Genetic diversity of laboratory strains and implications for research: The case of Aedes aegypti

Establishing Colonies from Field-Collected Mosquitoes: Special Accommodations for Wild Strains

A researcher may have many reasons for wanting to establish new laboratory colonies from field-collected mosquitoes. In particular, the ability to study the diversity found within and among natural populations in a controlled laboratory environment opens up a wide range of possibilities for understanding how and why burdens of vector-borne disease vary over space and time. However, field-collected mosquitoes are often more difficult to work with than established laboratory strains, and considerable logistical challenges are involved in safely transporting field-collected mosquitoes into the laboratory. Here, we provide advice for researchers working with Aedes aegypti, Anopheles gambiae, and Culex pipiens, as well as notes on other closely related species. We provide guidance on each stage of the life cycle and highlight the life stages for which it is easiest to initiate new laboratory colonies for each species. In accompanying protocols, we provide methods detailing Ae. aegypti egg collection and hatching as well as how to transport larvae and pupae from the field.

CRISPR-Cas9 and Cas12a target site richness reflects genomic diversity in natural populations of Anopheles gambiae and Aedes aegypti mosquitoes

Due to limitations in conventional disease vector control strategies including the rise of insecticide resistance in natural populations of mosquitoes, genetic control strategies using CRISPR gene drive systems have been under serious consideration. The identification of CRISPR target sites in mosquito populations is a key aspect for developing efficient genetic vector control strategies. While genome-wide Cas9 target sites have been explored in mosquitoes, a precise evaluation of target sites focused on coding sequence (CDS) is lacking. Additionally, target site polymorphisms have not been characterized for other nucleases such as Cas12a, which require a different DNA recognition site (PAM) and would expand the accessibility of mosquito genomes for genetic engineering. We undertook a comprehensive analysis of potential target sites for both Cas9 and Cas12a nucleases within the genomes of natural populations of Anopheles gambiae and Aedes aegypti from multiple continents. We demonstrate that using two nucleases increases the number of targets per gene. Also, we identified differences in nucleotide diversity between North American and African Aedes populations, impacting the abundance of good target sites with a minimal degree of polymorphisms that can affect the binding of gRNA. Lastly, we screened for gRNAs targeting sex-determination genes that could be widely applicable for developing field genetic control strategies. Overall, this work highlights the utility of employing both Cas9 and Cas12a nucleases and underscores the importance of designing universal genetic strategies adaptable to diverse mosquito populations.

Challenges of Robust RNAi-Mediated Gene Silencing in Aedes Mosquitoes

In this study, we report the complexities and challenges associated with achieving robust RNA interference (RNAi)-mediated gene knockdown in the mosquitoes Aedes aegypti and Aedes albopictus, a pivotal approach for genetic analysis and vector control. Despite RNAi's potential for species-specific gene targeting, our independent efforts to establish oral delivery of RNAi for identifying genes critical for mosquito development and fitness encountered significant challenges, failing to reproduce previously reported potent RNAi effects. We independently evaluated a range of RNAi-inducing molecules (siRNAs, shRNAs, and dsRNAs) and administration methods (oral delivery, immersion, and microinjection) in three different laboratories. We also tested various mosquito strains and utilized microorganisms for RNA delivery. Our results reveal a pronounced inconsistency in RNAi efficacy, characterized by minimal effects on larval survival and gene expression levels in most instances despite strong published effects for the tested targets. One or multiple factors, including RNase activity in the gut, the cellular internalization and processing of RNA molecules, and the systemic dissemination of the RNAi signal, could be involved in this variability, all of which are barely understood in mosquitoes. The challenges identified in this study highlight the necessity for additional research into the underlying mechanisms of mosquito RNAi to develop more robust RNAi-based methodologies. Our findings emphasize the intricacies of RNAi application in mosquitoes, which present a substantial barrier to its utilization in genetic control strategies.

The midgut epithelium of mosquitoes adjusts cell proliferation and endoreplication to respond to physiological challenges

BACKGROUND

Hematophagous mosquitoes transmit many pathogens that cause human diseases. Pathogen acquisition and transmission occur when female mosquitoes blood feed to acquire nutrients for reproduction. The midgut epithelium of mosquitoes serves as the point of entry for transmissible viruses and parasites.

RESULTS

We studied midgut epithelial dynamics in five major mosquito vector species by quantifying PH3-positive cells (indicative of mitotic proliferation), the incorporation of nucleotide analogs (indicative of DNA synthesis accompanying proliferation and/or endoreplication), and the ploidy (by flow cytometry) of cell populations in the posterior midgut epithelium of adult females. Our results show that the epithelial dynamics of post-emergence maturation and of mature sugar-fed guts were similar in members of the Aedes, Culex, and Anopheles genera. In the first three days post-emergence, ~ 20% of cells in the posterior midgut region of interest incorporated nucleotide analogs, concurrent with both proliferative activity and a broad shift toward higher ploidy. In mature mosquitoes maintained on sugar, an average of 3.5% of cells in the posterior midgut region of interest incorporated nucleotide analogs from five to eight days post-emergence, with a consistent presence of mitotic cells indicating constant cell turnover. Oral bacterial infection triggered a sharp increase in mitosis and nucleotide analog incorporation, suggesting that the mosquito midgut undergoes accelerated cellular turnover in response to damage. Finally, blood feeding resulted in an increase in cell proliferation, but the nature and intensity of the response varied by mosquito species and by blood source (human, bovine, avian or artificial). In An. gambiae, enterocytes appeared to reenter the cell cycle to increase ploidy after consuming blood from all sources except avian.

CONCLUSIONS

We saw that epithelial proliferation, differentiation, and endoreplication reshape the blood-fed gut to increase ploidy, possibly to facilitate increased metabolic activity. Our results highlight the plasticity of the midgut epithelium in mosquitoes' physiological responses to distinct challenges.

The influence of manure-based organic fertilisers on the oviposition behaviour of Anopheles arabiensis

The rice agroecosystem provides suitable breeding habitat for many malaria vector species, and rice-adjacent communities are consequently exposed to a greater malaria transmission risk than non-rice-associated communities. As part of efforts to expand rice production in Africa, sustainable and climate-adapted practices such as the System of Rice Intensification (SRI) are being promoted. SRI encourages the use of organic fertilisers (OFs) such as cow and chicken dung, as opposed to inorganic industrially produced fertilisers, due to their lower resource cost, apparent benefit to the rice agroecosystem and as a means to reduce the greenhouse gas emissions associated with the production of industrial fertilisers. However, the impact of OFs on mosquito fauna is not well documented and may have knock-on consequences on malaria transmission risk. Here, we demonstrate, using dual choice egg count assays, that both cow and chicken dung modulate the oviposition behaviour of Anopheles arabiensis, a major malaria vector in Sub-Saharan Africa. A significantly reduced proportion of eggs were laid in water treated with either cow or chicken dung compared to untreated water, with higher dung concentrations resulting in further reduced proportions. When presented in competition, significantly fewer eggs were laid in water treated with chicken dung than with cow dung. Moreover, there was no evidence of egg retention in any experiment, including in no-choice experiments where only dung-containing dishes were available. These results suggest both cow and chicken dung may act as oviposition deterrents to malaria vector species and that the application of manure-based OFs in rice agriculture may modulate the oviposition behaviour of An. gambiae s.l. within agroecosystems. Quantification of the ammonia present in dung-infused water showed higher concentrations were present in the chicken dung infusion, which may be one contributing factor to the difference in observed deterrence between the two dung types. Deterrence of mosquito oviposition in OF-treated farms may potentially affect the overall production of malaria vectors within rice fields and their contribution to local malaria transmission.

Relevant Day/Night Temperatures Simulating Belgian Summer Conditions Reduce Japanese Encephalitis Virus Dissemination and Transmission in Belgian Field-Collected Culex pipiens Mosquitoes

Japanese encephalitis virus (JEV), a zoonotic mosquito-borne Flavivirus, can be considered an emerging infectious disease. Therefore, vector competence studies with indigenous mosquitoes from regions where JEV is not yet endemic are of great importance. In our study, we compared the vector competence of Culex pipiens mosquitoes emerged from Belgian field-caught larvae under two different temperature conditions: a constant 25 °C and a 25/15 °C day/night temperature gradient representing typical summer temperatures in Belgium. Three- to seven-day-old F0-generation mosquitoes were fed on a JEV genotype 3 Nakayama strain spiked blood-meal and incubated for 14 days at the two aforementioned temperature conditions. Similar infection rates of 36.8% and 35.2% were found in both conditions. The observed dissemination rate in the gradient condition was, however, significantly lower compared to the constant temperature condition (8% versus 53.6%, respectively). JEV was detected by RT-qPCR in the saliva of 13.3% of dissemination positive mosquitoes in the 25 °C condition, and this transmission was confirmed by virus isolation in 1 out of 2 RT-qPCR positive samples. No JEV transmission to saliva was detected in the gradient condition. These results suggest that JEV transmission by Culex pipiens mosquitoes upon an accidental introduction in our region is unlikely under current climatic conditions. This could change in the future when temperatures increase due to climate change.

Overview of Aedes aegypti and Use in Laboratory Studies

The yellow fever mosquito Aedes aegypti is a prolific disease vector. This mosquito has been the subject of scientific investigation for more than a century. Continued research into Aedes aegypti biology is crucial for understanding how to halt the suite of major arthropod-borne viral diseases this mosquito transmits. Here, we provide an introductory overview of Aedes aegypti life cycle; evolutionary history, biology, and ecology; genetics and sex differences; vector competence; and laboratory colonization and considerations for rearing this robust mosquito species for use in laboratory research.

Population genetics of an invasive mosquito vector, Aedes albopictus in the Northeastern USA

The Asian tiger mosquito (Aedes albopictus) arrived in the USA in the 1980’s and rapidly spread throughout eastern USA within a decade. The predicted northern edge of its overwintering distribution on the East Coast of the USA roughly falls across New York, Connecticut, and Massachusetts, where the species has been recorded as early as 2000. It is unclear whether Ae. albopictus populations have become established and survive the cold winters in these areas or are recolonized every year. We genotyped and analyzed populations of Ae. albopictus from the northeast USA using 15 microsatellite markers and compared them with other populations across the country and to representatives of the major global genetic clades to investigate their connectivity and stability. Founder effects or bottlenecks were rare at the northern range of the Ae. albopictus distribution in the northeastern USA, with populations displaying high levels of genetic diversity and connectivity along the East Coast. There is no evidence of population turnover in Connecticut during the course of three consecutive years, with consistent genetic structure throughout this period. Overall, these results support the presence of established populations of Ae. albopictus in New York, Connecticut, and Massachusetts, successfully overwintering and migrating in large numbers. Given the stability and interconnectedness of these populations, Ae. albopictus has the potential to continue to proliferate and expand its range northward under mean warming conditions of climate change. Efforts to control Ae. albopictus in these areas should thus focus on vector suppression rather than eradication strategies, as local populations have become firmly established and are expected to reemerge every summer.

A CRISPR endonuclease gene drive reveals distinct mechanisms of inheritance bias

CRISPR/Cas gene drives can bias transgene inheritance through different mechanisms. Homing drives are designed to replace a wild-type allele with a copy of a drive element on the homologous chromosome. In Aedes aegypti, the sex-determining locus is closely linked to the white gene, which was previously used as a target for a homing drive element (wGDe). Here, through an analysis using this linkage we show that in males inheritance bias of wGDe did not occur by homing, rather through increased propagation of the donor drive element. We test the same wGDe drive element with transgenes expressing Cas9 with germline regulatory elements sds3, bgcn, and nup50. We only find inheritance bias through homing, even with the identical nup50-Cas9 transgene. We propose that DNA repair outcomes may be more context dependent than anticipated and that other previously reported homing drives may, in fact, bias their inheritance through other mechanisms.

A chromosome-level assembly of the widely used Rockefeller strain of Aedes aegypti, the yellow fever mosquito

Aedes aegypti is the vector of important human diseases, and genomic resources are crucial in facilitating the study of A. aegypti and its ecosystem interactions. Several laboratory-acclimated strains of this mosquito have been established, but the most used strain in toxicology studies is “Rockefeller,” which was originally collected and established in Cuba 130 years ago. A full-length genome assembly of another reference strain, “Liverpool,” was published in 2018 and is the reference genome for the species (AaegL5). However, genetic studies with the Rockefeller strain are complicated by the availability of only the Liverpool strain as the reference genome. Differences between Liverpool and Rockefeller have been known for decades, particularly in the expression of genes relevant to mosquito behavior and vector control (e.g. olfactory). These differences indicate that AaegL5 is likely not fully representative of the Rockefeller genome, presenting potential impediments to research. Here, we present a chromosomal-level assembly and annotation of the Rockefeller genome and a comparative characterization vs the Liverpool genome. Our results set the stage for a pan-genomic approach to understanding evolution and diversity within this important disease vector.

Saliva collection via capillary method may underestimate arboviral transmission by mosquitoes

BACKGROUND

Arthropod-borne viruses (arboviruses) impose a major health and economic burden on human populations globally, with mosquitoes serving as important vectors. Measuring the ability of a mosquito population to transmit an arbovirus is important in terms of evaluating its public health risk. In the laboratory, a variety of methods are used to estimate arboviral transmission by mosquitoes, including indirect methods involving viral detection from mosquito saliva collected by forced salivation. The accuracy of indirect methods to estimate arbovirus transmission to live animal hosts has not been fully evaluated.

METHODS

We compared three commonly used proxies of arboviral transmission, namely, the presence of virus in mosquito legs, in salivary glands (SG) and in saliva collected in capillary tubes using forced salivation, with direct transmission estimates from mosquitoes to suckling mice. We analyzed five vector-virus combinations, including Aedes aegypti infected with chikungunya virus, West Nile virus and Zika virus; Culex quinquefasciatus infected with West Nile virus; and Aedes triseriatus infected with La Crosse virus.

RESULTS

Comparatively, the methods of detecting virus infection in mosquito legs and in SG were equally accurate in predicting transmission. Overall, the presence of virus in mosquito legs was a more accurate predictor of transmission than the commonly implemented viral detection method using forced salivation into a capillary tube, and was subject to less technical variation.

CONCLUSIONS

These results suggest that, in general, forced salivation methods tend to underestimate virus transmission, and they provide confidence in the use of mosquito leg screens to evaluate the transmission potential of a mosquito population.

Olfaction-Related Gene Expression in the Antennae of Female Mosquitoes From Common Aedes aegypti Laboratory Strains

Adult female mosquitoes rely on olfactory cues like carbon dioxide and other small molecules to find vertebrate hosts to acquire blood. The molecular physiology of the mosquito olfactory system is critical for their host preferences. Many laboratory strains of the yellow fever mosquito Aedes aegypti have been established since the late 19th century. These strains have been used for most molecular studies in this species. Some earlier comparative studies have identified significant physiological differences between different laboratory strains. In this study, we used a Y-tube olfactometer to determine the attraction of females of seven different strains of Ae. aegypti to a human host: UGAL, Rockefeller, Liverpool, Costa Rica, Puerto Rico, and two odorant receptor co-receptor (Orco) mutants Orco2 and Orco16. We performed RNA-seq using antennae of Rockefeller, Liverpool, Costa Rica, and Puerto Rico females. Our results showed that female Aedes aegypti from the Puerto Rico strain had significantly reduced attraction rates toward human hosts compared to all other strains. RNA-seq analyses of the antenna transcriptomes of Rockefeller, Liverpool, Costa Rica, and Puerto Rico strains revealed distinct differences in gene expression between the four strains, but conservation in gene expression patterns of known human-sensing genes. However, we identified several olfaction-related genes that significantly vary between strains, including receptors with significantly different expression in mosquitoes from the Puerto Rico strain and the other strains.

Colonization and Authentication of the Pyrethroid-Resistant Anopheles gambiae s.s. Muleba-Kis Strain; an Important Test System for Laboratory Screening of New Insecticides

Simple Summary

Malaria control and prevention have traditionally relied on the use of insecticides in the form of treated bed nets or residual spraying in households. However, scaling up of these interventions—based on few available insecticide classes—resulted in the development and spread of insecticide resistance in malaria-transmitting mosquitoes. There is therefore an urgent need for introducing and applying new insecticides that are effective against these mosquitoes. Laboratories tasked with evaluating the efficacy of novel insecticides need to establish a large colony of resistant mosquitoes. In this study, we report the procedures used and challenges faced during the establishment and maintenance of a resistant mosquito strain in the laboratory which reflects the characteristics of the wild-resistant mosquito populations found in East Africa.

Abstract

Background: The emergence and spread of insecticide resistance in malaria vectors to major classes of insecticides call for urgent innovation and application of insecticides with novel modes of action. When evaluating new insecticides for public health, potential candidates need to be screened against both susceptible and resistant mosquitoes to determine efficacy and to identify potential cross-resistance to insecticides currently used for mosquito control. The challenges and lessons learned from establishing, maintaining, and authenticating the pyrethroid-resistant An. gambiae s.s. Muleba-Kis strain at the KCMUCo-PAMVERC Test Facility are described in this paper. Methods: Male mosquitoes from the F₁ generation of wild-pyrethroid resistant mosquitoes were cross-bred with susceptible female An. gambiae s.s. Kisumu laboratory strain followed by larval selection using a pyrethroid insecticide solution. Periodic screening for phenotypic and genotypic resistance was done. WHO susceptibility tests and bottle bioassays were used to assess the phenotypic resistance, while Taqman™ assays were used to screen for known target-site resistance alleles (kdr and ace-1). Additionally, the strains were periodically assessed for quality control by monitoring adult weight and wing length. Results: By out-crossing the wild mosquitoes with an established lab strain, a successful resistant insectary colony was established. Intermittent selection pressure using alphacypermethrin has maintained high kdr mutation (leucine-serine) frequencies in the selected colony. There was consistency in the wing length and weight measurements from the year 2016 to 2020, with the exception that one out of four years was significantly different. Mean annual wing length varied between 0.0142–0.0028 mm compared to values obtained in 2016, except in 2019 where it varied by 0.0901 mm. Weight only varied by approximately 0.001 g across four years, except in 2017 where it differed by 0.005 g. Routine phenotypic characterization on Muleba-Kis against pyrethroids using the WHO susceptibility test indicated high susceptibility when type I pyrethroids were used compared to type II pyrethroids. Dynamics on susceptibility status also depended on the lapse time when the selection was last done. Conclusions: This study described the procedure for introducing, colonizing, and maintaining a resistant An. gambiae s.s. strain in the laboratory with leucine to serine substitution kdr allele which reflects the features of the wild-resistant population in East Africa. Challenges in colonizing a wild-resistant mosquito strain were overcome by out-crossing between mosquito strains of desired traits followed by intermittent insecticide selection at the larval stage to select for the resistant phenotype.

Sexual selection theory meets disease vector control: Testing harmonic convergence as a "good genes" signal in Aedes aegypti mosquitoes

Background

The mosquito Aedes aegypti is a medically important, globally distributed vector of the viruses that cause dengue, yellow fever, chikungunya, and Zika. Although reproduction and mate choice are key components of vector population dynamics and control, our understanding of the mechanisms of sexual selection in mosquitoes remains poor. In “good genes” models of sexual selection, females use male cues as an indicator of both mate and offspring genetic quality. Recent studies in Ae. aegypti provide evidence that male wingbeats may signal aspects of offspring quality and performance during mate selection in a process known as harmonic convergence. However, the extent to which harmonic convergence may signal overall inherent quality of mates and their offspring remains unknown.

Methodology/Principal findings

To examine this, we measured the relationship between acoustic signaling and a broad panel of parent and offspring fitness traits in two generations of field-derived Ae. aegypti originating from dengue-endemic field sites in Thailand. Our data show that in this population of mosquitoes, harmonic convergence does not signal male fertility, female fecundity, or male flight performance traits, which despite displaying robust variability in both parents and their offspring were only weakly heritable.

Conclusions/Significance

Together, our findings suggest that vector reproductive control programs should treat harmonic convergence as an indicator of some, but not all aspects of inherent quality, and that sexual selection likely affects Ae. aegypti in a trait-, population-, and environment-dependent manner.

Mosquitoes transmit numerous pathogens that disproportionately impact developing countries. The mosquito Aedes aegypti, studied here, transmits viruses that cause neglected tropical diseases such as dengue, yellow fever, chikungunya, and Zika. Disease prevention programs rely heavily upon mosquito vector control. To successfully interrupt disease transmission, several control methods depend upon the ability of laboratory-modified male mosquitoes to successfully mate with wild females to suppress or replace natural populations. However, our understanding of what determines mating success in mosquitoes is far from complete. Our study addresses the question of whether female Ae. aegypti mosquitoes use male acoustic signals to select higher quality mates and improve their offspring’s fitness. We find that acoustic signals do not serve as universal indicators of fitness. Further, the fitness metrics we measured were only weakly heritable, suggesting that females that mate with high quality males do not necessarily produce fitter offspring. Our study provides a nuanced understanding of mate choice, mating acoustic signals, and parent and offspring reproductive fitness in a key disease-transmitting mosquito species. These discoveries improve our grasp of sexual selection in mosquitoes and can be leveraged by the vector control community to improve vitally important disease prevention programs.

Intracellular Interactions Between Arboviruses and Wolbachia in Aedes aegypti

Aedes aegypti is inherently susceptible to arboviruses. The geographical expansion of this vector host species has led to the persistence of Dengue, Zika, and Chikungunya human infections. These viruses take advantage of the mosquito’s cell to create an environment conducive for their growth. Arboviral infection triggers transcriptomic and protein dysregulation in Ae. aegypti and in effect, host antiviral mechanisms are compromised. Currently, there are no existing vaccines able to protect human hosts from these infections and thus, vector control strategies such as Wolbachia mass release program is regarded as a viable option. Considerable evidence demonstrates how the presence of Wolbachia interferes with arboviruses by decreasing host cytoskeletal proteins and lipids essential for arboviral infection. Also, Wolbachia strengthens host immunity, cellular regeneration and causes the expression of microRNAs which could potentially be involved in virus inhibition. However, variation in the magnitude of Wolbachia’s pathogen blocking effect that is not due to the endosymbiont’s density has been recently reported. Furthermore, the cellular mechanisms involved in this phenotype differs depending on Wolbachia strain and host species. This prompts the need to explore the cellular interactions between Ae. aegypti-arboviruses-Wolbachia and how different Wolbachia strains overall affect the mosquito’s cell. Understanding what happens at the cellular and molecular level will provide evidence on the sustainability of Wolbachia vector control.

Climate Mismatch between Introduced Biological Control Agents and Their Invasive Host Plants: Improving Biological Control of Tropical Weeds in Temperate Regions

Simple Summary

Mismatched distributions between biological control agents and their host plants occur for a variety of reasons but are often linked to climate, specifically differences in their low-temperature tolerances. How to measure and use low-temperature tolerances of control agents to inform agent prioritization, selection for redistribution, or predict efficacy is vitally important, but has not been previously synthesized in a single source. We discuss causes of climate mismatches between agents and target weeds, the traditional and non-traditional approaches that could be used to decrease the degree of mismatch and improve control, and regulatory issues to consider when taking such approaches. We also discuss the variety of cold tolerance metrics, their measurement and ecological value, and the types of modeling that can be carried out to improve predictions about potential distributions of agents. We also briefly touch on molecular bases for cold tolerance and opportunities for improving cold tolerance of agents using modern molecular tools.

Abstract

Many weed biological control programs suffer from large-scale spatial variation in success due to restricted distributions or abundances of agents in temperate climates. For some of the world’s worst aquatic weeds, agents are established but overwintering conditions limit their survival in higher latitudes or elevations. The resulting need is for new or improved site- or region-specific biological control tools. Here, we review this challenge with a focus on low-temperature limitations of agents and propose a roadmap for improving success. Investigations across spatial scales, from global (e.g., foreign exploration), to local (selective breeding), to individual organisms (molecular modification), are discussed. A combination of traditional (foreign) and non-traditional (introduced range) exploration may lead to the discovery and development of better-adapted agent genotypes. A multivariate approach using ecologically relevant metrics to quantify and compare cold tolerance among agent populations is likely required. These data can be used to inform environmental niche modeling combined with mechanistic modeling of species’ fundamental climate niches and life histories to predict where, when, and at what abundance agents will occur. Finally, synthetic and systems biology approaches in conjunction with advanced modern genomics, gene silencing and gene editing technologies may be used to identify and alter the expression of genes enhancing cold tolerance, but this technology in the context of weed biological control has not been fully explored.

Sex, age, and parental harmonic convergence behavior affect the immune performance of Aedes aegypti offspring

Harmonic convergence is a potential cue, female mosquitoes use to choose male mates. However, very little is known about the benefits this choice confers to offspring performance. Using Aedes aegypti (an important vector of human disease), we investigated whether offspring of converging parental pairs showed differences in immune competence compared to offspring derived from non-converging parental pairs. Here we show that harmonic convergence, along with several other interacting factors (sex, age, reproductive, and physiological status), significantly shaped offspring immune responses (melanization and response to a bacterial challenge). Harmonic convergence had a stronger effect on the immune response of male offspring than on female offspring. Further, female offspring from converging parental pairs disseminated dengue virus more quickly than offspring derived from non-converging parental pairs. Our results provide insight into a wide range of selective pressures shaping mosquito immune function and could have important implications for disease transmission and control.

Improving mosquito control strategies with population genomics

Mosquito control strategies increasingly apply knowledge from population genomics research. This review highlights recent applications to three research domains: mosquito invasions, insecticide resistance evolution, and rear and release programs. Current research trends follow developments in reference assemblies, either as improvements to existing assemblies (particularly Aedes) or assemblies for new taxa (particularly Anopheles). With improved assemblies, studies of invasive and rear and release target populations are better able to incorporate adaptive as well as demographic hypotheses. New reference assemblies are aiding comparisons of insecticide resistance across sister taxa while helping resolve taxon boundaries amidst frequent introgression. Anopheles gene drive deployments and improved Aedes genome assemblies should lead to a convergence in research aims for Anopheles and Aedes in the coming years.

Laboratory Oviposition Choice of Aedes aegypti (Diptera: Culicidae) From Kenya and Gabon: Effects of Conspecific Larvae, Salinity, Shading, and Microbiome

The mosquito Aedes aegypti (L.) is the primary vector of several arboviruses. Mosquito control and surveillance are essential to restrict disease transmission, the effectiveness of which depends on our understanding of the mosquito's behaviors, including oviposition. Previous studies have identified a variety of oviposition cues. However, most of these studies involved only Ae. aegypti outside of the species' native range, Africa. Populations outside Africa differ in their genetics and some behaviors from their African counterparts, suggesting possibly different oviposition preferences. Within Africa, Ae. aegypti can be found in both ancestral forest habitats and domestic habitats. The African domestic populations may represent an intermediate state between the forest and the truly domesticated non-African populations. Comparing mosquitoes from these three habitats (African forest, African domestic, and non-African domestic) might provide insight into the evolution of oviposition behavior. In this study, I examined the oviposition choices of multiple Ae. aegypti colonies from all three habitats in laboratory settings. I applied a two-choice assay to test four oviposition cues: the preexistence of conspecific larvae, salinity, shading, and microbiome. A subset of African colonies showed similar oviposition choices as their non-African counterparts, whereas the rest show little response to the factors tested. Within the African colonies, oviposition choices of the domestic colonies were significantly different from the forest colonies in most experiments. Yet, their preferences were not always intermediate between that of mosquitoes from the other two habitats. Collectively, this study adds to our understanding of Ae. aegypti oviposition, especially in previously understudied African populations.

Interpopulation variations in life history traits and reproductive tactics in Aedes aegypti: A test on populations 50 km apart

The interpopulation variation in life history traits of a species reflects evolutionary adaptation in response to a local environment regime. We examined the life history traits of Aedes aegypti populations from 2 cities in southern Taiwan separated by 50 km. Results revealed a high level of trait differentiation in immature developmental time and survival of Ae. aegypti between the 2 cities. The Kaohsiung populations exhibited total pupation of 40%-60% on day 8; this was significantly lower than that of the Tainan populations and laboratory-reared KHsm mosquitos, which exhibited a pupation rate of 70%-90%. The slow immature development of the Kaohsiung populations was reflected in the low percentage of adult emergence (22%-26%) on day 10. The prolonged immature development did not select larger adults with longer life spans because the Kaohsiung populations had a shorter life span (≈37 d) than that of the Tainan populations (≈42 d). By contrast, immature development and longevity did not differ between populations within each region, indicating weak local differentiation. Three field populations exhibited male-bias sex ratio because of differential mortality of female immatures. The effect of female size on adult life history was nonsignificant. Two reproduction tactics were detected, representing the balanced-mortality hypothesis and the bet-hedging hypothesis. Despite their differential life history strategies and reproductive tactics, these mosquito populations apparently counterbalanced any shortcomings in traits to produce similar population growth. Maintaining optimal population density is essential for Aedes mosquitos to increase the probability of encountering mates and reduce the Allee effect.

Evaluating the competence of the primary vector, Culex tritaeniorhynchus, and the invasive mosquito species, Aedes japonicus japonicus, in transmitting three Japanese encephalitis virus genotypes

Japanese encephalitis virus (JEV) is maintained in an enzootic cycle between swine, water birds, and mosquitoes. JEV has circulated indigenously in Asia, with Culex tritaeniorhynchus as the primary vector. In some areas where the primary vector is scarce or absent, sporadic cases of Japanese encephalitis have been reported, with Aedes japonicus japonicus presumed to have the potential as a secondary vector. As one of the world's most invasive culicid species, Ae. j. japonicus carries a considerable health risk for spreading diseases to wider areas, including Europe and North America. Thus, evaluation of its competency as a JEV vector, particularly in a native population, will be essential in preventing potential disease spread. In this study, the two mosquito species' vector competence in transmitting three JEV genotypes (I, III, and V) was assessed, with Cx. tritaeniorhynchus serving as a point of reference. The mosquitoes were virus-fed and the infection rate (IR), dissemination rate (DR), and transmission rate (TR) evaluated individually by either RT-qPCR or focus forming assay. Results showed striking differences between the two species, with IR of 95% (261/274) and 9% (16/177) in Cx. tritaeniorhynchus and Ae. j. japonicus, respectively. Both mosquitoes were susceptible to all three JEV genotypes with significant differences in IR and mean viral titer. Results confirm the primary vector's competence, but the fact that JEV was able to establish in Ae. j. japonicus is of public health significance, and with 2%-16% transmission rate it has the potential to successfully transmit JEV to the next host. This may explain the human cases and infrequent detection in primary vector-free areas. Importantly, Ae. j. japonicus could be a relevant vector spreading the disease into new areas, indicating the need for security measures in areas where the mosquito is distributed or where it may be introduced.

Genetic pest management and the background genetics of release strains

Genetic pest management (GPM) methods involve releasing modified versions of a pest species to mate with wild pests in the target area. Proposed for a wide range of applications in public health, agriculture and conservation, most progress has been made with pest insects. Offspring of the released modified insects and wild pests carry the modification-which might be transgenes, artificially introduced Wolbachia or genetic damage from radiation, for example-but they also carry a complete haploid genome from their laboratory-reared parent, as well as one from their wild parent. Unless these F₁ hybrids are completely unable to reproduce, further mating will lead to introgression of DNA sequences from the release strain into the wild population. We discuss issues around strain selection and the potential consequences of such introgression. We conclude that such introgression is probably harmless in almost all circumstances, and could, in theory, provide specific additional benefits to the release programme. We outline population monitoring approaches that could be used, going forward, to determine how background genetics may affect GPM. This article is part of the theme issue 'Novel control strategies for mosquito-borne diseases'.

Vector Competence of Aedes albopictus Populations from the Northeastern United States for Chikungunya, Dengue, and Zika Viruses

The Asian tiger mosquito (Aedes albopictus) is an important vector of a number of arboviruses, including Zika (ZIKV), dengue (DENV), and chikungunya (CHIKV) viruses, and has recently expanded its range in the eastern United States to southern New England and New York. Given the recent establishment and proliferation of Ae. albopictus in this region and the increasing amount of international travel between the United States and endemic countries, there is a need to elucidate the public health risk posed by this mosquito species in the Northeast. Accordingly, we evaluated the competence of four Ae. albopictus populations from Connecticut and New York, for two strains each of ZIKV, DENV serotype 2 (DENV-2), and CHIKV, currently circulating in the Americas, to evaluate the local transmission risk by this vector. We found that local Ae. albopictus populations are susceptible to infection by all three viruses but are most capable of transmitting CHIKV. Variation in competence was observed for ZIKV and CHIKV, driven by the virus strains and mosquito population, whereas competence was more homogeneous for the DENV-2 strains under evaluation. These results suggest that under optimal circumstances, Ae. albopictus could support localized transmission of these viruses and emphasize the importance of maintaining mosquito surveillance and control programs to suppress Ae. albopictus populations and limit further range expansion of this species.

Mapping Thermal Physiology of Vector-Borne Diseases in a Changing Climate: Shifts in Geographic and Demographic Risk of Suitability

PURPOSE OF REVIEW

To describe a collection of recent work published on thermal suitability for vector-borne diseases, in which mapping approaches illustrated the geographic shifts, and spatial approaches describe the demographic impact anticipated with a changing climate.

RECENT FINDINGS

While climate change predictions of warming indicate an expansion in VBD suitability risk in some parts of the globe, while in others, optimal temperatures for transmission may be exceeded, as seen for malaria in Western Africa, resulting in declining risk. The thermal suitability of specific vector-pathogen pairs can have large impacts on geographic range of risk, and changes in human demography itself will intersect with this risk to create different vulnerability profiles over the coming century. Using a physiological approach to describe the thermal suitability of transmission for vector-borne diseases allows us to illustrate the future risk as mapped information. This in turn can be coupled with demographic projections to anticipate changing risk, and even changing vulnerability within that population change.

Analysis of the transcription of genes encoding heat shock proteins (hsp) in Aedes aegypti Linnaeus, 1762 (Diptera: Culicidae), maintained under climatic conditions provided by the IPCC (Intergovernmental Panel On Climate Change) for the year 2100

Human actions intensify the greenhouse effect, aggravating climate changes in the Amazon and elsewhere in the world. The Intergovernmental Panel on Climate Change (IPCC) foresees a global increase of up to 4.5 °C and 850 ppm CO₂ (above current levels) by 2100. This will impact the biology of the Aedes aegypti mosquito, vector of Dengue, Zika, urban Yellow Fever and Chikungunya. Heat shock proteins are associated with adaptations to anthropic environments and the interaction of some viruses with the vector. The transcription of the hsp26, hsp83 and hsc70 genes of an A. aegypti population, maintained for more than forty-eight generations, in the Current, Intermediate and Extreme climatic scenario predicted by the IPCC was evaluated with qPCR. In females, highest levels of hsp26, hsp83 and hsc70 expression occurred in the Intermediate scenario, while in males, levels were high only for hsp26 gene in Current and Extreme scenarios. Expression of hsp83 and hsc70 genes in males was low under all climatic scenarios, while in the Extreme scenario females had lower expression than in the Current scenario. The data suggest compensatory or adaptive processes acting on heat shock proteins, which can lead to changes in the mosquito's biology, altering vectorial competence.

Climate and Urbanization Drive Mosquito Preference for Humans

The majority of mosquito-borne illness is spread by a few mosquito species that have evolved to specialize in biting humans, yet the precise causes of this behavioral shift are poorly understood. We address this gap in the arboviral vector Aedes aegypti. We first collect and characterize the behavior of mosquitoes from 27 sites scattered across the species' ancestral range in sub-Saharan Africa, revealing previously unrecognized variation in preference for human versus animal odor. We then use modeling to show that over 80% of this variation can be predicted by two ecological factors-dry season intensity and human population density. Finally, we integrate this information with whole-genome sequence data from 375 individual mosquitoes to identify a single underlying ancestry component linked to human preference. Genetic changes associated with human specialist ancestry were concentrated in a few chromosomal regions. Our findings suggest that human-biting in this important disease vector originally evolved as a by-product of breeding in human-stored water in areas where doing so provided the only means to survive the long, hot dry season. Our model also predicts that the rapid urbanization currently taking place in Africa will drive further mosquito evolution, causing a shift toward human-biting in many large cities by 2050.

Vector competence of Malaysian Aedes aegypti to Zika virus and impact of sequential arbovirus infections

Zika virus (ZIKV) is a mosquito-borne flavivirus with global impact since 2015. Although ZIKV was first isolated from Aedes aegypti in Malaysia in 1965, not much is known about the competency of Malaysian Ae. aegypti to ZIKV. To date only 9 cases of ZIKV have been reported in Malaysia despite the abundance of mosquito vectors. This study aimed to determine the susceptibility of Ae. aegypti to ZIKV, and the impact of sequential infections in Ae. aegypti mosquitoes with DENV serotype 2 (DENV-2) followed by ZIKV. Field-caught urban Ae. aegypti were orally challenged with a Martinique strain of ZIKV, and midgut, head/thorax and saliva were collected at 3, 7 and 14 days post-infection (dpi). At 14 dpi, ZIKV-exposed mosquitoes had infection and dissemination rates of 59% (n=10/17) and 90% (n=9/10), respectively. Average titres of 3.9 and 4.4 log pfu infectious ZIKV were recovered in midgut and head/thorax, respectively. In sequential infection, prior exposure of Ae. aegypti to DENV did not affect the subsequent ZIKV infection in head/thorax albeit with a low sample size. In conclusion, Malaysian urban Ae. aegypti is susceptible to the contemporary Asian lineage of ZIKV. The established and continuous DENV circulation in Ae. aegypti did not suppress ZIKV emergence in Malaysia. Other factors contributing to low level of ZIKV circulation in Malaysia remain to be explored.

Insecticide resistance, fitness and susceptibility to Zika infection of an interbred Aedes aegypti population from Rio de Janeiro, Brazil

Background

Aedes aegypti is a vector of high relevance, since it transmits several arboviruses, including dengue, chikungunya and Zika. Studies on vector biology are usually conducted with laboratory strains presenting a divergent genetic composition from field populations. This may impair vector control policies that were based on laboratory observations employing only long maintained laboratory strains. In the present study we characterized a laboratory strain interbreed with Ae. aegypti collected from five different localities in Rio de Janeiro (Aedes Rio), for insecticide resistance (IR), IR mechanisms, fitness and Zika virus infection.

Methods

We compared the recently established Aedes Rio with the laboratory reference strain Rockefeller. Insecticide resistance (deltamethrin, malathion and temephos), activity of metabolic resistance enzymes and kdr mutation frequency were determined. Some life table parameters (longevity, blood-feeding, number and egg viability) and Zika virus susceptibility was also determined.

Results

Aedes Rio showed resistance to deltamethrin (resistance ratio, RR₅₀ = 32.6) and temephos (RR₅₀ = 7.0) and elevated activity of glutathione S-transferase (GST) and esterases (α-EST and pNPA-EST), but not acetylcholinesterase (AChE). In total, 92.1% of males genotyped for kdr presented a “resistant” genotype. Weekly blood-fed females from both strains, presented reduced mortality compared to sucrose-fed mosquitoes; however, Aedes Rio blood-fed females did not live as long (mean lifespan: Rockefeller = 70 ± 3.07; Aedes Rio = 53.5 ± 2.16 days). There were no differences between strains in relation to blood-feeding and number of eggs, but Aedes Rio eggs presented reduced viability (mean hatch: Rockefeller = 77.79 ± 1.4%; Aedes Rio = 58.57 ± 1.77%). Zika virus infection (plaque-forming unit, PFU) was similar in both strains (mean PFU ± SE: Aedes Rio: 4.53 × 10⁴ ± 1.14 × 10⁴ PFU; Rockefeller: 2.02 × 10⁴ ± 0.71 × 10⁴ PFU).

Conclusion

Selected conditions in the field, such as IR mechanisms, may result in pleiotropic effects that interfere in general physiology of the insect. Therefore, it is important to well characterize field populations to be tested in parallel with laboratory reference strains. This practice would improve the significance of laboratory tests for vector control methods.

Linked-read sequencing identifies abundant microinversions and introgression in the arboviral vector Aedes aegypti

Background

Aedes aegypti is the principal mosquito vector of Zika, dengue, and yellow fever viruses. Two subspecies of Ae. aegypti exhibit phenotypic divergence with regard to habitat, host preference, and vectorial capacity. Chromosomal inversions have been shown to play a major role in adaptation and speciation in dipteran insects and would be of great utility for studies of Ae. aegypti. However, the large and highly repetitive genome of Ae. aegypti makes it difficult to detect inversions with paired-end short-read sequencing data, and polytene chromosome analysis does not provide sufficient resolution to detect chromosome banding patterns indicative of inversions.

Results

To characterize chromosomal diversity in this species, we have carried out deep Illumina sequencing of linked-read (10X Genomics) libraries in order to discover inversion loci as well as SNPs. We analyzed individuals from colonies representing the geographic limits of each subspecies, one contact zone between subspecies, and a closely related sister species. Despite genome-wide SNP divergence and abundant microinversions, we do not find any inversions occurring as fixed differences between subspecies. Many microinversions are found in regions that have introgressed and have captured genes that could impact behavior, such as a cluster of odorant-binding proteins that may play a role in host feeding preference.

Conclusions

Our study shows that inversions are abundant and widely shared among subspecies of Aedes aegypti and that introgression has occurred in regions of secondary contact. This library of 32 novel chromosomal inversions demonstrates the capacity for linked-read sequencing to identify previously intractable genomic rearrangements and provides a foundation for future population genetics studies in this species.