in C. quinquefasciatus to create a reproducible and straightforward method for CRISPR-Cas9-targeted mutagenesis in this vector. Here we describe methods to achieve high embryo survival and mutagenesis rates and we provide details on the injection supplies and procedures, embryo handling and guide RNA (gRNA) target designs. Through these efforts, we achieved embryo survival rates and germline mutagenesis rates that greatly exceed previously reported rates in this vector. This work is also the first to characterize the white gene marker in this species, which is a valuable phenotypic marker for future transgenesis or mutagenesis of this vector. Overall, these tools provide the framework for future functional genetic studies in this important disease vector and may support the development of future gene drive and genetic technologies that can be used to control this vector.

Functional genetic validation of key genes conferring insecticide resistance in the major African malaria vector, Anopheles gambiae

Insecticide resistance in Anopheles gambiae mosquitoes can derail malaria control programs, and to overcome it, we need to discover the underlying molecular basis. Here, we characterize 3 genes most often associated with insecticide resistance directly by their overproduction in genetically modified An. gambiae. We show that overexpression of each gene confers resistance to representatives of at least 1 insecticide class, and taken together, the 3 genes provide cross-resistance to all 4 major insecticide classes currently used in public health. These data validate the candidate genes as markers to monitor the spread of resistance in mosquito populations. The modified mosquitoes produced are also valuable tools to prescreen the efficacy of new insecticides against existing resistance mechanisms.

Resistance in Anopheles gambiae to members of all 4 major classes (pyrethroids, carbamates, organochlorines, and organophosphates) of public health insecticides limits effective control of malaria transmission in Africa. Increase in expression of detoxifying enzymes has been associated with insecticide resistance, but their direct functional validation in An. gambiae is still lacking. Here, we perform transgenic analysis using the GAL4/UAS system to examine insecticide resistance phenotypes conferred by increased expression of the 3 genes—Cyp6m2, Cyp6p3, and Gste2—most often found up-regulated in resistant An. gambiae. We report evidence in An. gambiae that organophosphate and organochlorine resistance is conferred by overexpression of GSTE2 in a broad tissue profile. Pyrethroid and carbamate resistance is bestowed by similar Cyp6p3 overexpression, and Cyp6m2 confers only pyrethroid resistance when overexpressed in the same tissues. Conversely, such Cyp6m2 overexpression increases susceptibility to the organophosphate malathion, presumably due to conversion to the more toxic metabolite, malaoxon. No resistant phenotypes are conferred when either Cyp6 gene overexpression is restricted to the midgut or oenocytes, indicating that neither tissue is involved in insecticide resistance mediated by the candidate P450s examined. Validation of genes conferring resistance provides markers to guide control strategies, and the observed negative cross-resistance due to Cyp6m2 gives credence to proposed dual-insecticide strategies to overcome pyrethroid resistance. These transgenic An. gambiae-resistant lines are being used to test the “resistance-breaking” efficacy of active compounds early in their development.

A transgenic tool to assess Anopheles mating competitiveness in the field

Background

Malaria parasites, transmitted by the bite of an anopheline mosquito, pose an immense public health burden on many tropical and subtropical regions. The most important malaria vectors in sub-Saharan Africa are mosquitoes of the Anopheles gambiae complex including An. gambiae (sensu stricto). Given the increasing rates of insecticide resistance in these mosquitoes, alternative control strategies based on the release of genetically modified males are being evaluated to stop transmission by these disease vectors. These strategies rely on the mating competitiveness of release males, however currently there is no method to determine male mating success without sacrificing the female. Interestingly, unlike other insects, during mating An. gambiae males transfer their male accessory glands (MAGs) seminal secretions as a coagulated mating plug which is deposited in the female atrium.

Results

Here we exploit this male reproductive feature and validate the use of a MAG-specific promoter to fluorescently label the mating plug and visualize the occurrence of insemination in vivo. We used the promoter region of the major mating plug protein, Plugin, to control the expression of a Plugin-tdTomato (PluTo) fusion protein, hypothesizing that this fusion protein could be incorporated into the plug for sexual transfer to the female. Anopheles gambiae PluTo transgenic males showed strong red fluorescence specifically in the MAGs and with a pattern closely matching endogenous Plugin expression. Moreover, the fusion protein was integrated into the mating plug and transferred to the female atrium during mating where it could be visualized microscopically in vivo without sacrificing the female. PluTo males were equally as competitive at mating as wild type males, and females mated to these males did not show any reduction in reproductive fitness.

Conclusion

The validation of the first MAG-specific promoter in transgenic An. gambiae facilitates the live detection of successful insemination hours after copulation has occurred. This provides a valuable tool for the assessment of male mating competitiveness not only in laboratory experiments but also in semi-field and field studies aimed at testing the feasibility of releasing genetically modified mosquitoes for disease control.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-3218-5) contains supplementary material, which is available to authorized users.

Multi-tissue GAL4-mediated gene expression in all Anopheles gambiae life stages using an endogenous polyubiquitin promoter

The ability to manipulate the Anopheles gambiae genome and alter gene expression effectively and reproducibly is a prerequisite for functional genetic analysis and for the development of novel control strategies in this important disease vector. However, in vivo transgenic analysis in mosquitoes is limited by the lack of promoters active ubiquitously. To address this, we used the GAL4/UAS system to investigate the promoter of the An. gambiae Polyubiquitin-c (PUBc) gene and demonstrated its ability to drive expression in mosquito cell culture before incorporation into An. gambiae transgenic driver lines. To generate such lines, piggyBac-mediated insertion was used to identify genomic regions able to sustain widespread expression and to create φC31 docking lines at these permissive sites. Patterns of expression induced by PUBc-GAL4 drivers carrying single intergenic insertions were assessed by crossing with a novel responder UAS-mCD8:mCherry line that was created by φC31-mediated integration. Amongst the drivers created at single, unique chromosomal integration loci, two were isolated that induced differential expression levels in a similar multiple-tissue spatial pattern throughout the mosquito life cycle. This work expands the tools available for An. gambiae functional analysis by providing a novel promoter for investigating phenotypes resulting from widespread multi-tissue expression, as well as identifying and tagging genomic sites that sustain broad transcriptional activity.

Highly Efficient Site-Specific Mutagenesis in Malaria Mosquitoes Using CRISPR

Anopheles mosquitoes transmit at least 200 million annual malaria infections worldwide. Despite considerable genomic resources, mechanistic understanding of biological processes in Anopheles has been hampered by a lack of tools for reverse genetics. Here, we report successful application of the CRISPR/Cas9 system for highly efficient, site-specific mutagenesis in the diverse malaria vectors Anopheles albimanus, A. coluzzii, and A. funestus. When guide RNAs (gRNAs) and Cas9 protein are injected at high concentration, germline mutations are common and usually biallelic, allowing for the rapid creation of stable mutant lines for reverse genetic analysis. Our protocol should enable researchers to dissect the molecular and cellular basis of anopheline traits critical to successful disease transmission, potentially exposing new targets for malaria control.

Tools for Anopheles gambiae Transgenesis

Transgenesis is an essential tool to investigate gene function and to introduce desired characters in laboratory organisms. Setting-up transgenesis in non-model organisms is challenging due to the diversity of biological life traits and due to knowledge gaps in genomic information. Some procedures will be broadly applicable to many organisms, and others have to be specifically developed for the target species. Transgenesis in disease vector mosquitoes has existed since the 2000s but has remained limited by the delicate biology of these insects. Here, we report a compilation of the transgenesis tools that we have designed for the malaria vector Anopheles gambiae, including new docking strains, convenient transgenesis plasmids, a puromycin resistance selection marker, mosquitoes expressing cre recombinase, and various reporter lines defining the activity of cloned promoters. This toolbox contributed to rendering transgenesis routine in this species and is now enabling the development of increasingly refined genetic manipulations such as targeted mutagenesis. Some of the reagents and procedures reported here are easily transferable to other nonmodel species, including other disease vector or agricultural pest insects.