Genetic diversity in vector populations influences the transmission efficiency of an important plant virus.

The transmission efficiency of aphid-vectored plant viruses can differ between aphid populations. Intra-species diversity (genetic variation, endosymbionts) is a key determinant of aphid phenotype; however, the extent to which intra-species diversity contributes towards variation in virus transmission efficiency is unclear. Here, we use multiple populations of two key aphid species that vector barley yellow dwarf virus (BYDV) strain PAV (BYDV-PAV), the grain aphid (Sitobion avenae) and the bird cherry-oat aphid (Rhopalosiphum padi), and examine how diversity in vector populations influences virus transmission efficiency. We use Illumina sequencing to characterize genetic and endosymbiont variation in multiple Si. avenae and Rh. padi populations and conduct BYDV-PAV transmission experiments to identify links between intra-species diversity in the vector and virus transmission efficiency. We observe limited variation in the transmission efficiency of Si. avenae, with transmission efficiency consistently low for this species. However, for Rh. padi, we observe a range of transmission efficiencies and show that BYDV transmission efficiency is influenced by genetic diversity within the vector, identifying 542 single nucleotide polymorphisms that potentially contribute towards variable transmission efficiency in Rh. padi. Our results represent an important advancement in our understanding of the relationship between genetic diversity, vector-virus interactions, and virus transmission efficiency.

Virus-vectoring thrips regulate the excessive multiplication of tomato spotted wilt virus using their antiviral immune responses.

The tomato spotted wilt virus (TSWV) is a member of the Tospoviridae family and has an negative/ambisense single-stranded RNA genome. Frankliniella occidentalis and F. intonsa are known to be dominant pests in Capsicum annuum (hot pepper) and can cause damage to the plant either directly by feeding, or indirectly by transmitting TSWV in a persistent and propagative manner, resulting in serious economic damage. This study compared the immune responses of two different thrips species against TSWV infection by transcriptome analysis, which then allowed the assessment of antiviral responses using RNA interference (RNAi). Both adult thrips shared about 90 % of the transcripts in non-viruliferous conditions. Most signal components of the immune pathways were shared by these two thrips species, and their expression levels fluctuated differentially in response to TSWV infection at early immature stages. The functional assays using RNAi treatments indicated that the Toll and JAK/STAT pathways were associated with the antiviral responses, but the IMD pathway was not. The upregulation of dorsal switch protein one supported its physiological role in recognizing TSWV infection and triggering the eicosanoid biosynthetic pathway, which mediates melanization and apoptosis in thrips. In addition, the signal components of the RNAi pathways fluctuated highly after TSWV infection. Individual RNAi treatments specific to the antiviral signalling and response components led to significant increases in the TSWV amount in the thrips, causing virus-induced mortality. These findings suggest that immune signalling pathways leading to antiviral responses are operating in the thrips to regulate TSWV litres to prevent a fatal viral overload. This study also indicates the differential antiviral responses between the TSWV-transmitting F. occidentalis and F. intonsa.

Seasonal change and influence of environmental variables on host-seeking activity of the biting midge Culicoides sonorensis at a southern California dairy, USA.

BACKGROUND: As a primary vector of bluetongue virus (BTV) in the US, seasonal abundance and diel flight activity of Culicoides sonorensis has been documented, but few studies have examined how time of host-seeking activity is impacted by environmental factors. This knowledge is essential for interpreting surveillance data and modeling pathogen transmission risk. METHODS: The diel host-seeking activity of C. sonorensis was studied on a California dairy over 3 years using a time-segregated trap baited with CO2. The relationship between environmental variables and diel host-seeking activity (start, peak, and duration of activity) of C. sonorensis was evaluated using multiple linear regression. Fisher's exact test and paired-sample z-test were used to evaluate the seasonal difference and parity difference on diel host-seeking activity. RESULTS: Host-seeking by C. sonorensis began and reached an activity peak before sunset at a higher frequency during colder months relative to warmer months. The time that host-seeking activity occurred was associated low and high daily temperature as well as wind speed at sunset. Colder temperatures and a greater diurnal temperature range were associated with an earlier peak in host-seeking. Higher wind speeds at sunset were associated with a delayed peak in host-seeking and a shortened duration of host-seeking. Parous midges reached peak host-seeking activity slightly later than nulliparous midges, possibly because of the need for oviposition by gravid females before returning to host-seeking. CONCLUSIONS: This study demonstrates that during colder months C. sonorensis initiates host-seeking and reaches peak host-seeking activity earlier relative to sunset, often even before sunset, compared to warmer months. Therefore, the commonly used UV light-baited traps are ineffective for midge surveillance before sunset. Based on this study, surveillance methods that do not rely on light trapping would provide a more accurate estimate of host-biting risk across seasons. The association of environmental factors to host-seeking shown in this study can be used to improve modeling or prediction of host-seeking activity. This study identified diurnal temperature range as associated with host-seeking activity, suggesting that Culicoides may respond to a rapidly decreasing temperature by shifting to an earlier host-seeking time, though this association needs further study.

Blowflies are potential vector for avian influenza virus at enzootic area in Japan.

High pathogenicity avian influenza (HPAI) poses a significant threat to both domestic and wild birds globally. The avian influenza virus, known for environmental contamination and subsequent oral infection in birds, necessitates careful consideration of alternative introduction routes during HPAI outbreaks. This study focuses on blowflies (genus Calliphora), in particular Calliphora nigribarbis, attracted to decaying animals and feces, which migrate to lowland areas of Japan from northern or mountainous regions in early winter, coinciding with HPAI season. Our investigation aims to delineate the role of blowflies as HPAI vectors by conducting a virus prevalence survey in a wild bird HPAI-enzootic area. In December 2022, 648 Calliphora nigribarbis were collected. Influenza virus RT-PCR testing identified 14 virus-positive samples (2.2% prevalence), with the highest occurrence observed near the crane colony (14.9%). Subtyping revealed the presence of H5N1 and HxN1 in some samples. Subsequent collections in December 2023 identified one HPAI virus-positive specimen from 608 collected flies in total, underscoring the potential involvement of blowflies in HPAI transmission. Our observations suggest C. nigribarbis may acquire the HPAI virus from deceased wild birds directly or from fecal materials from infected birds, highlighting the need to add blowflies as a target of HPAI vector control.

Establishment and partial characterisation of a new cell line derived from adult tissues of the tsetse fly Glossina morsitans morsitans.

BACKGROUND: Insect cell lines play a vital role in many aspects of research on disease vectors and agricultural pests. The tsetse fly Glossina morsitans morsitans is an important vector of salivarian trypanosomes in sub-Saharan Africa and, as such, is a major constraint on human health and agricultural development in the region. METHODS: Here, we report establishment and partial characterisation of a cell line, GMA/LULS61, derived from tissues of adult female G. m. morsitans. GMA/LULS61 cells, grown at 28 °C in L-15 (Leibovitz) medium supplemented with foetal bovine serum and tryptose phosphate broth, have been taken through 23 passages to date and can be split 1:1 at 2-week intervals. Karyotyping at passage 17 revealed a predominantly haploid chromosome complement. Species origin and absence of contaminating bacteria were confirmed by PCR amplification and sequencing of fragments of the COI gene and pan-bacterial 16S rRNA gene respectively. However, PCR screening of RNA extracted from GMA/LULS61 cells confirmed presence of the recently described Glossina morsitans morsitans iflavirus and Glossina morsitans morsitans negevirus, but absence of Glossina pallipides salivary gland hypertrophy virus. GMA/LULS61 cells supported infection and growth of 6/7 different insect-derived strains of the intracellular bacterial symbiont Wolbachia. CONCLUSIONS: The GMA/LULS61 cell line has potential for application in a variety of studies investigating the biology of G. m. morsitans and its associated pathogenic and symbiotic microorganisms.

Begomovirus Transmission to Tomato Plants Is Not Hampered by Plant Defenses Induced by Dicyphus hesperus Knight.

Plants can respond to insect infestation and virus infection by inducing plant defenses, generally mediated by phytohormones. Moreover, plant defenses alter host quality for insect vectors with consequences for the spread of viruses. In agricultural settings, other organisms commonly interact with plants, thereby inducing plant defenses that could affect plant-virus-vector interactions. For example, plant defenses induced by omnivorous insects can modulate insect behavior. This study focused on tomato yellow leaf curl virus (TYLCV), a plant virus of the family Geminiviridae and genus Begomovirus. It is transmitted in a persistent circulative manner by the whitefly Bemisia tabaci Gennadius (Hemiptera: Aleyrodidae), posing a global threat to tomato production. Mirids (Hemiptera: Miridae) are effective biological control agents of B. tabaci, but there is a possibility that their omnivorous nature could also interfere with the process of virus transmission. To test this hypothesis, this study first addressed to what extent the mirid bug Dicyphus hesperus Knight induces plant defenses in tomato. Subsequently, the impact of this plant-omnivore interaction on the transmission of TYLCV was evaluated. Controlled cage experiments were performed in a greenhouse setting to evaluate the impact of mirids on virus transmission and vector acquisition by B. tabaci. While we observed a reduced number of whiteflies settling on plants exposed to D. hesperus, the plant defenses induced by the mirid bug did not affect TYLCV transmission and accumulation. Additionally, whiteflies were able to acquire comparable amounts of TYLCV on mirid-exposed plants and control plants. Overall, the induction of plant defenses by D. hesperus did not influence TYLCV transmission by whiteflies on tomato.

Reduced cold tolerance of viral-infected leafhoppers attenuates viral persistent epidemics.

Most arthropod-borne viruses produce intermittent epidemics in infected plants. However, the underlying mechanisms of these epidemics are unclear. Here, we demonstrated that rice stripe mosaic virus (RSMV), a viral pathogen, significantly increases the mortality of its overwintering vector, the leafhopper species Recilia dorsalis. Cold-stress assays indicated that RSMV reduces the cold tolerance of leafhoppers, a process associated with the downregulation of leafhopper cuticular protein genes. An RSMV-derived small RNA (vsiR-t00355379) was found to facilitate the downregulation of a leafhopper endocuticle gene that is mainly expressed in the abdomen (named RdABD-5) and is conserved across dipteran species. The downregulation of RdABD-5 expression in R. dorsalis resulted in fewer and thinner endocuticle lamellae, leading to decreased cold tolerance. This effect was correlated with a reduced incidence rate of RSMV in early-planted rice plants. These findings contribute to our understanding of the mechanism by which viral pathogens reduce cold tolerance in arthropod vectors and suggest an approach to managing the fluctuating prevalence of arboviruses. IMPORTANCE: Increasing arthropod vector dispersal rates have increased the susceptibility of crop to epidemic viral diseases. However, the incidence of some viral diseases fluctuates annually. In this study, we demonstrated that a rice virus reduces the cold tolerance of its leafhopper vector, Recilia dorsalis. This effect is linked to the virus-derived small RNA-mediated downregulation of a gene encoding a leafhopper abdominal endocuticle protein. Consequently, the altered structural composition of the abdominal endocuticle reduces the overwinter survival of leafhoppers, resulting in a lower incidence of RSMV infection in early-planted rice plants. Our findings illustrate the important roles of RNA interference in virus-vector insect-environment interactions and help explain the annual fluctuations of viral disease epidemics in rice fields.

Current Status of La Crosse Virus in North America and Potential for Future Spread.

La Crosse virus (LACV) is an arthropod-borne RNA virus with substantial potential for future spread in North America. La Crosse virus is responsible for La Crosse encephalitis, a leading cause of arboviral encephalitis in children in the United States. Primarily transmitted by Aedes triseriatus (Eastern treehole) mosquitos and amplified by small mammal hosts, LACV has caused infections throughout the upper Midwest and, more recently, the mid-Atlantic and southeastern United States. Notably, in recent years, infections have also been identified increasingly in the Appalachian region. Anthropogenic and environmental factors have likely contributed to recent LACV spread, including the introduction of invasive vector species (especially Ae. albopictus), biotic interactions between and among vector and host species, land-use change, habitat disturbance, increased human travel and transport, and rising global temperatures. Prevention and control strategies, such as increased surveillance of vector and host populations, increased awareness among populations at risk for infection, and increased awareness among physicians are needed to limit future spread. Continued climate change with increases in global temperatures and erratic weather patterns may result in the expansion of competent mosquito vector species and thus could facilitate the geographic spread of LACV.

Replication-independent change in the frequencies of distinct genome segments of a multipartite virus during its transit within aphid vectors.

Multipartite viruses exhibit a fragmented genome composed of several nucleic acid segments individually packaged in distinct viral particles. The genome of all species of the genus Nanovirus holds eight segments, which accumulate at a very specific and reproducible relative frequency in the host plant tissues. In a given host species, the steady state pattern of the segments' relative frequencies is designated the genome formula and is thought to have an adaptive function through the modulation of gene expression. Nanoviruses are aphid-transmitted circulative non-propagative viruses, meaning that the virus particles are internalized into the midgut cells, transferred to the hemolymph, and then to the saliva, with no replication during this transit. Unexpectedly, a previous study on the faba bean necrotic stunt virus revealed that the genome formula changes after ingestion by aphids. We investigate here the possible mechanism inducing this change by first comparing the relative segment frequencies in different compartments of the aphid. We show that changes occur both in the midgut lumen and in the secreted saliva but not in the gut, salivary gland, or hemolymph. We further establish that the viral particles differentially resist physicochemical variations, in particular pH, ionic strength, and/or type of salt, depending on the encapsidated segment. We thus propose that the replication-independent genome formula changes within aphids are not adaptive, contrary to changes occurring in plants, and most likely reflect a fortuitous differential degradation of virus particles containing distinct segments when passing into extra-cellular media such as gastric fluid or saliva. IMPORTANCE: The genome of multipartite viruses is composed of several segments individually packaged into distinct viral particles. Each segment accumulates at a specific frequency that depends on the host plant species and regulates gene expression. Intriguingly, the relative frequencies of the genome segments also change when the octopartite faba bean necrotic stunt virus (FBNSV) is ingested by aphid vectors, despite the present view that this virus travels through the aphid gut and salivary glands without replicating. By monitoring the genomic composition of FBNSV populations during the transit in aphids, we demonstrate here that the changes take place extracellularly in the gut lumen and in the saliva. We further show that physicochemical factors induce differential degradation of viral particles depending on the encapsidated segment. We propose that the replication-independent changes within the insect vector are not adaptive and result from the differential stability of virus particles containing distinct segments according to environmental parameters.

The Incompetence of Mosquitoes-Can Zika Virus Be Adapted To Infect Culex tarsalis Cells?

The molecular evolutionary mechanisms underpinning virus-host interactions are increasingly recognized as key drivers of virus emergence, host specificity, and the likelihood that viruses can undergo a host shift that alters epidemiology and transmission biology. Zika virus (ZIKV) is mainly transmitted between humans by Aedes aegypti mosquitoes. However, the 2015 to 2017 outbreak stimulated discussion regarding the role of Culex spp. mosquitoes in transmission. Reports of ZIKV-infected Culex mosquitoes, in nature and under laboratory conditions, resulted in public and scientific confusion. We previously found that Puerto Rican ZIKV does not infect colonized Culex quinquefasciatus, Culex pipiens, or Culex tarsalis, but some studies suggest they may be competent ZIKV vectors. Therefore, we attempted to adapt ZIKV to Cx. tarsalis by serially passaging virus on cocultured Ae. aegypti (Aag2) and Cx. tarsalis (CT) cells to identify viral determinants of species specificity. Increasing fractions of CT cells resulted in decreased overall virus titer and no enhancement of Culex cell or mosquito infection. Next-generation sequencing of cocultured virus passages revealed synonymous and nonsynonymous variants throughout the genome that arose as CT cell fractions increased. We generated nine recombinant ZIKVs containing combinations of the variants of interest. None of these viruses showed increased infection of Culex cells or mosquitoes, demonstrating that variants associated with passaging were not specific to increased Culex infection. These results reveal the challenge of a virus adapting to a new host, even when pushed to adapt artificially. Importantly, they also demonstrate that while ZIKV may occasionally infect Culex mosquitoes, Aedes mosquitoes likely drive transmission and human risk. IMPORTANCE ZIKV is mainly transmitted between humans by Aedes mosquitoes. In nature, ZIKV-infected Culex mosquitoes have been found, and ZIKV infrequently infects Culex mosquitoes under laboratory conditions. Yet, most studies show that Culex mosquitoes are not competent vectors for ZIKV. We attempted to adapt ZIKV to Culex cells to identify viral determinants of species specificity. We sequenced ZIKV after it was passaged on a mixture of Aedes and Culex cells and found that it acquired many variants. We generated recombinant viruses containing combinations of the variants of interest to determine if any of these changes enhance infection in Culex cells or mosquitoes. Recombinant viruses did not show increased infection in Culex cells or mosquitoes, but some variants increased infection in Aedes cells, suggesting adaptation to those cells instead. These results reveal that arbovirus species specificity is complex, and that virus adaptation to a new genus of mosquito vectors likely requires multiple genetic changes.

Key role of exportin 6 in exosome-mediated viral transmission from insect vectors to plants.

Exosomes play a key role in virus exocytosis and transmission. The exportin family is usually responsible for cargo nucleocytoplasmic trafficking, and they are frequently found in exosomes. However, the function of exportins sorted in exosomes remains unknown. Here, we successfully isolated "cup holder"-like exosomes from the saliva of ∼30,000 small brown planthoppers, which are vectors of rice stripe virus (RSV). RSV virions were packed in comparatively large exosomes. Four viral genomic RNAs at a certain ratio were identified in the saliva exosomes. The virions contained in the saliva exosomes were capable of replicating and causing disease in rice plants. Interference with each phase of the insect exosome system affected the transmission of RSV from the insect vectors to rice plants. Fragmented exportin 6 was coimmunoprecipitated with viral nucleocapsid protein in saliva and sorted to exosomes via interactions with the cargo sorting protein VPS37a. When the expression of exportin 6 was knocked down, the amounts of RSV secreted in saliva and rice plants were reduced by 60% and 74%, respectively. These results showed that exportin 6 acted as a vehicle for transporting RSV into exosomes to overcome the barrier of insect salivary glands for horizontal transmission. Exportin 6 would represent an ideal target that could be manipulated to control the outbreak of insect-borne viruses in the future.

Nonconcomitant host-to-host transmission of multipartite virus genome segments may lead to complete genome reconstitution.

Because multipartite viruses package their genome segments in different viral particles, they face a potentially huge cost if the entire genomic information, i.e., all genome segments, needs to be present concomitantly for the infection to function. Previous work with the octapartite faba bean necrotic stunt virus (FBNSV; family Nanoviridae, genus Nanovirus) showed that this issue can be resolved at the within-host level through a supracellular functioning; all viral segments do not need to be present within the same host cell but may complement each other through intercellular trafficking of their products (protein or messenger RNA [mRNA]). Here, we report on whether FBNSV can as well decrease the genomic integrity cost during between-host transmission. Using viable infections lacking nonessential virus segments, we show that full-genome infections can be reconstituted and function through separate acquisition and/or inoculation of complementary sets of genome segments in recipient hosts. This separate acquisition/inoculation can occur either through the transmission of different segment sets by different individual aphid vectors or by the sequential acquisition by the same aphid of complementary sets of segments from different hosts. The possibility of a separate between-host transmission of different genome segments thus offers a way to at least partially resolve the genomic maintenance problem faced by multipartite viruses.

The Acquisition and Retention of Lumpy Skin Disease Virus by Blood-Feeding Insects Is Influenced by the Source of Virus, the Insect Body Part, and the Time since Feeding.

Lumpy skin disease virus (LSDV) is a poxvirus that causes severe systemic disease in cattle and is spread by mechanical arthropod-borne transmission. This study quantified the acquisition and retention of LSDV by four species of Diptera (Stomoxys calcitrans, Aedes aegypti, Culex quinquefasciatus, and Culicoides nubeculosus) from cutaneous lesions, normal skin, and blood from a clinically affected animal. The acquisition and retention of LSDV by Ae. aegypti from an artificial membrane feeding system was also examined. Mathematical models of the data were generated to identify the parameters which influence insect acquisition and retention of LSDV. For all four insect species, the probability of acquiring LSDV was substantially greater when feeding on a lesion compared with feeding on normal skin or blood from a clinically affected animal. After feeding on a skin lesion LSDV was retained on the proboscis for a similar length of time (around 9 days) for all four species and for a shorter time in the rest of the body, ranging from 2.2 to 6.4 days. Acquisition and retention of LSDV by Ae. aegypti after feeding on an artificial membrane feeding system that contained a high titer of LSDV was comparable to feeding on a skin lesion on a clinically affected animal, supporting the use of this laboratory model as a replacement for some animal studies. This work reveals that the cutaneous lesions of LSD provide the high-titer source required for acquisition of the virus by insects, thereby enabling the mechanical vector-borne transmission. IMPORTANCE Lumpy skin disease virus (LSDV) is a high consequence pathogen of cattle that is rapidly expanding its geographical boundaries into new regions such as Europe and Asia. This expansion is promoted by the mechanical transmission of the virus via hematogenous arthropods. This study quantifies the acquisition and retention of LSDV by four species of blood-feeding insects and reveals that the cutaneous lesions of LSD provide the high titer virus source necessary for virus acquisition by the insects. An artificial membrane feeding system containing a high titer of LSDV was shown to be comparable to a skin lesion on a clinically affected animal when used as a virus source. This promotes the use of these laboratory-based systems as replacements for some animal studies. Overall, this work advances our understanding of the mechanical vector-borne transmission of LSDV and provides evidence to support the design of more effective disease control programmes.

Flotillin 2 Facilitates the Infection of a Plant Virus in the Gut of Insect Vector.

Most plant viruses require insect vectors for transmission. One of the key steps for the transmission of persistent-circulative plant viruses is overcoming the gut barrier to enter epithelial cells. To date, little has been known about viral cofactors in gut epithelial cells of insect vectors. Here, we identified flotillin 2 as a plasma membrane protein that facilitates the infection of rice stripe virus (RSV) in its vector, the small brown planthopper. Flotillin 2 displayed a prominent plasma membrane location in midgut epithelial cells. The nucleocapsid protein of RSV and flotillin 2 colocalized on gut microvilli, and a nanomolar affinity existed between the two proteins. Knockout of flotillin 2 impeded the entry of virions into epithelial cells, resulting in a 57% reduction of RSV levels in planthoppers. The knockout of flotillin 2 decreased disease incidence in rice plants fed by viruliferous planthoppers from 40% to 11.7%. Furthermore, flotillin 2 mediated the infection of southern rice black-streaked dwarf virus in its vector, the white-backed planthopper. This work implies the potential of flotillin 2 as a target for controlling the transmission of rice stripe disease. IMPORTANCE Plant viral diseases are a major threat to world agriculture. The transmission of 80% of plant viruses requires vector insects, and 54% of vector-borne plant viruses are persistent-circulative viruses, which must overcome the barriers of gut cells with the help of proteins on the cell surface. Here, we identified flotillin 2 as a membrane protein that mediates the cell entry of rice stripe virus in its vector insect, small brown planthopper. Flotillin 2 displays a prominent cellular membrane location in midgut cells and can specifically bind to virions. The loss of flotillin 2 impedes the entry of virions into the midgut cells of vector insects and substantially suppresses viral transmission to rice. Therefore, flotillin 2 may be a promising target gene for manipulation in vector insects to control the transmission of rice stripe disease and perhaps that of other rice virus diseases in the future.

Ecological niche modeling predicting the potential distribution of African horse sickness virus from 2020 to 2060.

African horse sickness is a vector-borne, non-contagious and highly infectious disease of equines caused by African horse sickness viruses (AHSv) that mainly affect horses. The occurrence of the disease causes huge economic impacts because of its high fatality rate, trade ban and disease control costs. In the planning of vectors and vector-borne diseases like AHS, the application of Ecological niche models (ENM) used an enormous contribution in precisely delineating the suitable habitats of the vector. We developed an ENM to delineate the global suitability of AHSv based on retrospective outbreak data records from 2005 to 2019. The model was developed in an R software program using the Biomod2 package with an Ensemble modeling technique. Predictive environmental variables like mean diurnal range, mean precipitation of driest month(mm), precipitation seasonality (cv), mean annual maximum temperature (oc), mean annual minimum temperature (oc), mean precipitation of warmest quarter(mm), mean precipitation of coldest quarter (mm), mean annual precipitation (mm), solar radiation (kj /day), elevation/altitude (m), wind speed (m/s) were used to develop the model. From these variables, solar radiation, mean maximum temperature, average annual precipitation, altitude and precipitation seasonality contributed 36.83%, 17.1%, 14.34%, 7.61%, and 6.4%, respectively. The model depicted the sub-Sahara African continent as the most suitable area for the virus. Mainly Senegal, Burkina Faso, Niger, Nigeria, Ethiopia, Sudan, Somalia, South Africa, Zimbabwe, Madagascar and Malawi are African countries identified as highly suitable countries for the virus. Besides, OIE-listed disease-free countries like India, Australia, Brazil, Paraguay and Bolivia have been found suitable for the virus. This model can be used as an epidemiological tool in planning control and surveillance of diseases nationally or internationally.

PEBP balances apoptosis and autophagy in whitefly upon arbovirus infection.

Apoptosis and autophagy are two common forms of programmed cell death (PCD) used by host organisms to fight against virus infection. PCD in arthropod vectors can be manipulated by arboviruses, leading to arbovirus-vector coexistence, although the underlying mechanism is largely unknown. In this study, we find that coat protein (CP) of an insect-borne plant virus TYLCV directly interacts with a phosphatidylethanolamine-binding protein (PEBP) in its vector whitefly to downregulate MAPK signaling cascade. As a result, apoptosis is activated in the whitefly increasing viral load. Simultaneously, the PEBP4-CP interaction releases ATG8, a hallmark of autophagy initiation, which reduces arbovirus levels. Furthermore, apoptosis-promoted virus amplification is prevented by agonist-induced autophagy, whereas the autophagy-suppressed virus load is unaffected by manipulating apoptosis, suggesting that the viral load is predominantly determined by autophagy rather than by apoptosis. Our results demonstrate that a mild intracellular immune response including balanced apoptosis and autophagy might facilitate arbovirus preservation within its whitefly insect vector.

Diaphorina citri flavi-like virus localization, transmission, and association with Candidatus Liberibacter asiaticus in its psyllid host.

Huanglongbing is caused by Candidatus Liberibacter asiaticus (CLas) and transmitted by Diaphorina citri. D. citri harbors various insect-specific viruses, including the Diaphorina citri flavi-like virus (DcFLV). The distribution and biological role of DcFLV in its host and the relationship with CLas are unknown. DcFLV was found in various organs of D. citri, including the midgut and salivary glands, where it co-localized with CLas. CLas-infected nymphs had the highest DcFLV titers compared to the infected adults and CLas-free adults and nymphs. DcFLV was vertically transmitted to offspring from female D. citri and was temporarily detected in Citrus macrophylla and grapefruit leaves from greenhouse and field. The incidences of DcFLV and CLas were positively correlated in field-collected D. citri samples, suggesting that DcFLV might be associated with CLas in the vector. These results provide new insights on the interactions between DcFLV, the D. citri, and CLas.

Phlebotomine sand flies (Diptera: Psychodidae) of the Maghreb region: A systematic review of distribution, morphology, and role in the transmission of the pathogens.

BACKGROUND: Phlebotomine sand flies (Diptera: Psychodidae) are important vectors of various human and animal pathogens such as Bartonella bacilliformis, Phlebovirus, and parasitic protozoa of the genus Leishmania, causative agent of leishmaniases that account among most significant vector-borne diseases. The Maghreb countries Mauritania, Morocco, Algeria, Tunisia, and Libya occupy a vast area of North Africa and belong to most affected regions by these diseases. Locally varying climatic and ecological conditions support diverse sand fly fauna that includes many proven or suspected vectors. The aim of this review is to summarize often fragmented information and to provide an updated list of sand fly species of the Maghreb region with illustration of species-specific morphological features and maps of their reported distribution. MATERIALS AND METHODS: The literature search focused on scholar databases to review information on the sand fly species distribution and their role in the disease transmissions in Mauritania, Morocco, Algeria, Tunisia, and Libya, surveying sources from the period between 1900 and 2020. Reported distribution of each species was collated using Google Earth, and distribution maps were drawn using ArcGIS software. Morphological illustrations were compiled from various published sources. RESULTS AND CONCLUSIONS: In total, 32 species of the genera Phlebotomus (Ph.) and Sergentomyia (Se.) were reported in the Maghreb region (15 from Libya, 18 from Tunisia, 23 from Morocco, 24 from Algeria, and 9 from Mauritania). Phlebotomus mariae and Se. africana subsp. asiatica were recorded only in Morocco, Ph. mascitti, Se. hirtus, and Se. tiberiadis only in Algeria, whereas Ph. duboscqi, Se. dubia, Se. africana africana, Se. lesleyae, Se. magna, and Se. freetownensis were reported only from Mauritania. Our review has updated and summarized the geographic distribution of 26 species reported so far in Morocco, Algeria, Tunisia, and Libya, excluding Mauritania from a detailed analysis due to the unavailability of accurate distribution data. In addition, morphological differences important for species identification are summarized with particular attention to closely related species such as Ph. papatasi and Ph. bergeroti, Ph. chabaudi, and Ph. riouxi, and Se. christophersi and Se. clydei.

Effect of aphid biology and morphology on plant virus transmission.

Aphids severely affect crop production by transmitting many plant viruses. Viruses are obligate intracellular pathogens that mostly depend on vectors for their transmission and survival. A majority of economically important plant viruses are transmitted by aphids. They transmit viruses either persistently (circulative or non-circulative) or non-persistently. Plant virus transmission by insects is a process that has evolved over time and is strongly influenced by insect morphological features and biology. Over the past century, a large body of research has provided detailed knowledge of the molecular processes underlying virus-vector interactions. In this review, we discuss how aphid biology and morphology can affect plant virus transmission. © 2021 Society of Chemical Industry.

Alternative Splicing Landscape of Small Brown Planthopper and Different Response of JNK2 Isoforms to Rice Stripe Virus Infection.

Alternative splicing (AS) is a frequent posttranscriptional regulatory event occurring in response to various endogenous and exogenous stimuli in most eukaryotic organisms. However, little is known about the effects of insect-transmitted viruses on AS events in insect vectors. The present study used third-generation sequencing technology and RNA sequencing (RNA-Seq) to evaluate the AS response in the small brown planthopper Laodelphax striatellus to rice stripe virus (RSV). The full-length transcriptome of L. striatellus was obtained using single-molecule real-time sequencing technology (SMRT). Posttranscriptional regulatory events, including AS, alternative polyadenylation, and fusion transcripts, were analyzed. A total of 28,175 nonredundant transcript isoforms included 24,950 transcripts assigned to 8,500 annotated genes of L. striatellus, and 5,000 of these genes (58.8%) had AS events. RNA-Seq of the gut samples of insects infected by RSV for 8 d identified 3,458 differentially expressed transcripts (DETs); 2,185 of these DETs were transcribed from 1,568 genes that had AS events, indicating that 31.4% of alternatively spliced genes responded to RSV infection of the gut. One of the c-Jun N-terminal kinase (JNK) genes, JNK2, experienced exon skipping, resulting in three transcript isoforms. These three isoforms differentially responded to RSV infection during development and in various organs. Injection of double-stranded RNAs targeting all or two isoforms indicated that three or at least two JNK2 isoforms facilitated RSV accumulation in planthoppers. These results implied that AS events could participate in the regulation of complex relationships between viruses and insect vectors. IMPORTANCE Alternative splicing (AS) is a regulatory mechanism that occurs after gene transcription. AS events can enrich protein diversity to promote the reactions of the organisms to various endogenous and exogenous stimulations. It is not known how insect vectors exploit AS events to cope with transmitted viruses. The present study used third-generation sequencing technology to obtain the profile of AS events in the small brown planthopper Laodelphax striatellus, which is an efficient vector for rice stripe virus (RSV). The results indicated that 31.4% of alternatively spliced genes responded to RSV infection in the gut of planthoppers. One of the c-Jun N-terminal kinase (JNK) genes, JNK2, produced three transcript isoforms by AS. These three isoforms showed different responses to RSV infection, and at least two isoforms facilitated viral accumulation in planthoppers. These results implied that AS events could participate in the regulation of complex relationships between viruses and insect vectors.