Towards mosquito sterile insect technique programmes: exploring genetic, molecular, mechanical and behavioural methods of sex separation in mosquitoes

Establishing a dominant early larval sex-selection strain in the Asian malaria vector Anopheles stephensi

BACKGROUND

Genetic biocontrol interventions targeting mosquito-borne diseases require the release of male mosquitoes exclusively, as only females consume blood and transmit pathogens. Releasing only males eliminates the risk of increasing mosquito bites and spreading pathogens while enabling effective population control. The aim of this study is to develop robust sex-sorting methods for early larval stages in mosquitoes, enabling scalable male-only releases for genetic biocontrol interventions.

METHODS

To address the challenge of sex-sorting in the Asian malaria vector Anopheles stephensi, we engineer Sexing Element Produced by Alternative RNA-splicing of a Transgenic Observable Reporter (SEPARATOR). This dominant fluorescent-based method, previously proven effective in Aedes aegypti, exploits sex-specific alternative splicing of a reporter to ensure exclusive male-specific expression early in development. The sex-specific alternative RNA splicing of the doublesex gene was selected as a target for engineering SEPARATOR due to its evolutionary conservation in insects. To expand SEPARATOR's applicability for genetic sexing, we assessed the cross-species sex-specific RNA splicing activity of the An. gambiae doublesex (AngDsx) splicing module in An. stephensi. Male-specific enhanced green fluorescent protein (EGFP) expression was verified throughout the mosquito life cycle using a fluorescent stereomicroscope.

RESULTS

Our results confirm that SEPARATOR regulates male-specific EGFP expression in An. stephensi and enables reliable positive male selection from the first instar larval stages. Molecular analysis demonstrates that male-specific EGFP expression is dependent on doublesex sex-specific splicing events. Additionally, the splicing module from An. gambiae operates effectively in An. stephensi, demonstrating evolutionary conservation in sex-specific splicing events between these species.

CONCLUSIONS

SEPARATOR's independence from sex-chromosome linkage provides resistance to breakage that could be mediated by meiotic recombination and chromosomal rearrangements, making it highly suitable for mass male releases. By enabling precise male selection from the first instar larval stages, SEPARATOR represents a significant advancement that will aid in the genetic biocontrol for Anopheles mosquitoes.

Establishing a Male-Positive Genetic Sexing Strain in the Asian Malaria Vector Anopheles stephensi

Genetic biocontrol interventions targeting mosquito-borne diseases require the release of male mosquitoes exclusively, as only females consume blood and transmit human pathogens. This reduces the risk of spreading pathogens while enabling effective population control. Robust sex sorting methods to enable early larval sorting in mosquitoes need to be developed to allow for scalable sex sorting for genetic biocontrol interventions. This study applies the SEPARATOR (Sexing Element Produced by Alternative RNA-splicing of A Transgenic Observable Reporter) system, previously developed for Aedes aegypti, to the Asian malaria vector Anopheles stephensi. We hypothesized that the intron from the doublesex gene in Anopheles gambiae would function in An. stephensi due to evolutionary conservation. Our results confirm that the splicing module from An. gambiae operates effectively in An. stephensi, demonstrating evolutionary conservation in sex-specific splicing events between these species. This system enables reliable positive male selection from first instar larval to pupal stages. RT-PCR analysis demonstrates that male-specific EGFP expression is dependent on doublesex sex-specific splicing events. The SEPARATOR system's independence from sex-chromosome linkage confers resistance to meiotic recombination and chromosomal rearrangements. This approach may facilitate the mass release of males, and the cross-species portability of SEPARATOR establishes it as a valuable tool for genetic biocontrol interventions across various pest species.

Efficiency assessment of a novel automatic mosquito pupae sex separation system in support of area-wide male-based release strategies

This study provides a comparative analysis of two state-of-the-art automatic mosquito pupae sex sorters currently available: the ORINNO and the WOLBAKI Biotech pupae sex separation systems, which both exploit the sexual size dimorphism of pupae. In Aedes aegypti, the WOLBAKI sex sorter and the ORINNO with a sieve mesh size of 1.050 mm achieved sex separation with female contamination rates below 1%, low pupae mortality rates and high male flight capacity. However, in Ae. albopictus, there was more variability, with female contamination rates above the 1% threshold and pupae mortality reaching 27% when using the ORINNO sorter. On the other hand, the WOLBAKI sorter achieved a male pupae recovery of 47.99 ± 8.81% and 50.91 ± 11.77% in Ae. aegypti and Ae. albopictus, respectively, while the ORINNO sorter with a smaller sieve size achieved male pupae recoveries of 38.08 ± 9.69% and 40.16 ± 2.73% in Ae. aegypti and Ae. albopictus, respectively. This study provides valuable information for researchers and practitioners in the field, assisting in the selection of the most suitable system for mosquito control, management and research programs depending on their specific requirements.

Aedes aegypti Controls AE. Aegypti: SIT and IIT-An Overview

The sterile insect technique (SIT) and the incompatible insect technique (IIT) are emerging and potentially revolutionary tools for controlling Aedes aegypti (L.), a prominent worldwide mosquito vector threat to humans that is notoriously difficult to reduce or eliminate in intervention areas using traditional integrated vector management (IVM) approaches. Here we provide an overview of the discovery, development, and application of SIT and IIT to Ae. aegypti control, and innovations and advances in technology, including transgenics, that could elevate these techniques to a worldwide sustainable solution to Ae. aegypti when combined with other IVM practices.

Knockout of ovary serine protease Leads to Ovary Deformation and Female Sterility in the Asian Corn Borer, Ostrinia furnacalis

Oogenesis in insects is a carefully orchestrated process, facilitating the formation of female gametes, which is regulated by multiple extrinsic and intrinsic factors, including ovary serine protease (Osp). As a member of the serine protease family, Osp is a homolog of Nudel, a maternally required protease defining embryonic dorsoventral polarity in Drosophila. In this study, we used CRISPR/Cas9-mediated mutagenesis to functionally characterize Osp in the Asian corn borer, Ostrinia furnacalis, a devastating maize pest throughout Asia and Australia. Building on previous knowledge, we hypothesized that knockout of Osp would disrupt embryonic development in O. furnacalis females. To examine this overarching hypothesis, we (1) cloned and characterized Osp from O. furnacalis, (2) designed target sites on exons 1 and 4 to construct a CRISPR/Cas9 mutagenesis system, and (3) documented phenotypic impacts among O. furnacalis Osp mutants. As a result, we (1) examined the temporal-spatial expression profiles of OfOsp, which has an open reading frame of 5648 bp in length and encodes a protein of 1873 amino acids; (2) established O. furnacalis Osp mutants; and (3) documented recessive, female-specific sterility among OfOspF mutants, including absent or deformed oviducts and reduced fertility in female but not male mutants. Overall, the combined results support our initial hypothesis that Osp is required for embryonic development, specifically ovarian maturation, in O. furnacalis females. Given its substantial impacts on female sterility, Osp provides a potential target for the Sterile Insect Technique (SIT) to manage Lepidoptera pests in general and the species complex Ostrinia in particular.

Efficient sex separation by exploiting differential alternative splicing of a dominant marker in Aedes aegypti

Only female mosquitoes consume blood giving them the opportunity to transmit deadly human pathogens. Therefore, it is critical to remove females before conducting releases for genetic biocontrol interventions. Here we describe a robust sex-sorting approach termed SEPARATOR (Sexing Element Produced by Alternative RNA-splicing of A Transgenic Observable Reporter) that exploits sex-specific alternative splicing of an innocuous reporter to ensure exclusive dominant male-specific expression. Using SEPARATOR, we demonstrate reliable sex selection from early larval and pupal stages in Aedes aegypti, and use a Complex Object Parametric Analyzer and Sorter (COPAS) to demonstrate scalable high-throughput sex-selection of first instar larvae. Additionally, we use this approach to sequence the transcriptomes of early larval males and females and find several genes that are sex-specifically expressed. SEPARATOR can simplify mass production of males for release programs and is designed to be cross-species portable and should be instrumental for genetic biocontrol interventions.

Mosquitoes and Mosquito-Borne Diseases in Vietnam

Mosquito-borne diseases pose a significant threat to humans in almost every part of the world. Key factors such as global warming, climatic conditions, rapid urbanisation, frequent human relocation, and widespread deforestation significantly increase the number of mosquitoes and mosquito-borne diseases in Vietnam, and elsewhere around the world. In southeast Asia, and notably in Vietnam, national mosquito control programmes contribute to reducing the risk of mosquito-borne disease transmission, however, malaria and dengue remain a threat to public health. The aim of our review is to provide a complete checklist of all Vietnamese mosquitoes that have been recognised, as well as an overview of mosquito-borne diseases in Vietnam. A total of 281 mosquito species of 42 subgenera and 22 genera exist in Vietnam. Of those, Anopheles, Aedes, and Culex are found to be potential vectors for mosquito-borne diseases. Major mosquito-borne diseases in high-incidence areas of Vietnam include malaria, dengue, and Japanese encephalitis. This review may be useful to entomological researchers for future surveys of Vietnamese mosquitoes and to decision-makers responsible for vector control tactics.

Phillips GC, Greene KK, Poletto M, Sperry BD, Warner SA, Rose NR, Frandsen GK, Verza NC, Gorman KJ, Matzen KJ. New self-sexing Aedes aegypti strain eliminates barriers to scalable and sustainable vector control for governments and communities in dengue-prone environments

For more than 60 years, efforts to develop mating-based mosquito control technologies have largely failed to produce solutions that are both effective and scalable, keeping them out of reach of most governments and communities in disease-impacted regions globally. High pest suppression levels in trials have yet to fully translate into broad and effective Aedes aegypti control solutions. Two primary challenges to date-the need for complex sex-sorting to prevent female releases, and cumbersome processes for rearing and releasing male adult mosquitoes-present significant barriers for existing methods. As the host range of Aedes aegypti continues to advance into new geographies due to increasing globalisation and climate change, traditional chemical-based approaches are under mounting pressure from both more stringent regulatory processes and the ongoing development of insecticide resistance. It is no exaggeration to state that new tools, which are equal parts effective and scalable, are needed now more than ever. This paper describes the development and field evaluation of a new self-sexing strain of Aedes aegypti that has been designed to combine targeted vector suppression, operational simplicity, and cost-effectiveness for use in disease-prone regions. This conditional, self-limiting trait uses the sex-determination gene doublesex linked to the tetracycline-off genetic switch to cause complete female lethality in early larval development. With no female progeny survival, sex sorting is no longer required, eliminating the need for large-scale mosquito production facilities or physical sex-separation. In deployment operations, this translates to the ability to generate multiple generations of suppression for each mosquito released, while being entirely self-limiting. To evaluate these potential benefits, a field trial was carried out in densely-populated urban, dengue-prone neighbourhoods in Brazil, wherein the strain was able to suppress wild mosquito populations by up to 96%, demonstrating the utility of this self-sexing approach for biological vector control. In doing so, it has shown that such strains offer the critical components necessary to make these tools highly accessible, and thus they harbour the potential to transition mating-based approaches to effective and sustainable vector control tools that are within reach of governments and at-risk communities who may have only limited resources.

Aedes albopictus Sterile Male Production: Influence of Strains, Larval Diet and Mechanical Sexing Tools

The sterile insect technique (SIT) is a biologically based method of pest control, which relies on the mass production, sterilization, and release of sterile males of the target species. Since females can transmit viruses, it is important to develop a mass rearing system to produce a large number of males with a low presence of females. We evaluated the effects of different strains, larval diets and sexing tools on male productivity and residual female presence for the application of SIT against Aedes albopictus. Strains coming from Italy, Germany, Greece, and Montenegro, with different levels of colonization, were reared with three larval diets: IAEA-BY, BLP-B and SLP-BY. Developed pupae were sexed using two different mechanical methods: sieve or Fay-Morlan separator. The results proved that adoption of the Fay-Morlan separator increased the productivity and limited the female presence. The IAEA-BY diet showed the lowest female contamination. Strains with a high number of breeding generations showed a decreased productivity and an increased female presence. Increased female presence was found only in extensively reared strains and only when the sorting operation was conducted with sieves. We hypothesize that extensive colonization may determine a size reduction which limits the sexing tool efficiency itself.

Life-history traits of a fluorescent Anopheles arabiensis genetic sexing strain introgressed into South African genomic background

BACKGROUND

South Africa has set a mandate to eliminate local malaria transmission by 2023. In pursuit of this objective a Sterile Insect Technique programme targeting the main vector Anopheles arabiensis is currently under development. Significant progress has been made towards operationalizing the technology. However, one of the main limitations being faced is the absence of an efficient genetic sexing system. This study is an assessment of an An. arabiensis (AY-2) strain carrying the full Y chromosome from Anopheles gambiae, including a transgenic red fluorescent marker, being introgressed into a South African genetic background as a potential tool for a reliable sexing system.

METHODS

Adult, virgin males from the An. arabiensis AY-2 strain were outcrossed to virgin females from the South African, Kwazulu-Natal An. arabiensis (KWAG strain) over three generations. Anopheles arabiensis AY-2 fluorescent males were sorted as first instar larvae (L1) using the Complex Object Parametric Analyzer and Sorter (COPAS) and later screened as pupae to verify the sex. Life history traits of the novel hybrid KWAG-AY2 strain were compared to the original fluorescent AY-2 strain, the South African wild-type KWAG strain and a standard laboratory An. arabiensis (Dongola reference strain).

RESULTS

The genetic stability of the sex-linked fluorescent marker and the integrity and high level of sexing efficiency of the system were confirmed. No recombination events in respect to the fluorescent marker were detected over three rounds of introgression crosses. KWAG-AY2 had higher hatch rates and survival of L1 to pupae and L1 to adult than the founding strains. AY-2 showed faster development time of immature stages and larger adult body size, but lower larval survival rates. Adult KWAG males had significantly higher survival rates. There was no significant difference between the strains in fecundity and proportion of males. KWAG-AY2 males performed better than reference strains in flight ability tests.

CONCLUSION

The life history traits of KWAG-AY2, its rearing efficiency under laboratory conditions, the preservation of the sex-linked fluorescence and perfect sexing efficiency after three rounds of introgression crosses, indicate that it has potential for mass rearing. The potential risks and benefits associated to the use of this strain within the Sterile Insect Technique programme in South Africa are discussed.

Genetic Stability and Fitness of Aedes aegypti Red-Eye Genetic Sexing Strains With Pakistani Genomic Background for Sterile Insect Technique Applications

The mosquito species Aedes aegypti is the primary transmitter of viruses that cause endemic diseases like dengue in Pakistan. It is also a cause of other vector-borne diseases like yellow fever, Zika fever, and chikungunya, which significantly impact human health worldwide. In the absence of efficient vaccines (except for yellow fever) or drugs, vector control methods, such as the sterile insect technique (SIT), have been proposed as additional tools for the management of these diseases. Mosquito SIT programs are based on the release of sterile males and it is important female releases to be ideally zero or to be kept at a minimum, since females are the ones that bite, blood-feed and transmit pathogens. Recently, an Ae. aegypti genetic sexing strain (GSS), with and without a recombination-suppressing inversion (Inv35), was developed using the eye color as a selectable marker, with males having black eyes and females red eyes. In the present study, we introgressed the sexing features and the Inv35 of the Ae. aegypti red-eye GSS into the Pakistani genomic background aiming to their future use for SIT applications in the country. Both introgressed strains, the Red-eye GSS-PAK and the Red-eye GSS/Inv35-PAK, were evaluated in respect to their genetic stability and biological quality by assessing parameters like recombination rate, fecundity, fertility, pupal and adult recovery, time of development, pupal weight, survival, and flight ability in comparison with a wild Pakistani population (PAK). The results suggest that the sexing features and the recombination suppression properties of Inv35 were not affected after their introgression into the local genomic background; however, some biological traits of the two newly constructed strains were affected, positively or negatively, suggesting that a thorough quality control analysis should be performed after the introgression of a GSS into a new genomic background prior to its use in SIT field trials or applications. The importance of using GSS with local genomic background for SIT applications against Aedes aegypti is also discussed.

Reproductive interference and Satyrisation: mechanisms, outcomes and potential use for insect control

Reproductive Interference occurs when interactions between individuals from different species disrupt reproductive processes, resulting in a fitness cost to one or both parties involved. It is typically observed between individuals of closely related species, often upon secondary contact. In both vertebrates and invertebrates, Reproductive Interference is frequently referred to as 'Satyrisation'. It can manifest in various ways, ranging from blocking or reducing the efficacy of mating signals, through to negative effects of heterospecific copulations and the production of sterile or infertile hybrid offspring. The negative fitness effects of Satyrisation in reciprocal matings between species are often asymmetric and it is this aspect, which is most relevant to, and can offer utility in, pest management. In this review, we focus on Satyrisation and outline the mechanisms through which it can operate. We illustrate this by using test cases, and we consider the underlying reasons why the reproductive interactions that comprise Satyrisation occur. We synthesise the key factors affecting the expression of Satyrisation and explore how they have potential utility in developing new routes for the management and control of harmful insects. We consider how Satyrisation might interact with other control mechanisms, and conclude by outlining a framework for its use in control, highlighting some of the important next steps.

Introgression of the Aedes aegypti Red-Eye Genetic Sexing Strains Into Different Genomic Backgrounds for Sterile Insect Technique Applications

Aedes aegypti is an invasive mosquito species and major vector of human arboviruses. A wide variety of control methods have been employed to combat mosquito populations. One of them is the sterile insect technique (SIT) that has recently attracted considerable research efforts due to its proven record of success and the absence of harmful environmental footprints. The efficiency and cost-effectiveness of SIT is significantly enhanced by male-only releases. For mosquito SIT, male-only releases are ideally needed since females bite, blood-feed and transmit the pathogens. Ae. aegypti genetic sexing strains (GSS) have recently become available and are based on eye colour mutations that were chosen as selectable markers. These genetic sexing strains were developed through classical genetics and it was shown to be subjected to genetic recombination, a phenomenon that is not suppressed in males as is the case in many Diptera. The genetic stability of these GSS was strengthened by the induction and isolation of radiation-induced inversions. In this study, we used the red eye mutation and the inversion Inv35 line of the Ae. aegypti red-eye GSS s and introgressed them in six different genomic backgrounds to develop GSS with the respective local genomic backgrounds. Our goal was to assess whether the recombination frequencies in the strains with and without the inversion are affected by the different genomic backgrounds. In all cases the recombination events were suppressed in all Inv35 GSS strains, thus indicating that the genomic background does not negatively affect the inversion result. Absence of any effect that could be ascribed to genetic differences, enables the introgression of the key elements of the GSS into the local genomic background prior to release to the target areas. Maintaining the local background increases the chances for successful matings between released males and wild females and addresses potential regulatory concerns regarding biosafety and biosecurity.

The Potential for a Released Autosomal X-Shredder Becoming a Driving-Y Chromosome and Invasively Suppressing Wild Populations of Malaria Mosquitoes

Sex-ratio distorters based on X-chromosome shredding are more efficient than sterile male releases for population suppression. X-shredding is a form of sex distortion that skews spermatogenesis of XY males towards the preferential transmission of Y-bearing gametes, resulting in a higher fraction of sons than daughters. Strains harboring X-shredders on autosomes were first developed in the malaria mosquito Anopheles gambiae, resulting in strong sex-ratio distortion. Since autosomal X-shredders are transmitted in a Mendelian fashion and can be selected against, their frequency in the population declines once releases are halted. However, unintended transfer of X-shredders to the Y-chromosome could produce an invasive meiotic drive element, that benefits from its biased transmission to the predominant male-biased offspring and its effective shielding from female negative selection. Indeed, linkage to the Y-chromosome of an active X-shredder instigated the development of the nuclease-based X-shredding system. Here, we analyze mechanisms whereby an autosomal X-shredder could become unintentionally Y-linked after release by evaluating the stability of an established X-shredder strain that is being considered for release, exploring its potential for remobilization in laboratory and wild-type genomes of An. gambiae and provide data regarding expression on the mosquito Y-chromosome. Our data suggest that an invasive X-shredder resulting from a post-release movement of such autosomal transgenes onto the Y-chromosome is unlikely.

Sterile insect technique with accidental releases of sterile females. Impact on mosquito-borne diseases control when viruses are circulating

The sterile insect technique (SIT) is a technique to control some vectors of diseases by releasing sterile males. However, during these releases, sterilized females can be (accidentally) released and since only females are vectors of diseases, it is important to study their impact when arthropod viruses are circulating. To that aim, we develop and study an entomological-epidemiological model, considering either permanent or periodic releases. Qualitative analyses of the continuous and periodic models are conducted. We highlight a critical sterile males release rate, Λ_M^crit, above which the control of wild population is always effective, using massive releases. Estimating the basic reproduction number of the epidemiological model, R₀², we show that if it is above a certain threshold, R_0,∗², that depends on the basic offspring number, N, and the release rate of sterile females, the epidemiological risk can only be controlled using (very) massive releases. Otherwise, we can estimate the basic reproduction number of the SIT epidemiological model, R_0,SIT², that shapes the stability property of the (periodic) disease-free equilibrium. We show that it might be possible to take R_0,SIT² below 1 using non-massive, but large enough, releases. However, practically, it seems more efficient to consider massive releases, followed by small releases once the vector population is small enough. In addition to SIT, we also recommend mechanical control, i.e. the reduction of breeding sites, that greatly improves the efficacy of SIT, in terms of duration or size of the releases. Our results reveal that outside an epidemic period, the release of sterile females is not an issue, as long as the sterile males release rate is greater than Λ_M^crit. Within an epidemic period, we show that sterile females releases do not really impact the SIT efficiency, as long as the release rate, Λ_F, is lower than a critical value, Λ_F^crit, that depends on the mosquito and epidemiological threshold parameters, N, and R₀². To illustrate numerically our theoretical results, we consider Dengue parameters. We estimate all thresholds and also the effective reproduction number, R_eff², and highlight the importance of early permanent or periodic SIT control to prevent or mitigate the risk of a Dengue epidemic, with and without sterile females releases.

Sterile Insect Technique: Lessons From the Past

When E.F. Knipling conceived of the release of sexually sterile insects to suppress wild populations, he laid down several fundamental qualities that characterized suitable target species-some of which mosquitoes generally violate-including high reproductive rates and large population numbers. Regardless of this, their global importance in public health has led numerous research teams to attempt to use the mosquito sterile insect technique against several species. Because of the degree of financial commitment required for suppression programs, most releases have consisted of preliminary investigations of male performance, population characteristics, and production methods. Those that have accomplished suppression provide important insights regarding the challenges of production, dispersal, and immigration. Insights gained from these studies remain relevant today, regardless of the genetic control technology being applied. In this article, I highlight studies that were notable for the insights that were gained, the intrinsic difficulties that mosquitoes present, and synthesize these into recommendations for successful applications of the sterile insect technique and newer technologies to mosquitoes.

Aedes albopictus bionomics data collection by citizen participation on Procida Island, a promising Mediterranean site for the assessment of innovative and community-based integrated pest management methods

In the last decades, the colonization of Mediterranean Europe and of other temperate regions by Aedes albopictus created an unprecedented nuisance problem in highly infested areas and new public health threats due to the vector competence of the species. The Sterile Insect Technique (SIT) and the Incompatible Insect Technique (IIT) are insecticide-free mosquito-control methods, relying on mass release of irradiated/manipulated males, able to complement existing and only partially effective control tools. The validation of these approaches in the field requires appropriate experimental settings, possibly isolated to avoid mosquito immigration from other infested areas, and preliminary ecological and entomological data. We carried out a 4-year study in the island of Procida (Gulf of Naples, Italy) in strict collaboration with local administrators and citizens to estimate the temporal dynamics, spatial distribution, and population size of Ae. albopictus and the dispersal and survival of irradiated males. We applied ovitrap monitoring, geo-spatial analyses, mark-release-recapture technique, and a citizen-science approach. Results allow to predict the seasonal (from April to October, with peaks of 928-9,757 males/ha) and spatial distribution of the species, highlighting the capacity of Ae. albopictus population of Procida to colonize and maintain high frequencies in urban as well as in sylvatic inhabited environments. Irradiated males shown limited ability to disperse (mean daily distance travelled <60m) and daily survival estimates ranging between 0.80 and 0.95. Overall, the ecological characteristics of the island, the acquired knowledge on Ae. albopictus spatial and temporal distribution, the high human and Ae. albopictus densities and the positive attitude of the resident population in being active parts in innovative mosquito control projects provide the ground for evidence-based planning of the interventions and for the assessment of their effectiveness. In addition, the results highlight the value of creating synergies between research groups, local administrators, and citizens for affordable monitoring (and, in the future, control) of mosquito populations.

A functional requirement for sex-determination M/m locus region lncRNA genes in Aedes aegypti female larvae

Although many putative long non-coding RNA (lncRNA) genes have been identified in insect genomes, few of these genes have been functionally validated. A screen for female-specific larvicides that facilitate Aedes aegypti male sex separation uncovered multiple interfering RNAs with target sites in lncRNA genes located in the M/m locus region, including loci within or tightly linked to the sex determination locus. Larval consumption of a Saccharomyces cerevisiae (yeast) strain engineered to express interfering RNA corresponding to lncRNA transcripts resulted in significant female death, yet had no impact on male survival or fitness. Incorporation of the yeast larvicides into mass culturing protocols facilitated scaled production and separation of fit adult males, indicating that yeast larvicides could benefit mosquito population control strategies that rely on mass releases of male mosquitoes. These studies functionally verified a female-specific developmental requirement for M/m locus region lncRNA genes, suggesting that sexually antagonistic lncRNA genes found within this highly repetitive pericentromeric DNA sequence may be contributing to the evolution of A. aegypti sex chromosomes.

The Insect Pest Control Laboratory of the Joint FAO/IAEA Programme: Ten Years (2010-2020) of Research and Development, Achievements and Challenges in Support of the Sterile Insect Technique

The Joint FAO/IAEA Centre (formerly called Division) of Nuclear Techniques in Food and Agriculture was established in 1964 and its accompanying laboratories in 1961. One of its subprograms deals with insect pest control, and has the mandate to develop and implement the sterile insect technique (SIT) for selected key insect pests, with the goal of reducing the use of insecticides, reducing animal and crop losses, protecting the environment, facilitating international trade in agricultural commodities and improving human health. Since its inception, the Insect Pest Control Laboratory (IPCL) (formerly named Entomology Unit) has been implementing research in relation to the development of the SIT package for insect pests of crops, livestock and human health. This paper provides a review of research carried out between 2010 and 2020 at the IPCL. Research on plant pests has focused on the development of genetic sexing strains, characterizing and assessing the performance of these strains (e.g., Ceratitis capitata), elucidation of the taxonomic status of several members of the Bactrocera dorsalis and Anastrepha fraterculus complexes, the use of microbiota as probiotics, genomics, supplements to improve the performance of the reared insects, and the development of the SIT package for fruit fly species such as Bactrocera oleae and Drosophila suzukii. Research on livestock pests has focused on colony maintenance and establishment, tsetse symbionts and pathogens, sex separation, morphology, sterile male quality, radiation biology, mating behavior and transportation and release systems. Research with human disease vectors has focused on the development of genetic sexing strains (Anopheles arabiensis, Aedes aegypti and Aedes albopictus), the development of a more cost-effective larvae and adult rearing system, assessing various aspects of radiation biology, characterizing symbionts and pathogens, studying mating behavior and the development of quality control procedures, and handling and release methods. During the review period, 13 coordinated research projects (CRPs) were completed and six are still being implemented. At the end of each CRP, the results were published in a special issue of a peer-reviewed journal. The review concludes with an overview of future challenges, such as the need to adhere to a phased conditional approach for the implementation of operational SIT programs, the need to make the SIT more cost effective, to respond with demand driven research to solve the problems faced by the operational SIT programs and the use of the SIT to address a multitude of exotic species that are being introduced, due to globalization, and established in areas where they could not survive before, due to climate change.

The Sterile Insect Technique: Success and Perspectives in the Neotropics

The sterile insect technique (SIT), an environmentally friendly means of control, is currently used against plant, animal, and human pests under the area-wide integrated pest management. It consists in the mass production, sterilization, and release of insects in an affected area where sterile males mate with wild females leading to no reproduction. Here, we review SIT in the Neotropics and focus on particular recent successful cases of eradication of the Mediterranean fruit fly, Ceratitis capitata (Wiedemann), as well as effective programs used against the Mexican fruit fly Anastrepha ludens (Loew), the New World screwworm fly Cochliomyia hominivorax (Coquerel)), and the Cactus moth Cactoblastis cactorum (Berg). We examine when SIT does not work and innovations that have made SIT more efficient and also highlight complimentary techniques that can be used in conjunction. We address potential candidate species that could be controlled through SIT, for example Philornis downsi Dodge & Aitken. Finally, we consider the impact of climate change in the context of the use of the SIT against these pests. Given the recent dramatic decline in insect biodiversity, investing in environmentally friendly means of pest control should be a priority. We conclude that SIT should be promoted in the region, and leadership and political will is needed for continued success of SIT in the Neotropics.

Sterile Insect Technique in an Integrated Vector Management Program against Tiger Mosquito Aedes albopictus in the Valencia Region (Spain): Operating Procedures and Quality Control Parameters

Aedes albopictus and Aedes aegypti are the main vectors of arboviral diseases such as dengue, Zika and chikungunya viruses. About a third of the world population is currently at risk of contracting Aedes-borne epidemics. In recent years, A. albopictus has drastically increased its distribution in many countries. In the absence of efficient mosquito vector control methods, the sterile insect technique (SIT) is presented as a very promising and environment-friendly control tool. The Agriculture Department of the Valencian Region is promoting an ongoing pilot project to evaluate the efficacy of an integrated vector management program (IVM) based on the use of the SIT as the main method of control. The laboratory studies for evaluating the entomological efficacy of SIT through the phased conditional testing process recommended by World Health Organization and the International Atomic Energy Agency (WHO-IAEA) are addressed. This study describes the routine operating procedures and quality control parameters for the medium-scale rearing of sterile male A. albopictus. More than 15 million sterile males have been produced and released in an area of 80 ha between 2018 and 2020. Of the initial L1 larvae, we recovered 17.2% of male pupae after sex sorting to be sterilized and released on the field, while the rest of the pupae remained available to maintain the rearing colony. The residual percentage of females after sex sorting was on average 0.17%. The obtained values in terms of production and quality control as well as the proposed rearing methodology can be useful for designing a medium-scale mosquito-rearing pipeline.

Sterile Insect Technique (SIT) against Aedes Species Mosquitoes: A Roadmap and Good Practice Framework for Designing, Implementing and Evaluating Pilot Field Trials

Aedes albopictus and Aedes aegypti are invasive mosquito species that impose a substantial risk to human health. To control the abundance and spread of these arboviral pathogen vectors, the sterile insect technique (SIT) is emerging as a powerful complement to most commonly-used approaches, in part, because this technique is ecologically benign, specific, and non-persistent in the environment if releases are stopped. Because SIT and other similar vector control strategies are becoming of increasing interest to many countries, we offer here a pragmatic and accessible 'roadmap' for the pre-pilot and pilot phases to guide any interested party. This will support stakeholders, non-specialist scientists, implementers, and decision-makers. Applying these concepts will ensure, given adequate resources, a sound basis for local field trialing and for developing experience with the technique in readiness for potential operational deployment. This synthesis is based on the available literature, in addition to the experience and current knowledge of the expert contributing authors in this field. We describe a typical path to successful pilot testing, with the four concurrent development streams of Laboratory, Field, Stakeholder Relations, and the Business and Compliance Case. We provide a graphic framework with criteria that must be met in order to proceed.

Genetic sexing strains for the population suppression of the mosquito vector Aedes aegypti

Aedes aegypti is the primary vector of arthropod-borne viruses including dengue, chikungunya and Zika. Vector population control methods are reviving to impede disease transmission. An efficient sex separation for male-only releases is crucial for area-wide mosquito population suppression strategies. Here, we report on the construction of two genetic sexing strains using red- and white-eye colour mutations as selectable markers. Quality control analysis showed that the Red-eye genetic sexing strains (GSS) is better and more genetically stable than the White-eye GSS. The introduction of an irradiation-induced inversion (Inv35) increases genetic stability and reduces the probability of female contamination of the male release batches. Bi-weekly releases of irradiated males of both the Red-eye GSS and the Red-eye GSS/Inv35 fully suppressed target laboratory cage populations within six and nine weeks, respectively. An image analysis algorithm allowing sex determination based on eye colour identification at the pupal stage was developed. The next step is to automate the Red-eye-based genetic sexing and validate it in pilot trials prior to its integration in large-scale population suppression programmes. This article is part of the theme issue 'Novel control strategies for mosquito-borne diseases'.

Irradiation induced inversions suppress recombination between the M locus and morphological markers in Aedes aegypti

BACKGROUND

Aedes aegypti is the primary vector of arthropod-borne viruses and one of the most widespread and invasive mosquito species. Due to the lack of efficient specific drugs or vaccination strategies, vector population control methods, such as the sterile insect technique, are receiving renewed interest. However, availability of a reliable genetic sexing strategy is crucial, since there is almost zero tolerance for accidentally released females. Development of genetic sexing strains through classical genetics is hindered by genetic recombination that is not suppressed in males as is the case in many Diptera. Isolation of naturally-occurring or irradiation-induced inversions can enhance the genetic stability of genetic sexing strains developed through genetically linking desirable phenotypes with the male determining region.

RESULTS

For the induction and isolation of inversions through irradiation, 200 male pupae of the 'BRA' wild type strain were irradiated at 30 Gy and 100 isomale lines were set up by crossing with homozygous 'red-eye' (re) mutant females. Recombination between re and the M locus and the white (w) gene (causing a recessive white eye phenotype when mutated) and the M locus was tested in 45 and 32 lines, respectively. One inversion (Inv35) reduced recombination between both re and the M locus, and wand the M locus, consistent with the presence of a rather extended inversion between the two morphological mutations, that includes the M locus. Another inversion (Inv5) reduced recombination only between w and the M locus. In search of naturally-occurring, recombination-suppressing inversions, homozygous females from the red eye and the white eye strains were crossed with seventeen and fourteen wild type strains collected worldwide, representing either recently colonized or long-established laboratory populations. Despite evidence of varying frequencies of recombination, no combination led to the elimination or substantial reduction of recombination.

CONCLUSION

Inducing inversions through irradiation is a feasible strategy to isolate recombination suppressors either on the M or the m chromosome for Aedes aegypti. Such inversions can be incorporated in genetic sexing strains developed through classical genetics to enhance their genetic stability and support SIT or other approaches that aim to population suppression through male-delivered sterility.

CRISPR/Cas9 knockout of female-biased genes AeAct-4 or myo-fem in Ae. aegypti results in a flightless phenotype in female, but not male mosquitoes

Aedes aegypti is a vector of dengue, chikungunya, and Zika viruses. Current vector control strategies such as community engagement, source reduction, and insecticides have not been sufficient to prevent viral outbreaks. Thus, interest in novel strategies involving genetic engineering is growing. Female mosquitoes rely on flight to mate with males and obtain a bloodmeal from a host. We hypothesized that knockout of genes specifically expressed in female mosquitoes associated with the indirect flight muscles would result in a flightless female mosquito. Using CRISPR-Cas9 we generated loss-of-function mutations in several genes hypothesized to control flight in mosquitoes, including actin (AeAct-4) and myosin (myo-fem) genes expressed specifically in the female flight muscle. Genetic knockout of these genes resulted in 100% flightless females, with homozygous males able to fly, mate, and produce offspring, albeit at a reduced rate when compared to wild type males. Interestingly, we found that while AeAct-4 was haplosufficient, with most heterozygous individuals capable of flight, this was not the case for myo-fem, where about half of individuals carrying only one intact copy could not fly. These findings lay the groundwork for developing novel mechanisms of controlling Ae. aegypti populations, and our results suggest that this mechanism could be applicable to other vector species of mosquito.

Strategic Approach, Advances, and Challenges in the Development and Application of the SIT for Area-Wide Control of Aedes albopictus Mosquitoes in Reunion Island

Simple Summary

Aedes albopictus is a well-established competent arbovirus vector in Reunion Island, a French overseas territory in the Indian Ocean, occurring in a range of natural to urban environments where it represents a major threat to public health. Following the 2006 Chikungunya outbreak and periodic occurrence of dengue epidemics, the sterile insect technique (SIT) emerged as the most environment-friendly option for integration with the current vector control strategy that relies mainly on the elimination of breeding sites and insecticide applications. This paper describes the trajectory that has been followed in assessing the feasibility of SIT against Ae. albopictus in Reunion Island, and reviews some of the main achievements since 2009. These include essential scientific information so far obtained on the biology and ecology of Ae. albopictus, and the development of the requisite technological capabilities for the production and release of sexually competitive sterile males. Furthermore, it also draws attention to the strategies established to streamline the decision-making process, including an awareness campaign to enhance public understanding, efforts to secure public acceptance and regulatory validation of SIT pilot testing for small-scale suppression of wild Ae. albopictus in selected urban sites on the island.

Abstract

The global expansion of Aedes albopictus, together with the absence of specific treatment and vaccines for most of the arboviruses it transmits, has stimulated the development of more sustainable and ecologically acceptable methods for control of disease transmission through the suppression of natural vector populations. The sterile insect technique (SIT) is rapidly evolving as an additional tool for mosquito control, offering an efficient and more environment-friendly alternative to the use of insecticides. Following the devastating chikungunya outbreak, which affected 38% of the population on Reunion Island (a French overseas territory in the southwest of the Indian Ocean), there has been strong interest and political will to develop effective alternatives to the existing vector control strategies. Over the past 10 years, the French Research and Development Institute (IRD) has established an SIT feasibility program against Ae. albopictus on Reunion Island in collaboration with national and international partners. This program aimed to determine whether the SIT based on the release of radiation-sterilized males is scientifically and technically feasible, and socially acceptable as part of a control strategy targeting the local Ae. albopictus population. This paper provides a review of a multi-year and a particularly broad scoping process of establishing the scientific and technological feasibility of the SIT against Ae. albopictus on Reunion Island. It also draws attention to some prerequisites of the decision-making process, through awareness campaigns to enhance public understanding and support, social adoption, and regulatory validation of the SIT pilot tests.

Modelling the Wolbachia incompatible insect technique: strategies for effective mosquito population elimination

BACKGROUND

The Wolbachia incompatible insect technique (IIT) shows promise as a method for eliminating populations of invasive mosquitoes such as Aedes aegypti (Linnaeus) (Diptera: Culicidae) and reducing the incidence of vector-borne diseases such as dengue, chikungunya and Zika. Successful implementation of this biological control strategy relies on high-fidelity separation of male from female insects in mass production systems for inundative release into landscapes. Processes for sex-separating mosquitoes are typically error-prone and laborious, and IIT programmes run the risk of releasing Wolbachia-infected females and replacing wild mosquito populations.

RESULTS

We introduce a simple Markov population process model for studying mosquito populations subjected to a Wolbachia-IIT programme which exhibit an unstable equilibrium threshold. The model is used to study, in silico, scenarios that are likely to yield a successful elimination result. Our results suggest that elimination is best achieved by releasing males at rates that adapt to the ever-decreasing wild population, thus reducing the risk of releasing Wolbachia-infected females while reducing costs.

CONCLUSIONS

While very high-fidelity sex separation is required to avoid establishment, release programmes tend to be robust to the release of a small number of Wolbachia-infected females. These findings will inform and enhance the next generation of Wolbachia-IIT population control strategies that are already showing great promise in field trials.

Female-to-male sex conversion in Ceratitis capitata by CRISPR/Cas9 HDR-induced point mutations in the sex determination gene transformer-2

The Sterile Insect Technique (SIT) is based on the mass release of sterilized male insects to reduce the pest population size via infertile mating. Critical for all SIT programs is a conditional sexing strain to enable the cost-effective production of male-only populations. Compared to current female-elimination strategies based on killing or sex sorting, generating male-only offspring via sex conversion would be economically beneficial by doubling the male output. Temperature-sensitive mutations known from the D. melanogaster transformer-2 gene (tra2^ts) induce sex conversion at restrictive temperatures, while regular breeding of mutant strains is possible at permissive temperatures. Since tra2 is a conserved sex determination gene in many Diptera, including the major agricultural pest Ceratitis capitata, it is a promising candidate for the creation of a conditional sex conversion strategy in this Tephritid. Here, CRISPR/Cas9 homology-directed repair was used to induce the D. melanogaster-specific tra2^ts SNPs in Cctra2. 100% female to male conversion was successfully achieved in flies homozygous for the tra2^ts2 mutation. However, it was not possible, to identify a permissive temperature for the mutation allowing the rearing of a tra2^ts2 homozygous line, as lowering the temperature below 18.5 °C interferes with regular breeding of the flies.

Wolbachia pipientis Associated With Tephritid Fruit Fly Pests: From Basic Research to Applications

Members of the true fruit flies (family Tephritidae) are among the most serious agricultural pests worldwide, whose control and management demands large and costly international efforts. The need for cost-effective and environmentally friendly integrated pest management (IPM) has led to the development and implementation of autocidal control strategies. These approaches include the widely used sterile insect technique and the incompatible insect technique (IIT). IIT relies on maternally transmitted bacteria (namely Wolbachia) to cause a conditional sterility in crosses between released mass-reared Wolbachia-infected males and wild females, which are either uninfected or infected with a different Wolbachia strain (i.e., cytoplasmic incompatibility; CI). Herein, we review the current state of knowledge on Wolbachia-tephritid interactions including infection prevalence in wild populations, phenotypic consequences, and their impact on life history traits. Numerous pest tephritid species are reported to harbor Wolbachia infections, with a subset exhibiting high prevalence. The phenotypic effects of Wolbachia have been assessed in very few tephritid species, due in part to the difficulty of manipulating Wolbachia infection (removal or transinfection). Based on recent methodological advances (high-throughput DNA sequencing) and breakthroughs concerning the mechanistic basis of CI, we suggest research avenues that could accelerate generation of necessary knowledge for the potential use of Wolbachia-based IIT in area-wide integrated pest management (AW-IPM) strategies for the population control of tephritid pests.

A drug-inducible sex-separation technique for insects

Here, we describe a drug-inducible genetic system for insect sex-separation that demonstrates proof-of-principle for positive sex selection in D. melanogaster. The system exploits the toxicity of commonly used broad-spectrum antibiotics geneticin and puromycin to kill the non-rescued sex. Sex-specific rescue is achieved by inserting sex-specific introns into the coding sequences of antibiotic-resistance genes. When raised on geneticin-supplemented food, the sex-sorter line establishes 100% positive selection for female progeny, while the food supplemented with puromycin positively selects 100% male progeny. Since the described system exploits conserved sex-specific splicing mechanisms and reagents, it has the potential to be adaptable to other insect species of medical and agricultural importance.

Black soldier fly (Hermetia illucens) larvae powder as a larval diet ingredient for mass-rearing Aedes mosquitoes

The mass production of mosquitoes is becoming more wide-spread due to the increased application of the sterile insect technique (SIT) and other genetic control programmes. Due to the variable availability and high cost of the bovine liver powder (BLP) constituent of many current larval diets, there is an urgent demand for new ingredients in order to support sustainable and efficient mosquito production while reducing rearing cost, without affecting the quality of the insects produced. Two black soldier fly (BSF) powder-based diet formulations (50% tuna meal, 35% BSF powder, 15% brewer's yeast and 50% tuna meal + 50% BSF powder) were tested for their suitability to support the development of Aedes aegypti and Ae. albopictus mosquitoes in mass-rearing conditions. Overall, the results indicate that the use of the BSF powder did not negatively impact the development and quality of the produced insects in terms of time to pupation, adult production and male flight ability. Furthermore, depending on the species and diet formulations, there were improvements in some parameters such as female body size, egg production, egg hatch rate and male longevity. BSF powder is a valuable ingredient that can effectively replace costly BLP for the mass production of high quality Ae. aegypti and Ae. albopictus mosquitoes. Both diet formulations can be used for Ae. aegypti showing high plasticity to nutrition sources. However, for Ae. albopictus we recommend the combination including brewer's yeast.

Sex Sorting for Pest Control: It's Raining Men!

In the pursuit of better pest- and vector-control strategies, attention returns to an old proven technology, the sterile insect technique (SIT) and related insect population-suppression methods. A major obstacle for any of these approaches that involves the release of sterile males is the separation of males from females during the mass rearing stage, in order to improve the cost-efficiency of these methods and to prevent the release of biting and disease-vectoring females. This review describes recent sex-sorting developments in dipteran flies with an emphasis on assessing the suitability of these methods for large-scale rearing of male vectors for mass release.

Use of mechanical and behavioural methods to eliminate female Aedes aegypti and Aedes albopictus for sterile insect technique and incompatible insect technique applications

Background

Sex separation of mosquitoes at different stages is currently being attempted to ensure the successful release of male mosquitoes in novel vector control approaches. Mechanical and behavioral techniques have been tried most frequently.

Methods

Batches of (n = 300) Aedes aegypti and Ae. albopictus pupae were used for standard sieving (using sieves with 1.12, 1.25, 1.40 and 1.60 mm mesh sizes) and the Fay-Morlan glass plate separation methods. Male and female separation by each method was calculated. For behavioral separation, a spiked blood meal with different concentrations (0, 2, 4, 6, 8 and 10 ppm) of ivermectin and spinosad (spinosyn, 12% w/v), were provided to a batch (n = 300) of adult Ae. aegypti and Ae. albopictus (1:1 sex ratio) followed by observation of mortality. An additional “double feeding method” involved provision of a further blood meal after 24 h, with the same concentrations of ivermectin and spinosad as the initial feeding, followed by a 48-h observation of mortality. All experiments were repeated five times.

Results

In the standard sieving method, the percentage of males and females separated at different pore sizes differed significantly (P < 0.05). The majority of the male pupae were collected in the 1.12 mm pore sized sieve for both Ae. aegypti (73%) and Ae. albopictus (69%) while females were retained mainly in the sieve with the pore size of 1.25 mm. In the Fay-Morlan glass plate separation, 99.0% of the Ae. aegypti and 99.2% of the Ae. albopictus introduced male pupae could be separated, but with female contaminations of 16 and 12%, respectively. Provision of a blood meal spiked with 8 ppm of ivermectin under the “double feeding” was identified as the most effective way of achieving 100% female elimination for both Aedes species.

Conclusions

With 100% separation, use of a spiked blood meal is a more effective method of sex separation than the mechanical methods. Application of the spiked blood meal approach as a second separation level for sexes, after applying the Fay-Morlan glass plate method, could achieve 100% sex separation of sexes whilst allowing a reduction in the amount of toxicants required.

Transforming insect population control with precision guided sterile males with demonstration in flies

The sterile insect technique (SIT) is an environmentally safe and proven technology to suppress wild populations. To further advance its utility, a novel CRISPR-based technology termed precision guided SIT (pgSIT) is described. PgSIT mechanistically relies on a dominant genetic technology that enables simultaneous sexing and sterilization, facilitating the release of eggs into the environment ensuring only sterile adult males emerge. Importantly, for field applications, the release of eggs will eliminate burdens of manually sexing and sterilizing males, thereby reducing overall effort and increasing scalability. Here, to demonstrate efficacy, we systematically engineer multiple pgSIT systems in Drosophila which consistently give rise to 100% sterile males. Importantly, we demonstrate that pgSIT-generated sterile males are fit and competitive. Using mathematical models, we predict pgSIT will induce substantially greater population suppression than can be achieved by currently-available self-limiting suppression technologies. Taken together, pgSIT offers to potentially transform our ability to control insect agricultural pests and disease vectors.

Construction of a genetic sexing strain for Aedes albopictus: a promising tool for the development of sterilizing insect control strategies targeting the tiger mosquito

Background

Aedes albopictus is an invasive mosquito species of global medical concern as its distribution has recently expanded to Africa, the Americas and Europe. In the absence of prophylaxis protecting human populations from emerging arboviruses transmitted by this mosquito species, the most straightforward control measures rely on the suppression or manipulation of vector natural populations. A number of environmental-friendly methods using mass releases of sterilizing males are currently under development. However, these strategies are still lacking an efficient sexing method required for production of males at an industrial scale.

Results

We present the first Genetic Sexing Strain (GSS) in Ae. albopictus, hereafter referred as Tikok, obtained by sex linkage of the rdl gene conferring dieldrin resistance. Hatch rate, larval survival and sex ratio were followed during twelve generations. The use of dieldrin at the third larval stage allowed selecting 98 % of males on average.

Conclusion

A good production rate of Tikok males makes this GSS suitable for any control method based on mass production of Ae. albopictus males. Despite limitations resulting from reduced egg hatch as well as the nature of the used insecticide, the construction of this GSS paves the way for industrial sex separation of Ae. albopictus.

Pure early zygotic genes in the Asian malaria mosquito Anopheles stephensi

BACKGROUND

The Asian malaria mosquito, Anopheles stephensi, is a major urban malaria vector in the Middle East and on the Indian subcontinent. Early zygotic transcription, which marks the maternal-to-zygotic transition, has not been systematically studied in An. stephensi or any other Anopheles mosquitoes. Improved understanding of early embryonic gene expression in An. stephensi will facilitate genetic and evolutionary studies and help with the development of novel control strategies for this important disease vector.

RESULTS

We obtained RNA-seq data in biological triplicates from four early An. stephensi embryonic time points. Using these data, we identified 70 and 153 pure early zygotic genes (pEZGs) under stringent and relaxed conditions, respectively. We show that these pEZGs are enriched in functional groups related to DNA-binding transcription regulators, cell cycle modulators, proteases, transport, and cellular metabolism. On average these pEZGs are shorter and have less introns than other An. stephensi genes. Some of the pEZGs may arise de novo while others have clear non-pEZG paralogs. There is no or very limited overlap between An. stephensi pEZGs and Drosophila melanogaster or Aedes aegypti pEZGs. Interestingly, the upstream region of An. stephensi pEZGs lack significant enrichment of a previously reported TAGteam/VBRGGTA motif found in the regulatory region of pEZGs in D. melanogaster and Ae. aegypti. However, a GT-rich motif was found in An. stephensi pEZGs instead.

CONCLUSIONS

We have identified a number of pEZGs whose predicted functions and structures are consistent with their collective roles in the degradation of maternally deposited components, activation of the zygotic genome, cell division, and metabolism. The pEZGs appear to rapidly turn over within the Dipteran order and even within the Culicidae family. These pEZGs, and the shared regulatory motif, could provide the promoter or regulatory sequences to drive gene expression in the syncytial or early cellular blastoderm, a period when the developing embryo is accessible to genetic manipulation. In addition, these molecular resources may be used to achieve sex separation of mosquitoes for sterile insect technique.

The Nix locus on the male-specific homologue of chromosome 1 in Aedes albopictus is a strong candidate for a male-determining factor

Background

Global concern over the rapid expansion of the Asian tiger mosquito, Aedes albopictus, and its vector competence has highlighted an urgent need to improve currently available population control methods, like the Sterile Insect Technique. Knowledge of the sex determination cascade is a prerequisite for the development of early-stage sexing systems. To this end, we have characterised the putative sex determination gene, Nix, in this species. In Aedes species the chromosome complement consists of three pairs of chromosomes. The sex determination alleles are linked to the smallest homomorphic chromosome.

Results

We identified the male-specific chromosome 1 of Ae. albopictus that carries the putative male-determining gene Nix. We have also characterised the complete genomic sequence of the Nix gene which is composed of two exons and a short intron. The gene displays different levels of intron retention during development. Comparison of DNA sequences covering most of the Nix gene from individuals across the species range revealed no polymorphism.

Conclusions

Our characterisation of the Nix gene in Ae. albopictus represents an initial step in the analysis of the sex determination cascade in this species. We found evidence of intron retention (IR) in Nix. IR might play a role in regulating the expression of Nix during development. Our results provide the basis for the development of new genetic control strategies.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-3215-8) contains supplementary material, which is available to authorized users.

Functional analysis of the promoter of an early zygotic gene KLC2 in Aedes aegypti

Background

Aedes aegypti is an important mosquito vector that transmits arboviruses that cause devastating diseases including Zika, dengue fever, yellow fever and chikungunya. Improved understanding of gene regulation in the early development of Ae. aegypti will facilitate genetic studies and help the development of novel control strategies of this important disease vector.

Results

In this study, we demonstrated through transgenic assays that the promoter of an endogenous early zygotic gene KLC2 could drive gene expression in the syncytial blastoderm and early cellular blastoderm, which is a stage that the developing germline and the rest of embryo are accessible to genetic manipulation. An unexpected expression of the reporter gene in transgenic male testes was also observed. Further analysis confirmed the expression of the endogenous KLC2 in the testes, which was not detected in the previous RNA sequencing data.

Conclusions

Our finding provided a new promoter element that can be used in future genetic studies and applications in Ae. aegypti. Moreover, our transgenic reporter assays showed that cautions are needed when interpreting RNA sequencing data as transient or tissue-specific transcription may go undetected by RNAseq.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-3210-0) contains supplementary material, which is available to authorized users.

Combined sterile insect technique and incompatible insect technique: sex separation and quality of sterile Aedes aegypti male mosquitoes released in a pilot population suppression trial in Thailand

Background

The sterile insect technique (SIT), which is based on irradiation-induced sterility, and incompatible insect technique (IIT), which is based on Wolbachia-induced cytoplasmic incompatibility (a kind of male sterility), have been used as alternative methods to reduce mosquito vector populations. Both methods require the release of males to reduce fertile females and suppress the number of natural populations. Different techniques of sex separation to obtain only males have been investigated previously. Our work involves an application of mechanical larval-pupal glass separators to separate Wolbachia-infected Aedes aegypti males from females at the pupal stage, prior to irradiation, and for use in a pilot field release and to assess the quality of males and females before and after sex separation and sterilization.

Results

This study was the first to demonstrate the efficiency of mechanical glass separators in separating males for use in an Ae. aegypti suppression trial by a combined SIT/IIT approach. Our results indicated that male and female pupae of Wolbachia-infected Ae. aegypti mosquitoes were significantly different (p < 0.05) in weight, size, and emergence-time, which made it easier for sex separation by this mechanical method. During the pilot field release, the percentage of female contamination was detected to be quite low and significantly different between the first (0.10 ± 0.13) and the second (0.02 ± 0.02) twelve-week period. Both males and females were almost completely sterile after exposure to 70 Gy irradiation dose. We observed that both irradiated Wolbachia-infected males and females survived and lived longer than two weeks, but males could live longer than females (p < 0.05) when they were irradiated at the same irradiation dose. When comparing irradiated mosquitoes with non-irradiated ones, there was no significant difference in longevity and survival-rate between those males, but non-irradiated females lived longer than irradiated ones (p < 0.05).

Conclusion

Mechanical sex separation by using a larval-pupal glass separator was practically applied to obtain only males for further sterilization and open field release in a pilot population suppression trial of Ae. aegypti in Thailand. Female contamination was detected to be quite low, and skilled personnel can reduce the risk for female release. The irradiated Wolbachia-infected females accidentally released were found to be completely sterile, with shorter life span than males.

Molecular tools to create new strains for mosquito sexing and vector control

Vector control programs based on population reduction by matings with mass-released sterile insects require the release of only male mosquitoes, as the release of females, even if sterile, would increase the number of biting and potentially disease-transmitting individuals. While small-scale releases demonstrated the applicability of sterile males releases to control the yellow fever mosquito Aedes aegypti, large-scale programs for mosquitoes are currently prevented by the lack of efficient sexing systems in any of the vector species.Different approaches of sexing are pursued, including classical genetic and mechanical methods of sex separation. Another strategy is the development of transgenic sexing systems. Such systems already exist in other insect pests. Genome modification tools could be used to apply similar strategies to mosquitoes. Three major tools to modify mosquito genomes are currently used: transposable elements, site-specific recombination systems, and genome editing via TALEN or CRISPR/Cas. All three can serve the purpose of developing sexing systems and vector control strains in mosquitoes in two ways: first, via their use in basic research. A better understanding of mosquito biology, including the sex-determining pathways and the involved genes can greatly facilitate the development of sexing strains. Moreover, basic research can help to identify other regulatory elements and genes potentially useful for the construction of transgenic sexing systems. Second, these genome modification tools can be used to apply the gained knowledge to build and test mosquito sexing strains for vector control.

Sex determination and Aedes population control

The global economic cost of Aedes-borne diseases, such as dengue, is estimated to be in the billions of dollars annually. In this scenario, a sustained vector control strategy is the only alternative to control dengue, as well as other diseases transmitted by Aedes, including Zika and chikungunya. The use of transgenic mosquitoes is a promising weapon in the improvement of approaches currently applied in Aedes aegypti control. Field trials using genetically modified mosquitoes for population control have been conducted and offer an excellent opportunity to evaluate what can be improved. In a mass-rearing mosquito facility, the absence of a transgenic line that produces male-only progeny is undoubtedly a limiting factor; thus, being able to manipulate sex determination in this species is a fundamental step for the success of this strategy. Likewise, the possibility of manipulation of the sex determination pathway opens-up a new opportunity for disease control.

A perspective on the need and current status of efficient sex separation methods for mosquito genetic control

Major efforts are currently underway to develop novel, complementary methods to combat mosquito-borne diseases. Mosquito genetic control strategies (GCSs) have become an increasingly important area of research on account of their species-specificity, track record in targeting agricultural insect pests, and their environmentally non-polluting nature. A number of programs targeting Aedes and Anopheles mosquitoes, vectors of human arboviruses and malaria respectively, are currently being developed or deployed in many parts of the world. Operationally implementing these technologies on a large scale however, beyond proof-of-concept pilot programs, is hampered by the absence of adequate sex separation methods. Sex separation eliminates females in the laboratory from male mosquitoes prior to release. Despite the need for sex separation for the control of mosquitoes, there have been limited efforts in recent years in developing systems that are fit-for-purpose. In this special issue of Parasites and Vectors we report on the progress of the global Coordinated Research Program on “Exploring genetic, molecular, mechanical and behavioural methods for sex separation in mosquitoes” that is led by the Insect Pest Control Subprogramme of the Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture with the specific aim of building efficient sex separation systems for mosquito species. In an effort to overcome current barriers we briefly highlight what we believe are the three main reasons why progress has been so slow in developing appropriate sex separation systems: the availability of methods that are not scalable, the difficulty of building the ideal genetic systems and, finally, the lack of research efforts in this area.

Isolation and characterization of a temperature-sensitive lethal strain of Anopheles arabiensis for SIT-based application

Background

Malaria is still a global health problem and vector control is the cornerstone of disease control strategies using indoor residual insecticide spraying (IRS) and insecticide-treated nets. The situation is becoming acute with widespread resistance to the limited arsenal of available insecticide classes. Therefore, new and innovative tools to reduce Plasmodium transmission are in need and this situation raised considerable interest in using sterile insect technique (SIT) against human pest insects, particularly Anopheles malaria vectors. When considering a mosquito release programme, one of the first issues to be addressed is how to eliminate/separate the hematophagous vector females. In this paper, we report the development and evaluation of an Anopheles arabiensis temperature-sensitive lethal (tsl) strain towards its potential use for the development of a genetic sexing strain (GSS).

Anopheles arabiensis male mosquitoes originated from North Cameroon were treated with 0.05% ethyl methanesulfonate (EMS). The mutagen was added to 10% sucrose solution and mosquitoes fed ad-libitum for 24h and 48h on a cotton wool soaked with sucrose-mutagen solution placed at the top of the cup. Treated males were then allowed to mate with wild virgin females and their progenies were screened for heat sensitivity from F3 to F8, until the isolation and establishment of a tsl strain which was further characterized by assessing its productivity (fecundity and fertility), larval development, adult longevity as well as nature and inheritance pattern of the tsl.

Results

Observations showed that the number of eggs laid and their hatch rate were similar between females that mated with mutagenized males and those from the control suggesting that mutagenesis didn't affect An. arabiensis male fecundity and fertility. At F3, fourteen isofemale families out of 88 tested, which showed mortalities ranged between 50 and 80 %, were selected as lines potentially containing a tsl mutation. From F4 to F8, a tsl strain was isolated and established by screening L1 larvae at 41°C for 3 hours. This strain showed similar life history traits compared to the wild type strain in terms of fertility, larval development time and adult's emergence. Crossing experiments to further assess the nature and inheritance pattern of the tsl phenotype showed that it is due to a recessive allele located on an autosome.

Conclusion

The successful establishment of the An. arabiensis tsl strain is a valuable tool towards the development of a GSS for SIT applications against this species. This will be done by induction of a Y-autosome translocation to link the wild-type allele to the Y chromosome in such manner that males are wild-type (temperature resistant) and females are mutant (temperature sensitive).

Exploring protandry and pupal size selection for Aedes albopictus sex separation

BACKGROUND

We explored the possibility to improve male/female separation (sexing) in Aedes albopictus by selection of two strains, one toward increasing sex dimorphism and another toward increasing protandry. In the laboratory we selected and crossed small males with large females to exploit dimorphism, and early pupating males with late pupating females to exploit protandry.

RESULTS

While selection for enhanced dimorphism was not a profitable character, the selection for enhanced protandry up to F₁₀ produced significant improvement in the time interval between male and female pupation. By collecting the pupae at 24 h from the beginning of pupation, without any sieving operation, we obtained about 28.50% of pupae (calculated in relation to the estimated initial number of first instar larvae used), vs 26.49% we had in the control strain, and, more interestingly, when checking the sex ratio of these pupae we observed a presence of females of 0.92% vs 23.02% in the control strain. We also modified our egg hatching protocol from the previous standard procedure that required keeping the eggs in the glass hatching container overnight (for about 14-16 h) to a new protocol where eggs are kept in the hatching container for 4 h in order to obtain more synchronized larvae. This was possible without any reduction in the egg hatching rate.

CONCLUSIONS

In Aedes albopictus it is possible to develop hyper-protandric strains useful to produce male pupae without applying other sexing systems. This represents a considerable achievement assisting the Sterile Insect Technique application, allowing improvement of the current sexing method based on mechanical separation. More investigations are under way in order to further enhance the male productivity capacity of the strain and to determine whether the selection has any impact on the male fitness parameters.

A review on the progress of sex-separation techniques for sterile insect technique applications against Anopheles arabiensis

The feasibility of the sterile insect technique (SIT) as a malaria vector control strategy against Anopheles arabiensis has been under investigation over the past decade. One of the critical steps required for the application of this technique to mosquito control is the availability of an efficient and effective sex-separation system. Sex-separation systems eliminate female mosquitoes from the production line prior to irradiation and field release of sterile males. This is necessary because female mosquitoes can transmit pathogens such as malaria and, therefore, their release must be prevented. Sex separation also increases the efficiency of an SIT programme. Various sex-separation strategies have been explored including the exploitation of developmental and behavioural differences between male and female mosquitoes, and genetic approaches. Most of these are however species-specific and are not indicated for the major African malaria vectors such as An. arabiensis. As there is currently no reliable sex-separation method for An. arabiensis, various strategies were explored in an attempt to develop a robust system that can be applied on a mass-rearing scale. The progress and challenges faced during the development of a sexing system for future pilot and/or large-scale SIT release programmes against An. arabiensis are reviewed here. Three methods of sex separation were examined. The first is the use of pupal size for gender prediction. The second is the elimination of blood-feeding adult females through the addition of an endectocide to a blood meal source. The third is the establishment of a genetic sexing strain (GSS) carrying an insecticide resistance selectable marker (dieldrin-resistance rdl gene and/or other GABA receptor antagonists that can be used as alternative insecticides to dieldrin) or a temperature-sensitive lethal marker.

Exploring the potential of computer vision analysis of pupae size dimorphism for adaptive sex sorting systems of various vector mosquito species

Background

Several mosquito population suppression strategies based on the rearing and release of sterile males have provided promising results. However, the lack of an efficient male selection method has hampered the expansion of these approaches into large-scale operational programmes. Currently, most of these programmes targeting Aedes mosquitoes rely on sorting methods based on the sexual size dimorphism (SSD) at the pupal stage. The currently available sorting methods have not been developed based on biometric analysis, and there is therefore potential for improvement. We applied an automated pupal size estimator developed by Grupo Tragsa with laboratory samples of Anopheles arabiensis, Aedes albopictus, Ae. polynesiensis, and three strains of Ae. aegypti. The frequency distribution of the pupal size was analyzed. We propose a general model for the analysis of the frequency distribution of mosquito pupae in the context of SSD-sorting methods, which is based on a Gaussian mixture distribution functions, thus making possible the analysis of performance (% males recovery) and purity (% males on the sorted sample).

Results

For the three Aedes species, the distribution of the pupae size can be modeled by a mixture of two Gaussian distribution functions and the proposed model fitted the experimental data. For a given population, each size threshold is linked to a specific outcome of male recovery. Two dimensionless parameters that measure the suitability for SSD-based sorting of a specific batch of pupae are provided. The optimal sorting results are predicted for the highest values of SSD and lowest values of intra-batch variance. Rearing conditions have a strong influence in the performance of the SSD-sorting methods and non-standard rearing can lead to increase pupae size heterogeneity.

Conclusions

Sex sorting of pupae based on size dimorphism can be achieved with a high performance (% males recovery) and a reasonably high purity (% males on the sorted sample) for the different Aedes species and strains. The purity and performance of a sex sorting operation in the tested Aedes species are linked parameters whose relation can be modeled. The conclusions of this analysis are applicable to all the existing SSD-sorting methods. The efficiency of the SSD-sorting methods can be improved by reducing the heterogeneity of pupae size within rearing containers. The heterogeneity between batches does not strongly affect the quality of the sex sorting, as long as a specific separation threshold is not pre-set before the sorting process. For new developments, we recommend using adaptive and precise threshold selection methods applied individually to each batch or to a mix of batches. Adaptive and precise thresholds will allow the sex-sorting of mixed batches in operational conditions maintaining the target purity at the cost of a reduction in performance. We also recommend a strategy whereby an acceptable level of purity is pre-selected and remains constant across the different batches of pupae while the performance varies from batch to batch to fit with the desired purity.

Impact of irradiation on the reproductive traits of field and laboratory An. arabiensis mosquitoes

Background

The sterile insect technique (SIT) aims at suppressing or decreasing insect pest populations by introducing sterile insects into wild populations. SIT requires the mass-production of insects and their sterilization through, for example, radiation. However, both mass-rearing and radiation can affect the life history traits of insects making them less competitive than their wild counterparts. In the malaria mosquito Anopheles arabiensis, some progress has been made to improve the mating competitiveness of mass-reared irradiated males. However, to date, no study has explored the relative effects of colonization and irradiation on important reproductive traits in this species. Such data may help to focus research efforts more precisely to improve current techniques.

Methods

Two strains of An. arabiensis originating from the same locality were used: one reared in the laboratory for five generations and the second collected as late larval instars in the field prior to experimentation. Pupae were irradiated with 95 Gy and some adult reproductive traits, including insemination rate, fecundity, oviposition behavior, fertility and male survivorship, were assessed in different mating combinations.

Results

Our study revealed the different effects of mosquito strain and irradiation on reproductive processes. The insemination rate was higher in field (67.3%) than in laboratory (54.9%) females and was negatively affected by both female and male irradiation (un-irradiated vs irradiated: 70.2 vs 51.3% in females; 67.7 vs 53.7% in males). Irradiated females did not produce eggs and egg prevalence was lower in the field strain (75.4%) than in the laboratory strain (83.9%). The hatching rate was higher in the field strain (88.7%) than in the laboratory strain (70.6%) as well as in un-irradiated mosquitoes (96.5%) than in irradiated ones (49%). Larval viability was higher in the field strain (96.2%) than in the laboratory strain (78.5%) and in un-irradiated mosquitoes (97.6%) than irradiated ones (52%). Finally, field males lived longer than laboratory males (25.1 vs 20.5 days, respectively).

Conclusions

Our results revealed that both irradiation and colonization alter reproductive traits. However, different developmental stages are not equally affected. It is necessary to consider as many fitness traits as possible to evaluate the efficacy of the sterile insect technique.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-3228-3) contains supplementary material, which is available to authorized users.

The sequence of a male-specific genome region containing the sex determination switch in Aedes aegypti

Background

Aedes aegypti is the principal vector of several important arboviruses. Among the methods of vector control to limit transmission of disease are genetic strategies that involve the release of sterile or genetically modified non-biting males, which has generated interest in manipulating mosquito sex ratios. Sex determination in Ae. aegypti is controlled by a non-recombining Y chromosome-like region called the M locus, yet characterisation of this locus has been thwarted by the repetitive nature of the genome. In 2015, an M locus gene named Nix was identified that displays the qualities of a sex determination switch.

Results

With the use of a whole-genome bacterial artificial chromosome (BAC) library, we amplified and sequenced a ~200 kb region containing the male-determining gene Nix. In this study, we show that Nix is comprised of two exons separated by a 99 kb intron primarily composed of repetitive DNA, especially transposable elements.

Conclusions

Nix, an unusually large and highly repetitive gene, exhibits features in common with Y chromosome genes in other organisms. We speculate that the lack of recombination at the M locus has allowed the expansion of repeats in a manner characteristic of a sex-limited chromosome, in accordance with proposed models of sex chromosome evolution in insects.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-3090-3) contains supplementary material, which is available to authorized users.