Exploiting insect-specific viruses as a novel strategy to control vector-borne disease

Predicting novel mosquito-associated viruses from metatranscriptomic dark matter

The exponential growth of metatranscriptomic studies dedicated to arboviral surveillance in mosquitoes has yielded an unprecedented volume of unclassified sequences referred to as the virome dark matter. Mosquito-associated viruses are classified based on their host range into Mosquito-specific viruses (MSV) or Arboviruses. While MSV replication is restricted to mosquito cells, Arboviruses infect both mosquito vectors and vertebrate hosts. We developed the MosViR pipeline designed to identify complex genomic discriminatory patterns for predicting novel MSV or Arboviruses from viral contigs as short as 500 bp. The pipeline combines the predicted probability score from multiple predictive models, ensuring a robust classification with Area Under ROC (AUC) values exceeding 0.99 for test datasets. To assess the practical utility of MosViR in actual cases, we conducted a comprehensive analysis of 24 published mosquito metatranscriptomic datasets. By mining this metatranscriptomic dark matter, we identified 605 novel mosquito-associated viruses, with eight putative novel Arboviruses exhibiting high probability scores. Our findings highlight the limitations of current homology-based identification methods and emphasize the potentially transformative impact of the MosViR pipeline in advancing the classification of mosquito-associated viruses. MosViR offers a powerful and highly accurate tool for arboviral surveillance and for elucidating the complexities of the mosquito RNA virome.

The dichotomy between probiotic lactic acid bacteria and Plasmodium: A promising therapeutic avenue

Our understanding of gut microbial populations and their immense influence on host immunity, health, and diseases has increased deeply in recent years. Numerous reports have identified the role of mosquito and mammalian gut microbiota in the modulation of host susceptibility to Plasmodium infection. Artemisinin resistance in malaria-endemic regions necessitates the development of new, safer, and more affordable treatments to supplement existing therapies. In this review, we compiled a colossal amount of data from numerous studies that have assessed the roles played by gut microbial communities in Plasmodium infection, progression, transmission, and severity. Most interestingly, our study points to the overwhelming evidence from experimental studies in mural malaria to human trials, suggesting that the presence of lactic acid bacteria in the gut microbiota of mammalian hosts provides a great degree of protection against malaria. Therefore, our study provides a compelling narrative for probiotic administration as an adjunct therapy for combatting malaria.

Generating prophylactic immunity against arboviruses in vertebrates and invertebrates

The World Health Organization recently declared a global initiative to control arboviral diseases. These are mainly caused by pathogenic flaviviruses (such as dengue, yellow fever and Zika viruses) and alphaviruses (such as chikungunya and Venezuelan equine encephalitis viruses). Vaccines represent key interventions for these viruses, with licensed human and/or veterinary vaccines being available for several members of both genera. However, a hurdle for the licensing of new vaccines is the epidemic nature of many arboviruses, which presents logistical challenges for phase III efficacy trials. Furthermore, our ability to predict or measure the post-vaccination immune responses that are sufficient for subclinical outcomes post-infection is limited. Given that arboviruses are also subject to control by the immune system of their insect vectors, several approaches are now emerging that aim to augment antiviral immunity in mosquitoes, including Wolbachia infection, transgenic mosquitoes, insect-specific viruses and paratransgenesis. In this Review, we discuss recent advances, current challenges and future prospects in exploiting both vertebrate and invertebrate immune systems for the control of flaviviral and alphaviral diseases.

Highly divergent and diverse viral community infecting sylvatic mosquitoes from Northeast Brazil

Mosquitoes can transmit several pathogenic viruses to humans, but their natural viral community is also composed of a myriad of other viruses such as insect-specific viruses (ISVs) and those that infect symbiotic microorganisms. Besides a growing number of studies investigating the mosquito virome, the majority are focused on few urban species, and relatively little is known about the virome of sylvatic mosquitoes, particularly in high biodiverse biomes such as the Brazilian biomes. Here, we characterized the RNA virome of 10 sylvatic mosquito species from Atlantic forest remains at a sylvatic-urban interface in Northeast Brazil employing a metatranscriptomic approach. A total of 16 viral families were detected. The phylogenetic reconstructions of 14 viral families revealed that the majority of the sequences are putative ISVs. The phylogenetic positioning and, in most cases, the association with a high RNA-dependent RNA polymerase amino acid divergence from other known viruses suggests that the viruses characterized here represent at least 34 new viral species. Therefore, the sylvatic mosquito viral community is predominantly composed of highly divergent viruses highlighting the limited knowledge we still have about the natural virome of mosquitoes in general. Moreover, we found that none of the viruses recovered were shared between the species investigated, and only one showed high identity to a virus detected in a mosquito sampled in Peru, South America. These findings add further in-depth understanding about the interactions and coevolution between mosquitoes and viruses in natural environments.

IMPORTANCE

Mosquitoes are medically important insects as they transmit pathogenic viruses to humans and animals during blood feeding. However, their natural microbiota is also composed of a diverse set of viruses that cause no harm to the insect and other hosts, such as insect-specific viruses. In this study, we characterized the RNA virome of sylvatic mosquitoes from Northeast Brazil using unbiased metatranscriptomic sequencing and in-depth bioinformatic approaches. Our analysis revealed that these mosquitoes species harbor a diverse set of highly divergent viruses, and the majority comprises new viral species. Our findings revealed many new virus lineages characterized for the first time broadening our understanding about the natural interaction between mosquitoes and viruses. Finally, it also provided several complete genomes that warrant further assessment for mosquito and vertebrate host pathogenicity and their potential interference with pathogenic arboviruses.

Genomic characterization of Volzhskoe tick virus (Bunyaviricetes) from a Hyalomma marginatum tick, Hungary

Hyalomma marginatum, a vector for the high-consequence pathogen, the Crimean-Congo hemorrhagic fever virus (CCHFV), needs particular attention due to its impact on public health. Although it is a known vector for CCHFV, its general virome is largely unexplored. Here, we report findings from a citizen science monitoring program aimed to understand the prevalence and diversity of tick-borne pathogens, particularly focusing on Hyalomma ticks in Hungary. In 2021, we identified one adult specimen of Hyalomma marginatum and subjected it to Illumina-based viral metagenomic sequencing. Our analysis revealed sequences of the uncharacterized Volzhskoe tick virus, an unclassified member of the class Bunyaviricetes. The in silico analysis uncovered key genetic regions, including the glycoprotein and the RNA-dependent RNA polymerase (RdRp) coding regions. Phylogenetic analysis indicated a close relationship between our Volzhskoe tick virus sequences and other unclassified Bunyaviricetes species. These related species of unclassified Bunyaviricetes were detected in vastly different geolocations. These findings highlight the remarkable diversity of tick specific viruses and emphasize the need for further research to understand the transmissibility, seroreactivity or the potential pathogenicity of Volzhskoe tick virus and related species.

Sylvatic Mosquito Viromes in the Cerrado Biome of Minas Gerais, Brazil: Discovery of New Viruses and Implications for Arbovirus Transmission

Studies on animal virome have mainly concentrated on chordates and medically significant invertebrates, often overlooking sylvatic mosquitoes, constituting a major part of mosquito species diversity. Despite their potential role in arbovirus transmission, the viromes of sylvatic mosquitoes remain largely unexplored. These mosquitoes may also harbor insect-specific viruses (ISVs), affecting arboviral transmission dynamics. The Cerrado biome, known for rapid deforestation and its status as a biodiversity hotspot, offers an ideal setting for investigating mosquito viromes due to potential zoonotic spillover risks from land use changes. This study aimed to characterize the viromes of sylvatic mosquitoes collected from various locations within Minas Gerais state, Brazil. The total RNA was extracted from mosquito pools of Psorophora albipes, Sabethes albiprivus, Sa. chloropterus, Psorophora ferox, and Coquillettidia venezuelensis species, followed by high-throughput sequencing (HTS). Bioinformatic analysis included quality control, contig assembly, and viral detection. Sequencing data analysis revealed 11 near-complete viral genomes (new viruses are indicated with asterisks) across seven viral families and one unassigned genus. These included: Xinmoviridae (Ferox mosquito mononega-like virus* and Albipes mosquito Gordis-like virus*), Phasmaviridae (Sabethes albiprivus phasmavirus*), Lispiviridae (Pedras lispivirus variant MG), Iflaviridae (Sabethes albiprivus iflavivirus*), Virgaviridae (Buriti virga-like virus variant MG and Sabethes albiprivus virgavirus 1*), Flaviviridae (Psorophora ferox flavivirus*), Mesoniviridae (Alphamesonivirus cavallyense variant MG), and the genus Negevirus (Biggie virus variant MG virus and Coquillettidia venezuelensis negevirus*). Moreover, the presence of ISVs and potential novel arboviruses underscores the need for ongoing surveillance and control strategies to mitigate the risk of emerging infectious diseases.

Discovery of Two Novel Viruses of the Willow-Carrot Aphid, Cavariella aegopodii

The advancement of bioinformatics and sequencing technology has resulted in the identification of an increasing number of new RNA viruses. This study systematically identified the RNA virome of the willow-carrot aphid, Cavariella aegopodii (Hemiptera: Aphididae), using metagenomic sequencing and rapid amplification of cDNA ends (RACE) approaches. C. aegopodii is a sap-sucking insect widely distributed in Europe, Asia, North America, and Australia. The deleterious effects of C. aegopodii on crop growth primarily stem from its feeding activities and its role as a vector for transmitting plant viruses. The virome includes Cavariella aegopodii virga-like virus 1 (CAVLV1) and Cavariella aegopodii iflavirus 1 (CAIV1). Furthermore, the complete genome sequence of CAVLV1 was obtained. Phylogenetically, CAVLV1 is associated with an unclassified branch of the Virgaviridae family and is susceptible to host antiviral RNA interference (RNAi), resulting in the accumulation of a significant number of 22nt virus-derived small interfering RNAs (vsiRNAs). CAIV1, on the other hand, belongs to the Iflaviridae family, with vsiRNAs ranging from 18 to 22 nt. Our findings present a comprehensive analysis of the RNA virome of C. aegopodii for the first time, offering insights that could potentially aid in the future control of the willow-carrot aphid.

Impact of symbiotic insect-specific viruses on mosquito vector competence for arboviruses

Mosquitoes are vectors for arboviruses, such as dengue, Zika, and Chikungunya. Symbiotic interactions can affect the intrinsic ability of mosquitoes to acquire and transmit arboviruses, referred to as vector competence. Insect-specific viruses (ISVs) are commonly found in symbiotic associations with mosquitoes in the wild and can affect many aspects of mosquito biology. Here, we review current knowledge on the effects of symbiotic ISV-mosquito interactions on vector competence. We discuss potential mechanisms underlying these interactions and their implications for shaping new biological control strategies. Finally, we highlight the need for field data analyzing the circulation of ISVs in mosquitoes associated with mechanistic studies in the laboratory.

Complex interactions among insect viruses-insect vector-arboviruses

Insects are the host or vector of diverse viruses including those that infect vertebrates, plants, and fungi. Insect viruses reside inside their insect hosts and are vertically transmitted from parent to offspring. The insect virus-host relationship is intricate, as these viruses can impact various aspects of insect biology, such as development, reproduction, sex ratios, and immunity. Arthropod-borne viruses (arboviruses) that cause substantial global health or agricultural problems can also be vertically transmitted to insect vector progeny. Multiple infections with insect viruses and arboviruses are common in nature. Such coinfections involve complex interactions, including synergism, dependence, and antagonism. Recent studies have shed light on the influence of insect viruses on the competence of insect vectors for arboviruses. In this review, we focus on the biological effects of insect viruses on the transmission of arboviruses by insects. We also discuss the potential mechanisms by which insect viruses affect the ability of hosts to transmit arboviruses, as well as potential strategies for disease control through manipulation of insect viruses. Analyses of the interactions among insect vectors, insect viruses and arboviruses will provide new opportunities for development of innovative strategies to control arbovirus transmission.

Identification and Characterization of Three Novel Solemo-like Viruses in the White-Backed Planthopper, Sogatella furcifera

Agricultural insects play a crucial role in transmitting plant viruses and host a considerable number of insect-specific viruses (ISVs). Among these insects, the white-backed planthoppers (WBPH; Sogatella furcifera, Hemiptera: Delphacidae) are noteworthy rice pests and are responsible for disseminating the southern rice black-streaked dwarf virus (SRBSDV), a significant rice virus. In this study, we analyzed WBPH transcriptome data from public sources and identified three novel viruses. These newly discovered viruses belong to the plant-associated viral family Solemoviridae and were tentatively named Sogatella furcifera solemo-like virus 1-3 (SFSolV1-3). Among them, SFSolV1 exhibited a prevalent existence in different laboratory populations, and its complete genome sequence was obtained using rapid amplification of cDNA ends (RACE) approaches. To investigate the antiviral RNA interference (RNAi) response in WBPH, we conducted an analysis of virus-derived small interfering RNAs (vsiRNAs). The vsiRNAs of SFSolV1 and -2 exhibited typical patterns associated with the host's siRNA-mediated antiviral immunity, with a preference for 21- and 22-nt vsiRNAs derived equally from both the sense and antisense genomic strands. Furthermore, we examined SFSolV1 infection and distribution in WBPH, revealing a significantly higher viral load of SFSolV1 in nymphs' hemolymph compared to other tissues. Additionally, in adult insects, SFSolV1 exhibited higher abundance in male adults than in female adults.

Molecular and biological characterization of a bunyavirus infecting the brown planthopper (Nilaparvata lugens)

A negative-strand symbiotic RNA virus, tentatively named Nilaparvata lugens Bunyavirus (NLBV), was identified in the brown planthopper (BPH, Nilaparvata lugens). Phylogenetic analysis indicated that NLBV is a member of the genus Mobuvirus (family Phenuiviridae, order Bunyavirales). Analysis of virus-derived small interfering RNA suggested that antiviral immunity of BPH was successfully activated by NLBV infection. Tissue-specific investigation showed that NLBV was mainly accumulated in the fat-body of BPH adults. Moreover, NLBV was detected in eggs of viruliferous female BPHs, suggesting the possibility of vertical transmission of NLBV in BPH. Additionally, no significant differences were observed for the biological properties between NLBV-infected and NLBV-free BPHs. Finally, analysis of geographic distribution indicated that NLBV may be prevalent in Southeast Asia. This study provided a comprehensive characterization on the molecular and biological properties of a symbiotic virus in BPH, which will contribute to our understanding of the increasingly discovered RNA viruses in insects.

Negevirus Piura Suppresses Zika Virus Replication in Mosquito Cells

We investigated the interaction between the insect-specific virus, Piura virus (PIUV), and the arbovirus Zika virus (ZIKV) in Aedes albopictus cells. We performed coinfection experiments in C6/36 cells. Piura virus (Cor 33 strain, Colombia) and ZIKV (PRVABC58 strain, Puerto Rico) were co-inoculated into C6/36 cells using two multiplicity of infection (MOI) combinations: 0.1 for both viruses and 1.0 for ZIKV, 0.1 for PIUV. Wells were infected in triplicate with either PIUV and ZIKV coinfection, ZIKV-only, or PIUV-only. Mock infected cells served as control wells. The cell suspension was collected daily 7 days post-infection. Zika virus load was titrated by TCID50 on Vero 76 cells. The ZIKV-only infection and PIUV and ZIKV coinfection experiments were also quantified by RT-qPCR. We also investigated whether ZIKV interfered in the PIUV replication. PIUV suppressed the replication of ZIKV, resulting in a 10,000-fold reduction in ZIKV titers within 3 days post-infection. PIUV viral loads were not reduced in the presence of ZIKV. We conclude that, when concurrently infected, PIUV suppresses ZIKV in C6/36 cells while ZIKV does not interfere in PIUV replication.

Metatranscriptomic Sequencing of Medically Important Mosquitoes Reveals Extensive Diversity of RNA Viruses and Other Microbial Communities in Western Australia

Mosquitoes harbor a wide diversity of microorganisms, including viruses that are human pathogens, or that are insect specific. We used metatranscriptomics, an unbiased high-throughput molecular approach, to describe the composition of viral and other microbial communities in six medically important mosquito species from across Western Australia: Aedes vigilax, Culex annulirostris, Cx. australicus, Cx. globocoxitus, Cx. pipiens biotype molestus, and Cx. quinquefasciatus. We identified 42 viral species, including 13 novel viruses, from 19 families. Culex mosquitoes exhibited a significantly higher diversity of viruses than Aedes mosquitoes, and no virus was shared between the two genera. Comparison of mosquito populations revealed a heterogenous distribution of viruses between geographical regions and between closely related species, suggesting that geography and host species may play a role in shaping virome composition. We also detected bacterial and parasitic microorganisms, among which Wolbachia bacteria were detected in three members of the Cx. pipiens complex, Cx. australicus, Cx. pipiens biotype molestus, and Cx. quinquefasciatus. In summary, our unbiased metatranscriptomics approach provides important insights into viral and other microbial diversity in Western Australian mosquitoes that vector medically important viruses.

Diversity of RNA viruses in agricultural insects

Recent advancements in next-generation sequencing (NGS) technology and bioinformatics tools have revealed a vast array of viral diversity in insects, particularly RNA viruses. However, our current understanding of insect RNA viruses has primarily focused on hematophagous insects due to their medical importance, while research on the viromes of agriculturally relevant insects remains limited. This comprehensive review aims to address the gap by providing an overview of the diversity of RNA viruses in agricultural pests and beneficial insects within the agricultural ecosystem. Based on the NCBI Virus Database, over eight hundred RNA viruses belonging to 39 viral families have been reported in more than three hundred agricultural insect species. These viruses are predominantly found in the insect orders of Hymenoptera, Hemiptera, Thysanoptera, Lepidoptera, Diptera, Coleoptera, and Orthoptera. These findings have significantly enriched our understanding of RNA viral diversity in agricultural insects. While further virome investigations are necessary to expand our knowledge to more insect species, it is crucial to explore the biological roles of these identified RNA viruses within insects in future studies. This review also highlights the limitations and challenges for the effective virus discovery through NGS and their potential solutions, which might facilitate for the development of innovative bioinformatic tools in the future.

Microbiota in disease-transmitting vectors

Haematophagous arthropods, including mosquitoes, ticks, flies, triatomine bugs and lice (here referred to as vectors), are involved in the transmission of various pathogens to mammals on whom they blood feed. The diseases caused by these pathogens, collectively known as vector-borne diseases (VBDs), threaten the health of humans and animals. Although the vector arthropods differ in life histories, feeding behaviour as well as reproductive strategies, they all harbour symbiotic microorganisms, known as microbiota, on which they depend for completing essential aspects of their biology, such as development and reproduction. In this Review, we summarize the shared and unique key features of the symbiotic associations that have been characterized in the major vector taxa. We discuss the crosstalks between microbiota and their arthropod hosts that influence vector metabolism and immune responses relevant for pathogen transmission success, known as vector competence. Finally, we highlight how current knowledge on symbiotic associations is being explored to develop non-chemical-based alternative control methods that aim to reduce vector populations, or reduce vector competence. We conclude by highlighting the remaining knowledge gaps that stand to advance basic and translational aspects of vector-microbiota interactions.

Shotgun Metagenomic Sequencing Reveals Virome Composition of Mosquitoes from a Transition Ecosystem of North-Northeast Brazil

A wide diversity of pathogenic mosquito-borne viruses circulate in the Brazilian Amazon, and the intense deforestation can contribute to the spread of these viruses. In this context, this study aimed to investigate the viral diversity in mosquitoes of the genera Aedes, Culex, Haemagogus, and Sabethes from a transition area between the Amazon, Cerrado, and Caatinga biomes in Brazil. Metagenomic high-throughput sequencing was used to characterize the virome of 20 mosquito pools. A total of 15 virus-like genomes were identified, comprising species genomically close to insect-specific viruses of the families Iflaviridae, Metaviridae, Lispiviridae, Rhabdoviridae, Xinmoviridae, and Parvoviridae and species of plant viruses of the families Solemoviridae, Virgaviridae, and Partitiviridae. However, sequences of viruses associated with human and animal diseases were not detected. Most of the recovered genomes were divergent from those previously described. These findings reveal that there are a large number of unknown viruses to be explored in the middle-north of Brazil.

Structure-Activity Relationships Reveal Beneficial Selectivity Profiles of Inhibitors Targeting Acetylcholinesterase of Disease-Transmitting Mosquitoes

Insecticide resistance jeopardizes the prevention of infectious diseases such as malaria and dengue fever by vector control of disease-transmitting mosquitoes. Effective new insecticidal compounds with minimal adverse effects on humans and the environment are therefore urgently needed. Here, we explore noncovalent inhibitors of the well-validated insecticidal target acetylcholinesterase (AChE) based on a 4-thiazolidinone scaffold. The 4-thiazolidinones inhibit AChE1 from the mosquitoes Anopheles gambiae and Aedes aegypti at low micromolar concentrations. Their selectivity depends primarily on the substitution pattern of the phenyl ring; halogen substituents have complex effects. The compounds also feature a pendant aliphatic amine that was important for activity; little variation of this group is tolerated. Molecular docking studies suggested that the tight selectivity profiles of these compounds are due to competition between two binding sites. Three 4-thiazolidinones tested for in vivo insecticidal activity had similar effects on disease-transmitting mosquitoes despite a 10-fold difference in their in vitro activity.

Arboviruses and symbiotic viruses cooperatively hijack insect sperm-specific proteins for paternal transmission

Arboviruses and symbiotic viruses can be paternally transmitted by male insects to their offspring for long-term viral persistence in nature, but the mechanism remains largely unknown. Here, we identify the sperm-specific serpin protein HongrES1 of leafhopper Recilia dorsalis as a mediator of paternal transmission of the reovirus Rice gall dwarf virus (RGDV) and a previously undescribed symbiotic virus of the Virgaviridae family, Recilia dorsalis filamentous virus (RdFV). We show that HongrES1 mediates the direct binding of virions to leafhopper sperm surfaces and subsequent paternal transmission via interaction with both viral capsid proteins. Direct interaction of viral capsid proteins mediates simultaneously invasion of two viruses into male reproductive organs. Moreover, arbovirus activates HongrES1 expression to suppress the conversion of prophenoloxidase to active phenoloxidase, potentially producing a mild antiviral melanization defense. Paternal virus transmission scarcely affects offspring fitness. These findings provide insights into how different viruses cooperatively hijack insect sperm-specific proteins for paternal transmission without disturbing sperm functions.

Analysing inhibition of dengue virus in Wolbachia-infected mosquito cells following the removal of Wolbachia

Wolbachia pipientis is known to block replication of positive sense RNA viruses. Previously, we created an Aedes aegypti Aag2 cell line (Aag2.wAlbB) transinfected with the wAlbB strain of Wolbachia and a matching tetracycline-cured Aag2.tet cell line. While dengue virus (DENV) was blocked in Aag2.wAlbB cells, we found significant inhibition of DENV in Aag2.tet cells. RNA-Seq analysis of the cells confirmed removal of Wolbachia and lack of expression of Wolbachia genes that could have been due to lateral gene transfer in Aag2.tet cells. However, we noticed a substantial increase in the abundance of phasi charoen-like virus (PCLV) in Aag2.tet cells. When RNAi was used to reduce the PCLV levels, DENV replication was significantly increased. Further, we found significant changes in the expression of antiviral and proviral genes in Aag2.tet cells. Overall, the results reveal an antagonistic interaction between DENV and PCLV and how PCLV-induced changes could contribute to DENV inhibition.

Metatranscriptomic Sequencing Reveals Host Species as an Important Factor Shaping the Mosquito Virome

Mosquitoes are important vector hosts for numerous viral pathogens and harbor a large number of mosquito-specific viruses as well as human-infecting viruses. Previous studies have mainly focused on the discovery of mosquito viruses, and our understanding of major ecological factors associated with virome structure in mosquitoes remains limited. We utilized metatranscriptomic sequencing to characterize the viromes of five mosquito species sampled across eight locations in Yunnan Province, China. This revealed the presence of 52 viral species, of which 19 were novel, belonging to 15 viral families/clades. Of particular note was Culex hepacivirus 1, clustering within the avian clade of hepaciviruses. Notably, both the viromic diversity and abundance of Aedes genus mosquitoes were significantly higher than those of the Culex genus, while Aedes albopictus mosquitoes harbored a higher diversity than Aedes aegypti mosquitoes. Our findings thus point to discernible differences in viromic structure between mosquito genera and even between mosquito species within the same genus. Importantly, such differences were not attributable to differences in sampling between geographical location. Our study also revealed the ubiquitous presence of the endosymbiont bacterium Wolbachia, with the genetic diversity and abundance also varying between mosquito species. In conclusion, our results suggested that the mosquito host species play an important role in shaping the virome's structure. IMPORTANCE This study revealed the huge capability of mosquitoes in harboring a rich diversity of RNA viruses, although relevant studies have characterized the intensively unparalleled diversity of RNA viruses previously. Furthermore, our findings showed discernible differences not only in viromic structure between mosquito genera and even between mosquito species within the same genus but also in the genetic diversity and abundance of Wolbachia between different mosquito populations. These findings emphasize the importance of host genetic background in shaping the virome composition of mosquitoes.

Identifying the Gut Virome of Diaphorina citri from Florida Groves

Asian citrus psyllid (Diaphorina citri) transmits the bacterial pathogen Candidatus Liberibacter asiaticus (CLas), the putative causative agent of citrus Huanglongbing disease (HLB). Insect-specific viruses can act against insects as their natural enemies, and recently, several D. citri-associated viruses were discovered. The insect gut plays an important role as not only a pool for diverse microbes but also as a physical barrier to prevent the spread of pathogens such as CLas. However, there is little evidence of the presence of D. citri-associated viruses in the gut and of the interaction between them and CLas. Here, we dissected psyllid guts collected from five growing regions in Florida, and the gut virome was analyzed by high throughput sequencing. Four insect viruses, including D. citri-associated C virus (DcACV), D. citri densovirus (DcDV), D. citri reovirus (DcRV), and D. citri flavi-like virus (DcFLV), were identified, and their presence in the gut, including an additional D. citri cimodo-like virus (DcCLV), were confirmed with PCR-based assays. Microscopic analysis showed that DcFLV infection leads to morphological abnormalities in the nuclear structure in the infected psyllid gut cells. The complex and diverse composition of microbiota in the psyllid gut suggests a possible interaction and dynamics between CLas and the D. citri-associated viruses. Our study identified various D. citri-associated viruses that localized in the psyllid gut and provided more information that helps to evaluate the potential vectors for manipulating CLas in the psyllid gut.

Composition and global distribution of the mosquito virome - A comprehensive database of insect-specific viruses

Mosquitoes are vectors for emerging and re-emerging infectious viral diseases of humans, livestock and other animals. In addition to these arthropod-borne (arbo)viruses, mosquitoes are host to an array of insect-specific viruses, collectively referred to as the mosquito virome. Mapping the mosquito virome and understanding if and how its composition modulates arbovirus transmission is critical to understand arboviral disease emergence and outbreak dynamics. In recent years, next-generation sequencing as well as PCR and culture-based methods have been extensively used to identify mosquito-associated viruses, providing insights into virus ecology and evolution. Until now, the large amount of mosquito virome data, specifically those acquired by metagenomic sequencing, has not been comprehensively integrated. We have constructed a searchable database of insect-specific viruses associated with vector mosquitoes from 175 studies, published between October 2000 and February 2022. We identify the most frequently detected and widespread viruses of the Culex, Aedes and Anopheles mosquito genera and report their global distribution. In addition, we highlight the challenges of extracting and integrating published virome data and we propose that a standardized reporting format will facilitate data interpretation and re-use by other scientists. We expect our comprehensive database, summarizing mosquito virome data collected over 20 years, to be a useful resource for future studies.

Mosquito vector competence for dengue is modulated by insect-specific viruses

Aedes aegypti and A. albopictus mosquitoes are the main vectors for dengue virus (DENV) and other arboviruses, including Zika virus (ZIKV). Understanding the factors that affect transmission of arboviruses from mosquitoes to humans is a priority because it could inform public health and targeted interventions. Reasoning that interactions among viruses in the vector insect might affect transmission, we analysed the viromes of 815 urban Aedes mosquitoes collected from 12 countries worldwide. Two mosquito-specific viruses, Phasi Charoen-like virus (PCLV) and Humaita Tubiacanga virus (HTV), were the most abundant in A. aegypti worldwide. Spatiotemporal analyses of virus circulation in an endemic urban area revealed a 200% increase in chances of having DENV in wild A. aegypti mosquitoes when both HTV and PCLV were present. Using a mouse model in the laboratory, we showed that the presence of HTV and PCLV increased the ability of mosquitoes to transmit DENV and ZIKV to a vertebrate host. By transcriptomic analysis, we found that in DENV-infected mosquitoes, HTV and PCLV block the downregulation of histone H4, which we identify as an important proviral host factor in vivo.

An RNA Virome Analysis of the Pink-Winged Grasshopper Atractomorpha sinensis

A large number of RNA viruses have been discovered in most insect orders using high-throughput sequencing (HTS) and advanced bioinformatics methods. In this study, an RNA virome of the grasshopper was systematically identified in Atractomorpha sinensis (Orthoptera: Pyrgomorphidae), an important agricultural pest known as the pink-winged grasshopper. These insect viruses were classified as the nege-like virus, iflavirus, ollusvirus, and chu-like virus using HTS and phylogenetic analyses. Meanwhile, the full sequences of four novel RNA viruses were obtained with RACE and named Atractomorpha sinensis nege-like virus 1 (ASNV1), Atractomorpha sinensis iflavirus 1 (ASIV1), Atractomorpha sinensis ollusvirus 1 (ASOV1), and Atractomorpha sinensis chu-like virus 1 (ASCV1), respectively. Moreover, the analysis of virus-derived small interfering RNAs showed that most of the RNA viruses were targeted by the host antiviral RNA interference pathway. Moreover, our results provide a comprehensive analysis on the RNA virome of A. sinensis.

Risk assessment of urban yellow fever virus transmission in Kenya: is Aedes aegypti an efficient vector?

The absence of urban yellow fever epidemics in East Africa remains a mystery amidst the proliferation of Aedes aegypti in this region. To understand the transmission dynamics of the disease, we tested urban (Mombasa, Kisumu, and Nairobi) Aedes mosquito populations in Kenya for their susceptibility to an East African yellow fever virus (YFV) genotype. Overall, 22% (n = 805) of the Ae. aegypti that were orally challenged with an infectious dose of YFV had a midgut infection, with comparable rates for Mombasa and Kisumu (χ2 = 0.35, df = 1, P = 0.55), but significantly lower rates for Nairobi (χ2 ≥ 11.08, df = 1, P ≤ 0.0009). Variations in YFV susceptibility (midgut infection) among Ae. aegypti subspecies were not associated with discernable cytochrome c oxidase subunit 1 gene haplotypes. Remarkably, no YFV dissemination or transmission was observed among the orally challenged Ae. aegypti populations. Moreover, Ae. aegypti mosquitoes that were intrathoracically inoculated with YFV failed to transmit the virus via capillary feeding. In contrast, dissemination (oral exposure) and transmission (intrathoracic inoculation) of YFV was observed among a few peri-domestic Ae. bromeliae mosquitoes (n = 129) that were assessed from these urban areas. Our study highlights an inefficient urban Ae. aegypti population, and the potential for Ae. bromeliae in sustaining an urban YFV transmission in Kenya. An assessment of urban Ae. aegypti susceptibility to other YFV genotypes, and vector potential of urban Ae. bromeliae populations in Kenya is recommended to guide cost-effective vaccination.

Vertical and Horizontal Transmission of Cell Fusing Agent Virus in Aedes aegypti

Cell fusing agent virus (CFAV) is an insect-specific flavivirus (ISF) found in Aedes aegypti mosquitoes. ISFs have demonstrated the ability to modulate the infection or transmission of arboviruses such as dengue, West Nile, and Zika viruses. It is thought that vertical transmission is the main route for ISF maintenance in nature. This has been observed with CFAV, but there is evidence of horizontal and venereal transmission in other ISFs. Understanding the route of transmission can inform strategies to spread ISFs to vector populations as a method of controlling pathogenic arboviruses. We crossed individually reared male and female mosquitoes from both a naturally occurring CFAV-positive Ae. aegypti colony and its negative counterpart to provide information on maternal, paternal, and horizontal transmission. RT-PCR was used to detect CFAV in individual female pupal exuviae and was 89% sensitive, but only 42% in male pupal exuviae. This is a possible way to screen individuals for infection without destroying the adults. Female-to-male horizontal transmission was not observed during this study. However, there was a 31% transmission rate from mating pairs of CFAV-positive males to negative female mosquitoes. Maternal vertical transmission was observed with a filial infection rate of 93%. The rate of paternal transmission was 85% when the female remained negative, 61% when the female acquired CFAV horizontally, and 76% overall. Maternal and paternal transmission of CFAV could allow the introduction of this virus into wild Ae. aegypti populations through male or female mosquito releases, and thus provides a potential strategy for ISF-derived arbovirus control. IMPORTANCE Insect-specific flaviviruses (ISFs), are a group of nonpathogenic flaviviruses that only infect insects. ISFs can have a high prevalence in mosquito populations, but their transmission routes are not well understood. The results of this study confirm maternal transmission of cell fusing agent virus (CFAV) and demonstrate that paternal transmission is also highly efficient. Horizontal transmission of CFAV was also observed, aided by evaluation of the pupal infection status before mating with an infected individual. This technique of detecting infection in discarded pupae exuviae has not been evaluated previously and will be a useful tool for others in the field of studying viral transmission in mosquitoes. Identifying these routes of transmission provides information about how CFAV could be maintained in wild populations of mosquitoes and can aid future studies focusing on interactions of CFAV with their hosts and other viruses that infect mosquitoes.

Aedes aegypti and Ae. albopictus microbiome/virome: new strategies for controlling arboviral transmission?

Aedes aegypti and Aedes albopictus are the main vectors of highly pathogenic viruses for humans, such as dengue (DENV), chikungunya (CHIKV), and Zika (ZIKV), which cause febrile, hemorrhagic, and neurological diseases and remain a major threat to global public health. The high ecological plasticity, opportunistic feeding patterns, and versatility in the use of urban and natural breeding sites of these vectors have favored their dispersal and adaptation in tropical, subtropical, and even temperate zones. Due to the lack of available treatments and vaccines, mosquito population control is the most effective way to prevent arboviral diseases. Resident microorganisms play a crucial role in host fitness by preventing or enhancing its vectorial ability to transmit viral pathogens. High-throughput sequencing and metagenomic analyses have advanced our understanding of the composition and functionality of the microbiota of Aedes spp. Interestingly, shotgun metagenomics studies have established that mosquito vectors harbor a highly conserved virome composed of insect-specific viruses (ISV). Although ISVs are not infectious to vertebrates, they can alter different phases of the arboviral cycle, interfering with transmission to the human host. Therefore, this review focuses on the description of Ae. aegypti and Ae. albopictus as vectors susceptible to infection by viral pathogens, highlighting the role of the microbiota-virome in vectorial competence and its potential in control strategies for new emerging and re-emerging arboviruses.

Local-scale virome depiction in Medellín, Colombia, supports significant differences between Aedes aegypti and Aedes albopictus

Aedes spp. comprise the primary group of mosquitoes that transmit arboviruses such as dengue, Zika, and chikungunya viruses to humans, and thus these insects pose a significant burden on public health worldwide. Advancements in next-generation sequencing and metagenomics have expanded our knowledge on the richness of RNA viruses harbored by arthropods such as Ae. aegypti and Ae. albopictus. Increasing evidence suggests that vector competence can be modified by the microbiome (comprising both bacteriome and virome) of mosquitoes present in endemic zones. Using an RNA-seq-based metataxonomic approach, this study determined the virome structure, Wolbachia presence and mitochondrial diversity of field-caught Ae. aegypti and Ae. albopictus mosquitoes in Medellín, Colombia, a municipality with a high incidence of mosquito-transmitted arboviruses. The two species are sympatric, but their core viromes differed considerably in richness, diversity, and abundance; although the community of viral species identified was large and complex, the viromes were dominated by few virus species. BLAST searches of assembled contigs suggested that at least 17 virus species (16 of which are insect-specific viruses [ISVs]) infect the Ae. aegypti population. Dengue virus 3 was detected in one sample and it was the only pathogenic virus detected. In Ae. albopictus, up to 11 ISVs and one plant virus were detected. Therefore, the virome composition appears to be species-specific. The bacterial endosymbiont Wolbachia was identified in all Ae. albopictus samples and in some Ae. aegypti samples collected after 2017. The presence of Wolbachia sp. in Ae. aegypti was not related to significant changes in the richness, diversity, or abundance of this mosquito’s virome, although it was related to an increase in the abundance of Aedes aegypti To virus 2 (Metaviridae). The mitochondrial diversity of these mosquitoes suggested that the Ae. aegypti population underwent a change that started in the second half of 2017, which coincides with the release of Wolbachia-infected mosquitoes in Medellín, indicating that the population of wMel-infected mosquitoes released has introduced new alleles into the wild Ae. aegypti population of Medellín. However, additional studies are required on the dispersal speed and intergenerational stability of wMel in Medellín and nearby areas as well as on the introgression of genetic variants in the native mosquito population.

Reporter Flaviviruses as Tools to Demonstrate Homologous and Heterologous Superinfection Exclusion

Binjari virus (BinJV) is a lineage II or dual-host affiliated insect-specific flavivirus previously demonstrated as replication-deficient in vertebrate cells. Previous studies have shown that BinJV is tolerant to exchanging its structural proteins (prM-E) with pathogenic flaviviruses, making it a safe backbone for flavivirus vaccines. Here, we report generation by circular polymerase extension reaction of BinJV expressing zsGreen or mCherry fluorescent protein. Recovered BinJV reporter viruses grew to high titres (10⁷⁻⁸ FFU/mL) in Aedes albopictus C6/36 cells assayed using immunoplaque assays (iPA). We also demonstrate that BinJV reporters could be semi-quantified live in vitro using a fluorescence microplate reader with an observed linear correlation between quantified fluorescence of BinJV reporter virus-infected C6/36 cells and iPA-quantitated virus titres. The utility of the BinJV reporter viruses was then examined in homologous and heterologous superinfection exclusion assays. We demonstrate that primary infection of C6/36 cells with BinJV_zsGreen completely inhibits a secondary infection with homologous BinJV_mCherry or heterologous ZIKV_mCherry using fluorescence microscopy and virus quantitation by iPA. Finally, BinJV_zsGreen infections were examined in vivo by microinjection of Aedes aegypti with BinJV_zsGreen. At seven days post-infection, a strong fluorescence in the vicinity of salivary glands was detected in frozen sections. This is the first report on the construction of reporter viruses for lineage II insect-specific flaviviruses and establishes a tractable system for exploring flavivirus superinfection exclusion in vitro and in vivo.

Sexual transmission of Anopheles gambiae densovirus (AgDNV) leads to disseminated infection in mated females

Background

Anopheles gambiae densovirus (AgDNV) is an insect-specific, single-stranded DNA virus that infects An. gambiae sensu stricto (s.s.), the major mosquito species responsible for transmitting malaria parasites throughout sub-Saharan Africa. AgDNV is a benign virus that is very specific to its mosquito host and therefore has the potential to serve as a vector control tool via paratransgenesis (genetic modification of mosquito symbionts) to limit transmission of human pathogens. Prior to being engineered into a control tool, the natural transmission dynamics of AgDNV between An. gambiae mosquitoes needs to be fully understood. Additionally, improved knowledge of AgDNV infection in male mosquitoes is needed. In the study presented here, we examined the tissue tropism of AgDNV in the male reproductive tract and investigated both venereal and vertical transmission dynamics of the virus.

Methods

Anopheles gambiae s.s. adult males were infected with AgDNV via microinjection, and reproductive tissues were collected and assayed for AgDNV using qPCR. Next, uninfected females were introduced to AgDNV-infected or control males and, after several nights of mating, both the spermatheca and female carcass were assessed for venereally transmitted AgDNV. Finally, F1 offspring of this cross were collected and assayed to quantify vertical transmission of the virus.

Results

AgDNV infected the reproductive tract of male mosquitoes, including the testes and male accessory glands, without affecting mating rates. AgDNV-infected males venereally transmitted the virus to females, and these venereally infected females developed disseminated infection throughout the body. However, AgDNV was not vertically transmitted to the F1 offspring of this cross.

Conclusions

Infected male releases could be an effective strategy to introduce AgDNV-based paratransgenic tools into naïve populations of An. gambiae s.s. females.

Graphical Abstract

Characterization of Two Novel Insect-Specific Viruses Discovered in the Green Leafhopper, Cicadella viridis

Simple Summary

Insect-specific viruses (ISVs) have gained increasing attention for their potential use as biological agents. In this study, the full genomes of two ISVs (Cicadella viridis iflavirus 1, CvIfV1; Cicadella viridis nido-like virus 1, CvNiLV1) were revealed in green leafhoppers, using metatranscriptome, RT-PCR, and RACE approaches, respectively. CvIfV1 is a member of iflavirus and has a typical iflavirus genome organization. An antiviral RNA interference (RNAi) was triggered when host insects were challenged with CvIfV1, which resulted in an abundant accumulation of 21-nt virus-derived siRNAs (vsiRNAs). CvNiLV1 clusters are within a distinct unclassified clade of viruses in the order Nidovirales, and evolutionary related with viruses that infect vertebrate hosts. CvNiLV1 was also targeted by the host antiviral RNAi pathway, and generated the 21-nt vsiRNAs with a strong A/U bias in the 5′-terminal. Our study provided valuable information on ISVs in leafhoppers, and might prove useful in pest management.

Abstract

Insect-specific viruses (ISV) are one of the most promising agents for the biological control of insects. The green leafhopper, Cicadella viridis (Linnaeus), is an significant pest in agriculture, and causes economic losses to fruit trees, willows, and field crops. As a representative species of the large family Cicadellidae, ISVs in C. viridis have never been studied, to date. In this study, the full genome sequences of two ISVs, named Cicadella viridis iflavirus1 (CvIfV1), and Cicadella viridis nido-like virus 1 (CvNiLV1), were revealed using a metatranscriptomic approach. A homology search and phylogenetic analysis indicated that CvIfV1 is a new member in the family Iflaviridae (genus Iflavirus) with a typical iflavirus genome organization, whereas CvNiLV1 belongs to the unclassified clade/family of the order Nidovirales. In addition, analysis of virus-derived small interfering RNAs (vsiRNAs) was performed to investigate the antiviral RNA interference (RNAi) response of C. viridis. The vsiRNAs exhibit typical patterns produced by host siRNA-mediated antiviral immunity, including a preference of 21-nt vsiRNAs derived equally from the sense and antisense genomic strands, and a strong A/U bias in the 5′-terminus of the viral genomes. Our study provides valuable information for ISVs in leafhoppers for the first time, which might prove useful in the control of C. viridis in future.

Interspecies microbiome transplantation recapitulates microbial acquisition in mosquitoes

BACKGROUND

Mosquitoes harbor microbial communities that play important roles in their growth, survival, reproduction, and ability to transmit human pathogens. Microbiome transplantation approaches are often used to study host-microbe interactions and identify microbial taxa and assemblages associated with health or disease. However, no such approaches have been developed to manipulate the microbiota of mosquitoes.

RESULTS

Here, we developed an approach to transfer entire microbial communities between mosquito cohorts. We undertook transfers between (Culex quinquefasciatus to Aedes aegypti) and within (Ae. aegypti to Ae. aegypti) species to validate the approach and determine the number of mosquitoes required to prepare donor microbiota. After the transfer, we monitored mosquito development and microbiota dynamics throughout the life cycle. Typical holometabolous lifestyle-related microbiota structures were observed, with higher dynamics of microbial structures in larval stages, including the larval water, and less diversity in adults. Microbiota diversity in recipient adults was also more similar to the microbiota diversity in donor adults.

CONCLUSIONS

This study provides the first evidence for successful microbiome transplantation in mosquitoes. Our results highlight the value of such methods for studying mosquito-microbe interactions and lay the foundation for future studies to elucidate the factors underlying microbiota acquisition, assembly, and function in mosquitoes under controlled conditions. Video Abstract.

Monitoring of synthetic insecticides resistance and mechanisms among malaria vector mosquitoes in Iran: A systematic review

BACKGROUND

In Iran, the prospect of malaria control relies mainly on insecticides used against the genus Anopheles (Diptera: Culicidae) as important vectors of malaria, arboviruses, and so on. Only eight out of 30 malaria mosquito vectors (Anopheles species) have been examined for insecticide resistance in Iran. This study aimed to review articles related to the incremental trend in insecticide resistance and their mechanisms among anopheline malaria vectors in Iran.

METHODS

A literature review was conducted based on such search engines as Iran doc, Web of Science, SID, PubMed, Scopus, and Google Scholar websites using the following keywords: "Anopheles," "Malaria," "Resistance," "Vectors," "Insecticide Resistance," and "Iran" for data collection. Published papers in English or Persian covering 1980 to 2020 were reviewed.

RESULTS

A total of 1125 articles were screened, only 16 of which were filtered to be pertinent in this review. While most of the mosquito vectors of malaria, such as Anopheles stephensi, were resistant to DDT, dieldrin, malathion, and becoming less susceptible to deltamethrin and other synthetic pyrethroid insecticides, few like Anopheles fluviatilis s. l. were susceptible to all insecticides. A disseminating trend in insecticide resistance among different anopheline mosquito vector species was evident. Metabolic and insecticide target-site resistance mechanisms were involved with organochlorines and pyrethroids, respectively.

CONCLUSIONS

Insecticide resistance is becoming a severe scourge to the effectiveness of vector-borne disease management measures. This event is especially critical in developing and marginalized communities that applied chemical-based vector elimination programs for malaria; therefore, it is crucial to monitor insecticide resistance in malaria vectors in Iran using biochemical and molecular tools.

Interactions of the Insect-Specific Palm Creek Virus with Zika and Chikungunya Viruses in Aedes Mosquitoes

Palm Creek virus (PCV) is an insect-specific flavivirus that can interfere with the replication of mosquito-borne flaviviruses in Culex mosquitoes, thereby potentially reducing disease transmission. We examined whether PCV could interfere with arbovirus replication in Aedes (Ae.) aegypti and Ae. albopictus mosquitoes, major vectors for many prominent mosquito-borne viral diseases. We infected laboratory colonies of Ae. aegypti and Ae. albopictus with PCV to evaluate infection dynamics. PCV infection was found to persist to at least 21 days post-infection and could be detected in the midguts and ovaries. We then assayed for PCV-arbovirus interference by orally challenging PCV-infected mosquitoes with Zika and chikungunya viruses. For both arboviruses, PCV infection had no effect on infection and transmission rates, indicating limited potential as a method of intervention for Aedes-transmitted arboviruses. We also explored the hypothesis that PCV-arbovirus interference is mediated by the small interfering RNA pathway in silico. Our findings indicate that RNA interference is unlikely to underlie the mechanism of arbovirus inhibition and emphasise the need for empirical examination of individual pairs of insect-specific viruses and arboviruses to fully understand their impact on arbovirus transmission.

Negeviruses Reduce Replication of Alphaviruses during Coinfection

Negeviruses are a group of insect-specific viruses (ISVs) that have been found in many arthropods. Their presence in important vector species led us to examine their interactions with arboviruses during coinfections. Wild-type negeviruses reduced the replication of several alphaviruses during coinfections in mosquito cells. Negev virus (NEGV) isolates were also used to express green fluorescent protein (GFP) and anti-chikungunya virus (CHIKV) antibody fragments during coinfections with CHIKV. NEGV expressing anti-CHIKV antibody fragments was able to further reduce replication of CHIKV during coinfections, while reductions of CHIKV with NEGV expressing GFP were similar to titers with wild-type NEGV alone. These results are the first to show that negeviruses induce superinfection exclusion of arboviruses and to demonstrate a novel approach to deliver antiviral antibody fragments with paratransgenic ISVs. The ability to inhibit arbovirus replication and express exogenous proteins in mosquito cells makes negeviruses a promising platform for control of arthropod-borne pathogens. IMPORTANCE Negeviruses are a group of insect-specific viruses (ISVs), viruses known to infect only insects. They have been discovered over a wide geographical and species range. Their ability to infect mosquito species that transmit dangerous arboviruses makes negeviruses a candidate for a pathogen control platform. Coinfections of mosquito cells with a negevirus and an alphavirus demonstrated that negeviruses can inhibit the replication of alphaviruses. Additionally, modifying Negev virus (NEGV) to express a fragment of an anti-CHIKV antibody further reduced the replication of CHIKV in coinfected cells. This is the first evidence to demonstrate that negeviruses can inhibit the replication of important arboviruses in mosquito cells. The ability of a modified NEGV to drive the expression of antiviral proteins also highlights a method for negeviruses to target specific pathogens and limit the incidence of vector-borne diseases.

A survey of mosquito-borne and insect-specific viruses in hospitals and livestock markets in western Kenya

Aedes aegypti and Culex pipiens complex mosquitoes are prolific vectors of arboviruses that are a global threat to human and animal health. Increased globalization and ease of travel have facilitated the worldwide dissemination of these mosquitoes and the viruses they transmit. To assess disease risk, we determined the frequency of arboviruses in western Kenyan counties bordering an area of high arboviral activity. In addition to pathogenic viruses, insect-specific flaviviruses (ISFs), some of which are thought to impair the transmission of specific pathogenic arboviruses, were also evaluated. We trapped mosquitoes in the short and long rainy seasons in 2018 and 2019 at livestock markets and hospitals. Mosquitoes were screened for dengue, chikungunya and other human pathogenic arboviruses, ISFs, and their blood-meal sources as determined by high-resolution melting analysis of (RT-)PCR products. Of 6,848 mosquitoes collected, 89% were trapped during the long rainy season, with A. aegypti (59%) and Cx. pipiens sensu lato (40%) being the most abundant. Most blood-fed mosquitoes were Cx. pipiens s.l. with blood-meals from humans, chicken, and sparrow (Passer sp.). We did not detect dengue or chikungunya viruses. However, one Culex poicilipes female was positive for Sindbis virus, 30 pools of Ae. aegypti had cell fusing agent virus (CFAV; infection rate (IR) = 1.27%, 95% CI = 0.87%-1.78%); 11 pools of Ae. aegypti had Aedes flavivirus (AeFV; IR = 0.43%, 95% CI = 0.23%-0.74%); and seven pools of Cx. pipiens s.l. (IR = 0.23%, 95% CI = 0.1%-0.45%) and one pool of Culex annulioris had Culex flavivirus. Sindbis virus, which causes febrile illness in humans, can complicate the diagnosis and prognosis of patients with fever. The presence of Sindbis virus in a single mosquito from a population of mosquitoes with ISFs calls for further investigation into the role ISFs may play in blocking transmission of other arboviruses in this region.

The microbiome and mosquito vectorial capacity: rich potential for discovery and translation

Microbiome research has gained considerable interest due to the emerging evidence of its impact on human and animal health. As in other animals, the gut-associated microbiota of mosquitoes affect host fitness and other phenotypes. It is now well established that microbes can alter pathogen transmission in mosquitoes, either positively or negatively, and avenues are being explored to exploit microbes for vector control. However, less attention has been paid to how microbiota affect phenotypes that impact vectorial capacity. Several mosquito and pathogen components, such as vector density, biting rate, survival, vector competence, and the pathogen extrinsic incubation period all influence pathogen transmission. Recent studies also indicate that mosquito gut-associated microbes can impact each of these components, and therefore ultimately modulate vectorial capacity. Promisingly, this expands the options available to exploit microbes for vector control by also targeting parameters that affect vectorial capacity. However, there are still many knowledge gaps regarding  mosquito-microbe interactions  that need to be addressed in order to exploit them efficiently. Here, we review current evidence of impacts of the microbiome on aspects of vectorial capacity, and we highlight likely opportunities for novel vector control strategies and areas where further studies are required. Video abstract.

Diversity and infectivity of the RNA virome among different cryptic species of an agriculturally important insect vector: whitefly Bemisia tabaci

A large number of insect-specific viruses (ISVs) have recently been discovered, mostly from hematophagous insect vectors because of their medical importance, but little attention has been paid to important plant virus vectors such as the whitefly Bemisia tabaci, which exists as a complex of cryptic species. Public SRA datasets of B. tabaci and newly generated transcriptomes of three Chinese populations are here comprehensively investigated to characterize the whitefly viromes of different cryptic species. Twenty novel ISVs were confidently identified, mostly associated with a particular cryptic species while different cryptic species harbored one or more core ISVs. Microinjection experiments showed that some ISVs might cross-infect between the two invasive whitefly cryptic species, Middle East Asia Minor 1 (MEAM1) and Mediterranean (MED), but others appeared to have a more restricted host range, reflecting the possibility of distinct long-term coevolution of these ISVs and whitefly hosts. Moreover, analysis of the profiles of virus-derived small-interfering RNAs indicated that some of the ISVs can successfully replicate in whitefly and the antiviral RNAi pathway of B. tabaci is actively involved in response to ISV infections. Our study provides a comprehensive analysis of the RNA virome, the distinct relationships and cross-cryptic species infectivity of ISVs in an agriculturally important insect vector.

The virome of vector mosquitoes

Mosquitoes are the major vectors for arthropod-borne viruses (arboviruses) of medical importance. Aedes aegypti and A. albopictus are the most prolific and widespread mosquito vectors being responsible for global transmission of dengue, Zika and Chikungunya viruses. Characterizing the collection of viruses circulating in mosquitoes, the virome, has long been of special interest. In addition to arboviruses, mosquitoes carry insect-specific viruses (ISVs) that do not directly infect vertebrates. Mounting evidence indicates that ISVs interact with arboviruses and may affect mosquito vector competence. Here, we review our current knowledge about the virome of vector mosquitoes and discuss the challenges for the field that may lead to novel strategies to prevent outbreaks of arboviruses.

The Bunyavirales: The Plant-Infecting Counterparts

Negative-strand (-) RNA viruses (NSVs) comprise a large and diverse group of viruses that are generally divided in those with non-segmented and those with segmented genomes. Whereas most NSVs infect animals and humans, the smaller group of the plant-infecting counterparts is expanding, with many causing devastating diseases worldwide, affecting a large number of major bulk and high-value food crops. In 2018, the taxonomy of segmented NSVs faced a major reorganization with the establishment of the order Bunyavirales. This article overviews the major plant viruses that are part of the order, i.e., orthospoviruses (Tospoviridae), tenuiviruses (Phenuiviridae), and emaraviruses (Fimoviridae), and provides updates on the more recent ongoing research. Features shared with the animal-infecting counterparts are mentioned, however, special attention is given to their adaptation to plant hosts and vector transmission, including intra/intercellular trafficking and viral counter defense to antiviral RNAi.

Screening for Viruses and Lemur-Associated Filara in Wild-Caught Mosquitoes From Madagascar

Madagascar is a hotspot of biodiversity, but poverty and population growth provoke a high risk of conflict between food security and biodiversity conservation in this tropical country. Numerous vector-borne diseases, including viral infections, affect public health in Madagascar and a continuous expansion of anthropogenically used areas intensifies contact on the human-wildlife interface. However, data on human and animal pathogens in potential insect vectors is limited. Therefore, we conducted a parasitological and virological survey of 785 adult female mosquitoes between March and May 2016 at the Ankarafantsika National Park in northwestern Madagascar. Screening included Alpha-, Phlebo-, and Flaviviridae and the recently described filarial nematode species, Lemurfilaria lemuris. The predominant mosquito genus was Culex (91%), followed by Mansonia (4.1%), Anopheles (3.4%), and Aedes (0.9%). Viral screening revealed no arboviruses, but an insect-specific flavivirus in two Culex sitiens pools. No pools screened positive for the lemur-specific filarial nematode L. lemuris.

Discovery of a Novel Flavivirus (Flaviviridae) From the Horse Fly, Tabanus rufidens (Diptera: Tabanidae): The Possible Coevolutionary Relationships Between the Classical Insect-Specific Flaviviruses and Host Dipteran Insects

Tabanid flies (Tabanidae: Diptera) are common hematophagous insects known to transmit some pathogens mechanically or biologically to animals; they are widely distributed throughout the world. However, no tabanid-borne viruses, except mechanically transmitted viruses, have been reported to date. In this study, we conducted RNA virome analysis of several human-biting tabanid species in Japan, to discover and characterize viruses associated with tabanids. A novel flavivirus was encountered during the study in the Japanese horse fly, Tabanus rufidens (Bigot, 1887). The virus was detected only in T. rufidens, but not in other tabanid species, and as such was designated Tabanus rufidens flavivirus (TrFV). TrFV could not be isolated using a mammalian cell line and showed a closer phylogenetic relationship to the classical insect-specific flaviviruses (cISFs) rather than the vertebrate-infecting flaviviruses (VIFs), suggesting that it is a novel member of the cISFs. The first discovery of a cISF from Brachycera provides new insight into the evolutionary history and dynamics of flaviviruses.

Diverse mosquito-specific flaviviruses in the Bolivian Amazon basin

The genus Flavivirus includes a range of mosquito-specific viruses in addition to well-known medically important arboviruses. Isolation and comprehensive genomic analyses of viruses in mosquitoes collected in Bolivia resulted in the identification of three novel flavivirus species. Psorophora flavivirus (PSFV) was isolated from Psorophora albigenu. The coding sequence of the PSFV polyprotein shares 60 % identity with that of the Aedes-associated lineage II insect-specific flavivirus (ISF), Marisma virus. Isolated PSFV replicates in both Aedes albopictus- and Aedes aegypti-derived cells, but not in mammalian Vero or BHK-21 cell lines. Two other flaviviruses, Ochlerotatus scapularis flavivirus (OSFV) and Mansonia flavivirus (MAFV), which were identified from Ochlerotatus scapularis and Mansonia titillans, respectively, group with the classical lineage I ISFs. The protein coding sequences of these viruses share only 60 and 40 % identity with the most closely related of known lineage I ISFs, including Xishuangbanna aedes flavivirus and Sabethes flavivirus, respectively. Phylogenetic analysis suggests that MAFV is clearly distinct from the groups of the current known Culicinae-associated lineage I ISFs. Interestingly, the predicted amino acid sequence of the MAFV capsid protein is approximately two times longer than that of any of the other known flaviviruses. Our results indicate that flaviviruses with distinct features can be found at the edge of the Bolivian Amazon basin at sites that are also home to dense populations of human-biting mosquitoes.

Detection of Cell-Fusing Agent virus across ecologically diverse populations of Aedes aegypti on the Caribbean island of Saint Lucia

Background. Outbreaks of mosquito-borne arboviral diseases including dengue virus (DENV), Zika virus (ZIKV), yellow fever virus (YFV) and chikungunya virus (CHIKV) have recently occurred in the Caribbean. The geographical range of the principal vectors responsible for transmission, Aedes (Ae.) aegypti and Ae. albopictus are increasing and greater mosquito surveillance is needed in the Caribbean given international tourism is so prominent. The island of Saint Lucia has seen outbreaks of DENV and CHIKV in the past five years but vector surveillance has been limited with the last studies dating back to the late 1970s. Natural disasters have changed the landscape of Saint Lucia and the island has gone through significant urbanisation. Methods. In this study, we conducted an entomological survey of Ae. aegypti and Ae. albopictus distribution across the island and analysed environmental parameters associated with the presence of these species in addition to screening for medically important arboviruses and other flaviviruses. Results. Although we collected Ae. aegypti across a range of sites across the island, no Ae. albopictus were collected despite traps being placed in diverse ecological settings. The number of Ae. aegypti collected was significantly associated with higher elevation, and semi-urban settings yielded female mosquito counts per trap-day that were five-fold lower than urban settings. Screening for arboviruses revealed a high prevalence of cell-fusing agent virus (CFAV). Conclusions. Outbreaks of arboviruses transmitted by Ae. aegypti and Ae. albopictus have a history of occurring in small tropical islands and Saint Lucia is particularly vulnerable given the limited resources available to undertake vector control and manage outbreaks. Surveillance strategies can identify risk areas for predicting future outbreaks. Further research is needed to determine the diversity of current mosquito species, investigate insect-specific viruses, as well as pathogenic arboviruses, and this should also be extended to the neighbouring smaller Caribbean islands.

Transmission competence of a new mesonivirus, Yichang virus, in mosquitoes and its interference with representative flaviviruses

Advances in technology have greatly stimulated the understanding of insect-specific viruses (ISVs). Unfortunately, most of these findings are based on sequencing technology, and laboratory data are scarce on the transmission dynamics of ISVs in nature and the potential effects of these viruses on arboviruses. Mesonivirus is a class of ISVs with a wide geographical distribution. Recently, our laboratory reported the isolation of a novel strain of mesonivirus, Yichang virus (YCV), from Culex mosquitoes, China. In this study, the experimental infection of YCV by the oral route for adult and larvae mosquitoes, and the vertical transmission has been conducted, which suggests that YCV could adopt a mixed-mode transmission. Controlled experiments showed that the infectivity of YCV depends on the mosquito species, virus dose, and infection route. The proliferation curve and tissue distribution of YCV in Cx. quinquefasciatus and Ae. albopictus showed that YCV is more susceptible to Ae. albopictus and is located in the midgut. Furthermore, we also assessed the interference of YCV with flaviviruses both in vitro and in vivo. YCV significantly inhibited the proliferation of DENV-2 and ZIKV, in cell culture, and reduced transmission rate of DENV-2 in Ae. albopictus. Our work provides insights into the transmission of ISVs in different mosquito species during ontogeny and their potential ability to interact with mosquito-borne viruses.

A Novel Anphevirus in Aedes albopictus Mosquitoes Is Distributed Worldwide and Interacts with the Host RNA Interference Pathway

The Asian tiger mosquito Aedes albopictus is a competent vector for several human arboviruses including dengue, chikungunya and Zika viruses. Mosquitoes also harbor insect-specific viruses (ISVs) that may modulate host physiology and potentially affect the transmission of viruses that are pathogenic to vertebrates, thus representing a potential tool for vector control strategies. In Ae. albopictus we identified a novel anphevirus (family Xinmoviridae; order Mononegavirales) provisionally designated here as Aedes albopictus anphevirus (AealbAV). AealbAV contains a ~12.4 kb genome that is highly divergent from currently known viruses but displays gene content and genomic organization typical of known anpheviruses. We identified AealbAV in several publicly available RNA-Seq datasets from different geographical regions both in laboratory colonies and field collected mosquitoes. Coding-complete genomes of AealbAV strains are highly similar worldwide (>96% nucleotide identity) and cluster according to the geographical origin of their hosts. AealbAV appears to be present in various body compartments and mosquito life stages, including eggs. We further detected AealbAV-derived vsiRNAs and vpiRNAs in publicly available miRNA-Seq libraries of Ae. albopictus and in samples experimentally coinfected with chikungunya virus. This suggests that AealbAV is targeted by the host RNA interference (RNAi) response, consistent with persistent virus replication. The discovery and characterization of AealbAV in Ae. albopictus will now allow us to identify its infection in mosquito populations and laboratory strains, and to assess its potential impact on Ae. albopictus physiology and ability to transmit arboviruses.

Wolbachia modulates prevalence and viral load of Culex pipiens densoviruses in natural populations

The inadequacy of standard mosquito control strategies calls for ecologically safe novel approaches, for example the use of biological agents such as the endosymbiotic α-proteobacteria Wolbachia or insect-specific viruses (ISVs). Understanding the ecological interactions between these "biocontrol endosymbionts" is thus a fundamental step. Wolbachia are transmitted vertically from mother to offspring and modify their hosts' phenotypes, including reproduction (e.g., cytoplasmic incompatibility) and survival (e.g., viral interference). In nature, Culex pipiens (sensu lato) mosquitoes are always found infected with genetically diverse Wolbachia called wPip that belong to five phylogenetic groups. In recent years, ISVs have also been discovered in these mosquito species, although their interactions with Wolbachia in nature are unknown. Here, we studied the interactions between a widely prevalent ISV, the Culex pipiens densovirus (CpDV, Densovirinae), and Wolbachia in northern Tunisian C. pipiens populations. We showed an influence of different Wolbachia groups on CpDV prevalence and a general positive correlation between Wolbachia and CpDV loads. By investigating the putative relationship between CpDV diversification and wPip groups in the different sites, we detected a signal linked to wPip groups in CpDV phylogeny in sites where all larvae were infected by the same wPip group. However, no such signal was detected where the wPip groups coexisted, suggesting CpDV horizontal transfer between hosts. Overall, our results provide good evidence for an ecological influence of Wolbachia on an ISV, CpDV, in natural populations and highlight the importance of integrating Wolbachia in our understanding of ISV ecology in nature.

Microbial interactions in the mosquito gut determine Serratia colonization and blood-feeding propensity

How microbe–microbe interactions dictate microbial complexity in the mosquito gut is unclear. Previously we found that, Serratia, a gut symbiont that alters vector competence and is being considered for vector control, poorly colonized Aedes aegypti yet was abundant in Culex quinquefasciatus reared under identical conditions. To investigate the incompatibility between Serratia and Ae. aegypti, we characterized two distinct strains of Serratia marcescens from Cx. quinquefasciatus and examined their ability to infect Ae. aegypti. Both Serratia strains poorly infected Ae. aegypti, but when microbiome homeostasis was disrupted, the prevalence and titers of Serratia were similar to the infection in its native host. Examination of multiple genetically diverse Ae. aegypti lines found microbial interference to S. marcescens was commonplace, however, one line of Ae. aegypti was susceptible to infection. Microbiome analysis of resistant and susceptible lines indicated an inverse correlation between Enterobacteriaceae bacteria and Serratia, and experimental co-infections in a gnotobiotic system recapitulated the interference phenotype. Furthermore, we observed an effect on host behavior; Serratia exposure to Ae. aegypti disrupted their feeding behavior, and this phenotype was also reliant on interactions with their native microbiota. Our work highlights the complexity of host–microbe interactions and provides evidence that microbial interactions influence mosquito behavior.

Understanding the Mechanisms Underlying Host Restriction of Insect-Specific Viruses

Arthropod-borne viruses contribute significantly to global mortality and morbidity in humans and animals. These viruses are mainly transmitted between susceptible vertebrate hosts by hematophagous arthropod vectors, especially mosquitoes. Recently, there has been substantial attention for a novel group of viruses, referred to as insect-specific viruses (ISVs) which are exclusively maintained in mosquito populations. Recent discoveries of novel insect-specific viruses over the past years generated a great interest not only in their potential use as vaccine and diagnostic platforms but also as novel biological control agents due to their ability to modulate arbovirus transmission. While arboviruses infect both vertebrate and invertebrate hosts, the replication of insect-specific viruses is restricted in vertebrates at multiple stages of virus replication. The vertebrate restriction factors include the genetic elements of ISVs (structural and non-structural genes and the untranslated terminal regions), vertebrate host factors (agonists and antagonists), and the temperature-dependent microenvironment. A better understanding of these bottlenecks is thus warranted. In this review, we explore these factors and the complex interplay between ISVs and their hosts contributing to this host restriction phenomenon.