Optimized CRISPR tools and site-directed transgenesis towards gene drive development in Culex quinquefasciatus mosquitoes

Genome-wide identification of yellow gene family in Hermetia illucens and functional analysis of yellow-y by CRISPR/Cas9

The yellow gene family plays a crucial role in insect pigmentation. It has potential for use as a visible marker gene in genetic manipulation and transgenic engineering in several model and non-model insects. Sadly, yellow genes have rarely been identified in Stratiomyidae species and the functions of yellow genes are relatively unknown. In the present study, we first manually annotated and curated 10 yellow genes in the black soldier fly (BSF), Hermetia illucens (Stratiomyidae). Then, the conserved amino acids in the major royal jelly proteins (MRJPs) domain, structural architecture and phylogenetic relationship of yellow genes in BSF were analyzed. We found that the BSF yellow-y, yellow-c and yellow-f genes are expressed at all developmental stages, especially in the prepupal stage. Using the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) system, we successfully disrupted yellow-y, yellow-c and yellow-f in the BSF. Consequently, the mutation of yellow-y clearly resulted in a pale-yellow body color in prepupae, pupae and adults, instead of the typical black body color of the wild type. However, the mutation of yellow-c or yellow-f genes did not result in any change in color of the insects, when compared with the wild type. Our study indicates that the BSF yellow-y gene plays a role in body pigmentation, providing an optimal marker gene for the genetic manipulation of BSF.

Identification and functional analysis of Cochliomyia hominivorax U6 gene promoters

The New World screwworm, Cochliomyia hominivorax, is an obligate parasite, which is a major pest of livestock. While the sterile insect technique was used very successfully to eradicate C. hominivorax from North and Central America, more cost-effective genetic methods will likely be needed in South America. The recent development of CRISPR/Cas9-based genetic approaches, such as homing gene drive, could provide a very efficient means for the suppression of C. hominivorax populations. One component of a drive system is the guide RNA(s) driven by a U6 gene promoter. Here, we have developed an in vivo assay to evaluate the activity of the promoters from seven C. hominivorax U6 genes. Embryos from the related blowfly Lucilia cuprina were injected with plasmid DNA containing a U6-promoter-guide RNA construct and a source of Cas9, either protein or plasmid DNA. Activity was assessed by the number of site-specific mutations in the targeted gene in hatched larvae. One promoter, Chom U6_b, showed the highest activity. These U6 gene promoters could be used to build CRISPR/Cas9-based genetic systems for the control of C. hominivorax.

CRISPR-based gene drives generate super-Mendelian inheritance in the disease vector Culex quinquefasciatus

Culex mosquitoes pose a significant public health threat as vectors for a variety of diseases including West Nile virus and lymphatic filariasis, and transmit pathogens threatening livestock, companion animals, and endangered birds. Rampant insecticide resistance makes controlling these mosquitoes challenging and necessitates the development of new control strategies. Gene drive technologies have made significant progress in other mosquito species, although similar advances have been lagging in Culex. Here we test a CRISPR-based homing gene drive for Culex quinquefasciatus, and show that the inheritance of two split-gene-drive transgenes, targeting different loci, are biased in the presence of a Cas9-expressing transgene although with modest efficiencies. Our findings extend the list of disease vectors where engineered homing gene drives have been demonstrated to include Culex alongside Anopheles and Aedes, and pave the way for future development of these technologies to control Culex mosquitoes.

Next-generation CRISPR gene-drive systems using Cas12a nuclease

One method for reducing the impact of vector-borne diseases is through the use of CRISPR-based gene drives, which manipulate insect populations due to their ability to rapidly propagate desired genetic traits into a target population. However, all current gene drives employ a Cas9 nuclease that is constitutively active, impeding our control over their propagation abilities and limiting the generation of alternative gene drive arrangements. Yet, other nucleases such as the temperature sensitive Cas12a have not been explored for gene drive designs in insects. To address this, we herein present a proof-of-concept gene-drive system driven by Cas12a that can be regulated via temperature modulation. Furthermore, we combined Cas9 and Cas12a to build double gene drives capable of simultaneously spreading two independent engineered alleles. The development of Cas12a-mediated gene drives provides an innovative option for designing next-generation vector control strategies to combat disease vectors and agricultural pests.

Establishment of highly efficient transgenic system for black soldier fly (Hermetia illucens)

The black soldier fly (BSF), Hermetia illucens, is a promising insect for mitigating solid waste problems as its larvae are able to bioconvert organic waste into valuable biomass. We recently reported a high-quality genome assembly of the BSF; analysis of this genome sequence will further the understanding of insect biology and identify genes that can be manipulated to improve efficiency of bioconversion. To enable genetic manipulation of the BSF, we have established the first transgenic methods for this economically important insect. We cloned and identified the ubiquitous actin5C promoter (Hiactin5C-p3k) and 3 endogenous U6 promoters (HiU6:1, HiU6:2, and HiU6:3). The Hiactin5C promoter was used to drive expression of a hyperactive variant of the piggyBac transposase, which exhibited up to 6-fold improvement in transformation rate when compared to the wild-type transposase. Furthermore, we evaluated the 3 HiU6 promoters using this transgenic system. HiU6:1 and HiU6:2 promoters provided the highest knockdown efficiency with RNAi and are thus promising candidates for future Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) development. Overall, our findings provide valuable genetic engineering toolkits for basic research and genetic manipulation of the BSF.

CRISPR-based gene drives generate super-Mendelian inheritance in the disease vector Culex quinquefasciatus

Culex mosquitoes pose a significant public health threat as vectors for a variety of diseases including West Nile virus and lymphatic filariasis, and transmit pathogens threatening livestock, companion animals, and endangered birds. Rampant insecticide resistance makes controlling these mosquitoes challenging and necessitates the development of new control strategies. Gene drive technologies have made significant progress in other mosquito species, although similar advances have been lagging in Culex. Here we test the first CRISPR-based homing gene drive for Culex quinquefasciatus, demonstrating the possibility of using this technology to control Culex mosquitoes. Our results show that the inheritance of two split-gene-drive transgenes, targeting different loci, are biased in the presence of a Cas9-expressing transgene although with modest efficiencies. Our findings extend the list of disease vectors where engineered homing gene drives have been demonstrated to include Culex alongside Anopheles and Aedes, and pave the way for future development of these technologies to control Culex mosquitoes.

Use of Insect Promoters in Genetic Engineering to Control Mosquito-Borne Diseases

Mosquito transgenesis and gene-drive technologies provide the basis for developing promising new tools for vector-borne disease prevention by either suppressing wild mosquito populations or reducing their capacity from transmitting pathogens. Many studies of the regulatory DNA and promoters of genes with robust sex-, tissue- and stage-specific expression profiles have supported the development of new tools and strategies that could bring mosquito-borne diseases under control. Although the list of regulatory elements available is significant, only a limited set of those can reliably drive spatial-temporal expression. Here, we review the advances in our ability to express beneficial and other genes in mosquitoes, and highlight the information needed for the development of new mosquito-control and anti-disease strategies.

A perspective on the expansion of the genetic technologies to support the control of neglected vector-borne diseases and conservation

Genetic-based technologies are emerging as promising tools to support vector population control. Vectors of human malaria and dengue have been the main focus of these development efforts, but in recent years these technologies have become more flexible and adaptable and may therefore have more wide-ranging applications. Culex quinquefasciatus, for example, is the primary vector of avian malaria in Hawaii and other tropical islands. Avian malaria has led to the extinction of numerous native bird species and many native bird species continue to be threatened as climate change is expanding the range of this mosquito. Genetic-based technologies would be ideal to support avian malaria control as they would offer alternatives to interventions that are difficult to implement in natural areas, such as larval source reduction, and limit the need for chemical insecticides, which can harm beneficial species in these natural areas. This mosquito is also an important vector of human diseases, such as West Nile and Saint Louis encephalitis viruses, so genetic-based control efforts for this species could also have a direct impact on human health. This commentary will discuss the current state of development and future needs for genetic-based technologies in lesser studied, but important disease vectors, such as C. quinquefasciatus, and make comparisons to technologies available in more studied vectors. While most current genetic control focuses on human disease, we will address the impact that these technologies could have on both disease and conservation focused vector control efforts and what is needed to prepare these technologies for evaluation in the field. The versatility of genetic-based technologies may result in the development of many important tools to control a variety of vectors that impact human, animal, and ecosystem health.

Optimized In Vitro CRISPR/Cas9 Gene Editing Tool in the West Nile Virus Mosquito Vector, Culex quinquefasciatus

Culex quinquefasciatus mosquitoes are a globally widespread vector of multiple human and animal pathogens, including West Nile virus, Saint Louis encephalitis virus, and lymphatic filariasis. Since the introduction of West Nile virus to the United States in 1999, a cumulative 52,532 cases have been reported to the CDC, including 25,849 (49.2%) neuroinvasive cases and 2456 (5%) deaths. Viral infections elicit immune responses in their mosquito vectors, including the RNA interference (RNAi) pathway considered to be the cornerstone antiviral response in insects. To investigate mosquito host genes involved in pathogen interactions, CRISPR/Cas9-mediated gene-editing can be used for functional studies of mosquito-derived cell lines. Yet, the tools available for the study of Cx. quinquefasciatus-derived (Hsu) cell lines remain largely underdeveloped compared to other mosquito species. In this study, we constructed and characterized a Culex-optimized CRISPR/Cas9 plasmid for use in Hsu cell cultures. By comparing it to the original Drosophila melanogaster CRISPR/Cas9 plasmid, we showed that the Culex-optimized plasmid demonstrated highly efficient editing of the genomic loci of the RNAi proteins Dicer-2 and PIWI4 in Hsu cells. These new tools support our ability to investigate gene targets involved in mosquito antiviral response, and thus the future development of gene-based vector control strategies.

Transcriptional response of individual Hawaiian Culex quinquefasciatus mosquitoes to the avian malaria parasite Plasmodium relictum

Background

Plasmodium parasites that cause bird malaria occur in all continents except Antarctica and are primarily transmitted by mosquitoes in the genus Culex. Culex quinquefasciatus, the mosquito vector of avian malaria in Hawaiʻi, became established in the islands in the 1820s. While the deadly effects of malaria on endemic bird species have been documented for many decades, vector-parasite interactions in avian malaria systems are relatively understudied.

Methods

To evaluate the gene expression response of mosquitoes exposed to a Plasmodium infection intensity known to occur naturally in Hawaiʻi, offspring of wild-collected Hawaiian Cx. quinquefasciatus were fed on a domestic canary infected with a fresh isolate of Plasmodium relictum GRW4 from a wild-caught Hawaiian honeycreeper. Control mosquitoes were fed on an uninfected canary. Transcriptomes of five infected and three uninfected individual mosquitoes were sequenced at each of three stages of the parasite life cycle: 24 h post feeding (hpf) during ookinete invasion; 5 days post feeding (dpf) when oocysts are developing; 10 dpf when sporozoites are released and invade the salivary glands.

Results

Differential gene expression analyses showed that during ookinete invasion (24 hpf), genes related to oxidoreductase activity and galactose catabolism had lower expression levels in infected mosquitoes compared to controls. Oocyst development (5 dpf) was associated with reduced expression of a gene with a predicted innate immune function. At 10 dpf, infected mosquitoes had reduced expression levels of a serine protease inhibitor, and further studies should assess its role as a Plasmodium agonist in C. quinquefasciatus. Overall, the differential gene expression response of Hawaiian Culex exposed to a Plasmodium infection intensity known to occur naturally in Hawaiʻi was low, but more pronounced during ookinete invasion.

Conclusions

This is the first analysis of the transcriptional responses of vectors to malaria parasites in non-mammalian systems. Interestingly, few similarities were found between the response of Culex infected with a bird Plasmodium and those reported in Anopheles infected with human Plasmodium. The relatively small transcriptional changes observed in mosquito genes related to immune response and nutrient metabolism support conclusions of low fitness costs often documented in experimental challenges of Culex with avian Plasmodium.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-022-04271-x.

Natural and Engineered Sex Ratio Distortion in Insects

Insects have evolved highly diverse genetic sex-determination mechanisms and a relatively balanced male to female sex ratio is generally expected. However, selection may shift the optimal sex ratio while meiotic drive and endosymbiont manipulation can result in sex ratio distortion (SRD). Recent advances in sex chromosome genomics and CRISPR/Cas9-mediated genome editing brought significant insights into the molecular regulators of sex determination in an increasing number of insects and provided new ways to engineer SRD. We review these advances and discuss both naturally occurring and engineered SRD in the context of the Anthropocene. We emphasize SRD-mediated biological control of insects to help improve One Health, sustain agriculture, and conserve endangered species.

Cas9-mediated maternal-effect and derived resistance alleles in a gene-drive strain of the African malaria vector mosquito, Anopheles gambiae

CRISPR/Cas9 technologies are important tools for the development of gene-drive systems to modify mosquito vector populations to control the transmission of pathogens that cause diseases such as malaria. However, one of the challenges for current Cas9-based drive systems is their ability to produce drive-resistant alleles resulting from insertions and deletions (indels) caused principally by nonhomologous end-joining following chromosome cleavage. Rapid increases in the frequency of such alleles may impair gene-drive dynamics. We explored the generation of indels in the germline and somatic cells in female gene-drive lineages using a series of selective crosses between a gene-drive line, AgNosCd-1, and wild-type mosquitoes. We find that potential drive-resistant mutant alleles are generated largely during embryonic development, most likely caused by deposition of the Cas9 endonuclease and guide RNAs in oocytes and resulting embryos by homozygous and hemizygous gene-drive mothers.

Comparison of Mosquito Larvicidal Formulations of Diflubenzuron on Culex Pipiens Mosquitoes in Belgrade, Serbia

Culex mosquitos are important infectious diseases vectors in temperate and tropical regions of the World, affecting nearly 350 million people in both developed and developing countries. Our approach was to “recycle” the well-established larvicide, and by studying the tablets, pellets and granules as floating or sinking formulations, we found a method to optimise the use of diflubenzuron against Culex pipiens mosquitoes in field conditions.

A standard WHO procedure was used to test the larvicide efficacy. The combined efficacy of all floating formulations was 10.7% higher than sinking preparations (p-value =0.002) and that maximised throughout the experiment on days 14 and 21, (p-values 0.012 and 0.008, respectively). All floating formulations kept their efficacies above 70% until day 21 of the experiment, while the mortality of sinking designs dropped significantly after day 14. The lowest efficacy was observed when sinking granules were used and the highest when floating tablets were applied in the canals. Only the floating tablets showed no significant changes in efficacy from day 1 to 21, but then that efficacy drops sharp until day 35.

Since the larvae spend most of their time on the surface of the slow-moving waters to breathe, and floating pellets and tablets are made of materials that can serve as food sources, we conclude that these formulations have a higher efficacy, at least in conditions of non- or slow-moving waters. This study shows the importance of a systematic approach to reformulation of old, already proven and environmentally safe larvicides which can control the mosquito populations and their spreading of various pathogens.

Beyond the eye: Kynurenine pathway impairment causes midgut homeostasis dysfunction and survival and reproductive costs in blood-feeding mosquitoes

Insect ommochrome biosynthesis pathways metabolize tryptophan to generate eye-color pigments and wild-type alleles of pathway genes are useful phenotypic markers in transgenesis studies. Pleiotropic effects of mutations in some genes exert a load on both survival and reproductive success in blood-feeding species. Here, we investigated the challenges imposed on mosquitoes by the increase of tryptophan metabolites resulting from blood meal digestion and the impact of disruptions of the ommochrome biosynthesis pathway. Female mosquitoes with spontaneous and induced mutations in the orthologs of the genes encoding kynurenine hydroxylase in Aedes aegypti, Anopheles stephensi and Culex quinquefasciatus exhibited impaired survival and reproductive phenotypes that varied in type and severity among the species. A compromised midgut permeability barrier function was also observed in An. stephensi. Surprisingly, mutant mosquitoes displayed an increase in microbiota compared to controls that was not accompanied by a general induction of immune genes. Antibiotic treatment rescued some deleterious traits implicating a role for the kynurenine pathway (KP) in midgut homeostasis. Supplemental xanthurenic acid, a KP end-product, rescued lethality and limited microbiota proliferation in Ae. aegypti. These data implicate the KP in the regulation of the host/microbiota interface. These pleiotropic effects on mosquito physiology are important in the development of genetic strategies targeting vector mosquitoes.

Gene drives gaining speed

Gene drives are selfish genetic elements that are transmitted to progeny at super-Mendelian (>50%) frequencies. Recently developed CRISPR-Cas9-based gene-drive systems are highly efficient in laboratory settings, offering the potential to reduce the prevalence of vector-borne diseases, crop pests and non-native invasive species. However, concerns have been raised regarding the potential unintended impacts of gene-drive systems. This Review summarizes the phenomenal progress in this field, focusing on optimal design features for full-drive elements (drives with linked Cas9 and guide RNA components) that either suppress target mosquito populations or modify them to prevent pathogen transmission, allelic drives for updating genetic elements, mitigating strategies including trans-complementing split-drives and genetic neutralizing elements, and the adaptation of drive technology to other organisms. These scientific advances, combined with ethical and social considerations, will facilitate the transparent and responsible advancement of these technologies towards field implementation.

Bioinformatic and cell-based tools for pooled CRISPR knockout screening in mosquitos

Mosquito-borne diseases present a worldwide public health burden. Current efforts to understand and counteract them have been aided by the use of cultured mosquito cells. Moreover, application in mammalian cells of forward genetic approaches such as CRISPR screens have identified essential genes and genes required for host-pathogen interactions, and in general, aided in functional annotation of genes. An equivalent approach for genetic screening of mosquito cell lines has been lacking. To develop such an approach, we design a new bioinformatic portal for sgRNA library design in several mosquito genomes, engineer mosquito cell lines to express Cas9 and accept sgRNA at scale, and identify optimal promoters for sgRNA expression in several mosquito species. We then optimize a recombination-mediated cassette exchange system to deliver CRISPR sgRNA and perform pooled CRISPR screens in an Anopheles cell line. Altogether, we provide a platform for high-throughput genome-scale screening in cell lines from disease vector species.

EXTRACTION AND PURITY DNA OF Culex spp MOSQUITO IN KEMELAK VILLAGE, BINDUNG LANGIT, OGAN KOMERING ULU

Culex spp are mosquito vectors that have a very wide distribution capability and are carriers of pathogens that can interfere with human and animal health. The wide distribution makes Culex spp a dangerous threat. DNA extraction is one of the important steps in obtaining genetic information and genetic analysis. Good quality DNA is used for activities such as the use of molecular markers, genome library creation, and sequencing. This study aims to determine the quality, concentration and purity of Culex spp mosquito DNA in Kemelak Bindung Langit Village, OKU Regency. It is hoped that the sample can be used for further research analysis on Mitochondria D-Loop Sequences in Culex spp mosquitoes. Quantitative measurement of DNA in the form of concentration and purity of DNA using Nanodrop Thermo cycle while qualitative DNA using electrophoresis technique. The results of the isolation of the mosquito genome DNA, obtained clear DNA bands without any degradation (smear) and the concentration results for the four samples ranged from 10-100 ng/µL and the DNA purity was good, ranging from 1.8 to 2.00.

Suppressed expression of oxidoreductin-like protein, Oxidor, increases follicle degeneration and decreases survival during the overwintering diapause of the mosquito Culex pipiens

Throughout diapause in mosquitoes, stress resistance and subsequent prolonged lifespan are a few important features of diapause that are crucial for overwintering success. In the mosquito Culex pipiens, we suggest that oxidoreductin-like protein is involved with these diapause characteristics for overwintering survival. Expression of oxidor was more than two-fold higher in early stage diapausing females compared to their non-diapausing counterparts. Suppression of the gene that encodes oxidoreductin-like protein by RNAi significantly increased the proportion of degenerating follicles in early-stage adult diapausing females. Inhibition of oxidor also significantly reduced the survivability of diapausing females which indicates that this protein plays a key role in protecting multiple tissues during early diapause.