Differential gene expression related to Nora virus infection of Drosophila melanogaster

Naturally occurring viruses of Drosophila reduce offspring number and lifespan

Drosophila remains a pre-eminent insect model system for host-virus interaction, but the host range and fitness consequences of the drosophilid virome are poorly understood. Metagenomic studies have reported approximately 200 viruses associated with Drosophilidae, but few isolates are available to characterize the Drosophila immune response, and most characterization has relied on injection and systemic infection. Here, we use a more natural infection route to characterize the fitness effects of infection and to study a wider range of viruses. We exposed laboratory Drosophila melanogaster to 23 naturally occurring viruses from wild-collected drosophilids. We recorded transmission rates along with two components of female fitness: survival and the lifetime number of adult offspring produced. Nine different viruses transmitted during contact with laboratory D. melanogaster, although for the majority, rates of transmission were less than 20%. Five virus infections led to a significant decrease in lifespan (D. melanogaster Nora virus, D. immigrans Nora virus, Muthill virus, galbut virus and Prestney Burn virus), and three led to a reduction in the total number of offspring. Our findings demonstrate the utility of the Drosophila model for community-level studies of host-virus interactions, and suggest that viral infection could be a substantial fitness burden on wild flies.

Single domain von Willebrand factor type C "cytokines" and the regulation of the stress/immune response in insects

The single domain von Willebrand factor type C (SVWC) appears in small secreted peptides that are arthropod-specific and are produced following environmental stress or pathogen exposure. Most research has focused on proteins with SVWC domain that are induced after virus infection and are hypothesized to function as "cytokines" to regulate the innate immune response. The expansion of SVWC genes in insect species indicates that many other functions remain to be discovered. Research in shrimp has elucidated the adaptability of Vago-like peptides in the innate immune response against bacteria, fungi and viruses after activation by Jak-STAT and/or Toll/Imd pathways in which they can act as pathogen-recognition receptors or cytokine-like signaling molecules. SVWC factors also appear in scorpion venoms and tick saliva, underlining their versatility to acquire new functions. This review discusses the discovery and function of SVWC peptides from insects to crustaceans and chelicerates and reveals the enormous gaps in knowledge that remain to be filled to understand this enigmatic group of secreted peptides.

Viruses in Laboratory Drosophila and Their Impact on Host Gene Expression

Drosophila melanogaster has one of the best characterized antiviral immune responses among invertebrates. However, relatively few easily transmitted natural virus isolates are available, and so many Drosophila experiments have been performed using artificial infection routes and artificial host-virus combinations. These may not reflect natural infections, especially for subtle phenotypes such as gene expression. Here, to explore the laboratory virus community and to better understand how natural virus infections induce changes in gene expression, we have analysed seven publicly available D. melanogaster transcriptomic sequencing datasets that were originally sequenced for projects unrelated to virus infection. We have found ten known viruses-including five that have not been experimentally isolated-but no previously unknown viruses. Our analysis of host gene expression revealed that numerous genes were differentially expressed in flies that were naturally infected with a virus. For example, flies infected with nora virus showed patterns of gene expression consistent with intestinal vacuolization and possible host repair via the upd3 JAK/STAT pathway. We also found marked sex differences in virus-induced differential gene expression. Our results show that natural virus infection in laboratory Drosophila does indeed induce detectable changes in gene expression, suggesting that this may form an important background condition for experimental studies in the laboratory.

Characterization of Cross-Species Transmission of Drosophila melanogaster Nora Virus

Drosophila melanogaster Nora virus (DmNV) is a novel picorna-like virus first characterized in 2006. Since then, Nora virus has been detected in several non-Drosophila species, including insects in the Orders Hymenoptera, Lepidoptera, Coleoptera, and Orthoptera. The objective of this study was to determine if DmNV could infect individuals of other species of invertebrates besides D. melanogaster. The presence of DmNV in native invertebrates and commercially available stocks was determined. Laboratory-reared D. yakuba, D. mercatorum, Gryllodes sigillatus, Tenebrio molitor, Galleria mellonella, and Musca domestica were intentionally infected with DmNV. In addition, native invertebrates were collected and D. melanogaster stocks were purchased and screened for DmNV presence using reverse transcription-polymerase chain reaction (RT-PCR) before being intentionally infected for study. All Drosophila species and other invertebrates, except M. domestica, that were intentionally infected with DmNV ended up scoring positive for the virus via RT-PCR. DmNV infection was also detected in three native invertebrates (Spilosoma virginica, Diplopoda, and Odontotaenius disjunctus) and all commercially available stocks tested. These findings suggest that DmNV readily infects individuals of other species of invertebrates, while also appearing to be an endemic virus in both wild and laboratory D. melanogaster populations. The detection of DmNV in commercially available stocks presents a cautionary message for scientists using these stocks in studies of virology and immunology.

The Novel Agrotis ipsilon Nora Virus Confers Deleterious Effects to the Fitness of Spodoptera frugiperda (Lepidoptera: Noctuidae)

In the present study, we identified a novel, positive-sense single-stranded RNA virus in the Chinese black cutworm, Agrotis ipsilon. It has a genome length of 11,312 nucleotides, excluding the poly(A) tails, and contains five open reading frames. The ORF2 encodes the conserved domains of RNA helicase and RNA-dependent RNA polymerase, while ORF4 and 5 encode three viral proteins. Herein, the A. ipsilon virus was clustered with a Helicoverpa armigera Nora virus and was thus provisionally named “Agrotis ipsilon Nora virus” (AINV). AINV was successfully transmitted into a novel host, Spodoptera frugiperda, through injection, causing a stable infection. This found the possibility of horizontal AINV transmission among moths belonging to the same taxonomic family. Nonetheless, AINV infection was deleterious to S. frugiperda and mainly mediated by antiviral and amino acid metabolism-related pathways. Furthermore, the infection significantly increased the S. frugiperda larval period but significantly reduced its moth eclosion rate. It suggests that AINV is probably to be a parasitic virus of S. frugiperda.

The potential use of Drosophila as an in vivo model organism for COVID-19-related research: a review

The world urgently needs effective antiviral approaches against emerging viruses, as shown by the coronavirus disease 2019 (COVID-19) pandemic, which has become an exponentially growing health crisis. Scientists from diverse backgrounds have directed their efforts towards identifying key features of SARS-CoV-2 and clinical manifestations of COVID-19 infection. Reports of more transmissible variants of SARS-CoV-2 also raise concerns over the possibility of an explosive trajectory of the pandemic, so scientific attention should focus on developing new weapons to help win the fight against coronaviruses that may undergo further mutations in the future. Drosophila melanogaster offers a powerful and potential in vivo model that can significantly increase the efficiency of drug screening for viral and bacterial infections. Thanks to its genes with functional human homologs, Drosophila could play a significant role in such gene-editing studies geared towards designing vaccines and antiviral drugs for COVID-19. It can also help rectify current drawbacks of CRISPR-based therapeutics like off-target effects and delivery issues, representing another momentous step forward in healthcare. Here I present an overview of recent literature and the current state of knowledge, explaining how it can open up new avenues for Drosophila in our battle against infectious diseases.

Heterogeneity in the Response of Different Subtypes of Drosophila melanogaster Midgut Cells to Viral Infections

Single-cell RNA sequencing (scRNA-seq) offers the possibility to monitor both host and pathogens transcriptomes at the cellular level. Here, public scRNA-seq datasets from Drosophila melanogaster midgut cells were used to compare the differences in replication strategy and cellular responses between two fly picorna-like viruses, Thika virus (TV) and D. melanogaster Nora virus (DMelNV). TV exhibited lower levels of viral RNA accumulation but infected a higher number of cells compared to DMelNV. In both cases, viral RNA accumulation varied according to cell subtype. The cellular heat shock response to TV and DMelNV infection was cell-subtype- and virus-specific. Disruption of bottleneck genes at later stages of infection in the systemic response, as well as of translation-related genes in the cellular response to DMelNV in two cell subtypes, may affect the virus replication.

Effect of Nora virus infection on native gut bacterial communities of Drosophila melanogaster

Gastrointestinal microflora is a key component in the maintenance of health and longevity across many species. In humans and mice, nonpathogenic viruses present in the gastrointestinal tract enhance the effects of the native bacterial microbiota. However, it is unclear whether nonpathogenic gastrointestinal viruses, such as Nora virus that infects Drosophila melanogaster, lead to similar observations. Longevity analysis of Nora virus infected (NV+) and uninfected (NV-) D. melanogaster in relationship to presence (B+) or absence (B-) of the native gut bacteria using four different treatment groups, NV+/B+, NV+/B-, NV-/B+, and NV-/B-, was conducted. Data from the longevity results were tested via Kaplan-Meier analysis and demonstrated that Nora virus can be detrimental to the longevity of the organism, whereas bacterial presence is beneficial. These data led to the hypothesis that gastrointestinal bacterial composition varies from NV+ to NV- flies. To test this, NV+ and NV- virgin female flies were collected and aged for 4 days. Surface sterilization followed by dissections of the fat body and the gastrointestinal tract, divided into crop (foregut), midgut, and hindgut, were performed. Ribosomal 16S DNA samples were sequenced to determine the bacterial communities that comprise the microflora in the gastrointestinal tract of NV+ and NV- D. melanogaster. When analyzing operational taxonomic units (OTUs), the data demonstrate that the NV+ samples consist of more OTUs than NV- samples. The NV+ samples were both more rich and diverse in OTUs compared to NV-. When comparing whole body samples to specific organs and organ sections, the whole fly was more diverse in OTUs, whereas the crop was the most rich. These novel data are pertinent in describing where Nora virus infection may be occurring within the gastrointestinal tract, as well as continuing discussion between the relationship of persistent viral and bacterial interaction.

Transcriptomic identification and characterization of genes commonly responding to sublethal concentrations of six different insecticides in the common fruit fly, Drosophila melanogaster

Pretreatment with sublethal concentrations (LC₁₀) of six insecticides (chlorantraniliprole, cypermethrin, dinotefuran, indoxacarb, ivermectin, and spinosad) significantly elevated tolerance of the common fruit fly Drosophila melanogaster to lethal concentration of the respective insecticide. Commonly responding genes to sublethal treatments of the six insecticides were identified by transcriptome analysis based on a fold change >1.5 or < -1.5, and p < 0.05 as selection criteria. Following treatment with all the six insecticides, 26 transcripts were commonly over-transcribed, whereas 30 transcripts were commonly under-transcribed. Reliability of the transcriptome data was confirmed by quantitative PCR. A majority of the over-transcribed genes included those related to olfactory behavior, such as odorant-binding proteins, as well as immune-related genes, including attacin, diptericin, and immune-induced molecule 18. In contrast, genes belonging to the mitochondrial respiratory chain, such as mitochondrial NADH-ubiquinone oxidoreductase chain 1/3/4/5 and mitochondrial cytochrome b/c, were commonly under-transcribed. Furthermore, genes related to eggshell formation and motion were also under-transcribed, which may indicate a possible energy trade-off for xenobiotic stress. In summary, most of the differentially expressed genes were not directly related to well-known detoxification genes, suggesting that the roles of commonly expressed tolerance-related genes are not likely related to direct metabolic detoxification, but rather are associated with restoration of homeostasis.

Proteomic Polyphenism in Color Morphotypes of Diaphorina citri, Insect Vector of Citrus Greening Disease

Diaphorina citri is a vector of "Candidatus Liberibacter asiaticus" (CLas), associated with citrus greening disease. D. citri exhibit at least two color morphotypes, blue and non-blue, the latter including gray and yellow morphs. Blue morphs have a greater capacity for long-distance flight and transmit CLas less efficiently as compared to non-blue morphs. Differences in physiology and immunity between color morphs of the insect vector may influence disease epidemiology and biological control strategies. We evaluated the effect of CLas infection on color morph and sex-specific proteomic profiles of D. citri. Immunity-associated proteins were more abundant in blue morphs as compared to non-blue morphs but were upregulated at a higher magnitude in response to CLas infection in non-blue insects. To test for differences in color morph immunity, we measured two phenotypes: (1) survival of D. citri when challenged with the entomopathogenic fungus Beauveria bassiana and (2) microbial load of the surface and internal microbial communities. Non-blue color morphs showed higher mortality at four doses of B. bassinana, but no differences in microbial load were observed. Thus, color morph polyphenism is associated with two distinct proteomic immunity phenotypes in D. citri that may impact transmission of CLas and resistance to B. bassiana under some conditions.

Structure of Nora virus at 2.7 Å resolution and implications for receptor binding, capsid stability and taxonomy

Nora virus, a virus of Drosophila, encapsidates one of the largest single-stranded RNA virus genomes known. Its taxonomic affinity is uncertain as it has a picornavirus-like cassette of enzymes for virus replication, but the capsid structure was at the time for genome publication unknown. By solving the structure of the virus, and through sequence comparison, we clear up this taxonomic ambiguity in the invertebrate RNA virosphere. Despite the lack of detectable similarity in the amino acid sequences, the 2.7 Å resolution cryoEM map showed Nora virus to have T = 1 symmetry with the characteristic capsid protein β-barrels found in all the viruses in the Picornavirales order. Strikingly, α-helical bundles formed from the extended C-termini of capsid protein VP4B and VP4C protrude from the capsid surface. They are similar to signalling molecule folds and implicated in virus entry. Unlike other viruses of Picornavirales, no intra-pentamer stabilizing annulus was seen, instead the intra-pentamer stability comes from the interaction of VP4C and VP4B N-termini. Finally, intertwining of the N-termini of two-fold symmetry-related VP4A capsid proteins and RNA, provides inter-pentamer stability. Based on its distinct structural elements and the genetic distance to other picorna-like viruses we propose that Nora virus, and a small group of related viruses, should have its own family within the order Picornavirales.

Nora Virus VP4b and ORF1 Circulate in Hemolymph of Infected D. melanogaster with Coordinate Expression of Vago and Vir-1

Study of the novel RNA virus, Nora virus, which is a persistent, picorna-like virus that replicates in the gut of Drosophila melanogaster offers insight into human innate immunity and other picorna-like viruses. Nora virus infection leads to a locomotor abnormality and upregulation of two candidate target proteins, Vago and Virus-induced RNA 1 (Vir-1). These proteins are uncharacterized in response to Nora virus. We hypothesize that Nora virus is circulating in the hemolymph of Nora virus-infected D. melanogaster, allowing for migration beyond the primary site of replication in the gut. Analysis by qRT-PCR demonstrated biphasic viral load and corresponding vago and vir-1 transcription levels, suggesting transcription of vago and vir-1 occurs in response to viral infection. However, Vir-1 is also present in virus-free D. melanogaster suggesting basal expression or alternative functions. Presence of Nora virus RNA and the Viral Protein 4b (VP4b), in hemolymph of infected D. melanogaster supports the hypothesized circulation of Nora virus in the hemolymph. The study suggests that impaired locomotor function may be due to transport of Nora virus from the gut to the brain via the hemolymph.

Impaired Geotaxis as a Novel Phenotype of Nora Virus Infection of Drosophila melanogaster

Nora virus (NV) is a picorna-like virus that contains a positive-sense, single-stranded RNA genome. The virus infects Drosophila melanogaster with no known characterized phenotype. In this study, geotaxis assays and longevity analyses were used to determine if Nora virus infection affects D. melanogaster's locomotor ability. In addition, Drosophila C virus (DCV), a well-characterized D. melanogaster virus, was used as a positive control, as it has previously shown a locomotor defect in infected flies. Stocks infected with NV (NV+) and DCV (DCV+) and virus-free (NV-/DCV-) stocks were established. Over a 3-year period, approximately 2,500 virgin female flies were tested for geotaxis and longevity using Kaplan-Meier analyses, as well as the Cox Proportional Hazards regression for survivorship. There was a significant decrease in the geotaxis when the D. melanogaster flies were infected with Nora virus compared to uninfected controls, but no difference was found between DCV+ and NV+ trials. There were not significant differences in longevity between the three groups. This is the first time that a phenotype has been recorded in association with Nora virus infection. Overall, the data demonstrate that geotaxis dysfunction may be a phenotypic hallmark of Nora virus infection.

Single-cell RNA sequencing identifies novel cell types in Drosophila blood

Drosophila has been extensively used to model the human blood-immune system, as both systems share many developmental and immune response mechanisms. However, while many human blood cell types have been identified, only three were found in flies: plasmatocytes, crystal cells and lamellocytes. To better understand the complexity of fly blood system, we used single-cell RNA sequencing technology to generate comprehensive gene expression profiles for Drosophila circulating blood cells. In addition to the known cell types, we identified two new Drosophila blood cell types: thanacytes and primocytes. Thanacytes, which express many stimulus response genes, are involved in distinct responses to different types of bacteria. Primocytes, which express cell fate commitment and signaling genes, appear to be involved in keeping stem cells in the circulating blood. Furthermore, our data revealed four novel plasmatocyte subtypes (Ppn⁺, CAH7⁺, Lsp⁺ and reservoir plasmatocytes), each with unique molecular identities and distinct predicted functions. We also identified cross-species markers from Drosophila hemocytes to human blood cells. Our analysis unveiled a more complex Drosophila blood system and broadened the scope of using Drosophila to model human blood system in development and disease.

An insight into the sialome, mialome and virome of the horn fly, Haematobia irritans

BACKGROUND

The horn fly (Haematobia irritans) is an obligate blood feeder that causes considerable economic losses in livestock industries worldwide. The control of this cattle pest is mainly based on insecticides; unfortunately, in many regions, horn flies have developed resistance. Vaccines or biological control have been proposed as alternative control methods, but the available information about the biology or physiology of this parasite is rather scarce.

RESULTS

We present a comprehensive description of the salivary and midgut transcriptomes of the horn fly (Haematobia irritans), using deep sequencing achieved by the Illumina protocol, as well as exploring the virome of this fly. Comparison of the two transcriptomes allow for identification of uniquely salivary or uniquely midgut transcripts, as identified by statistically differential transcript expression at a level of 16 x or more. In addition, we provide genomic highlights and phylogenetic insights of Haematobia irritans Nora virus and present evidence of a novel densovirus, both associated to midgut libraries of H. irritans.

CONCLUSIONS

We provide a catalog of protein sequences associated with the salivary glands and midgut of the horn fly that will be useful for vaccine design. Additionally, we discover two midgut-associated viruses that infect these flies in nature. Future studies should address the prevalence, biological effects and life cycles of these viruses, which could eventually lead to translational work oriented to the control of this economically important cattle pest.

Conservation of the behavioral and transcriptional response to social experience among Drosophilids

While social experience has been shown to significantly alter behaviors in a wide range of species, comparative studies that uniformly measure the impact of a single experience across multiple species have been lacking, limiting our understanding of how plastic traits evolve. To address this, we quantified variations in social feeding behaviors across 10 species of Drosophilids, tested the effect of altering rearing context on these behaviors (reared in groups or in isolation) and correlated observed behavioral shifts to accompanying transcriptional changes in the heads of these flies. We observed significant variability in the extent of aggressiveness, the utilization of social cues during food search, and social space preferences across species. The sensitivity of these behaviors to rearing experience also varied: socially naive flies were more aggressive than their socialized conspecifics in some species, and more reserved or identical in others. Despite these differences, the mechanism of socialization appeared to be conserved within the melanogaster subgroup as species could cross‐socialize each other, and the transcriptional response to social exposure was significantly conserved. The expression levels of chemosensory‐perception genes often varied between species and rearing conditions, supporting a growing body of evidence that behavioral evolution is driven by the differential regulation of this class of genes. The clear differences in behavioral responses to socialization observed in Drosophilids make this an ideal system for continued studies on the genetic basis and evolution of socialization and behavioral plasticity.

Analysis of immune-related genes during Nora virus infection of Drosophila melanogaster using next generation sequencing

Drosophila melanogaster depends upon the innate immune system to regulate and combat viral infection. This is a complex, yet widely conserved process that involves a number of immune pathways and gene interactions. In addition, expression of genes involved in immunity are differentially regulated as the organism ages. This is particularly true for viruses that demonstrate chronic infection, as is seen with Nora virus. Nora virus is a persistent non-pathogenic virus that replicates in a horizontal manner in D. melanogaster. The genes involved in the regulation of the immune response to Nora virus infection are largely unknown. In addition, the temporal response of immune response genes as a result of infection has not been examined. In this study, D. melanogaster either infected with Nora virus or left uninfected were aged for 2, 10, 20 and 30 days. The RNA from these samples was analyzed by next generation sequencing (NGS) and the resulting immune-related genes evaluated by utilizing both the PANTHER and DAVID databases, as well as comparison to lists of immune related genes and FlyBase. The data demonstrate that Nora virus infected D. melanogaster exhibit an increase in immune related gene expression over time. In addition, at day 30, the data demonstrate that a persistent immune response may occur leading to an upregulation of specific immune response genes. These results demonstrate the utility of NGS in determining the potential immune system genes involved in Nora virus replication, chronic infection and involvement of antiviral pathways.

Complex Coding and Regulatory Polymorphisms in a Restriction Factor Determine the Susceptibility of Drosophila to Viral Infection

It is common to find that major-effect genes are an important cause of variation in susceptibility to infection. Here we have characterized natural variation in a gene called pastrel that explains over half of the genetic variance in susceptibility to the Drosophila C virus (DCV) in populations of Drosophila melanogaster We found extensive allelic heterogeneity, with a sample of seven alleles of pastrel from around the world conferring four phenotypically distinct levels of resistance. By modifying candidate SNPs in transgenic flies, we show that the largest effect is caused by an amino acid polymorphism that arose when an ancestral threonine was mutated to alanine, greatly increasing resistance to DCV. Overexpression of the ancestral, susceptible allele provides strong protection against DCV; indicating that this mutation acted to improve an existing restriction factor. The pastrel locus also contains complex structural variation and cis-regulatory polymorphisms altering gene expression. We find that higher expression of pastrel is associated with increased survival after DCV infection. To understand why this variation is maintained in populations, we investigated genetic variation surrounding the amino acid variant that is causing flies to be resistant. We found no evidence of natural selection causing either recent changes in allele frequency or geographical variation in frequency, suggesting that this is an old polymorphism that has been maintained at a stable frequency. Overall, our data demonstrate how complex genetic variation at a single locus can control susceptibility to a virulent natural pathogen.

Advances in Myeloid-Like Cell Origins and Functions in the Model Organism Drosophila melanogaster

Drosophila has long served as a valuable model for deciphering many biological processes, including immune responses. Indeed, the genetic tractability of this organism is particularly suited for large-scale analyses. Studies performed during the last 3 decades have proven that the signaling pathways that regulate the innate immune response are conserved between Drosophila and mammals. This review summarizes the recent advances on Drosophila hematopoiesis and immune cellular responses, with a particular emphasis on phagocytosis.

A genome-wide scan for genes under balancing selection in Drosophila melanogaster

BACKGROUND

In the history of population genetics balancing selection has been considered as an important evolutionary force, yet until today little is known about its abundance and its effect on patterns of genetic diversity. Several well-known examples of balancing selection have been reported from humans, mice, plants, and parasites. However, only very few systematic studies have been carried out to detect genes under balancing selection. We performed a genome scan in Drosophila melanogaster to find signatures of balancing selection in a derived (European) and an ancestral (African) population. We screened a total of 34 genomes searching for regions of high genetic diversity and an excess of SNPs with intermediate frequency.

RESULTS

In total, we found 183 candidate genes: 141 in the European population and 45 in the African one, with only three genes shared between both populations. Most differences between both populations were observed on the X chromosome, though this might be partly due to false positives. Functionally, we find an overrepresentation of genes involved in neuronal development and circadian rhythm. Furthermore, some of the top genes we identified are involved in innate immunity.

CONCLUSION

Our results revealed evidence of genes under balancing selection in European and African populations. More candidate genes have been found in the European population. They are involved in several different functions.

Characterization of Nora Virus Structural Proteins via Western Blot Analysis

Nora virus is a single stranded RNA picorna-like virus with four open reading frames (ORFs). The coding potentials of the ORFs are not fully characterized, but ORF3 and ORF4 are believed to encode the capsid proteins (VP3, VP4a, VP4b, and VP4c) comprising the virion. To determine the polypeptide composition of Nora virus virions, polypeptides from purified virus were compared to polypeptides detected in Nora virus infected Drosophila melanogaster. Nora virus was purified from infected flies and used to challenge mice for the production of antisera. ORF3, ORF4a, ORF4b, and ORF4c were individually cloned and expressed in E. coli; resultant recombinant proteins purified and were used to make monospecific antisera. Antisera were evaluated via Western blot against whole virus particles and Nora virus infected fly lysates. Viral purification yielded two particle types with densities of ~1.31 g/mL (empty particles) and ~1.33 g/mL (complete virions). Comparison of purified virus polypeptide composition to Nora virus infected D. melanogaster lysate showed the number of proteins in infected cell lysates is less than purified virus. Our results suggest the virion is composed of 6 polypeptides, VP3, VP4a, two forms of VP4b, and two forms of VP4c. This polypeptide composition is similar to other small RNA insect viruses.

Dynamic changes in host-virus interactions associated with colony founding and social environment in fire ant queens (Solenopsis invicta)

The dynamics of host-parasite interactions can change dramatically over the course of a chronic infection as the internal (physiological) and external (environmental) conditions of the host change. When queens of social insects found a colony, they experience changes in both their physiological state (they develop their ovaries and begin laying eggs) and the social environment (they suddenly stop interacting with the other members of the mother colony), making this an excellent model system for examining how these factors interact with chronic infections. We investigated the dynamics of host-viral interactions in queens of Solenopsis invicta (fire ant) as they transition from mating to colony founding/brood rearing to the emergence of the first workers. We examined these dynamics in naturally infected queens in two different social environments, where queens either founded colonies as individuals or as pairs. We hypothesized that stress associated with colony founding plays an important role in the dynamics of host-parasite interactions. We also hypothesized that different viruses have different modalities of interaction with the host that can be quantified by physiological measures and genomic analysis of gene expression in the host. We found that the two most prevalent viruses, SINV-1 and SINV-2, are associated with different fitness costs that are mirrored by different patterns of gene expression in the host. In fact SINV-2, the virus that imposes the significant reduction of a queen's reproductive output is also associated with larger changes of global gene expression in the host. These results show the complexity of interactions between S. invicta and two viral parasites. Our findings also show that chronic infections by viral parasites in insects are dynamic processes that may pose different challenges in the host, laying the groundwork for interesting ecological and evolutionary considerations.

Antiviral responses of arthropod vectors: an update on recent advances

Arthropod vectors, such as mosquitoes, ticks, biting midges and sand flies, transmit many viruses that can cause outbreaks of disease in humans and animals around the world. Arthropod vector species are invading new areas due to globalisation and environmental changes, and contact between exotic animal species, humans and arthropod vectors is increasing, bringing with it the regular emergence of new arboviruses. For future strategies to control arbovirus transmission, it is important to improve our understanding of virus-vector interactions. In the last decade knowledge of arthropod antiviral immunity has increased rapidly. RNAi has been proposed as the most important antiviral response in mosquitoes and it is likely to be the most important antiviral response in all arthropods. However, other newly-discovered antiviral strategies such as melanisation and the link between RNAi and the JAK/STAT pathway via the cytokine Vago have been characterised in the last few years. This review aims to summarise the most important and most recent advances made in arthropod antiviral immunity.

Induced antiviral innate immunity in Drosophila

Immunity to viral infections in the model organism Drosophila melanogaster involves both RNA interference and additional induced responses. The latter include not only cellular mechanisms such as programmed cell death and autophagy, but also the induction of a large set of genes, some of which contribute to the control of viral replication and resistance to infection. This induced response to infection is complex and involves both virus-specific and cell-type specific mechanisms. We review here recent developments, from the sensing of viral infection to the induction of signaling pathways and production of antiviral effector molecules. Our current understanding, although still partial, validates the Drosophila model of antiviral induced immunity for insect pests and disease vectors, as well as for mammals.

Human pathogenic bacteria, fungi, and viruses in Drosophila: disease modeling, lessons, and shortcomings

Drosophila has been the invertebrate model organism of choice for the study of innate immune responses during the past few decades. Many Drosophila-microbe interaction studies have helped to define innate immunity pathways, and significant effort has been made lately to decipher mechanisms of microbial pathogenesis. Here we catalog 68 bacterial, fungal, and viral species studied in flies, 43 of which are relevant to human health. We discuss studies of human pathogens in flies revealing not only the elicitation and avoidance of immune response but also mechanisms of tolerance, host tissue homeostasis, regeneration, and predisposition to cancer. Prominent among those is the emerging pattern of intestinal regeneration as a defense response induced by pathogenic and innocuous bacteria. Immunopathology mechanisms and many microbial virulence factors have been elucidated, but their relevance to human health conventionally necessitates validation in mammalian models of infection.

Insect antiviral innate immunity: pathways, effectors, and connections

Insects are infected by a wide array of viruses some of which are insect restricted and pathogenic, and some of which are transmitted by biting insects to vertebrates. The medical and economic importance of these viruses heightens the need to understand the interaction between the infecting pathogen and the insect immune system in order to develop transmission interventions. The interaction of the virus with the insect host innate immune system plays a critical role in the outcome of infection. The major mechanism of antiviral defense is the small, interfering RNA pathway that responds through the detection of virus-derived double-stranded RNA to suppress virus replication. However, other innate antimicrobial pathways such as Imd, Toll, and Jak-STAT and the autophagy pathway have also been shown to play important roles in antiviral immunity. In this review, we provide an overview of the current understanding of the main insect antiviral pathways and examine recent findings that further our understanding of the roles of these pathways in facilitating a systemic and specific response to infecting viruses.