A nucleopolyhedrovirus from Uranotaenia sapphirina (Diptera: Culicidae)

Impact of different diets on the survival, pupation, and adult emergence of Culex pipiens biotype molestus larvae, and infectability with the insect-specific Culex Y virus

The current rapidly advancing climate change will affect the transmission of arthropod-borne viruses (arboviruses), mainly through changes in vector populations. Mosquitos of the Culex pipiens complex play a particularly prominent role in virus transmission in central Europe. Factors that contribute to the vector population density and the ability of those vectors to transmit viral pathogens (vector competence) can include nutrition during the larval stages. To test the influence of larval diet on larval survival and adult emergence, as well as vector competence, several diets varying in their nutritional composition were compared using a newly established assay. We tested the effects of 17 diets or diet combinations on the fitness of third-instar larvae of Culex pipiens biotype molestus. Larval survival rates at day 7 ranged from 43.33% to 94.44%. We then selected 3 of the 17 diets (Tetra Pleco, as the routine feed; JBL NovoTab, as the significantly inferior feed; and KG, as the significantly superior feed) and tested the effect of these diets, in combination with Culex Y virus infection, on larval survival rate. All Culex Y virus-infected larvae showed significantly lower larval survival, as well as low pupation and adult emergence rates. However, none of the tested diets in our study had a significant impact on larval survival in combination with viral infection. Furthermore, we were able to correlate several water quality parameters, such as phosphate, nitrate, and ammonium concentration, electrical conductivity, and low O2 saturations, with reduced larval survival. Thus, we were able to demonstrate that Culex Y virus could be a suitable agent to reduce mosquito population density by reducing larval density, pupation rate, and adult emergence rate. When combined with certain water quality parameters, these effects can be further enhanced, leading to a reduced mosquito population density, and reduce the cycle of transmission. Furthermore, we demonstrate, for the first time, the infection of larvae of the mosquito Culex pipiens biotype molestus with a viral pathogen.

Identification of Three Novel Genes in Phenuiviridae Detected from Aedes Mosquitoes in Hokkaido, Japan

Mosquitoes are important arthropod vectors of arboviruses. The family Phenuiviridae includes several medically important arboviruses, such as the Rift Valley fever phlebovirus and Toscana phlebovirus. Recent comprehensive genetic analyses have identified many novel mosquito-specific viruses that are phylogenetically related to Phenuiviridae. We collected mosquitoes from Hokkaido in northern Japan, and conducted reverse transcription polymerase chain reactions (RT-PCRs) targeting the RNA-dependent RNA polymerase (RdRp) gene of Phenuiviridae. A total of 285 pools, comprising 3,082 mosquitoes from 2 genera and 8 species, were collected. Partial RdRp sequences were detected in 97 pools, which allowed us to classify the viruses into 3 clusters provisionally designated as Etutanne virus (ETTV) 1, 2, and 3. The virus most closely related to ETTVs is Narangue virus (family Phenuiviridae, genus Mobuvirus), which was detected in Mansonia mosquitoes; the nucleotide and amino acid sequences of the Narangue virus are 58.4-66.2% and 64.7-86.7% similar, respectively, to those of ETTVs. PCR and RT-PCR using DNA and RNase digestion methods showed that the ETTVs are RNA viruses that do not form non-retroviral integrated RNA virus sequences in the mosquito genome.

Chemical Treatments for Insect Cell Differentiation: The Effects of 20-Hydroxyecdysone and Veratridine on Cultured Spodoptera frugiperda (Sf21) Insect Cell Ultrastructure

Simple Summary

Cultured insect Sf21 cells treated with the hormone 20-hydroxyecdysone grow long processes and resemble neurons. They also make physical contact with one another and appear to have the potential to form synapses, areas in which nerve cells are in close contact and communicate with one another electrically or by the release of chemical transmitters. This study uses electron microscopy to look for structural evidence of synapses in 20-hydroxyexdysone treated Sf21 cell cultures. Unfortunately, no evidence of synaptic structures were observed, suggesting that other factors are required for the formation of functional synapses in these cultures.

Abstract

Previous studies have shown that insect cell cultures stop dividing, form clumps, and can be induced to grow processes reminiscent of axons, when the culture medium is supplemented with 20-hydroxyecdysone, insulin, or an agent that mimics their action, such as the ecdysone agonist, methoxyfenozide. Those cell growing processes resemble nerve cells, and the present study evaluates the ultrastructure of these cultures by transmission electron microscopy. Sf21 cells treated with 20-hydroxyecdysone (with or without veratridine amendment) and subjected to ultrastructural analysis had a similar somatic appearance to control cells, with slight changes in organelles and organization, such as a greater number of cytoplasmic vacuoles and mitochondrial granules. Finger-like projections were observed between control and treated cells. However, no structural markers of synaptic contacts (e.g., vesicles or synaptic thickenings) were observed in controls, 20-hydroxyecdysone, or 20-hydroxyecdysone + veratridine treated cells. It is concluded that additional agents would be required to induce functional synaptogenesis in Sf21 cells.

Symbiotic Interactions Between Mosquitoes and Mosquito Viruses

Mosquitoes not only transmit human and veterinary pathogens called arboviruses (arthropod-borne viruses) but also harbor mosquito-associated insect-specific viruses (mosquito viruses) that cannot infect vertebrates. In the past, studies investigating mosquito viruses mainly focused on highly pathogenic interactions that were easier to detect than those without visible symptoms. However, the recent advances in viral metagenomics have highlighted the abundance and diversity of viruses which do not generate mass mortality in host populations. Over the last decade, this has facilitated the rapid growth of virus discovery in mosquitoes. The circumstances around the discovery of mosquito viruses greatly affected how they have been studied so far. While earlier research mainly focused on the pathogenesis caused by DNA and some double-stranded RNA viruses during larval stages, more recently discovered single-stranded RNA mosquito viruses were heavily studied for their putative interference with arboviruses in female adults. Thus, many aspects of mosquito virus interactions with their hosts and host-microbiota are still unknown. In this context, considering mosquito viruses as endosymbionts can help to identify novel research areas, in particular in relation to their long-term interactions with their hosts (e.g. relationships during all life stages, the stability of the associations at evolutionary scales, transmission routes and virulence evolution) and the possible context-dependent range of interactions (i.e. beneficial to antagonistic). Here, we review the symbiotic interactions of mosquito viruses considering different aspects of their ecology, such as transmission, host specificity, host immune system and interactions with other symbionts within the host cellular arena. Finally, we highlight related research gaps in mosquito virus research.

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.

Mosquito-Specific Viruses-Transmission and Interaction

Mosquito-specific viruses (MSVs) are a subset of insect-specific viruses that are found to infect mosquitoes or mosquito derived cells. There has been an increase in discoveries of novel MSVs in recent years. This has expanded our understanding of viral diversity and evolution but has also sparked questions concerning the transmission of these viruses and interactions with their hosts and its microbiome. In fact, there is already evidence that MSVs interact with the immune system of their host. This is especially interesting, since mosquitoes can be infected with both MSVs and arthropod-borne (arbo) viruses of public health concern. In this review, we give an update on the different MSVs discovered so far and describe current data on their transmission and interaction with the mosquito immune system as well as the effect MSVs could have on an arboviruses-co-infection. Lastly, we discuss potential uses of these viruses, including vector and transmission control.

Urban and semi-urban mosquitoes of Mexico City: A risk for endemic mosquito-borne disease transmission

Since past century, vector-borne diseases have been a major public health concern in several states of Mexico. However, Mexico City continues to be free of endemic mosquito-borne viral diseases. The city is the most important politic and economic state of Mexico and one of the most important city of Latin America. Its subtropical highland climate and high elevation (2240 masl) had historically made the occurrence of Aedes species unlikely. However, the presence of other potential disease vectors (Culex spp, Culiseta spp), and the current intermittent introductions of Aedes aegypti, have revealed that control programs must adopt routine vector surveillance in the city. In this study, we provide an updated species list from a five-years of vector surveillance performed in Mexico City. A total of 18,553 mosquito larvae were collected. Twenty-two species from genus Culex, Aedes, Culiseta, Anopheles, Lutzia and Uranotaenia were observed. Nine new mosquito records for the city were found. Ae. albopictus was recorded for the first time in Mexico City. Interestingly, a new record, Ae. epactius was the most frequent species reported. Cx. pipiens quinquefasciatus exhibited the highest number of individuals collected. We detected six areas which harbor the highest mosquito species records in the city. Cemeteries included 68.9% of our collection sites. Temporarily ponds showed the highest species diversity. We detected an increasing presence of Ae. aegypti, which was detected for three consecutive years (2015–2017), predominantly in the warmer microclimates of the city. We found a possible correlation between increasing temperature and Ae. aegypti and Ae. albopictus expanding range. This study provides a starting point for developing strategies related to environmental management for mosquito control. The promotion of mosquito control practices through community participation, mass media and education programmes in schools should be introduced in the city.

Biological and molecular studies of a cypovirus from the black fly Simulium ubiquitum (Diptera: Simuliidae)

A cypovirus from the black fly Simulium ubiquitum (SuCPV) was isolated and examined using biological and molecular techniques. SuCPV produces small (typically 0.25mum), polyhedral shaped inclusion bodies (polyhedra), in which the virus particles become multiply embedded. SuCPV is the third cypovirus isolated from Diptera, but the first from Simuliidae that has been characterized using molecular analyses. SuCPV has a genome composed of 10 segments of dsRNA, with an electrophoretic migration pattern that is different from those of recent UsCPV-17 and CrCPV-17 isolates from the mosquitoes Uranotaenia sapphirina and Culex restuans, respectively. The SuCPV electropherotype appears to show significant differences from those of the previously characterized lepidopteran cypoviruses. Sequence analysis of SuCPV segment 10 shows that it is unrelated to either of the two CPV isolates from Diptera or to the CPV species for which Seg-10 has been previously characterized from Lepidoptera. A comparison of the terminal regions of SuCPV genome segments to those of CPV-1, 2, 4, 5 14, 15, 16, 17, 18, and 19 also revealed only low levels of conservation. We therefore, propose that SuCPV is classified within a new Cypovirus species, which we have tentatively identified as Cypovirus-20. We have therefore referred to this virus isolate as S. ubiquitum CPV-20 (SuCPV-20).

Mosquito pathogenic viruses--the last 20 years

There are several types of viral pathogens that cause disease in mosquitoes with most belonging to 4 major groups. The most common viruses of mosquitoes are the baculoviruses (NPVs) (Baculoviridae: Nucleopolyhedrovirus) and cytoplasmic polyhedrosis viruses (CPVs) (Reoviridae: Cypovirus). The other major types of viruses in mosquitoes are represented by the densoviruses (DNVs) (Parvoviridae: Brevidensovirus) and the iridoviruses (MIVs) (Iridoviridae: Chloriridovirus). Baculoviruses, densoviruses and iridoviruses are DNA viruses while cypoviruses are the main RNA viruses in mosquitoes. This chapter presents an overview of the recent advancements in the study of mosquito pathogenic viruses and discusses how this new understanding of virus-mosquito interactions can be used to develop novel research and control strategies.

Prospects for the mosquito baculovirus CuniNPV as a tool for mosquito control

CuniNPV is a pathogen of Culex mosquitoes, vectors of West Nile virus and other forms of encephalitis. Successful development of CuniNPV requires an efficient production system and formulated product that incorporates magnesium, an essential component for transmission. It may be possible to develop mosquito baculoviruses as a new type of biopesticide by microencapsulating the virus and magnesium into formulations that would be effective regardless of the water quality. In addition, this new insight on transmission may facilitate the discovery and development of additional baculoviruses for the control of other important mosquito vectors. Biological mining of the CuniNPV genome and investigations to understand virus-mosquito interactions at the molecular level offer exciting possibilities for the development of novel mosquito control strategies and tools. Understanding the molecular mechanisms of infection will provide the opportunity to devise new control strategies, for example, compromising the defensive systems of the mosquito (proteases for the peritrophic matrix) or exploiting receptors used by the virus to specifically deliver toxins to mosquito larvae via the midgut. As additional baculovirus genomes become available, comparative genomics could lead to a more informed understanding of how the virus exploits its host as well as the factors responsible for the genus-specific host range of most known mosquito baculoviruses.

Transmission of viruses to mosquito larvae mediated by divalent cations

The two major groups of pathogenic viruses in mosquitoes are the occluded viruses, represented by baculoviruses and cypoviruses, and the non-occluded viruses, represented by the densoviruses and the iridoviruses. Baculoviruses, densoviruses, and iridoviruses are DNA viruses, while cypoviruses are the major group of RNA viruses reported from mosquitoes. Research on mosquito pathogenic viruses has been limited, in part, due to the inability to effectively transmit them to the larval mosquito host. Recently, there have been tremendous advancements in the ability to transmit mosquito baculoviruses and cypoviruses with the finding that transmission is mediated by divalent cations. Oral transmission of both baculoviruses and cypoviruses to mosquito larvae is enhanced by magnesium and inhibited by calcium ions. The current status of transmission for each of the major groups is reviewed with emphasis on the common role of divalent cations in transmission of the distantly related baculoviruses and cypoviruses.

Molecular and biological characterization of a Cypovirus from the mosquito Culex restuans

A cypovirus from the mosquito Culex restuans (named CrCPV) was isolated and its biology, morphology, and molecular characteristics were investigated. CrCPV is characterized by small (0.1-1.0 microm), irregularly shaped inclusion bodies that are multiply embedded. Laboratory studies demonstrated that divalent cations influenced transmission of CrCPV to Culex quinquefasciatus larvae; magnesium enhanced CrCPV transmission by approximately 30% while calcium inhibited transmission. CrCPV is the second cypovirus from a mosquito that has been confirmed by using molecular analysis. CrCPV has a genome composed of 10 dsRNA segments with an electropherotype similar to the recently discovered UsCPV-17 from the mosquito Uranotaenia sapphirina, but distinct from the lepidopteran cypoviruses BmCPV-1 (Bombyx mori) and TnCPV-15 (Trichoplusia ni). Nucleotide and deduced amino acid sequence analysis of CrCPV segment 10 (polyhedrin) suggests that CrCPV is closely related (83% nucleotide sequence identity and 87% amino acid sequence identity) to the newly characterized UsCPV-17 but is unrelated to the 16 remaining CPV species from lepidopteran hosts. A comparison of the terminal segment regions of CrCPV and UsCPV-17, an additional method for differentiating various Cypovirus species, revealed a high level of conservation. Therefore, we propose that CrCPV is a member of the Cypovirus-17 group and designate this species as CrCPV-17.

Morphological and molecular characterization of a Cypovirus (Reoviridae) from the mosquito Uranotaenia sapphirina (Diptera: Culicidae)

A novel cypovirus has been isolated from the mosquito Uranotaenia sapphirina (UsCPV) and shown to cause a chronic infection confined to the cytoplasm of epithelial cells of the gastric ceca and posterior stomach. The production of large numbers of virions and inclusion bodies and their arrangement into paracrystalline arrays gives the gut of infected insects a distinctive blue iridescence. The virions, which were examined by electron microscopy, are icosahedral (55 to 65 nm in diameter) with a central core that is surrounded by a single capsid layer. They are usually packaged individually within cubic inclusion bodies (polyhedra, approximately 100 nm across), although two to eight virus particles were sometimes occluded together. The virus was experimentally transmitted per os to several mosquito species. The transmission rate was enhanced by the presence of magnesium ions but was inhibited by calcium ions. Most of the infected larvae survived to adulthood, and the adults retained the infection. Electrophoretic analysis of the UsCPV genome segments (using 1% agarose gels) generated a migration pattern (electropherotype) that is different from those of the 16 Cypovirus species already recognized. UsCPV genome segment 10 (Seg-10) showed no significant nucleotide sequence similarity to the corresponding segment of the other cypoviruses that have previously been analyzed, and it has different "conserved" termini. A BLAST search of the UsCPV deduced amino acid sequence also showed little similarity to Antheraea mylitta CPV-4 (67 of 290 [23%]) or Choristoneura fumiferana CPV-16 (33 of 111 [29%]). We conclude that UsCPV should be recognized as a member of a new Cypovirus species (Cypovirus 17, strain UsCPV-17).