Genome analysis of a Glossina pallidipes salivary gland hypertrophy virus reveals a novel, large, double-stranded circular DNA virus

A novel and diverse family of filamentous DNA viruses associated with parasitic wasps

Large dsDNA viruses from the Naldaviricetes class are currently composed of four viral families infecting insects and/or crustaceans. Since the 1970s, particles described as filamentous viruses (FVs) have been observed by electronic microscopy in several species of Hymenoptera parasitoids but until recently, no genomic data was available. This study provides the first comparative morphological and genomic analysis of these FVs. We analyzed the genomes of seven FVs, six of which were newly obtained, to gain a better understanding of their evolutionary history. We show that these FVs share all genomic features of the Naldaviricetes while encoding five specific core genes that distinguish them from their closest relatives, the Hytrosaviruses. By mining public databases, we show that FVs preferentially infect Hymenoptera with parasitoid lifestyle and that these viruses have been repeatedly integrated into the genome of many insects, particularly Hymenoptera parasitoids, overall suggesting a long-standing specialization of these viruses to parasitic wasps. Finally, we propose a taxonomical revision of the class Naldaviricetes in which FVs related to the Leptopilina boulardi FV constitute a fifth family. We propose to name this new family, Filamentoviridae.

Interactions between Glossina pallidipes salivary gland hypertrophy virus and tsetse endosymbionts in wild tsetse populations

BACKGROUND

Tsetse control is considered an effective and sustainable tactic for the control of cyclically transmitted trypanosomosis in the absence of effective vaccines and inexpensive, effective drugs. The sterile insect technique (SIT) is currently used to eliminate tsetse fly populations in an area-wide integrated pest management (AW-IPM) context in Senegal. For SIT, tsetse mass rearing is a major milestone that associated microbes can influence. Tsetse flies can be infected with microorganisms, including the primary and obligate Wigglesworthia glossinidia, the commensal Sodalis glossinidius, and Wolbachia pipientis. In addition, tsetse populations often carry a pathogenic DNA virus, the Glossina pallidipes salivary gland hypertrophy virus (GpSGHV) that hinders tsetse fertility and fecundity. Interactions between symbionts and pathogens might affect the performance of the insect host.

METHODS

In the present study, we assessed associations of GpSGHV and tsetse endosymbionts under field conditions to decipher the possible bidirectional interactions in different Glossina species. We determined the co-infection pattern of GpSGHV and Wolbachia in natural tsetse populations. We further analyzed the interaction of both Wolbachia and GpSGHV infections with Sodalis and Wigglesworthia density using qPCR.

RESULTS

The results indicated that the co-infection of GpSGHV and Wolbachia was most prevalent in Glossina austeni and Glossina morsitans morsitans, with an explicit significant negative correlation between GpSGHV and Wigglesworthia density. GpSGHV infection levels > 10^3.31 seem to be absent when Wolbachia infection is present at high density (> 10^7.36), suggesting a potential protective role of Wolbachia against GpSGHV.

CONCLUSION

The result indicates that Wolbachia infection might interact (with an undefined mechanism) antagonistically with SGHV infection protecting tsetse fly against GpSGHV, and the interactions between the tsetse host and its associated microbes are dynamic and likely species specific; significant differences may exist between laboratory and field conditions.

Conserved Viral Transcription Plays a Key Role in Virus-Like Particle Production of the Parasitoid Wasp Venturia canescens

Nudiviruses are large double-stranded DNA viruses related to baculoviruses known to be endogenized in the genomes of certain parasitic wasp species. These wasp-virus associations allow the production of viral particles or virus-like particles that ensure wasp parasitism success within lepidopteran hosts. Venturia canescens is an ichneumonid wasp belonging to the Campopleginae subfamily that has endogenized nudivirus genes belonging to the Alphanudivirus genus to produce "virus-like particles" (Venturia canescens virus-like particles [VcVLPs]), which package proteic virulence factors. The main aim of this study was to determine whether alphanudivirus gene functions have been conserved following endogenization. The expression dynamics of alphanudivirus genes was monitored by a high throughput transcriptional approach, and the functional role of lef-4 and lef-8 genes predicted to encode viral RNA polymerase components was investigated by RNA interference. As described for baculovirus infections and for endogenized nudivirus genes in braconid wasp species producing bracoviruses, a transcriptional cascade involving early and late expressed alphanudivirus genes could be observed. The expression of lef-4 and lef-8 was also shown to be required for the expression of alphanudivirus late genes allowing correct particle formation. Together with previous literature, the results show that endogenization of nudiviruses in parasitoid wasps has repeatedly led to the conservation of the viral RNA polymerase function, allowing the production of viruses or viral-like particles that differ in composition but enable wasp parasitic success. IMPORTANCE This study shows that endogenization of a nudivirus genome in a Campopleginae parasitoid wasp has led to the conservation, as for endogenized nudiviruses in braconid parasitoid wasps, of the viral RNA polymerase function, required for the transcription of genes encoding viral particles involved in wasp parasitism success. We also showed for the first time that RNA interference (RNAi) can be successfully used to downregulate gene expression in this species, a model in behavioral ecology. This opens the opportunity to investigate the function of genes involved in other traits important for parasitism success, such as reproductive strategies and host choice. Fundamental data acquired on gene function in Venturia canescens are likely to be transferable to other parasitoid wasp species used in biological control programs. This study also renders possible the investigation of other nudivirus gene functions, for which little data are available.

The complete genome sequence of an alphabaculovirus from the brown tussock moth, Olene mendosa Hübner, expands our knowledge of lymantriine baculovirus diversity and evolution

The complete genome sequence was determined for an apparent alphabaculovirus isolated from larval cadavers of the brown tussock moth, Olene mendosa Hübner, collected during an epizootic in Coimbatore, India. The genome was determined to be a circular 142,291 bp molecule, and 147 ORFs and nine homologous regions were annotated for the sequence. Analysis of the sequence confirmed that this virus, Olene mendosa nucleopolyhedrovirus (OlmeNPV), was a member of genus Alphabaculovirus in family Baculoviridae. Phylogenies inferred from nucleotide and amino acid alignments indicated that OlmeNPV was part of a group of viruses that infect moths of genus Lymantria, suggesting that OlmeNPV may have shifted hosts from a Lymantria species to an ancestral Olene species at some point during its evolutionary history. OlmeNPV was most closely related to Lymantria xylina multiple nucleopolyhedrovirus isolate 5 (LyxyMNPV-5). The genomes of OlmeNPV and LyxyMNPV-5 were distinguished not only by differences in ORF content, but by a 27 kbp region of the genome that is inverted in LyxyMNPV-5 relative to OlmeNPV. Pairwise nucleotide distances between OlmeNPV and other Lymantria spp. alphabaculoviruses indicate that OlmeNPV represents a new baculovirus species.

Characterization and Tissue Tropism of Newly Identified Iflavirus and Negeviruses in Glossina morsitans morsitans Tsetse Flies

Tsetse flies cause major health and economic problems as they transmit trypanosomes causing sleeping sickness in humans (Human African Trypanosomosis, HAT) and nagana in animals (African Animal Trypanosomosis, AAT). A solution to control the spread of these flies and their associated diseases is the implementation of the Sterile Insect Technique (SIT). For successful application of SIT, it is important to establish and maintain healthy insect colonies and produce flies with competitive fitness. However, mass production of tsetse is threatened by covert virus infections, such as the Glossina pallidipes salivary gland hypertrophy virus (GpSGHV). This virus infection can switch from a covert asymptomatic to an overt symptomatic state and cause the collapse of an entire fly colony. Although the effects of GpSGHV infections can be mitigated, the presence of other covert viruses threaten tsetse mass production. Here we demonstrated the presence of two single-stranded RNA viruses isolated from Glossina morsitans morsitans originating from a colony at the Seibersdorf rearing facility. The genome organization and the phylogenetic analysis based on the RNA-dependent RNA polymerase (RdRp) revealed that the two viruses belong to the genera Iflavirus and Negevirus, respectively. The names proposed for the two viruses are Glossina morsitans morsitans iflavirus (GmmIV) and Glossina morsitans morsitans negevirus (GmmNegeV). The GmmIV genome is 9685 nucleotides long with a poly(A) tail and encodes a single polyprotein processed into structural and non-structural viral proteins. The GmmNegeV genome consists of 8140 nucleotides and contains two major overlapping open reading frames (ORF1 and ORF2). ORF1 encodes the largest protein which includes a methyltransferase domain, a ribosomal RNA methyltransferase domain, a helicase domain and a RdRp domain. In this study, a selective RT-qPCR assay to detect the presence of the negative RNA strand for both GmmIV and GmmNegeV viruses proved that both viruses replicate in G. m. morsitans. We analyzed the tissue tropism of these viruses in G. m. morsitans by RNA-FISH to decipher their mode of transmission. Our results demonstrate that both viruses can be found not only in the host’s brain and fat bodies but also in their reproductive organs, and in milk and salivary glands. These findings suggest a potential horizontal viral transmission during feeding and/or a vertically viral transmission from parent to offspring. Although the impact of GmmIV and GmmNegeV in tsetse rearing facilities is still unknown, none of the currently infected tsetse species show any signs of disease from these viruses.

A Behavior-Manipulating Virus Relative as a Source of Adaptive Genes for Drosophila Parasitoids

Some species of parasitic wasps have domesticated viral machineries to deliver immunosuppressive factors to their hosts. Up to now, all described cases fall into the Ichneumonoidea superfamily, which only represents around 10% of hymenoptera diversity, raising the question of whether such domestication occurred outside this clade. Furthermore, the biology of the ancestral donor viruses is completely unknown. Since the 1980s, we know that Drosophila parasitoids belonging to the Leptopilina genus, which diverged from the Ichneumonoidea superfamily 225 Ma, do produce immunosuppressive virus-like structure in their reproductive apparatus. However, the viral origin of these structures has been the subject of debate. In this article, we provide genomic and experimental evidence that those structures do derive from an ancestral virus endogenization event. Interestingly, its close relatives induce a behavior manipulation in present-day wasps. Thus, we conclude that virus domestication is more prevalent than previously thought and that behavior manipulation may have been instrumental in the birth of such associations.

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

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

Baculovirus ODV-E66 degrades larval peritrophic membrane to facilitate baculovirus oral infection

ODV-E66 is a major envelope proteins of baculovirus occlusion derived virus (ODV) with chondroitinase activity. Here, we studied the roles of ODV-E66 during Helicoverpa armigera nucleopolyhedrovirus (HearNPV) primary infection. ODV-E66 is a late viral protein dispensable for BV production and ODV morphogenesis. Deletion of odv-e66 had a profound effect on HearNPV oral infectivity in 4th instar larvae with a 50% lethal concentration (LC₅₀) value of 26 fold higher than that of the repaired virus, compared to in 3rd instar larvae. Calcofluor white, an agent which destroys the peritrophic membrane (PM), could rescue the oral infectivity of odv-e66 deleted HearNPV, implying the PM may be the target of ODV-E66. In vitro assays showed HearNPV ODV-E66 has chondroitinase activity. Electron microscopy demonstrated that odv-e66 deletion alleviated the damage to the PM caused by HearNPV infection. These data suggest an important role of ODV-E66 in the penetration of the PM during oral infection.

Coevolution of hytrosaviruses and host immune responses

BACKGROUND

Hytrosaviruses (SGHVs; Hytrosaviridae family) are double-stranded DNA (dsDNA) viruses that cause salivary gland hypertrophy (SGH) syndrome in flies. Two structurally and functionally distinct SGHVs are recognized; Glossina pallidipes SGHV (GpSGHV) and Musca domestica SGHV (MdSGHV), that infect the hematophagous tsetse fly and the filth-feeding housefly, respectively. Genome sizes and gene contents of GpSGHV (~ 190 kb; 160-174 genes) and MdSGHV (~ 124 kb; 108 genes) may reflect an evolution with the SGHV-hosts resulting in differences in pathobiology. Whereas GpSGHV can switch from asymptomatic to symptomatic infections in response to certain unknown cues, MdSGHV solely infects symptomatically. Overt SGH characterizes the symptomatic infections of SGHVs, but whereas MdSGHV induces both nuclear and cellular hypertrophy (enlarged non-replicative cells), GpSGHV induces cellular hyperplasia (enlarged replicative cells). Compared to GpSGHV's specificity to Glossina species, MdSGHV infects other sympatric muscids. The MdSGHV-induced total shutdown of oogenesis inhibits its vertical transmission, while the GpSGHV's asymptomatic and symptomatic infections promote vertical and horizontal transmission, respectively. This paper reviews the coevolution of the SGHVs and their hosts (housefly and tsetse fly) based on phylogenetic relatedness of immune gene orthologs/paralogs and compares this with other virus-insect models.

RESULTS

Whereas MdSGHV is not vertically transmitted, GpSGHV is both vertically and horizontally transmitted, and the balance between the two transmission modes may significantly influence the pathogenesis of tsetse virus. The presence and absence of bacterial symbionts (Wigglesworthia and Sodalis) in tsetse and Wolbachia in the housefly, respectively, potentially contributes to the development of SGH symptoms. Unlike MdSGHV, GpSGHV contains not only host-derived proteins, but also appears to have evolutionarily recruited cellular genes from ancestral host(s) into its genome, which, although may be nonessential for viral replication, potentially contribute to the evasion of host's immune responses. Whereas MdSGHV has evolved strategies to counteract both the housefly's RNAi and apoptotic responses, the housefly has expanded its repertoire of immune effector, modulator and melanization genes compared to the tsetse fly.

CONCLUSIONS

The ecologies and life-histories of the housefly and tsetse fly may significantly influence coevolution of MdSGHV and GpSGHV with their hosts. Although there are still many unanswered questions regarding the pathogenesis of SGHVs, and the extent to which microbiota influence expression of overt SGH symptoms, SGHVs are attractive 'explorers' to elucidate the immune responses of their hosts, and the transmission modes of other large DNA viruses.

Hytrosavirus genetic diversity and eco-regional spread in Glossina species

BACKGROUND

The management of the tsetse species Glossina pallidipes (Diptera; Glossinidae) in Africa by the sterile insect technique (SIT) has been hindered by infections of G. pallidipes production colonies with Glossina pallidipes salivary gland hypertrophy virus (GpSGHV; Hytrosaviridae family). This virus can significantly decrease productivity of the G. pallidipes colonies. Here, we used three highly diverged genes and two variable number tandem repeat regions (VNTRs) of the GpSGHV genome to identify the viral haplotypes in seven Glossina species obtained from 29 African locations and determine their phylogenetic relatedness.

RESULTS

GpSGHV was detected in all analysed Glossina species using PCR. The highest GpSGHV prevalence was found in G. pallidipes colonized at FAO/IAEA Insect Pest Control Laboratory (IPCL) that originated from Uganda (100%) and Tanzania (88%), and a lower prevalence in G. morsitans morsitans from Tanzania (58%) and Zimbabwe (20%). Whereas GpSGHV was detected in 25-40% of G. fuscipes fuscipes in eastern Uganda, the virus was not detected in specimens of neighboring western Kenya. Most of the identified 15 haplotypes were restricted to specific Glossina species in distinct locations. Seven haplotypes were found exclusively in G. pallidipes. The reference haplotype H1 (GpSGHV-Uga; Ugandan strain) was the most widely distributed, but was not found in G. swynnertoni GpSGHV. The 15 haplotypes clustered into three distinct phylogenetic clades, the largest contained seven haplotypes, which were detected in six Glossina species. The G. pallidipes-infecting haplotypes H10, H11 and H12 (from Kenya) clustered with H7 (from Ethiopia), which presumably corresponds to the recently sequenced GpSGHV-Eth (Ethiopian) strain. These four haplotypes diverged the most from the reference H1 (GpSGHV-Uga). Haplotypes H1, H5 and H14 formed three main genealogy hubs, potentially representing the ancestors of the 15 haplotypes.

CONCLUSION

These data identify G. pallidipes as a significant driver for the generation and diversity of GpSGHV variants. This information may provide control guidance when new tsetse colonies are established and hence, for improved management of the virus in tsetse rearing facilities that maintain multiple Glossina species.

RNA interference-based antiviral immune response against the salivary gland hypertrophy virus in Glossina pallidipes

Background

Glossina pallidipes salivary gland hypertrophy virus (GpSGHV; Hytrosaviridae) is a non-occluded dsDNA virus that specifically infects the adult stages of the hematophagous tsetse flies (Glossina species, Diptera: Glossinidae). GpSGHV infections are usually asymptomatic, but unknown factors can result to a switch to acute symptomatic infection, which is characterized by the salivary gland hypertrophy (SGH) syndrome associated with decreased fecundity that can ultimately lead to a colony collapse. It is uncertain how GpSGHV is maintained amongst Glossina spp. populations but RNA interference (RNAi) machinery, a conserved antiviral defense in insects, is hypothesized to be amongst the host’s mechanisms to maintain the GpSGHV in asymptomatic (persistent or latent) infection state. Here, we investigated the involvement of RNAi during GpSGHV infections by comparing the expression of three key RNAi machinery genes, Dicer (DCR), Argonaute (AGO) and Drosha, in artificially virus injected, asymptomatic and symptomatic infected G. pallidipes flies compared to PBS injected (controls) individuals. We further assessed the impact of AGO2 knockdown on virus infection by RT-qPCR quantification of four selected GpSGHV genes, i.e. odv-e66, dnapol, maltodextrin glycosyltransferase (a tegument gene) and SGHV091 (a capsid gene).

Results

We show that in response to hemocoelic injections of GpSGHV into G. pallidipes flies, increased virus replication was accompanied by significant upregulation of the expression of three RNAi key genes; AGO1, AGO2 and DCR2, and a moderate increase in the expression of Drosha post injection compared to the PBS-injected controls. Furthermore, compared to asymptomatically infected individuals, symptomatic flies showed significant downregulation of AGO1, AGO2 and Drosha, but a moderate increase in the expression of DCR2. Compared to the controls, knockdown of AGO2 did not have a significant impact on virus infection in the flies as evidenced by unaltered transcript levels of the selected GpSGHV genes.

Conclusion

The upregulation of the expression of the RNAi genes implicate involvement of this machinery in controlling GpSGHV infections and the establishment of symptomatic GpSGHV infections in Glossina. These findings provide a strategic foundation to understand GpSGHV infections and to control latent (asymptomatic) infections in Glossina spp. and thereby control SGHVs in insect production facilities.

Electronic supplementary material

The online version of this article (10.1186/s12866-018-1298-1) contains supplementary material, which is available to authorized users.

Expression Profile of Glossina pallidipes MicroRNAs During Symptomatic and Asymptomatic Infection With Glossina pallidipes Salivary Gland Hypertrophy Virus (Hytrosavirus)

The Glossina pallidipes salivary gland hypertrophy virus (GpSGHV) infects tsetse flies predominantly asymptomatically and occasionally symptomatically. Symptomatic infections are characterized by overt salivary gland hypertrophy (SGH) in mass reared tsetse flies, which causes reproductive dysfunctions and colony collapse, thus hindering tsetse control via sterile insect technique (SIT). Asymptomatic infections have no apparent cost to the fly's fitness. Here, small RNAs were sequenced and profiles in asymptomatically and symptomatically infected G. pallidipes flies determined. Thirty-eight host-encoded microRNAs (miRNAs) were present in both the asymptomatic and symptomatic fly profiles, while nine host miRNAs were expressed specifically in asymptomatic flies versus 10 in symptomatic flies. Of the shared 38 miRNAs, 15 were differentially expressed when comparing asymptomatic with symptomatic flies. The most up-regulated host miRNAs in symptomatic flies was predicted to target immune-related mRNAs of the host. Six GpSGHV-encoded miRNAs were identified, of which five of them were only in symptomatic flies. These virus-encoded miRNAs may not only target host immune genes but may also participate in viral immune evasion. This evidence of differential host miRNA profile in Glossina in symptomatic flies advances our understanding of the GpSGHV-Glossina interactions and provides potential new avenues, for instance by utilization of particular miRNA inhibitors or mimics to better manage GpSGHV infections in tsetse mass-rearing facilities, a prerequisite for successful SIT implementation.

Genome Sequencing of the Behavior Manipulating Virus LbFV Reveals a Possible New Virus Family

Parasites are sometimes able to manipulate the behavior of their hosts. However, the molecular cues underlying this phenomenon are poorly documented. We previously reported that the parasitoid wasp Leptopilina boulardi which develops from Drosophila larvae is often infected by an inherited DNA virus. In addition to being maternally transmitted, the virus benefits from horizontal transmission in superparasitized larvae (Drosophila that have been parasitized several times). Interestingly, the virus forces infected females to lay eggs in already parasitized larvae, thus increasing the chance of being horizontally transmitted. In a first step towards the identification of virus genes responsible for the behavioral manipulation, we present here the genome sequence of the virus, called LbFV. The sequencing revealed that its genome contains an homologous repeat sequence (hrs) found in eight regions in the genome. The presence of this hrs may explain the genomic plasticity that we observed for this genome. The genome of LbFV encodes 108 ORFs, most of them having no homologs in public databases. The virus is however related to Hytrosaviridae, although distantly. LbFV may thus represent a member of a new virus family. Several genes of LbFV were captured from eukaryotes, including two anti-apoptotic genes. More surprisingly, we found that LbFV captured from an ancestral wasp a protein with a Jumonji domain. This gene was afterwards duplicated in the virus genome. We hypothesized that this gene may be involved in manipulating the expression of wasp genes, and possibly in manipulating its behavior.

Structural features of the salivary gland hypertrophy virus of the tsetse fly revealed by cryo-electron microscopy and tomography

Glossina palipides salivary gland hypertrophy virus (GpSGHV) infects tsetse flies, which are vectors for African trypanosomosis. This virus represents a major challenge in insect mass rearing and has hampered the implementation of the sterile insect technique programs in the Member States of the International Atomic Energy Agency. GpSGHV virions consist of long rod-shaped particles over 9000Å in length, but little is known about their detailed structural organization. We show by cryo electron microscopy and cryo electron tomography that the GpSGHV virion has a unique, non-icosahedral helical structure. Its envelope exhibits regularly spaced spikes that protrude from the lipid bilayer and are arranged on a four-start helix. This study provides a detailed insight into the 3D architecture of GpSGHV, which will help to understand the viral life cycle and possibly allow the design of antiviral strategies in the context of tsetse fly infections.

Responses of the Housefly, Musca domestica, to the Hytrosavirus Replication: Impacts on Host's Vitellogenesis and Immunity

Hytrosaviridae family members replicate in the salivary glands (SGs) of their adult dipteran hosts and are transmitted to uninfected hosts via saliva during feeding. Despite inducing similar gross symptoms (SG hypertrophy; SGH), hytrosaviruses (SGHVs) have distinct pathobiologies, including sex-ratio distortions in tsetse flies and refusal of infected housefly females to copulate. Via unknown mechanism(s), SGHV replication in other tissues results in reduced fecundity in tsetse flies and total shutdown of vitellogenesis and sterility in housefly females. We hypothesized that vitellogenesis shutdown was caused by virus-induced modulation of hormonal titers. Here, we used RNA-Seq to investigate virus-induced modulation of host genes/pathways in healthy and virus-infected houseflies, and we validated expression of modulated genes (n = 23) by RT-qPCR. We also evaluated the levels and activities of hemolymph AMPs, levels of endogenous sesquiterpenoids, and impacts of exogenous hormones on ovarian development in viremic females. Of the 973 housefly unigenes that were significantly modulated (padj ≤ 0.01, log2FC ≤ -2.0 or ≥ 2.0), 446 and 527 genes were downregulated and upregulated, respectively. While the most downregulated genes were related to reproduction (embryogenesis/oogenesis), the repertoire of upregulated genes was overrepresented by genes related to non-self recognition, ubiquitin-protease system, cytoskeletal traffic, cellular proliferation, development and movement, and snRNA processing. Overall, the virus, Musca domestica salivary gland hytrosavirus (MdSGHV), induced the upregulation of various components of the siRNA, innate antimicrobial immune, and autophagy pathways. We show that MdSGHV undergo limited morphogenesis in the corpora allata/corpora cardiaca (CA/CC) complex of M. domestica. MdSGHV replication in CA/CC potentially explains the significant reduction of hemolymph sesquiterpenoids levels, the refusal to mate, and the complete shutdown of egg development in viremic females. Notably, hormonal rescue of vitellogenesis did not result in egg production. The mechanism underlying MdSGHV-induced sterility has yet to be resolved.

Comparative Analysis of Salivary Gland Proteomes of Two Glossina Species that Exhibit Differential Hytrosavirus Pathologies

Glossina pallidipes salivary gland hypertrophy virus (GpSGHV; family Hytrosaviridae) is a dsDNA virus exclusively pathogenic to tsetse flies (Diptera; Glossinidae). The 190 kb GpSGHV genome contains 160 open reading frames and encodes more than 60 confirmed proteins. The asymptomatic GpSGHV infection in flies can convert to symptomatic infection that is characterized by overt salivary gland hypertrophy (SGH). Flies with SGH show reduced general fitness and reproductive dysfunction. Although the occurrence of SGH is an exception rather than the rule, G. pallidipes is thought to be the most susceptible to expression of overt SGH symptoms compared to other Glossina species that are largely asymptomatic. Although Glossina salivary glands (SGs) play an essential role in GpSGHV transmission, the functions of the salivary components during the virus infection are poorly understood. In this study, we used mass spectrometry to study SG proteomes of G. pallidipes and G. m. morsitans, two Glossina model species that exhibit differential GpSGHV pathologies (high and low incidence of SGH, respectively). A total of 540 host proteins were identified, of which 23 and 9 proteins were significantly up- and down-regulated, respectively, in G. pallidipes compared to G. m. morsitans. Whereas 58 GpSGHV proteins were detected in G. pallidipes F₁ progenies, only 5 viral proteins were detected in G. m. morsitans. Unlike in G. pallidipes, qPCR assay did not show any significant increase in virus titers in G. m. morsitans F₁ progenies, confirming that G. m. morsitans is less susceptible to GpSGHV infection and replication compared to G. pallidipes. Based on our results, we speculate that in the case of G. pallidipes, GpSGHV employs a repertoire of host intracellular signaling pathways for successful infection. In the case of G. m. morsitans, antiviral responses appeared to be dominant. These results are useful for designing additional tools to investigate the Glossina-GpSGHV interactions.

Comprehensive annotation of Glossina pallidipes salivary gland hypertrophy virus from Ethiopian tsetse flies: a proteogenomics approach

Glossina pallidipes salivary gland hypertrophy virus (GpSGHV; family Hytrosaviridae) can establish asymptomatic and symptomatic infection in its tsetse fly host. Here, we present a comprehensive annotation of the genome of an Ethiopian GpSGHV isolate (GpSGHV-Eth) compared with the reference Ugandan GpSGHV isolate (GpSGHV-Uga; GenBank accession number EF568108). GpSGHV-Eth has higher salivary gland hypertrophy syndrome prevalence than GpSGHV-Uga. We show that the GpSGHV-Eth genome has 190 291 nt, a low G+C content (27.9 %) and encodes 174 putative ORFs. Using proteogenomic and transcriptome mapping, 141 and 86 ORFs were mapped by transcripts and peptides, respectively. Furthermore, of the 174 ORFs, 132 had putative transcriptional signals [TATA-like box and poly(A) signals]. Sixty ORFs had both TATA-like box promoter and poly(A) signals, and mapped by both transcripts and peptides, implying that these ORFs encode functional proteins. Of the 60 ORFs, 10 ORFs are homologues to baculovirus and nudivirus core genes, including three per os infectivity factors and four RNA polymerase subunits (LEF4, 5, 8 and 9). Whereas GpSGHV-Eth and GpSGHV-Uga are 98.1 % similar at the nucleotide level, 37 ORFs in the GpSGHV-Eth genome had nucleotide insertions (n = 17) and deletions (n = 20) compared with their homologues in GpSGHV-Uga. Furthermore, compared with the GpSGHV-Uga genome, 11 and 24 GpSGHV ORFs were deleted and novel, respectively. Further, 13 GpSGHV-Eth ORFs were non-canonical; they had either CTG or TTG start codons instead of ATG. Taken together, these data suggest that GpSGHV-Eth and GpSGHV-Uga represent two different lineages of the same virus. Genetic differences combined with host and environmental factors possibly explain the differential GpSGHV pathogenesis observed in different G. pallidipes colonies.

Disruption of the salivary gland muscle in tsetse, Glossina pallidipes Austen, as a result of salivary gland hypertrophy virus infection

The secretory region of the salivary glands in Glossina pallidipes Austen (Diptera: Glossinidae) is characterized by an external muscle layer. Scanning electron microscopy and transmission electron microscopy investigations provide a detailed description of the longitudinal muscle fibres and a comparison of their structure when affected by salivary gland hypertrophy virus. The virus is responsible for hypertrophy of the salivary glands in symptomatic flies, specifically of the muscle fibres, the cytoarchitecture of which is completely altered. Although observations did not reveal viral particles in the muscle cells of either asymptomatic or symptomatic flies, muscle fibres were enlarged and detached from one another and their associated basement membrane only in symptomatic flies. A decrease in type IV collagen labelling in the basement membrane of the muscles in symptomatic flies is reported and is considered a potential cause of the salivary gland muscle alteration and, possibly, myopathy. The maintenance of an organized muscular layer is essential for the normal secretion of saliva and hence its pathology in symptomatic tsetse flies could affect the normal transmission of the trypanosome that develops inside the salivary gland epithelium. Therefore, a better understanding of the possible role of the virus is essential in order to elucidate its impact on salivary deployment in symptomatic flies.

In search of pathogens: transcriptome-based identification of viral sequences from the pine processionary moth (Thaumetopoea pityocampa)

Thaumetopoea pityocampa (pine processionary moth) is one of the most important pine pests in the forests of Mediterranean countries, Central Europe, the Middle East and North Africa. Apart from causing significant damage to pinewoods, T. pityocampa occurrence is also an issue for public and animal health, as it is responsible for dermatological reactions in humans and animals by contact with its irritating hairs. High throughput sequencing technologies have allowed the fast and cost-effective generation of genetic information of interest to understand different biological aspects of non-model organisms as well as the identification of potential pathogens. Using these technologies, we have obtained and characterized the transcriptome of T. pityocampa larvae collected in 12 different geographical locations in Turkey. cDNA libraries for Illumina sequencing were prepared from four larval tissues, head, gut, fat body and integument. By pooling the sequences from Illumina platform with those previously published using the Roche 454-FLX and Sanger methods we generated the largest reference transcriptome of T. pityocampa. In addition, this study has also allowed identification of possible viral pathogens with potential application in future biocontrol strategies.

The genome of the nucleopolyhedrosis-causing virus from Tipula oleracea sheds new light on the Nudiviridae family

A large double-stranded DNA (dsDNA) virus that produces occlusion bodies, typical of baculoviruses, has been described to infect crane fly larvae of the genus Tipula (Diptera, Tipulidae). Because of a lack of genomic data, this virus has remained unclassified. Electron microscopy of an archival virus isolated from Tipula oleracea, T. oleracea nudivirus (ToNV), showed irregularly shaped occlusion bodies measuring from 2 to 5 μm in length and 2 μm in middiameter, filled with rod-shape virions containing single nucleocapsids within a bilayer envelope. Whole-genome amplification and Roche 454 sequencing revealed a complete circular genome sequence of 145.7 kb, containing five direct repeat regions. We predicted 131 open reading frames, including a homolog of the polyhedrin gene encoding the major occlusion body protein of T. paludosa nucleopolyhedrovirus (NPV). BLAST searches demonstrated that ToNV had 21 of the 37 baculovirus core genes but shared 52 genes with nudiviruses (NVs). Phylogenomic analyses indicated that ToNV clearly belongs to the Nudiviridae family but should probably be assigned to a new genus. Among nudiviruses, ToNV was most closely related to the Penaeus monodon NV and Heliothis zea NV clade but distantly related to Drosophila innubia NV, the other nudivirus infecting a Diptera. Lastly, ToNV was found to be most closely related to the nuvidirus ancestor of bracoviruses. This was also reflected in terms of gene content, as ToNV was the only known exogenous virus harboring homologs of the Cc50C22.6 and 27b (Cc50C22.7) genes found in the nudiviral genomic cluster involved in bracovirus particle production.

IMPORTANCE

The Nudiviridae is a family of arthropod dsDNA viruses from which striking cases of endogenization have been reported (i.e., symbiotic bracoviruses deriving from a nudivirus and the endogenous nudivirus of the brown planthopper). Although related to baculoviruses, relatively little is known about the genomic diversity of exogenous nudiviruses. Here, we characterized, morphologically and genetically, an archival sample of the Tipula oleracea nudivirus (ToNV), which has the particularity of forming occlusion bodies. Comparative genomic and phylogenomic analyses showed ToNV to be to date the closest known relative of the exogenous ancestor of bracoviruses and that ToNV should be assigned to a new genus. Moreover, we revised the homology relationships of nudiviral genes and identified a new set of 32 core genes for the Nudiviridae, of which 21 were also baculovirus core genes. These findings provide important insights into the evolutionary history of large arthropod dsDNA viruses.

Genome sequence of the tsetse fly (Glossina morsitans): vector of African trypanosomiasis

Tsetse flies are the sole vectors of human African trypanosomiasis throughout sub-Saharan Africa. Both sexes of adult tsetse feed exclusively on blood and contribute to disease transmission. Notable differences between tsetse and other disease vectors include obligate microbial symbioses, viviparous reproduction, and lactation. Here, we describe the sequence and annotation of the 366-megabase Glossina morsitans morsitans genome. Analysis of the genome and the 12,308 predicted protein-encoding genes led to multiple discoveries, including chromosomal integrations of bacterial (Wolbachia) genome sequences, a family of lactation-specific proteins, reduced complement of host pathogen recognition proteins, and reduced olfaction/chemosensory associated genes. These genome data provide a foundation for research into trypanosomiasis prevention and yield important insights with broad implications for multiple aspects of tsetse biology.

Brown planthopper nudivirus DNA integrated in its host genome

The brown planthopper (BPH), Nilaparvata lugens (Hemiptera:Delphacidae), is one of the most destructive insect pests of rice crops in Asia. Nudivirus-like sequences were identified during the whole-genome sequencing of BPH. PCR examination showed that the virus sequences were present in all of the 22 BPH populations collected from East, Southeast, and South Asia. Thirty-two of the 33 nudivirus core genes were identified, including 20 homologues of baculovirus core genes. In addition, several gene clusters that were arranged collinearly with those of other nudiviruses were found in the partial virus genome. In a phylogenetic tree constructed using the supermatrix method, the original virus was grouped with other nudiviruses and was closely related to polydnavirus. Taken together, these data indicated that the virus sequences belong to a new member of the family Nudiviridae. More specifically, the virus sequences were integrated into the chromosome of its insect host during coevolution. This study is the first report of a large double-stranded circular DNA virus genome in a sap-sucking hemipteran insect.

IMPORTANCE

This is the first report of a large double-stranded DNA virus integrated genome in the planthopper, a plant sap-sucking hemipteran insect. It is an exciting addition to the evolutionary story of bracoviruses (polydnaviruses), nudiviruses, and baculoviruses. The results on the virus sequences integrated in the chromosomes of its insect host also represent a story of successful coevolution of an invertebrate virus and a plant sap-sucking insect.

Virology, Epidemiology and Pathology of Glossina Hytrosavirus, and Its Control Prospects in Laboratory Colonies of the Tsetse Fly, Glossina pallidipes (Diptera; Glossinidae)

The Glossina hytrosavirus (family Hytrosaviridae) is a double-stranded DNA virus with rod-shaped, enveloped virions. Its 190 kbp genome encodes 160 putative open reading frames. The virus replicates in the nucleus, and acquires a fragile envelope in the cell cytoplasm. Glossina hytrosavirus was first isolated from hypertrophied salivary glands of the tsetse fly, Glossina pallidipes Austen (Diptera; Glossinidae) collected in Kenya in 1986. A certain proportion of laboratory G. pallidipes flies infected by Glossina hytrosavirus develop hypertrophied salivary glands and midgut epithelial cells, gonadal anomalies and distorted sex-ratios associated with reduced insemination rates, fecundity and lifespan. These symptoms are rare in wild tsetse populations. In East Africa, G. pallidipes is one of the most important vectors of African trypanosomosis, a debilitating zoonotic disease that afflicts 37 sub-Saharan African countries. There is a large arsenal of control tactics available to manage tsetse flies and the disease they transmit. The sterile insect technique (SIT) is a robust control tactic that has shown to be effective in eradicating tsetse populations when integrated with other control tactics in an area-wide integrated approach. The SIT requires production of sterile male flies in large production facilities. To supply sufficient numbers of sterile males for the SIT component against G. pallidipes, strategies have to be developed that enable the management of the Glossina hytrosavirus in the colonies. This review provides a historic chronology of the emergence and biogeography of Glossina hytrosavirus, and includes researches on the infectomics (defined here as the functional and structural genomics and proteomics) and pathobiology of the virus. Standard operation procedures for viral management in tsetse mass-rearing facilities are proposed and a future outlook is sketched.

Managing hytrosavirus infections in Glossina pallidipes colonies: feeding regime affects the prevalence of salivary gland hypertrophy syndrome

Many species of tsetse flies are infected by a virus that causes salivary gland hypertrophy (SGH) syndrome and the virus isolated from Glossina pallidipes (GpSGHV) has recently been sequenced. Flies with SGH have a reduced fecundity and fertility. Due to the deleterious impact of SGHV on G. pallidipes colonies, several approaches were investigated to develop a virus management strategy. Horizontal virus transmission is the major cause of the high prevalence of the GpSGHV in tsetse colonies. Implementation of a “clean feeding” regime (fresh blood offered to each set of flies so that there is only one feed per membrane), instead of the regular feeding regime (several successive feeds per membrane), was among the proposed approaches to reduce GpSGHV infections. However, due to the absence of disposable feeding equipment (feeding trays and silicone membranes), the implementation of a clean feeding approach remains economically difficult. We developed a new clean feeding approach applicable to large-scale tsetse production facilities using existing resources. The results indicate that implementing this approach is feasible and leads to a significant reduction in virus load from 10⁹ virus copies in regular colonies to an average of 10^2.5 and eliminates the SGH syndrome from clean feeding colonies by28 months post implementation of this approach. The clean feeding approach also reduced the virus load from an average of 10⁸ virus copy numbers to an average of 10³ virus copies and SGH prevalence of 10% to 4% in flies fed after the clean fed colony. Taken together, these data indicate that the clean feeding approach is applicable in large-scale G. pallidipes production facilities and eliminates the deleterious effects of the virus and the SGH syndrome in these colonies.

Transgenerational transmission of the Glossina pallidipes hytrosavirus depends on the presence of a functional symbiome

The vertically transmitted endosymbionts (Sodalis glossinidius and Wigglesworthia glossinidia) of the tsetse fly (Diptera: Glossinidae) are known to supplement dietary deficiencies and modulate the reproductive fitness and the defense system of the fly. Some tsetse fly species are also infected with the bacterium, Wolbachia and with the Glossina hytrosavirus (GpSGHV). Laboratory-bred G. pallidipes exhibit chronic asymptomatic and acute symptomatic GpSGHV infection, with the former being the most common in these colonies. However, under as yet undefined conditions, the asymptomatic state can convert to the symptomatic state, leading to detectable salivary gland hypertrophy (SGH(+)) syndrome. In this study, we investigated the interplay between the bacterial symbiome and GpSGHV during development of G. pallidipes by knocking down the symbionts with antibiotic. Intrahaemocoelic injection of GpSGHV led to high virus titre (10(9) virus copies), but was not accompanied by either the onset of detectable SGH(+), or release of detectable virus particles into the blood meals during feeding events. When the F1 generations of GpSGHV-challenged mothers were dissected within 24 h post-eclosion, SGH(+) was observed to increase from 4.5% in the first larviposition cycle to >95% in the fourth cycle. Despite being sterile, these F1 SGH(+) progeny mated readily. Removal of the tsetse symbiome, however, suppressed transgenerational transfer of the virus via milk secretions and blocked the ability of GpSGHV to infect salivary glands of the F1 progeny. Whereas GpSGHV infects and replicates in salivary glands of developing pupa, the virus is unable to induce SGH(+) within fully differentiated adult salivary glands. The F1 SGH(+) adults are responsible for the GpSGHV-induced colony collapse in tsetse factories. Our data suggest that GpSGHV has co-evolved with the tsetse symbiome and that the symbionts play key roles in the virus transmission from mother to progeny.

Improving Sterile Insect Technique (SIT) for tsetse flies through research on their symbionts and pathogens

Tsetse flies (Diptera: Glossinidae) are the cyclical vectors of the trypanosomes, which cause human African trypanosomosis (HAT) or sleeping sickness in humans and African animal trypanosomosis (AAT) or nagana in animals. Due to the lack of effective vaccines and inexpensive drugs for HAT, and the development of resistance of the trypanosomes against the available trypanocidal drugs, vector control remains the most efficient strategy for sustainable management of these diseases. Among the control methods used for tsetse flies, Sterile Insect Technique (SIT), in the frame of area-wide integrated pest management (AW-IPM), represents an effective tactic to suppress and/or eradicate tsetse flies. One constraint in implementing SIT is the mass production of target species. Tsetse flies harbor obligate bacterial symbionts and salivary gland hypertrophy virus which modulate the fecundity of the infected flies. In support of the future expansion of the SIT for tsetse fly control, the Joint FAO/IAEA Programme of Nuclear Techniques in Food and Agriculture implemented a six year Coordinated Research Project (CRP) entitled "Improving SIT for Tsetse Flies through Research on their Symbionts and Pathogens". The consortium focused on the prevalence and the interaction between the bacterial symbionts and the virus, the development of strategies to manage virus infections in tsetse colonies, the use of entomopathogenic fungi to control tsetse flies in combination with SIT, and the development of symbiont-based strategies to control tsetse flies and trypanosomosis. The results of the CRP and the solutions envisaged to alleviate the constraints of the mass rearing of tsetse flies for SIT are presented in this special issue.

Proteomic footprints of a member of Glossinavirus (Hytrosaviridae): an expeditious approach to virus control strategies in tsetse factories

The Glossinavirus (Glossina pallidipes salivary gland hypertrophy virus (GpSGHV)) is a rod-shaped enveloped insect virus containing a 190,032 bp-long, circular dsDNA genome. The virus is pathogenic for the tsetse fly Glossina pallidipes and has been associated with the collapse of selected mass-reared colonies. Maintenance of productive fly colonies is critical to tsetse and trypanosomiasis eradication in sub-Saharan Africa using the Sterile Insect Technique. Proteomics, an approach to define the expressed protein complement of a genome, was used to further our understanding of the protein composition, morphology, morphogenesis and pathology of GpSGHV. Additionally, this approach provides potential targets for novel and sustainable molecular-based antiviral strategies to control viral infections in tsetse colonies. To achieve this goal, identification of key protein partners involved in virus transmission is required. In this review, we integrate the available data on GpSGHV proteomics to assess the impact of viral infections on host metabolism and to understand the contributions of such perturbations to viral pathogenesis. The relevance of the proteome findings to tsetse and trypanosomiasis management in sub-Sahara Africa is also considered.

Prevalence of SGHV among tsetse species of economic importance in Tanzania and their implication for SIT application

Sterile Insect technique is an important component in area-wide integrated tsetse control. The presence of the salivary glands hypertrophy virus (SGHV) in the wild tsetse, which are the seeds for colony adaptations in the laboratory has become a stumbling block in establishing and maintaining colonies in the laboratory. The virus is transmitted both vertically (in the wild) and horizontally (in the laboratory). However, its prevalence is magnified in the laboratory as a result of the use of in vitro membrane feeding regimen. Fly species of Glossina fuscipes fuscipes, G. pallidipes, G. morsitans and G. swynnertoni were collected from the coastal and inland areas of Tanzania and virus infection rates were assessed microscopically and by PCR. The data showed that in a period of 4years, the virus was present in all species tested irrespective of their ages, sex, and season of the year. However, infection levels differed among species and from one location to another. Symptomatic infection determined by dissection was 1.2% (25/2164) from the coast as compared to 0.4% (6/1725) for inland collected flies. PCR analysis indicated a higher infection rate of 19.81% (104/525) of asymptomatic flies. From these observations, we conclude that care should be taken when planning to initiate tsetse laboratory colonies for use in SIT eradication program. All efforts should be made to select non-infected flies when initiating laboratory colonies and to try to minimize the infection with SGHV. Also management of SGHV infection in the established colony should be applied.

Phylogeny and evolution of Hytrosaviridae

The Hytrosaviridae comprises a family of dsDNA viruses with a circular genome of 120-190 kb p. They are exclusively associated with Diptera, such as the tsetse fly, the house fly and the Narcissus bulb fly. Hytrosaviruses cause a very unique pathology including hypertrophy of salivary glands as well as testicular and ovarian malformation. On the other hand these viruses share a significant number of gene homologues with other dsDNA viruses, esp. baculoviruses and nudiviruses. These gene homologues include twelve so-called baculovirus core genes involved in transcription, DNA replication and the infection process. Most strikingly, the Musca domestica salivary gland hypertrophy virus (MdSGHV) encodes a homologue of a polyhedrin/granulin gene of Alpha-, Beta-, Gammabaculoviruses. Hence, it is proposed that hytrosaviruses are phylogenetically related to baculoviruses but evolved in a very close association with their dipteran hosts.

Alimentary Canal of the Adult Blow Fly, Chrysomya megacephala (F.) (Diptera: Calliphoridae)-Part I: Ultrastructure of Salivary Glands

The salivary gland ultrastructure of the adult male blow fly, Chrysomya megacephala (F.) (Diptera: Calliphoridae), was investigated at the ultrastructural level using light microscopy (LM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The salivary glands are paired structures composed of a single median deferent duct bifurcated into two long, narrow efferent ducts connected to the coiled tubular glands. The SEM image of the gland surface revealed that the basal lamina is relatively smooth in general, but the whole surface appeared as a trace of rough swollen insertion by intense tracheal ramification. Ultrastructurally, the salivary gland is enclosed within the basal lamina, and interdigitation cytoplasmic extensions were apparent between the adjacent gland cells. The basement membrane appeared infoldings that is similar to the complex of the labyrinth channel. The cytoplasm characteristic of the gland revealed high activity, based on the abundance of noticeable secretory granules, either singly or in an aggregated reservoir. In addition, mitochondria were found to intersperse among rich parallel of arrays rough endoplasmic reticulum. Thick cuticle, which was well-delineated and electron dense, apically lined the gland compartments, with discontinuity of the double-layer cuticle revealing a trace of secretion discharged into the lumen. Gross anatomy of the adult salivary gland was markedly different from that of the third instar of the same species, and structural dissimilarity is discussed briefly.

The antiviral drug valacyclovir successfully suppresses salivary gland hypertrophy virus (SGHV) in laboratory colonies of Glossina pallidipes

Many species of tsetse flies are infected with a virus that causes salivary gland hypertrophy (SGH) symptoms associated with a reduced fecundity and fertility. A high prevalence of SGH has been correlated with the collapse of two laboratory colonies of Glossina pallidipes and colony maintenance problems in a mass rearing facility in Ethiopia. Mass-production of G. pallidipes is crucial for programs of tsetse control including the sterile insect technique (SIT), and therefore requires a management strategy for this virus. Based on the homology of DNA polymerase between salivary gland hypertrophy virus and herpes viruses at the amino acid level, two antiviral drugs, valacyclovir and acyclovir, classically used against herpes viruses were selected and tested for their toxicity on tsetse flies and their impact on virus replication. While long term per os administration of acyclovir resulted in a significant reduction of productivity of the colonies, no negative effect was observed in colonies fed with valacyclovir-treated blood. Furthermore, treatment of a tsetse colony with valacyclovir for 83 weeks resulted in a significant reduction of viral loads and consequently suppression of SGH symptoms. The combination of initial selection of SGHV-negative flies by non-destructive PCR, a clean feeding system, and valacyclovir treatment resulted in a colony that was free of SGH syndromes in 33 weeks. This is the first report of the use of a drug to control a viral infection in an insect and of the demonstration that valacyclovir can be used to suppress SGH in colonies of G. pallidipes.

Prevalence and genetic variation of salivary gland hypertrophy virus in wild populations of the tsetse fly Glossina pallidipes from southern and eastern Africa

The Glossina pallidipes salivary gland hypertrophy virus (GpSGHV) is a rod-shaped, non-occluded double-stranded DNA virus that causes salivary gland hypertrophy (SGH) and reduced fecundity in the tsetse fly G. pallidipes. High GpSGHV prevalence (up to 80%) makes it impossible to mass-rear G. pallidipes colonies for the sterile insect technique (SIT). To evaluate the feasibility of molecular-based GpSGHV management strategies, we investigated the prevalence and genetic diversity of GpSGHV in wild populations of G. pallidipes collected from ten geographical locations in eastern and southern Africa. Virus diversity was examined using a total sequence of 1497 nucleotides (≈ 1% of the GpSGHV genome) from five putative conserved ORFs, p74, pif1, pif2, pif3 and dnapol. Overall, 34.08% of the analyzed flies (n=1972) tested positive by nested PCR. GpSGHV prevalence varied from 2% to 100% from one location to another but phylogenetic and gene genealogy analyses using concatenated sequences of the five putative ORFs revealed low virus diversity. Although no correlation of the virus diversity to geographical locations was detected, the GpSGHV haplotypes could be assigned to one of two distinct clades. The reference (Tororo) haplotype was the most widely distributed, and was shared by 47 individuals in seven of the 11 locations. The Ethiopian haplotypes were restricted to one clade, and showed the highest divergence (with 14-16 single nucleotide mutation steps) from the reference haplotype. The current study suggests that the proposed molecular-based virus management strategies have a good prospect of working throughout eastern and southern Africa due to the low diversity of the GpSGHV strains.

Analysis of the structural proteins from the Musca domestica hytrosavirus with an emphasis on the major envelope protein

The Musca domestica hytrosavirus (MdHV), a member of the family Hyrosaviridae, is a large, dsDNA, enveloped virus that infects adult house flies and causes a diagnostic hypertrophy of the salivary gland. Herein, studies were directed at identifying key structural components of the viral envelope and nucleocapsid. SDS-PAGE of detergent-treated virus fractions identified protein bands unique to the envelope and nucleocapsid components. Using prior LC-MSMS data we identified the viral ORF associated with the major envelope band, cloned and expressed recombinant viral antigens, and prepared a series of polyclonal sera. Western blots confirmed that antibodies recognized the target viral antigen and provided evidence that the viral protein MdHV96 underwent post-translational processing; antibodies bound to the target high molecular weight parent molecule as well as distinct sets of smaller bands. Immuno gold electron microscopy demonstrated that the anti-MdHV96 sera recognized target antigens associated with the envelope. The nucleocapsids migrated from the virogenic stroma in the nucleus through the nuclear membrane into the cytoplasm, where they acquired an initial envelope that contained MdHV96. This major envelope protein, appeared to incorporate into intracellular membranes of both the caniculi and rough endoplasmic reticulum membranes and mediate binding to the nucleocapsids. Oral infection bioassays demonstrated that the anti-HV96 polyclonal sera acted as neutralizing agents in suppressing the levels of orally acquired infections.

The salivary secretome of the tsetse fly Glossina pallidipes (Diptera: Glossinidae) infected by salivary gland hypertrophy virus

BACKGROUND

The competence of the tsetse fly Glossina pallidipes (Diptera; Glossinidae) to acquire salivary gland hypertrophy virus (SGHV), to support virus replication and successfully transmit the virus depends on complex interactions between Glossina and SGHV macromolecules. Critical requisites to SGHV transmission are its replication and secretion of mature virions into the fly's salivary gland (SG) lumen. However, secretion of host proteins is of equal importance for successful transmission and requires cataloging of G. pallidipes secretome proteins from hypertrophied and non-hypertrophied SGs.

METHODOLOGY/PRINCIPAL FINDINGS

After electrophoretic profiling and in-gel trypsin digestion, saliva proteins were analyzed by nano-LC-MS/MS. MaxQuant/Andromeda search of the MS data against the non-redundant (nr) GenBank database and a G. morsitans morsitans SG EST database, yielded a total of 521 hits, 31 of which were SGHV-encoded. On a false discovery rate limit of 1% and detection threshold of least 2 unique peptides per protein, the analysis resulted in 292 Glossina and 25 SGHV MS-supported proteins. When annotated by the Blast2GO suite, at least one gene ontology (GO) term could be assigned to 89.9% (285/317) of the detected proteins. Five (∼1.8%) Glossina and three (∼12%) SGHV proteins remained without a predicted function after blast searches against the nr database. Sixty-five of the 292 detected Glossina proteins contained an N-terminal signal/secretion peptide sequence. Eight of the SGHV proteins were predicted to be non-structural (NS), and fourteen are known structural (VP) proteins.

CONCLUSIONS/SIGNIFICANCE

SGHV alters the protein expression pattern in Glossina. The G. pallidipes SG secretome encompasses a spectrum of proteins that may be required during the SGHV infection cycle. These detected proteins have putative interactions with at least 21 of the 25 SGHV-encoded proteins. Our findings opens venues for developing novel SGHV mitigation strategies to block SGHV infections in tsetse production facilities such as using SGHV-specific antibodies and phage display-selected gut epithelia-binding peptides.

Next generation sequencing technologies for insect virus discovery

Insects are commonly infected with multiple viruses including those that cause sublethal, asymptomatic, and latent infections. Traditional methods for virus isolation typically lack the sensitivity required for detection of such viruses that are present at low abundance. In this respect, next generation sequencing technologies have revolutionized methods for the discovery and identification of new viruses from insects. Here we review both traditional and modern methods for virus discovery, and outline analysis of transcriptome and small RNA data for identification of viral sequences. We will introduce methods for de novo assembly of viral sequences, identification of potential viral sequences from BLAST data, and bioinformatics for generating full-length or near full-length viral genome sequences. We will also discuss implications of the ubiquity of viruses in insects and in insect cell lines. All of the methods described in this article can also apply to the discovery of viruses in other organisms.

Paleozoic origin of insect large dsDNA viruses

To understand how extant viruses interact with their hosts, we need a historical framework of their evolutionary association. Akin to retrovirus or hepadnavirus viral fossils present in eukaryotic genomes, bracoviruses are integrated in braconid wasp genomes and are transmitted by Mendelian inheritance. However, unlike viral genomic fossils, they have retained functional machineries homologous to those of large dsDNA viruses pathogenic to arthropods. Using a phylogenomic approach, we resolved the relationships between bracoviruses and their closest free relatives: baculoviruses and nudiviruses. The phylogeny showed that bracoviruses are nested within the nudivirus clade. Bracoviruses establish a bridge between the virus and animal worlds. Their inclusion in a virus phylogeny allowed us to relate free viruses to fossils. The ages of the wasps were used to calibrate the virus phylogeny. Bayesian analyses revealed that insect dsDNA viruses first evolved at ∼310 Mya in the Paleozoic Era during the Carboniferous Period with the first insects. Furthermore the virus diversification time frame during the Mesozoic Era appears linked to the diversification of insect orders; baculoviruses that infect larvae evolved at the same period as holometabolous insects. These results imply ancient coevolution by resource tracking between several insect dsDNA virus families and their hosts, dating back to 310 Mya.

Tsetse salivary gland hypertrophy virus: hope or hindrance for tsetse control?

Many species of tsetse flies (Diptera: Glossinidae) are infected with a virus that causes salivary gland hypertrophy (SGH), and flies with SGH symptoms have a reduced fecundity and fertility. The prevalence of SGH in wild tsetse populations is usually very low (0.2%–5%), but higher prevalence rates (15.2%) have been observed occasionally. The successful eradication of a Glossina austeni population from Unguja Island (Zanzibar) using an area-wide integrated pest management approach with a sterile insect technique (SIT) component (1994–1997) encouraged several African countries, including Ethiopia, to incorporate the SIT in their national tsetse control programs. A large facility to produce tsetse flies for SIT application in Ethiopia was inaugurated in 2007. To support this project, a Glossina pallidipes colony originating from Ethiopia was successfully established in 1996, but later up to 85% of adult flies displayed symptoms of SGH. As a result, the colony declined and became extinct by 2002. The difficulties experienced with the rearing of G. pallidipes, epitomized by the collapse of the G. pallidipes colony originating from Ethiopia, prompted the urgent need to develop management strategies for the salivary gland hypertrophy virus (SGHV) for this species. As a first step to identify suitable management strategies, the virus isolated from G. pallidipes (GpSGHV) was recently sequenced and research was initiated on virus transmission and pathology. Different approaches to prevent virus replication and its horizontal transmission during blood feeding have been proposed. These include the use of antiviral drugs such as acyclovir and valacyclovir added to the blood for feeding or the use of antibodies against SGHV virion proteins. In addition, preliminary attempts to silence the expression of an essential viral protein using RNA interference will be discussed.

Two hytrosaviruses, MdSGHV and GpSGHV, induce distinct cytopathologies in their respective host insects

Recently, a new virus family (Hytrosaviridae) was proposed for double-stranded DNA viruses that cause salivary gland hypertrophy in their dipteran hosts. The two type species, MdSGHV and GpSGHV, induce similar gross pathology and share several morphological, biological, and molecular characteristics. This histological study revealed profound differences in the cytopathology of these viruses supporting their previously proposed placement in different genera.

Viruses associated with ovarian degeneration in Apis mellifera L. queens

Queen fecundity is a critical issue for the health of honeybee (Apis mellifera L.) colonies, as she is the only reproductive female in the colony and responsible for the constant renewal of the worker bee population. Any factor affecting the queen's fecundity will stagnate colony development, increasing its susceptibility to opportunistic pathogens. We discovered a pathology affecting the ovaries, characterized by a yellow discoloration concentrated in the apex of the ovaries resulting from degenerative lesions in the follicles. In extreme cases, marked by intense discoloration, the majority of the ovarioles were affected and these cases were universally associated with egg-laying deficiencies in the queens. Microscopic examination of the degenerated follicles showed extensive paracrystal lattices of 30 nm icosahedral viral particles. A cDNA library from degenerated ovaries contained a high frequency of deformed wing virus (DWV) and Varroa destructor virus 1 (VDV-1) sequences, two common and closely related honeybee Iflaviruses. These could also be identified by in situ hybridization in various parts of the ovary. A large-scale survey for 10 distinct honeybee viruses showed that DWV and VDV-1 were by far the most prevalent honeybee viruses in queen populations, with distinctly higher prevalence in mated queens (100% and 67%, respectively for DWV and VDV-1) than in virgin queens (37% and 0%, respectively). Since very high viral titres could be recorded in the ovaries and abdomens of both functional and deficient queens, no significant correlation could be made between viral titre and ovarian degeneration or egg-laying deficiency among the wider population of queens. Although our data suggest that DWV and VDV-1 have a role in extreme cases of ovarian degeneration, infection of the ovaries by these viruses does not necessarily result in ovarian degeneration, even at high titres, and additional factors are likely to be involved in this pathology.

Salivary gland hypertrophy viruses: a novel group of insect pathogenic viruses

Salivary gland hypertrophy viruses (SGHVs) are a unique, unclassified group of entomopathogenic, double-stranded DNA viruses that have been reported from three genera of Diptera. These viruses replicate in nuclei of salivary gland cells in adult flies, inducing gland enlargement with little obvious external disease symptoms. Viral infection inhibits reproduction by suppressing vitellogenesis, causing testicular aberrations, and/or disrupting mating behavior. Historical and present research findings support a recent proposal of a new virus family, the Hytrosaviridae. This review describes the discovery and prevalence of different SGHVs, summarizes their biochemical characterization and taxonomy, compares morphological and histopathological properties, and details transmission routes and the influence of infection on host biology and reproduction. In addition, the potential use of SGHVs as sterilizing agents for house fly control and the deleterious impact of SGHVs on colonized tsetse flies reared for sterile insect technique are discussed.

Universal primers for rapid detection of hytrosaviruses

Hytrosaviridae is a proposed virus family encompassing viruses that cause salivary gland hypertrophy (SGH) syndrome in infected insects and reduce the fertility in their dipteran insect hosts. They contain a large, double stranded DNA genome of 120-190 kbp. To date, these viruses have been detected only in adult Diptera. These include hytrosaviruses detected in various tsetse fly species (Glossina spp.), the narcissus bulb fly Merodon equestris and the house fly Musca domestica. The limited number of hytrosaviruses reported to date may be a reflection of the frequent absence of external symptoms in infected adult flies and the fact that the virus does not cause rapid mortality. Based on the complete genome sequence of Glossinia pallidipes (GpSGHV) and Musca domestica (MdSGHV) salivary gland hypertrophy viruses, a PCR based methodology was developed to detect the viruses in these species. To be able to detect hytrosaviruses in other Diptera, five degenerate primer pairs were designed and tested on GpSGHV and MdSGHV DNA using gradient PCR with annealing temperatures from 37 to 61°C. Two pairs of primers were selected from p74, two pairs from PIF-1 and one pair from ODV-e66 homologous proteins. Four primer pairs generated a virus specific PCR product on both MdSGHV and GpSGHV at all tested annealing temperatures, while the ODV-e66 based primers did not generate a virus specific product with annealing temperatures higher that 47°C. No non-specific PCR product was found when using genomic DNA of infected flies as template DNA. These results offer new sets of primers that could be used to detect hytrosaviruses in other insects.

Proteomic analysis of Glossina pallidipes salivary gland hypertrophy virus virions for immune intervention in tsetse fly colonies

Many species of tsetse flies (Diptera: Glossinidae) can be infected by a virus that causes salivary gland hypertrophy (SGH). The genomes of viruses isolated from Glossina pallidipes (GpSGHV) and Musca domestica (MdSGHV) have recently been sequenced. Tsetse flies with SGH have reduced fecundity and fertility which cause a serious problem for mass rearing in the frame of sterile insect technique (SIT) programmes to control and eradicate tsetse populations in the wild. A potential intervention strategy to mitigate viral infections in fly colonies is neutralizing of the GpSGHV infection with specific antibodies against virion proteins. Two major GpSGHV virion proteins of about 130 and 50 kDa, respectively, were identified by Western analysis using a polyclonal rabbit antibody raised against whole GpSHGV virions. The proteome of GpSGHV, containing the antigens responsible for the immune-response, was investigated by liquid chromatography tandem mass spectrometry and 61 virion proteins were identified by comparison with the genome sequence. Specific antibodies were produced in rabbits against seven candidate proteins, including the ORF10/C-terminal fragment, ORF47 and ORF96 as well as proteins involved in peroral infectivity PIF-1 (ORF102), PIF-2 (ORF53), PIF-3 (ORF76) and P74 (ORF1). Antiserum against ORF10 specifically reacted to the 130 kDa protein in a Western blot analysis and to the envelope protein of GpSGHV, detected by using immunogold-electron microscopy. This result suggests that immune intervention of viral infections in colonies of G. pallidipes is a realistic option.

Non-canonical DNA transcription enzymes and the conservation of two-barrel RNA polymerases

DNA transcription depends on multimeric RNA polymerases that are exceptionally conserved in all cellular organisms, with an active site region of >500 amino acids mainly harboured by their Rpb1 and Rpb2 subunits. Together with the distantly related eukaryotic RNA-dependent polymerases involved in gene silencing, they form a monophyletic family of ribonucleotide polymerases with a similarly organized active site region based on two double-Psi barrels. Recent viral and phage genome sequencing have added a surprising variety of putative nucleotide polymerases to this protein family. These proteins have highly divergent subunit composition and amino acid sequences, but always contain eight invariant amino acids forming a universally conserved catalytic site shared by all members of the two-barrel protein family. Moreover, the highly conserved 'funnel' and 'switch 2' components of the active site region are shared by all putative DNA-dependent RNA polymerases and may thus determine their capacity to transcribe double-stranded DNA templates.