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

Evaluating the Effect of Irradiation on the Densities of Two RNA Viruses in Glossina morsitans morsitans

Tsetse flies are cyclic vectors of Trypanosoma parasites, which cause debilitating diseases in humans and animals. To decrease the disease burden, the number of flies is reduced using the sterile insect technique (SIT), where male flies are sterilized through irradiation and released into the field. This procedure requires the mass rearing of high-quality male flies able to compete with wild male flies for mating with wild females. Recently, two RNA viruses, an iflavirus and a negevirus, were discovered in mass-reared Glossina morsitans morsitans and named GmmIV and GmmNegeV, respectively. The aim of this study was to evaluate whether the densities of these viruses in tsetse flies are affected by the irradiation treatment. Therefore, we exposed tsetse pupae to various doses (0-150 Gy) of ionizing radiation, either in air (normoxia) or without air (hypoxia), for which oxygen was displaced by nitrogen. Pupae and/or emerging flies were collected immediately afterwards, and at three days post irradiation, virus densities were quantified through RT-qPCR. Generally, the results show that irradiation exposure had no significant impact on the densities of GmmIV and GmmNegeV, suggesting that the viruses are relatively radiation-resistant, even at higher doses. However, sampling over a longer period after irradiation would be needed to verify that densities of these insect viruses are not changed by the sterilisation treatment.

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.

Interactions Between Tsetse Endosymbionts and Glossina pallidipes Salivary Gland Hypertrophy Virus in Glossina Hosts

Tsetse flies are the sole cyclic vector for trypanosomosis, the causative agent for human African trypanosomosis or sleeping sickness and African animal trypanosomosis or nagana. Tsetse population control is the most efficient strategy for animal trypanosomosis control. Among all tsetse control methods, the Sterile Insect Technique (SIT) is one of the most powerful control tactics to suppress or eradicate tsetse flies. However, one of the challenges for the implementation of SIT is the mass production of target species. Tsetse flies have a highly regulated and defined microbial fauna composed of three bacterial symbionts (Wigglesworthia, Sodalis and Wolbachia) and a pathogenic Glossina pallidipes Salivary Gland Hypertrophy Virus (GpSGHV) which causes reproduction alterations such as testicular degeneration and ovarian abnormalities with reduced fertility and fecundity. Interactions between symbionts and GpSGHV might affect the performance of the insect host. In the present study, we assessed the possible impact of GpSGHV on the prevalence of tsetse endosymbionts under laboratory conditions to decipher the bidirectional interactions on six Glossina laboratory species. The results indicate that tsetse symbiont densities increased over time in tsetse colonies with no clear impact of the GpSGHV infection on symbionts density. However, a positive correlation between the GpSGHV and Sodalis density was observed in Glossina fuscipes species. In contrast, a negative correlation between the GpSGHV density and symbionts density was observed in the other taxa. It is worth noting that the lowest Wigglesworthia density was observed in G. pallidipes, the species which suffers most from GpSGHV infection. In conclusion, the interactions between GpSGHV infection and tsetse symbiont infections seems complicated and affected by the host and the infection density of the GpSGHV and tsetse symbionts.

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.

Impact of Glossina pallidipes salivary gland hypertrophy virus (GpSGHV) on a heterologous tsetse fly host, Glossina fuscipes fuscipes

BACKGROUND

Tsetse flies (Diptera: Glossinidae) are the vectors of African trypanosomosis, the causal agent of sleeping sickness in humans and nagana in animals. Glossina fuscipes fuscipes is one of the most important tsetse vectors of sleeping sickness, particularly in Central Africa. Due to the development of resistance of the trypanosomes to the commonly used trypanocidal drugs and the lack of effective vaccines, vector control approaches remain the most effective strategies for sustainable management of those diseases. The Sterile Insect Technique (SIT) is an effective, environment-friendly method for the management of tsetse flies in the context of area-wide integrated pest management programs (AW-IPM). This technique relies on the mass-production of the target insect, its sterilization with ionizing radiation and the release of sterile males in the target area where they will mate with wild females and induce sterility in the native population. It has been shown that Glossina pallidipes salivary gland hypertrophy virus (GpSGHV) infection causes a decrease in fecundity and fertility hampering the maintenance of colonies of the tsetse fly G. pallidipes. This virus has also been detected in different species of tsetse files. In this study, we evaluated the impact of GpSGHV on the performance of a colony of the heterologous host G. f. fuscipes, including the flies' productivity, mortality, survival, flight propensity and mating ability and insemination rates.

RESULTS

Even though GpSGHV infection did not induce SGH symptoms, it significantly reduced all examined parameters, except adult flight propensity and insemination rate.

CONCLUSION

These results emphasize the important role of GpSGHV management strategy in the maintenance of G. f. fuscipes colonies and the urgent need to implement measures to avoid virus infection, to ensure the optimal mass production of this tsetse species for use in AW-IPM programs with an SIT component.

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.

Prevalence of trypanosomes, salivary gland hypertrophy virus and Wolbachia in wild populations of tsetse flies from West Africa

BACKGROUND

Tsetse flies are vectors of African trypanosomes, protozoan parasites that cause sleeping sickness (or human African trypanosomosis) in humans and nagana (or animal African trypanosomosis) in livestock. In addition to trypanosomes, four symbiotic bacteria Wigglesworthia glossinidia, Sodalis glossinidius, Wolbachia, Spiroplasma and one pathogen, the salivary gland hypertrophy virus (SGHV), have been reported in different tsetse species. We evaluated the prevalence and coinfection dynamics between Wolbachia, trypanosomes, and SGHV in four tsetse species (Glossina palpalis gambiensis, G. tachinoides, G. morsitans submorsitans, and G. medicorum) that were collected between 2008 and 2015 from 46 geographical locations in West Africa, i.e. Burkina Faso, Mali, Ghana, Guinea, and Senegal.

RESULTS

The results indicated an overall low prevalence of SGHV and Wolbachia and a high prevalence of trypanosomes in the sampled wild tsetse populations. The prevalence of all three infections varied among tsetse species and sample origin. The highest trypanosome prevalence was found in Glossina tachinoides (61.1%) from Ghana and in Glossina palpalis gambiensis (43.7%) from Senegal. The trypanosome prevalence in the four species from Burkina Faso was lower, i.e. 39.6% in Glossina medicorum, 18.08%; in Glossina morsitans submorsitans, 16.8%; in Glossina tachinoides and 10.5% in Glossina palpalis gambiensis. The trypanosome prevalence in Glossina palpalis gambiensis was lowest in Mali (6.9%) and Guinea (2.2%). The prevalence of SGHV and Wolbachia was very low irrespective of location or tsetse species with an average of 1.7% for SGHV and 1.0% for Wolbachia. In some cases, mixed infections with different trypanosome species were detected. The highest prevalence of coinfection was Trypanosoma vivax and other Trypanosoma species (9.5%) followed by coinfection of T. congolense with other trypanosomes (7.5%). The prevalence of coinfection of T. vivax and T. congolense was (1.0%) and no mixed infection of trypanosomes, SGHV and Wolbachia was detected.

CONCLUSION

The results indicated a high rate of trypanosome infection in tsetse wild populations in West African countries but lower infection rate of both Wolbachia and SGHV. Double or triple mixed trypanosome infections were found. In addition, mixed trypanosome and SGHV infections existed however no mixed infections of trypanosome and/or SGHV with Wolbachia were found.

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.

Hytrosaviruses: current status and perspective

Salivary gland hytrosaviruses (SGHVs) are entomopathogenic dsDNA, enveloped viruses that replicate in the salivary glands (SGs) of the adult dipterans, Glossina spp (GpSGHV) and Musca domestica (MdSGHV). Although belonging to the same virus family (Hytrosaviridae), SGHVs have distinct morphologies and pathobiologies. Two GpSGHV strains potentially account for the differential pathologies in lab-bred tsetse. New data suggest incorporation of host-derived cellular proteins and lipids into mature SGHVs. In addition to within the SGs, MdSGHV undergoes limited replication in the corpora allata, potentially disrupting hormone biosynthesis, and GpSGHV replicates in the milk glands providing a transmission conduit to progeny tsetse. Whereas MdSGHV is a potential biocontrol agent, the vertically transmitted GpSGHV is unsuitable for tsetse vector control but does jeopardize tsetse mass rearing.

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.

Viruses of insects reared for food and feed

The use of insects as food for humans or as feed for animals is an alternative for the increasing high demand for meat and has various environmental and social advantages over the traditional intensive production of livestock. Mass rearing of insects, under insect farming conditions or even in industrial settings, can be the key for a change in the way natural resources are utilized in order to produce meat, animal protein and a list of other valuable animal products. However, because insect mass rearing technology is relatively new, little is known about the different factors that determine the quality and yield of the production process. Obtaining such knowledge is crucial for the success of insect-based product development. One of the issues that is likely to compromise the success of insect rearing is the outbreak of insect diseases. In particular, viral diseases can be devastating for the productivity and the quality of mass rearing systems. Prevention and management of viral diseases imply the understanding of the different factors that interact in insect mass rearing. This publication provides an overview of the known viruses in insects most commonly reared for food and feed. Nowadays with large-scale sequencing techniques, new viruses are rapidly being discovered. We discuss factors affecting the emergence of viruses in mass rearing systems, along with virus transmission routes. Finally we provide an overview of the wide range of measures available to prevent and manage virus outbreaks in mass rearing systems, ranging from simple sanitation methods to highly sophisticated methods including RNAi and transgenics.

More than one rabbit out of the hat: Radiation, transgenic and symbiont-based approaches for sustainable management of mosquito and tsetse fly populations

Mosquitoes (Diptera: Culicidae) and tsetse flies (Diptera: Glossinidae) are bloodsucking vectors of human and animal pathogens. Mosquito-borne diseases (malaria, filariasis, dengue, zika, and chikungunya) cause severe mortality and morbidity annually, and tsetse fly-borne diseases (African trypanosomes causing sleeping sickness in humans and nagana in livestock) cost Sub-Saharan Africa an estimated US$ 4750 million annually. Current reliance on insecticides for vector control is unsustainable: due to increasing insecticide resistance and growing concerns about health and environmental impacts of chemical control there is a growing need for novel, effective and safe biologically-based methods that are more sustainable. The integration of the sterile insect technique has proven successful to manage crop pests and disease vectors, particularly tsetse flies, and is likely to prove effective against mosquito vectors, particularly once sex-separation methods are improved. Transgenic and symbiont-based approaches are in development, and more advanced in (particularly Aedes) mosquitoes than in tsetse flies; however, issues around stability, sustainability and biosecurity have to be addressed, especially when considering population replacement approaches. Regulatory issues and those relating to intellectual property and economic cost of application must also be overcome. Standardised methods to assess insect quality are required to compare and predict efficacy of the different approaches. Different combinations of these three approaches could be integrated to maximise their benefits, and all have the potential to be used in tsetse and mosquito area-wide integrated pest management programmes.

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.

Microbiome frequency and their association with trypanosome infection in male Glossina morsitans centralis of Western Zambia

Tsetse flies (Diptera: Glossinidae) are considered primary cyclical vectors that transmit pathogenic trypanosomes in Africa. They harbour a variety of microbes including Wolbachia, Sodalis and the salivary gland hypertrophy virus (SGHV) which are all vertically transmitted. Knowledge on tsetse microbiome and their interactions may identify novel strategies for tsetse fly and trypanosomiasis control. Area-wide application of such strategies requires an understanding of the natural microbiome frequency in the different species and subspecies of Glossina in their geographical populations. Consequently, this study determined the prevalence of Sodalis, Wolbachia, SGHV and trypanosome infections in Glossina morsitanscentralis from two sites of Western Zambia. We also explored possible associations of the microbes with trypanosome infections. Male G. morsitanscentralis samples were collected from two sites (Lyoni and Lusinina) in Western Zambia. The age structure of the flies at each site was determined using the wing fray method. DNA was extracted from the samples and analyzed for Wolbachia, Sodalis, SGHV and trypanosome presence using PCR. Associations and measures of associations between trypanosome infection and microbes in the fly were determined. The flies from the two locations (Lusinina, n=45 and Lyoni, n=24) had a similar age structure with their median fray category not being significantly different (p=0.698). The overall prevalence of Wolbachia was 72.5% (95% CI: 61.6-83.3%), Sodalis was 15.9% (95% CI: 7.1-24.8%), SGHV was 31.9% (95% CI: 20.6-43.2%) and Trypanosoma species was 23.2% (95% CI: 13-33.4%). The prevalence of Wolbachia was significantly higher in Lusinina than Lyoni (p=0.000). However this was not the case for Sodalis, SGHV and Trypanosoma species. Despite the low number of flies that were positive for both trypanosome and Sodalis (6; 8.7%), a statistically significant association (p=0.013; AOR 6.2; 95% CI: 1.5-25.8) was observed in G. morsitanscentralis. The study showed that the prevalence of microbiota may vary within the same species of the tsetse depending on the geographical location as was the case of Wolbachia. Further it showed that infection with Sodalis could affect vector competence. The study concludes that Sodalis could be an ideal candidate for symbiont-mediated trypanosomiasis control interventions in G. morsitanscentralis.

Antiviral drug valacyclovir treatment combined with a clean feeding system enhances the suppression of salivary gland hypertrophy in laboratory colonies of Glossina pallidipes

BACKGROUND

Hytrosaviridae cause salivary gland hypertrophy (SGH) syndrome in some infected tsetse flies (Diptera: Glossinidae). Infected male and female G. pallidipes with SGH have a reduced fecundity and fertility. Due to the deleterious impact of the virus on G. pallidipes colonies, adding the antiviral drug valacyclovir to the blood diet and changing the feeding regime to a clean feeding system (each fly receives for each feeding a fresh clean blood meal) have been investigated to develop virus management strategies. Although both approaches used alone successfully reduced the virus load and the SGH prevalence in small experimental groups, considerable time was needed to obtain the desired SGH reduction and both systems were only demonstrated with colonies that had a low initial virus prevalence (SGH ≤ 10%). As problems with SGH are often only recognized once the incidence is already high, it was necessary to demonstrate that this combination would also work for high prevalence colonies.

FINDINGS

Combining both methods at colony level successfully suppressed the SGH in G. pallidipes colonies that had a high initial virus prevalence (average SGH of 24%). Six months after starting the combined treatment SGH symptoms were eliminated from the treated colony, in contrast to 28 months required to obtain the same results using clean feeding alone and 21 months using antiviral drug alone.

CONCLUSIONS

Combining valacyclovir treatment with the clean feeding system provides faster control of SGH in tsetse than either method alone and is effective even when the initial SGH prevalence is high.

Laboratory colonisation and genetic bottlenecks in the tsetse fly Glossina pallidipes

Background

The IAEA colony is the only one available for mass rearing of Glossina pallidipes, a vector of human and animal African trypanosomiasis in eastern Africa. This colony is the source for Sterile Insect Technique (SIT) programs in East Africa. The source population of this colony is unclear and its genetic diversity has not previously been evaluated and compared to field populations.

Methodology/Principal Findings

We examined the genetic variation within and between the IAEA colony and its potential source populations in north Zimbabwe and the Kenya/Uganda border at 9 microsatellites loci to retrace the demographic history of the IAEA colony. We performed classical population genetics analyses and also combined historical and genetic data in a quantitative analysis using Approximate Bayesian Computation (ABC). There is no evidence of introgression from the north Zimbabwean population into the IAEA colony. Moreover, the ABC analyses revealed that the foundation and establishment of the colony was associated with a genetic bottleneck that has resulted in a loss of 35.7% of alleles and 54% of expected heterozygosity compared to its source population. Also, we show that tsetse control carried out in the 1990's is likely reduced the effective population size of the Kenya/Uganda border population.

Conclusions/Significance

All the analyses indicate that the area of origin of the IAEA colony is the Kenya/Uganda border and that a genetic bottleneck was associated with the foundation and establishment of the colony. Genetic diversity associated with traits that are important for SIT may potentially have been lost during this genetic bottleneck which could lead to a suboptimal competitiveness of the colony males in the field. The genetic diversity of the colony is lower than that of field populations and so, studies using colony flies should be interpreted with caution when drawing general conclusions about G. pallidipes biology.

There is only one mass reared laboratory colony of Glossina pallidipes, a vector of human African trypanosomiasis and arguably the main vector of animal African trypanosomiasis in eastern Africa. This colony is the main one used for basic research on this species and is intended to be used for Sterile Insect Technique (SIT) programs for control of field populations. The origins of this colony are not clear and the extent to which it is genetically representative of the species is unknown. Using population genetics analyses to compare the current colony with two potential source populations we have shown that the colony is from the Kenya/Uganda border and that its foundation and establishment in the laboratory were associated with a genetic bottleneck, i.e. reduction of genetic variation due to increased genetic drift in a population of reduced size. As a consequence, the genetic diversity of the colony is lower than that of G. pallidipes field populations.

Tsetse fly microbiota: form and function

Tsetse flies are the primary vectors of African trypanosomes, which cause Human and Animal African trypanosomiasis in 36 countries in sub-Saharan Africa. These flies have also established symbiotic associations with bacterial and viral microorganisms. Laboratory-reared tsetse flies harbor up to four vertically transmitted organisms—obligate Wigglesworthia, commensal Sodalis, parasitic Wolbachia and Salivary Gland Hypertrophy Virus (SGHV). Field-captured tsetse can harbor these symbionts as well as environmentally acquired commensal bacteria. This microbial community influences several aspects of tsetse's physiology, including nutrition, fecundity and vector competence. This review provides a detailed description of tsetse's microbiome, and describes the physiology underlying host-microbe, and microbe-microbe, interactions that occur in this fly.

Wolbachia, Sodalis and trypanosome co-infections in natural populations of Glossina austeni and Glossina pallidipes

BACKGROUND

Tsetse flies harbor at least three bacterial symbionts: Wigglesworthia glossinidia, Wolbachia pipientis and Sodalis glossinidius. Wigglesworthia and Sodalis reside in the gut in close association with trypanosomes and may influence establishment and development of midgut parasite infections. Wolbachia has been shown to induce reproductive effects in infected tsetse. This study was conducted to determine the prevalence of these endosymbionts in natural populations of G. austeni and G. pallidipes and to assess the degree of concurrent infections with trypanosomes.

METHODS

Fly samples analyzed originated from Kenyan coastal forests (trapped in 2009-2011) and South African G. austeni collected in 2008. The age structure was estimated by standard methods. G. austeni (n=298) and G. pallidipes (n= 302) were analyzed for infection with Wolbachia and Sodalis using PCR. Trypanosome infection was determined either by microscopic examination of dissected organs or by PCR amplification.

RESULTS

Overall we observed that G. pallidipes females had a longer lifespan (70 d) than G. austeni (54 d) in natural populations. Wolbachia infections were present in all G. austeni flies analysed, while in contrast, this symbiont was absent from G. pallidipes. The density of Wolbachia infections in the Kenyan G. austeni population was higher than that observed in South African flies. The infection prevalence of Sodalis ranged from 3.7% in G. austeni to about 16% in G. pallidipes. Microscopic examination of midguts revealed an overall trypanosome infection prevalence of 6% (n = 235) and 5% (n = 552), while evaluation with ITS1 primers indicated a prevalence of about 13% (n = 296) and 10% (n = 302) in G. austeni and G. pallidipes, respectively. The majority of infections (46%) were with T. congolense. Co-infection with all three organisms was observed at 1% and 3.3% in G. austeni and G. pallidipes, respectively. Eleven out of the thirteen (85%) co-infected flies harboured T. congolense and T. simiae parasites. While the association between trypanosomes and Sodalis infection was statistically significant in G. pallidipes (P = 0.0127), the number of co-infected flies was too few for a definite conclusion.

CONCLUSIONS

The tsetse populations analyzed differed in the prevalence of symbionts, despite being sympatric and therefore exposed to identical environmental factors. The density of infections with Wolbachia also differed between G. austeni populations. There were too few natural co-infections detected with the Sodalis and trypanosomes to suggest extensive inter-relations between these infections in natural populations. We discuss these findings in the context of potential symbiont-mediated control interventions to reduce parasite infections and/or fly populations.

Glossina fuscipes populations provide insights for human African trypanosomiasis transmission in Uganda

Uganda has both forms of human African trypanosomiasis (HAT): the chronic gambiense disease in the northwest and the acute rhodesiense disease in the south. The recent spread of rhodesiense into central Uganda has raised concerns given the different control strategies the two diseases require. We present knowledge on the population genetics of the major vector species Glossina fuscipes fuscipes in Uganda with a focus on population structure, measures of gene flow between populations, and the occurrence of polyandry. The microbiome composition and diversity is discussed, focusing on their potential role on trypanosome infection outcomes. We discuss the implications of these findings for large-scale tsetse control programs, including suppression or eradication, being undertaken in Uganda, and potential future genetic applications.

Interwoven biology of the tsetse holobiont

Microbial symbionts can be instrumental to the evolutionary success of their hosts. Here, we discuss medically significant tsetse flies (Diptera: Glossinidae), a group comprised of over 30 species, and their use as a valuable model system to study the evolution of the holobiont (i.e., the host and associated microbes). We first describe the tsetse microbiota, which, despite its simplicity, harbors a diverse range of associations. The maternally transmitted microbes consistently include two Gammaproteobacteria, the obligate mutualists Wigglesworthia spp. and the commensal Sodalis glossinidius, along with the parasitic Alphaproteobacteria Wolbachia. These associations differ in their establishment times, making them unique and distinct from previously characterized symbioses, where multiple microbial partners have associated with their host for a significant portion of its evolution. We then expand into discussing the functional roles and intracommunity dynamics within this holobiont, which enhances our understanding of tsetse biology to encompass the vital functions and interactions of the microbial community. Potential disturbances influencing the tsetse microbiome, including salivary gland hypertrophy virus and trypanosome infections, are highlighted. While previous studies have described evolutionary consequences of host association for symbionts, the initial steps facilitating their incorporation into a holobiont and integration of partner biology have only begun to be explored. Research on the tsetse holobiont will contribute to the understanding of how microbial metabolic integration and interdependency initially may develop within hosts, elucidating mechanisms driving adaptations leading to cooperation and coresidence within the microbial community. Lastly, increased knowledge of the tsetse holobiont may also contribute to generating novel African trypanosomiasis disease control strategies.

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.

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.

Correlation between structure, protein composition, morphogenesis and cytopathology of Glossina pallidipes salivary gland hypertrophy virus

The Glossina pallidipes salivary gland hypertrophy virus (GpSGHV) is a dsDNA virus with rod-shaped, enveloped virions. Its 190 kb genome contains 160 putative protein-coding ORFs. Here, the structural components, protein composition and associated aspects of GpSGHV morphogenesis and cytopathology were investigated. Four morphologically distinct structures: the nucleocapsid, tegument, envelope and helical surface projections, were observed in purified GpSGHV virions by electron microscopy. Nucleocapsids were present in virogenic stroma within the nuclei of infected salivary gland cells, whereas enveloped virions were located in the cytoplasm. The cytoplasm of infected cells appeared disordered and the plasma membranes disintegrated. Treatment of virions with 1 % NP-40 efficiently partitioned the virions into envelope and nucleocapsid fractions. The fractions were separated by SDS-PAGE followed by in-gel trypsin digestion and analysis of the tryptic peptides by liquid chromatography coupled to electrospray and tandem mass spectrometry. Using the MaxQuant program with Andromeda as a database search engine, a total of 45 viral proteins were identified. Of these, ten and 15 were associated with the envelope and the nucleocapsid fractions, respectively, whilst 20 were detected in both fractions, most likely representing tegument proteins. In addition, 51 host-derived proteins were identified in the proteome of the virus particle, 13 of which were verified to be incorporated into the mature virion using a proteinase K protection assay. This study provides important information about GpSGHV biology and suggests options for the development of future anti-GpSGHV strategies by interfering with virus-host interactions.

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.

Muscavirus (MdSGHV) disease dynamics in house fly populations--how is this virus transmitted and has it potential as a biological control agent?

The newly classified family Hytrosaviridae comprises several double-stranded DNA viruses that have been isolated from various dipteran species. These viruses cause characteristic salivary gland hypertrophy and suppress gonad development in their hosts. One member, Muscavirus or MdSGHV, exclusively infects adult house flies (Musca domestica) and, owing to its massive reproduction in and release from the salivary glands, is believed to be transmitted orally among feeding flies. However, results from recent experiments suggest that additional transmission routes likely are involved in the maintenance of MdSGHV in field populations of its host. Firstly, several hours before newly emerged feral flies begin feeding activities, the fully formed peritrophic matrix (PM) constitutes an effective barrier against oral infection. Secondly, flies are highly susceptible to topical virus treatments and intrahemocoelic injections. Thirdly, disease transmission is higher when flies are maintained in groups with infected conspecifics than when flies have access to virus-contaminated food. We hypothesize that interactions between flies may lead to cuticular damage, thereby providing an avenue to viral particles for direct access to the hemocoel. Based on our current knowledge, two options seem plausible for developing Muscavirus as a sterilizing agent to control house fly populations: The virus may either be formulated with PM-disrupting materials to facilitate oral infection from a feeding bait system, or amended with abrasive materials to enhance infection through a damaged cuticle after topical aerosol applications.

Enhancing tsetse fly refractoriness to trypanosome infection--a new IAEA coordinated research project

To date, IAEA-supported Sterile Insect Technique (SIT) projects for tsetse and trypanosomiasis control have been in areas without human sleeping sickness, but future projects could include areas of actual or potential human disease transmission. In this context it would be imperative that released sterile tsetse flies are incompetent to transmit the disease-causing trypanosome parasite. Therefore, development of tsetse fly strains refractory to trypanosome infection is highly desirable as a simple and effective method of ensuring vector incompetence of the released flies. This new IAEA Coordinated Research Project (CRP) focuses on gaining a deeper knowledge of the tripartite interactions between the tsetse fly vectors, their symbionts and trypanosome parasites. The objective of this CRP is to acquire a better understanding of mechanisms that limit the development of trypanosome infections in tsetse and how these may be enhanced.

The bacterial flora of tsetse fly midgut and its effect on trypanosome transmission

The tsetse fly, Glossina palpalis is a vector of the trypanosome that causes sleeping sickness in humans and nagana in cattle along with associated human health problems and massive economic losses. The insect is also known to carry a number of symbionts such as Sodalis, Wigglesworthia, Wolbachia whose effects on the physiology of the insect have been studied in depth. However, effects of other bacterial flora on the physiology of the host and vector competence have received little attention. Epidemiological studies on tsetse fly populations from different geographic sites revealed the presence of a variety of bacteria in the midgut. The most common of the flora belong to the genera Entrobacter (most common), Enterococcus, and Acinetobacter. It was a little surprising to find such diversity in the tsetse midgut since the insect is monophagous consuming vertebrate blood only. Diversity of bacteria is normally associated with polyphagous insects. In contrast to the symbionts, the role of resident midgut bacterial flora on the physiology of the fly and vector competence remains to be elucidated. With regard, Sodalis glossinidius, our data showed that flies harbouring this symbiont have three times greater probability of being infected by trypanosomes than flies without the symbiont. The data delineated in these studies under score the need to carry out detailed investigations on the role of resident bacteria on the physiology of the fly and vector competence.

RNAi: future in insect management

RNA interference is a post- transcriptional, gene regulation mechanism found in virtually all plants and animals including insects. The demonstration of RNAi in insects and its successful use as a tool in the study of functional genomics opened the door to the development of a variety of novel, environmentally sound approaches for insect pest management. Here the current understanding of the biogenesis of the two RNAi classes in insects is reviewed. These are microRNAs (miRNAs) and short interfering RNAs (siRNAs). Several other key approaches in RNAi -based for insect control, as well as for the prevention of diseases in insects are also reviewed. The problems and prospects for the future use of RNAi in insects are presented.

Tsetse-Wolbachia symbiosis: comes of age and has great potential for pest and disease control

Tsetse flies (Diptera: Glossinidae) are the sole vectors of African trypanosomes, the causative agent of sleeping sickness in human and nagana in animals. Like most eukaryotic organisms, Glossina species have established symbiotic associations with bacteria. Three main symbiotic bacteria have been found in tsetse flies: Wigglesworthia glossinidia, an obligate symbiotic bacterium, the secondary endosymbiont Sodalis glossinidius and the reproductive symbiont Wolbachia pipientis. In the present review, we discuss recent studies on the detection and characterization of Wolbachia infections in Glossina species, the horizontal transfer of Wolbachia genes to tsetse chromosomes, the ability of this symbiont to induce cytoplasmic incompatibility in Glossina morsitans morsitans and also how new environment-friendly tools for disease control could be developed by harnessing Wolbachia symbiosis.

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.

Intercommunity effects on microbiome and GpSGHV density regulation in tsetse flies

Tsetse flies have a highly regulated and defined microbial fauna made of 3 bacterial symbionts (obligate Wigglesworthia glossinidia, commensal Sodalis glossinidius and parasitic Wolbachia pipientis) in addition to a DNA virus (Glossina pallidipes Salivary gland Hypertrophy Virus, GpSGHV). It has been possible to rear flies in the absence of either Wigglesworthia or in totally aposymbiotic state by dietary supplementation of tsetse's bloodmeal. In the absence of Wigglesworthia, tsetse females are sterile, and adult progeny are immune compromised. The functional contributions for Sodalist are less known, while Wolbachia cause reproductive manupulations known as cytoplasmic incompatibility (CI). High GpSGHV virus titers result in reduced fecundity and lifespan, and have compromised efforts to colonize flies in the insectary for large rearing purposes. Here we investigated the within community effects on the density regulation of the individual microbiome partners in tsetse lines with different symbiotic compositions. We show that absence of Wigglesworthia results in loss of Sodalis in subsequent generations possibly due to nutritional dependancies between the symbiotic partners. While an initial decrease in Wolbachia and GpSGHV levels are also noted in the absence of Wigglesworthia, these infections eventually reach homeostatic levels indicating adaptations to the new host immune environment or nutritional ecology. Absence of all bacterial symbionts also results in an initial reduction of viral titers, which recover in the second generation. Our findings suggest that in addition to the host immune system, interdependencies between symbiotic partners result in a highly tuned density regulation for tsetse's microbiome.