Discovery of two novel totiviruses from Culex tritaeniorhynchus classifiable in a distinct clade with arthropod-infecting viruses within the family Totiviridae

Identification and Characterization of Four Novel Viruses in Balclutha incisa

Balclutha incisa (Cicadellidae: Deltocephalinae), a leafhopper prevalent in tropical and temperate regions, is notably abundant in grasses and rice. The virome of B. incisa was investigated using deep transcriptome sequencing, leading to the first identification of four viruses belonging to the families Aliusviridae, Iflaviridae, and Totiviridae in B. incisa. These viruses have been provisionally named B. incisa ollusvirus 1 (BiOV1), B. incisa ollusvirus 2 (BiOV2), B. incisa iflavirus 1 (BiIV1), and B. incisa totivirus 1 (BiTV1). The complete genome sequences of these viruses were obtained through rapid amplification of cDNA ends (RACE). BiOV1 has a linear genome of 15,125 nucleotides (nt), while BiOV2 possesses a circular genome of 14,853 nt. The BiIV1 genome, excluding the poly(A) tail, is 10,903 nt in length and encodes a single open reading frame (ORF) for a polyprotein consisting of 3194 amino acids (aa). The BiTV1 genome is 4357 nt long and contains two overlapping ORFs, with the viral RNA-dependent RNA polymerase (RdRp) translated via a -1 ribosomal frameshift. Phylogenetic and sequence identity analyses suggest that all these viruses are novel members of their respective families. This study significantly expands our understanding of the virome associated with B. incisa by reporting and characterizing these novel viruses.

Detection of Known and Novel Virus Sequences in the Black Soldier Fly and Expression of Host Antiviral Pathways

The mass rearing of animals in close quarters can be highly conducive to microbe transmission, including pathogens. This has been shown multiple times in the case of important industrial insects such as crickets, silkworms, and honeybees. One industrial insect of increasing importance is the black soldier fly (Diptera: Hermetia illucens), as it can convert organic waste into high-quality protein and fatty acids. Along with this, they take up far less space than traditional protein sources, as millions of black soldier flies can be reared in a relatively small facility. Because of this, there is a growing interest in the pathogens that could impact black soldier fly-rearing efforts. So far, only three black soldier fly-associated viruses have been identified. We used metatranscriptomic sequencing to survey black soldier fly guts, frass, and diet for viruses. We detected sequences from two novel viruses. One, which we name Hermetia illucens sigma-like virus 1, is phylogenetically related to viruses of the genus Sigmavirus, which have been highly studied in Drosophila. The other novel virus, which we name Hermetia illucens inse-like virus 1, is the second double-stranded RNA virus of the order Ghabrivirales described in the black soldier fly, and groups within a new family of insect viruses called the Inseviridae. We also detected two black soldier fly-associated viruses previously identified by our group: BSF nairo-like virus 1 and BSF uncharacterized bunyavirus-like 1. Consistent with our previous study, these two viruses are found primarily in frass samples and occur together more often than expected at random. When analyzing host transcription, we found significant differences in gene expression for eight candidate antiviral genes in the black soldier fly when comparing samples with and without viral sequences. Our results suggest that black soldier fly-virus interactions are ongoing, and they could be of interest to black soldier fly producers.

Co-option of a non-retroviral endogenous viral element in planthoppers

Non-retroviral endogenous viral elements (nrEVEs) are widely dispersed throughout the genomes of eukaryotes. Although nrEVEs are known to be involved in host antiviral immunity, it remains an open question whether they can be domesticated as functional proteins to serve cellular innovations in arthropods. In this study, we found that endogenous toti-like viral elements (ToEVEs) are ubiquitously integrated into the genomes of three planthopper species, with highly variable distributions and polymorphism levels in planthopper populations. Three ToEVEs display exon‒intron structures and active transcription, suggesting that they might have been domesticated by planthoppers. CRISPR/Cas9 experiments revealed that one ToEVE in Nilaparvata lugens, NlToEVE14, has been co-opted by its host and plays essential roles in planthopper development and fecundity. Large-scale analysis of ToEVEs in arthropod genomes indicated that the number of arthropod nrEVEs is currently underestimated and that they may contribute to the functional diversity of arthropod genes.

Molecular Characterization of Two Totiviruses from the Commensal Yeast Geotrichum candidum

Mycoviruses can infect many of the major taxa of fungi including yeasts. Mycoviruses in the yeast fungus Geotrichum candidum are not well studied with only three G. candidum-associated viral species characterized to date, all of which belong to the Totiviridae genus Totivirus. In this study, we report the molecular characteristics of another two totiviruses co-infecting isolate Gc6 of G. candidum. The two totiviruses were tentatively named Geotrichum candidum totivirus 2 isolate Gc6 (GcTV2-Gc6) and Geotrichum candidum totivirus 4 isolate Gc6 (GcTV4-Gc6). Both viruses have the typical genome organization of totiviruses comprising two ORFs encoding capsid protein (CP) and RNA-dependent RNA polymerase (RdRp) at the N and C termini, respectively. The genomes of GcTV2-Gc6 and GcTV4-Gc6 are 4592 and 4530 bp long, respectively. Both viruses contain the-frameshifting elements and their proteins could be expressed as a single fusion protein. GcTV2-Gc6 is closely related to a totivirus isolated from the same host whereas GcTV4-Gc6 is related to insect-associated totiviruses. The phylogenetic analysis indicated that GcTV2-Gc6 and GcTV4-Gc6 belong to two different sister clades, I-A and I-B, respectively. It is interesting that all viruses identified from G. candidum belong to the genus Totivirus; however, this might be due to the lack of research reporting the characterization of mycoviruses from this fungal host. It is possible that the RNA interference (RNAi) mechanism cannot actively suppress totivirus accumulation in G. candidum Gc6.

Aedes aegypti Totivirus identified in mosquitoes in the Brazilian Amazon region

The totiviridae family contains viruses with double-stranded RNA genomes of 4.6-7.0 kpb, which encode a capsid protein (CP) and RNA-dependent RNA polymerase (RdRp), and they are approximately 40 nm in diameter with icosahedral symmetry. Totiviruses were first isolated from mosquitoes collected in Shaanxi Province (China). Here, we report a new Aedes aegypti Totivirus (AaTV) identified in mosquitoes from the Amazon rainforest. Mosquitoes (Diptera: Culicidae) were collected from a forest reserve belonging to the Amazon forest in the city of Macapá, Amapá state, Northern Brazil. A viral sequence with a 5748 nucleotide length that was nearly identical to Aedes aegypti Totivirus (AaTV), here named Aedes aegypti Totivirus BR59AP, was detected. A detailed molecular analysis was performed and shows that AaTV-BR59AP is highly related to the AaTV strain from the Caribbean region. We emphasize the importance of the characterization of new viruses in mosquitoes to deepen our understanding of viral diversity in insects and their potential role in disease.

Is the Intergenic Region of Aedes aegypti Totivirus a Recombination Hotspot?

The genus totivirus in the family Totiviridae contains double-stranded RNA viruses. Their genome has two open reading frames (ORFs) that encode capsid protein (CP) and RNA-dependent RNA polymerase (RdRp). The toti-like viruses recently identified in Anopheles sp. and Aedes aegypti mosquitoes (AaTV) share the same genome organization as other totiviruses. The AaTVs that have been described in distinct geographical regions are monophyletic. In this study, we show that AaTV sequences can be grouped into at least three phylogenetic clades (named A, B, and C). Clades A and B are composed of AaTV sequences from mosquitoes collected in the Caribbean region (Guadeloupe), and clade C contains sequences from the USA. These clades may represent AaTV lineages that are locally adapted to their host populations. We also identified three recombinant AaTV strains circulating in mosquitoes in Guadeloupe. Although these strains have different chimeric patterns, the position of the recombination breakpoint was identical in all strains. Interestingly, this breakpoint is located in a hairpin-like structure in the intergenic region of the AaTV genome. This RNA structure may stall RNA polymerase processivity and consequently induce template switching. In vitro studies should be conducted to further investigate the biological significance of AaTV's intergenic region as a recombination hotspot.

Aedes aegypti and Ae. albopictus microbiome/virome: new strategies for controlling arboviral transmission?

Aedes aegypti and Aedes albopictus are the main vectors of highly pathogenic viruses for humans, such as dengue (DENV), chikungunya (CHIKV), and Zika (ZIKV), which cause febrile, hemorrhagic, and neurological diseases and remain a major threat to global public health. The high ecological plasticity, opportunistic feeding patterns, and versatility in the use of urban and natural breeding sites of these vectors have favored their dispersal and adaptation in tropical, subtropical, and even temperate zones. Due to the lack of available treatments and vaccines, mosquito population control is the most effective way to prevent arboviral diseases. Resident microorganisms play a crucial role in host fitness by preventing or enhancing its vectorial ability to transmit viral pathogens. High-throughput sequencing and metagenomic analyses have advanced our understanding of the composition and functionality of the microbiota of Aedes spp. Interestingly, shotgun metagenomics studies have established that mosquito vectors harbor a highly conserved virome composed of insect-specific viruses (ISV). Although ISVs are not infectious to vertebrates, they can alter different phases of the arboviral cycle, interfering with transmission to the human host. Therefore, this review focuses on the description of Ae. aegypti and Ae. albopictus as vectors susceptible to infection by viral pathogens, highlighting the role of the microbiota-virome in vectorial competence and its potential in control strategies for new emerging and re-emerging arboviruses.

First Evidence of Past and Present Interactions between Viruses and the Black Soldier Fly, Hermetia illucens

Black soldier flies (BSFs, Hermetia illucens) are becoming a prominent research model encouraged by the insect as food and feed and waste bioconversion industries. Insect mass-rearing facilities are at risk from the spread of viruses, but so far, none have been described in BSFs. To fill this knowledge gap, a bioinformatic approach was undertaken to discover viruses specifically associated with BSFs. First, BSF genomes were screened for the presence of endogenous viral elements (EVEs). This led to the discovery and mapping of seven orthologous EVEs integrated into three BSF genomes originating from five viral families. Secondly, a virus discovery pipeline was used to screen BSF transcriptomes. This led to detecting a new exogenous totivirus that we named hermetia illucens totivirus 1 (HiTV1). Phylogenetic analyses showed this virus belongs to a clade of insect-specific totiviruses and is closely related to the largest EVE located on chromosome 1 of the BSF genome. Lastly, this EVE was found to express a small transcript in some BSFs infected by HiTV1. Altogether, this data mining study showed that far from being unscathed from viruses, BSFs bear traces of past interactions with several viral families and of present interactions with the exogenous HiTV1.

Molecular characterization of a novel totivirus infecting the basal fungus Conidiobolus heterosporus

Mycoviruses are widely distributed in fungi, but only a few mycoviruses have been reported in basal fungi to date. Here, we characterized a novel totivirus isolated from the basal fungus Conidiobolus heterosporus, and we designated this virus as "Conidiobolus heterosporus totivirus 1" (ChTV1). The complete genome of ChTV1 contains two discontinuous open reading frames (ORFs), ORF1 and ORF2, encoding a putative coat protein (CP) and a putative RNA-dependent RNA polymerase (RdRP), respectively. Phylogenetic analysis based on RdRP sequences showed that ChTV1 clustered with members of the genus Totivirus. The RdRP of ChTV1 has 51% sequence identity to that of Trichoderma koningiopsis totivirus 1 (TkTV1), which is the highest among mycoviruses. However, TkTV1 formed a distinct cluster with Wuhan insect virus 27, with 63% RdRP sequence identity, although Wuhan insect virus 27 has not been described, and its host represents a different kingdom. Therefore, we propose that ChTV1 is a new member of the genus Totivirus, family Totiviridae.

Insect-specific viruses and arboviruses in adult male culicids from Midwestern Brazil

Viruses were identified from male anthropophilic mosquitoes from Mato Grosso (MT) State, Midwest Brazil from February 2017 to January 2018. Mosquitoes tested included Aedes (Stegomyia) aegypti (1139 males; 84 pools), Culex quinquefasciatus (9426 males; 179 pools), Culex sp. (3 males; 3 pools) and Psorophora albigenu (1 male; 1 pool) collected from four cities of MT. Pools were subjected to viral RNA extraction followed by RT-PCRs specific for ten flaviviruses, five alphaviruses and Simbu serogroup of orthobunyaviruses. Positive pools were passaged three times in VERO cells (alphavirus and orthobunyavirus) or C6/36 cells (flavivirus), with isolates confirmed through RT-PCR and nucleotide sequencing. We detected pools positive for Ilhéus (1 pool), dengue serotype 4 (1), Mayaro (12), equine encephalitis virus (1) yellow fever (1), Oropouche (2), Zika (4) and chikungunya (12) viruses. High throughput sequencing of arbovirus positive pools identified 35 insect-specific viruses (ISVs) from the families Circoviridae (2), Parvoviridae (2), Totiviridae (1), Flaviviridae (1), Iflaviridae (2), Mesoniviridae (4), Nodaviridae (2), Luteoviridae (1), Phasmaviridae (1) Phenuiviridae (2), Rhabdoviridae (2), Orthomyxoviridae (1), Xinmoviridae (1), and unclassified Bunyavirales (1), unclassified Picornavirales (3), unclassified Riboviria (4) and taxon Negevirus (5). From these, five novel viruses were tentatively named Mojica circovirus, Kuia iflavirus, Muxirum negevirus, Lambada picorna-like virus and Tacuru picorna-like virus. Our findings underscore the diversity and wide geographical distribution of ISVs and arboviruses infecting male culicids.

Identification and genetic analysis of a totivirus isolated from the Culex tritaeniorhynchus in northern China

Totiviridae, a viral family of double-stranded RNA (dsRNA) viruses, contain a single dsRNA genome 4.6-7.0 kb in length. Totiviridae were initially only known to infect fungi and other eukaryotes as well as plants, but an increase in totiviruses has been detected in insects, mosquitoes, and bats. Here, we describe the isolation and characterization of a strain belonging to the family Totiviridae isolated from Culex tritaeniorhynchus in Kenli, China, in 2016. We isolated a totivirus from field-collected mosquitoes in China by cell culture in Aedes albopictus C6/36 cells, identified the virus by morphological observation and complete genome sequencing, and characterized it by phylogenetic analysis. Transmission electron microscopy identified icosahedral, non-enveloped virus particles with a mean diameter of 35-40 nm. The genome was 7612 bp in length, including two open reading frames (ORFs). ORF1 (5058 nt) encodes the capsid protein, while ORF2 (2216 nt) encodes the viral RNA-dependent RNA polymerase (RdRp). Nucleotide and amino acid homology analysis of isolate showed higher levels of sequence identity with isolate CTV_NJ2 (China, 2010) with 94.87% nucleic acid identity and 97.32% amino acid identity. The isolate was designated C. tritaeniorhynchus totivirus KL (CTV-KL). This is the first identification of a totivirus in a C. tritaeniorhynchus in northern China. Analysis of the virus's morphology, characteristic and genome organization will further enrich our understanding of the molecular and biological characteristics of dsRNA Totiviridae viruses.

Characterization of Cronartium ribicola dsRNAs reveals novel members of the family Totiviridae and viral association with fungal virulence

Background

Mycoviruses were recently discovered in the white pine blister rust (WPBR) fungus Cronartium ribicola (J.C. Fisch.). Detection and characterization of their double stranded RNA (dsRNA) would facilitate understanding of pathogen virulence and disease pathogenesis in WPBR systems.

Methods

Full-length cDNAs were cloned from the dsRNAs purified from viral-infected C. ribicola, and their cDNA sequences were determined by DNA sequencing. Evolutionary relationships of the dsRNAs with related mycoviruses were determined by phylogenetic analysis. Dynamic distributions of the viral RNAs within samples of their fungal host C. ribicola were investigated by measurement of viral genome prevalence and viral gene expression.

Results

In this study we identified and characterized five novel dsRNAs from C. ribicola, designated as Cronartium ribicola totivirus 1–5 (CrTV1 to CrTV5). These dsRNA sequences encode capsid protein and RNA-dependent RNA polymerase with significant homologies to dsRNA viruses of the family Totiviridae. Phylogenetic analysis showed that the CrTVs were grouped into two distinct clades. CrTV2 through CrTV5 clustered within the genus Totivirus. CrTV1 along with a few un-assigned dsRNAs constituted a distinct phyletic clade that is genetically distant from presently known genera in the Totiviridae family, indicating that CrTV1 represents a novel genus in the Totiviridae family. The CrTVs were prevalent in fungal samples obtained from infected western white pine, whitebark pine, and limber pines. Viral RNAs were generally expressed at higher levels during in planta mycelium growth than in aeciospores and urediniospores. CrTV4 was significantly associated with C. ribicola virulent pathotype and specific C. ribicola host tree species, suggesting dsRNAs as potential tools for dissection of pathogenic mechanisms of C. ribicola and diagnosis of C. ribicola pathotypes.

Conclusion

Phylogenetic and expression analyses of viruses in the WPBR pathogen, C. ribicola, have enchanced our understanding of virus diversity in the family Totiviridae, and provided a potential strategy to utilize pathotype-associated mycoviruses to control fungal forest diseases.