Comprehensive meta-analysis of severe fever with thrombocytopenia syndrome virus infections in humans, vertebrate hosts and questing ticks.

BACKGROUND: Severe fever with thrombocytopenia syndrome (SFTS) is an emerging tick-borne zoonosis caused by the SFTS virus (SFTSV). Understanding the prevalence of SFTSV RNA in humans, vertebrate hosts and ticks is crucial for SFTS control. METHODS: A systematic review and meta-analysis were conducted to determine the prevalence of SFTSV RNA in humans, vertebrate hosts and questing ticks. Nine electronic databases were searched for relevant publications, and data on SFTSV RNA prevalence were extracted. Pooled prevalence was estimated using a random effects model. Subgroup analysis and multivariable meta-regression were performed to investigate sources of heterogeneity. RESULTS: The pooled prevalence of SFTSV RNA in humans was 5.59% (95% confidence interval [CI] 2.78-9.15%) in those in close contact (close contacts) with infected individuals (infected cases) and 0.05% (95% CI 0.00-0.65%) in healthy individuals in endemic areas. The SFTSV infection rates in artiodactyls (5.60%; 95% CI 2.95-8.96%) and carnivores (6.34%; 95% CI 3.27-10.23%) were higher than those in rodents (0.45%; 95% CI 0.00-1.50%). Other animals, such as rabbits, hedgehogs and birds, also played significant roles in SFTSV transmission. The genus Haemaphysalis was the primary transmission vector, with members of Ixodes, Dermacentor, and Amblyomma also identified as potential vectors. The highest pooled prevalence was observed in adult ticks (1.03%; 95% CI 0.35-1.96%), followed by nymphs (0.66%; 95% CI 0.11-1.50%) and larvae (0.01%; 95% CI 0.00-0.46%). The pooled prevalence in ticks collected from endemic areas (1.86%; 95% CI 0.86-3.14%) was higher than that in ticks collected in other regions (0.41%; 95% CI 0.12-0.81%). CONCLUSIONS: Latent SFTSV infections are present in healthy individuals residing in endemic areas, and close contacts with SFTS cases are at a significantly higher risk of infection. The type of animal is linked to infection rates in vertebrate hosts, while infection rates in ticks are associated with the developmental stage. Further research is needed to investigate the impact of various environmental factors on SFTSV prevalence in vertebrate hosts and ticks.

A novel approach to exploring the dark genome and its application to mapping of the vertebrate virus fossil record.

BACKGROUND: Genomic regions that remain poorly understood, often referred to as the dark genome, contain a variety of functionally relevant and biologically informative features. These include endogenous viral elements (EVEs)-virus-derived sequences that can dramatically impact host biology and serve as a virus fossil record. In this study, we introduce a database-integrated genome screening (DIGS) approach to investigate the dark genome in silico, focusing on EVEs found within vertebrate genomes. RESULTS: Using DIGS on 874 vertebrate genomes, we uncover approximately 1.1 million EVE sequences, with over 99% originating from endogenous retroviruses or transposable elements that contain EVE DNA. We show that the remaining 6038 sequences represent over a thousand distinct horizontal gene transfer events across 10 virus families, including some that have not previously been reported as EVEs. We explore the genomic and phylogenetic characteristics of non-retroviral EVEs and determine their rates of acquisition during vertebrate evolution. Our study uncovers novel virus diversity, broadens knowledge of virus distribution among vertebrate hosts, and provides new insights into the ecology and evolution of vertebrate viruses. CONCLUSIONS: We comprehensively catalog and analyze EVEs within 874 vertebrate genomes, shedding light on the distribution, diversity, and long-term evolution of viruses and reveal their extensive impact on vertebrate genome evolution. Our results demonstrate the power of linking a relational database management system to a similarity search-based screening pipeline for in silico exploration of the dark genome.

Deep mining of the Sequence Read Archive reveals major genetic innovations in coronaviruses and other nidoviruses of aquatic vertebrates.

Virus discovery by genomics and metagenomics empowered studies of viromes, facilitated characterization of pathogen epidemiology, and redefined our understanding of the natural genetic diversity of viruses with profound functional and structural implications. Here we employed a data-driven virus discovery approach that directly queries unprocessed sequencing data in a highly parallelized way and involves a targeted viral genome assembly strategy in a wide range of sequence similarity. By screening more than 269,000 datasets of numerous authors from the Sequence Read Archive and using two metrics that quantitatively assess assembly quality, we discovered 40 nidoviruses from six virus families whose members infect vertebrate hosts. They form 13 and 32 putative viral subfamilies and genera, respectively, and include 11 coronaviruses with bisegmented genomes from fishes and amphibians, a giant 36.1 kilobase coronavirus genome with a duplicated spike glycoprotein (S) gene, 11 tobaniviruses and 17 additional corona-, arteri-, cremega-, nanhypo- and nangoshaviruses. Genome segmentation emerged in a single evolutionary event in the monophyletic lineage encompassing the subfamily Pitovirinae. We recovered the bisegmented genome sequences of two coronaviruses from RNA samples of 69 infected fishes and validated the presence of poly(A) tails at both segments using 3'RACE PCR and subsequent Sanger sequencing. We report a genetic linkage between accessory and structural proteins whose phylogenetic relationships and evolutionary distances are incongruent with the phylogeny of replicase proteins. We rationalize these observations in a model of inter-family S recombination involving at least five ancestral corona- and tobaniviruses of aquatic hosts. In support of this model, we describe an individual fish co-infected with members from the families Coronaviridae and Tobaniviridae. Our results expand the scale of the known extraordinary evolutionary plasticity in nidoviral genome architecture and call for revisiting fundamentals of genome expression, virus particle biology, host range and ecology of vertebrate nidoviruses.

In Vitro and In Vivo Coinfection and Superinfection Dynamics of Mayaro and Zika Viruses in Mosquito and Vertebrate Backgrounds.

Globalization and climate change have contributed to the simultaneous increase and spread of arboviral diseases. Cocirculation of several arboviruses in the same geographic region provides an impetus to study the impacts of multiple concurrent infections within an individual vector mosquito. Here, we describe coinfection and superinfection with the Mayaro virus (Togaviridae, Alphavirus) and Zika virus (Flaviviridae, Flavivirus) in vertebrate and mosquito cells, as well as Aedes aegypti adult mosquitoes, to understand the interaction dynamics of these pathogens and effects on viral infection, dissemination, and transmission. Aedes aegypti mosquitoes were able to be infected with and transmit both pathogens simultaneously. However, whereas Mayaro virus was largely unaffected by coinfection, it had a negative impact on infection and dissemination rates for Zika virus compared to single infection scenarios. Superinfection of Mayaro virus atop a previous Zika virus infection resulted in increased Mayaro virus infection rates. At the cellular level, we found that mosquito and vertebrate cells were also capable of being simultaneously infected with both pathogens. Similar to our findings in vivo, Mayaro virus negatively affected Zika virus replication in vertebrate cells, displaying complete blocking under certain conditions. Viral interference did not occur in mosquito cells. IMPORTANCE Epidemiological and clinical studies indicate that multiple arboviruses are cocirculating in human populations, leading to some individuals carrying more than one arbovirus at the same time. In turn, mosquitoes can become infected with multiple pathogens simultaneously (coinfection) or sequentially (superinfection). Coinfection and superinfection can have synergistic, neutral, or antagonistic effects on viral infection dynamics and ultimately have impacts on human health. Here we investigate the interaction between Zika virus and Mayaro virus, two emerging mosquito-borne pathogens currently circulating together in Latin America and the Caribbean. We find a major mosquito vector of these viruses-Aedes aegypti-can carry and transmit both arboviruses at the same time. Our findings emphasize the importance of considering co- and superinfection dynamics during vector-pathogen interaction studies, surveillance programs, and risk assessment efforts in epidemic areas.

ICTV Virus Taxonomy Profile: Retroviridae 2021.

Viruses in the family Retroviridae are found in a wide variety of vertebrate hosts. Enveloped virions are 80-100 nm in diameter with an inner core containing the viral genome and replicative enzymes. Core morphology is often characteristic for viruses within the same genus. Replication involves reverse transcription and integration into host cell DNA, resulting in a provirus. Integration into germline cells can result in a heritable provirus known as an endogenous retrovirus. This is a summary of the International Committee on Taxonomy of Viruses (ICTV) Report on the family Retroviridae, which is available at ictv.global/report/retroviridae.

Flavivirus infections induce a Golgi stress response in vertebrate and mosquito cells.

The stress of the Golgi apparatus is an autoregulatory mechanism that is induced to compensate for greater demand in the Golgi functions. No examples of Golgi stress responses due to physiological stimuli are known. Furthermore, the impact on this organelle of viral infections that occupy the vesicular transport during replication is unknown. In this work, we evaluated if a Golgi stress response is triggered during dengue and Zika viruses replication, two flaviviruses whose replicative cycle is heavily involved with the Golgi complex, in vertebrate and mosquito cells. Using GM-130 as a Golgi marker, and treatment with monensin as a positive control for the induction of the Golgi stress response, a significant expansion of the Golgi cisternae was observed in BHK-21, Vero E6 and mosquito cells infected with either virus. Activation of the TFE3 pathway was observed in the infected cells as indicated by the translocation from the cytoplasm to the nucleus of TFE3 and increased expression of pathway targeted genes. Of note, no sign of activation of the stress response was observed in CRFK cells infected with Feline Calicivirus (FCV), a virus released by cell lysis, not requiring vesicular transport. Finally, dilatation of the Golgi complex and translocation of TFE3 was observed in vertebrate cells expressing dengue and Zika viruses NS1, but not NS3. These results indicated that infections by dengue and Zika viruses induce a Golgi stress response in vertebrate and mosquito cells due to the increased demand on the Golgi complex imposed by virion and NS1 processing and secretion.

Isolation of a novel insect-specific flavivirus with immunomodulatory effects in vertebrate systems.

We describe the isolation and characterization of a novel insect-specific flavivirus (ISFV), tentatively named Aripo virus (ARPV), that was isolated from Psorophora albipes mosquitoes collected in Trinidad. The ARPV genome was determined and phylogenetic analyses showed that it is a dual host associated ISFV, and clusters with the main mosquito-borne flaviviruses. ARPV antigen was significantly cross-reactive with Japanese encephalitis virus serogroup antisera, with significant cross-reactivity to Ilheus and West Nile virus (WNV). Results suggest that ARPV replication is limited to mosquitoes, as it did not replicate in the sandfly, culicoides or vertebrate cell lines tested. We also demonstrated that ARPV is endocytosed into vertebrate cells and is highly immunomodulatory, producing a robust innate immune response despite its inability to replicate in vertebrate systems. We show that prior infection or coinfection with ARPV limits WNV-induced disease in mouse models, likely the result of a robust ARPV-induced type I interferon response.

Discovery and Characterization of Actively Replicating DNA and Retro-Transcribing Viruses in Lower Vertebrate Hosts Based on RNA Sequencing.

In a previous study, a metatranscriptomics survey of RNA viruses in several important lower vertebrate host groups revealed huge viral diversity, transforming the understanding of the evolution of vertebrate-associated RNA virus groups. However, the diversity of the DNA and retro-transcribing viruses in these host groups was left uncharacterized. Given that RNA sequencing is capable of revealing viruses undergoing active transcription and replication, we collected previously generated datasets associated with lower vertebrate hosts, and searched them for DNA and retro-transcribing viruses. Our results revealed the complete genome, or "core gene sets", of 18 vertebrate-associated DNA and retro-transcribing viruses in cartilaginous fishes, ray-finned fishes, and amphibians, many of which had high abundance levels, and some of which showed systemic infections in multiple organs, suggesting active transcription or acute infection within the host. Furthermore, these new findings recharacterized the evolutionary history in the families Hepadnaviridae, Papillomaviridae, and Alloherpesviridae, confirming long-term virus-host codivergence relationships for these virus groups. Collectively, our results revealed reliable and sufficient information within metatranscriptomics sequencing to characterize not only RNA viruses, but also DNA and retro-transcribing viruses, and therefore established a key methodology that will help us to understand the composition and evolution of the total "infectome" within a diverse range of vertebrate hosts.

Generation and preliminary characterization of vertebrate-specific replication-defective Zika virus.

Zika virus (ZIKV) is a mosquito-borne flavivirus that replicates in both vertebrate and insect cells, whereas insect-specific flaviviruses (ISF) replicate only in insect cells. We sought to convert ZIKV, from a dual-tropic flavivirus, into an insect-specific virus for the eventual development of a safe ZIKV vaccine. Reverse genetics was used to introduce specific mutations into the furin cleavage motif within the ZIKV pre-membrane protein (prM). Mutant clones were selected, which replicated well in C6/36 insect cells but exhibited reduced replication in non-human primate (Vero) cells. Further characterization of the furin cleavage site mutants indicated they replicated poorly in both human (HeLa, U251), and baby hamster kidney (BHK-21) cells. One clone with the induced mutation in the prM protein and at positions 291and 452 within the NS3 protein was totally and stably replication-defective in vertebrate cells (VSRD-ZIKV). Preliminary studies in ZIKV sensitive, immunodeficient mice demonstrated that VSRD-ZIKV-infected mice survived and were virus-negative. Our study indicates that a reverse genetic approach targeting the furin cleavage site in prM can be used to select an insect-specific ZIKV with the potential utility as a vaccine strain.

Vertebrate-Aedes aegypti and Culex quinquefasciatus (Diptera)-arbovirus transmission networks: Non-human feeding revealed by meta-barcoding and next-generation sequencing.

BACKGROUND: Aedes aegypti mosquito-borne viruses including Zika (ZIKV), dengue (DENV), yellow fever (YFV), and chikungunya (CHIKV) have emerged and re-emerged globally, resulting in an elevated burden of human disease. Aedes aegypti is found worldwide in tropical, sub-tropical, and temperate areas. The characterization of mosquito blood meals is essential to understand the transmission dynamics of mosquito-vectored pathogens. METHODOLOGY/PRINCIPAL FINDINGS: Here, we report Ae. aegypti and Culex quinquefasciatus host feeding patterns and arbovirus transmission in Northern Mexico using a metabarcoding-like approach with next-generation deep sequencing technology. A total of 145 Ae. aegypti yielded a blood meal analysis result with 107 (73.8%) for a single vertebrate species and 38 (26.2%) for two or more. Among the single host blood meals for Ae. aegypti, 28.0% were from humans, 54.2% from dogs, 16.8% from cats, and 1.0% from tortoises. Among those with more than one species present, 65.9% were from humans and dogs. For Cx. quinquefasciatus, 388 individuals yielded information with 326 (84%) being from a single host and 63 (16.2%) being from two or more hosts. Of the single species blood meals, 77.9% were from dogs, 6.1% from chickens, 3.1% from house sparrows, 2.4% from humans, while the remaining 10.5% derived from other 12 host species. Among those which had fed on more than one species, 11% were from dogs and humans, and 89% of other host species combinations. Forage ratio analysis revealed dog as the most over-utilized host by Ae. aegypti (= 4.3) and Cx. quinquefasciatus (= 5.6) and the human blood index at 39% and 4%, respectively. A total of 2,941 host-seeking female Ae. aegypti and 3,536 Cx. quinquefasciatus mosquitoes were collected in the surveyed area. Of these, 118 Ae. aegypti pools and 37 Cx. quinquefasciatus pools were screened for seven arboviruses (ZIKV, DENV 1-4, CHIKV, and West Nile virus (WNV)) using qRT-PCR and none were positive (point prevalence = 0%). The 95%-exact upper limit confidence interval was 0.07% and 0.17% for Ae. aegypti and Cx. quinquefasciatus, respectively. CONCLUSIONS/SIGNIFICANCE: The low human blood feeding rate in Ae. aegypti, high rate of feeding on mammals by Cx. quinquefasciatus, and the potential risk to transmission dynamics of arboviruses in highly urbanized areas of Northern Mexico is discussed.

Divergent Influenza-Like Viruses of Amphibians and Fish Support an Ancient Evolutionary Association.

Influenza viruses (family Orthomyxoviridae) infect a variety of vertebrates, including birds, humans, and other mammals. Recent metatranscriptomic studies have uncovered divergent influenza viruses in amphibians, fish and jawless vertebrates, suggesting that these viruses may be widely distributed. We sought to identify additional vertebrate influenza-like viruses through the analysis of publicly available RNA sequencing data. Accordingly, by data mining, we identified the complete coding segments of five divergent vertebrate influenza-like viruses. Three fell as sister lineages to influenza B virus: salamander influenza-like virus in Mexican walking fish (Ambystoma mexicanum) and plateau tiger salamander (Ambystoma velasci), Siamese algae-eater influenza-like virus in Siamese algae-eater fish (Gyrinocheilus aymonieri) and chum salmon influenza-like virus in chum salmon (Oncorhynchus keta). Similarly, we identified two influenza-like viruses of amphibians that fell as sister lineages to influenza D virus: cane toad influenza-like virus and the ornate chorus frog influenza-like virus, in the cane toad (Rhinella marina) and ornate chorus frog (Microhyla fissipes), respectively. Despite their divergent phylogenetic positions, these viruses retained segment conservation and splicing consistent with transcriptional regulation in influenza B and influenza D viruses, and were detected in respiratory tissues. These data suggest that influenza viruses have been associated with vertebrates for their entire evolutionary history.

[Transmission of plant and vertebrate viruses by arthropods]. / La transmission des virus de plantes et de vertébrés par arthropodes.

Many plant and vertebrate viruses use mobile vectors to be transmitted between hosts. These vectors, mainly arthropods, acquire or inoculate the virus by feeding on plant extract or vertebrate blood. Several virus transmission modes have been characterized based on the tight interactions between the virus and the vector. Some viruses are internalized into cells and migrate through different tissues and organs before being released. In the vector, the virus can replicate in some cases. Other viruses are retained, specifically or non-specifically, on the vector mouthparts. Acquiring knowledge on the molecular mechanisms of virus transmission by arthropods consists in studying (i) virus receptors in the vectors, (ii) the mode of virus uptake into vector cells, (iii) virus localization and transport in the vector, and (iv) viral determinants required for transmission. This review, although non exhaustive, presents a state-of-the-art of plant and vertebrate virus transmission by arthropods, notably by pointing to their similarities and differences.

Agua Salud alphavirus defines a novel lineage of insect-specific alphaviruses discovered in the New World.

The genus Alphavirus harbours mostly insect-transmitted viruses that cause severe disease in humans, livestock and wildlife. Thus far, only three alphaviruses with a host range restricted to insects have been found in mosquitoes from the Old World, namely Eilat virus (EILV), Taï Forest alphavirus (TALV) and Mwinilunga alphavirus (MWAV). In this study, we found a novel alphavirus in one Culex declarator mosquito sampled in Panama. The virus was isolated in C6/36 mosquito cells, and full genome sequencing revealed an 11 468 nt long genome with maximum pairwise nucleotide identity of 62.7 % to Sindbis virus. Phylogenetic analyses placed the virus as a solitary deep rooting lineage in a basal relationship to the Western equine encephalitis antigenic complex and to the clade comprising EILV, TALV and MWAV, indicating the detection of a novel alphavirus, tentatively named Agua Salud alphavirus (ASALV). No growth of ASALV was detected in vertebrate cell lines, including cell lines derived from ectothermic animals, and replication of ASALV was strongly impaired above 31 °C, suggesting that ASALV represents the first insect-restricted alphavirus of the New World.

Virus structures constrain transmission modes.

Here we investigate links between the structures of viruses and routes of transmission. Viruses show a wide range of different structures, and the transmission of viruses between vertebrate hosts can take place through many different routes. We compiled a database of 243 virus-host combinations and report a statistical analysis that documents the associations between structures and routes of transmission-for example, viruses that are transmitted by the faecal-oral mode of infection are rarely enclosed in a lipid envelope.

An Ancient Lineage of Highly Divergent Parvoviruses Infects both Vertebrate and Invertebrate Hosts.

Chapparvoviruses (ChPVs) comprise a divergent, recently identified group of parvoviruses (family Parvoviridae), associated with nephropathy in immunocompromised laboratory mice and with prevalence in deep sequencing results of livestock showing diarrhea. Here, we investigate the biological and evolutionary characteristics of ChPVs via comparative in silico analyses, incorporating sequences derived from endogenous parvoviral elements (EPVs) as well as exogenous parvoviruses. We show that ChPVs are an ancient lineage within the Parvoviridae, clustering separately from members of both currently established subfamilies. Consistent with this, they exhibit a number of characteristic features, including several putative auxiliary protein-encoding genes, and capsid proteins with no sequence-level homology to those of other parvoviruses. Homology modeling indicates the absence of a ß-A strand, normally part of the luminal side of the parvoviral capsid protein core. Our findings demonstrate that the ChPV lineage infects an exceptionally broad range of host species, including both vertebrates and invertebrates. Furthermore, we observe that ChPVs found in fish are more closely related to those from invertebrates than they are to those of amniote vertebrates. This suggests that transmission between distantly related host species may have occurred in the past and that the Parvoviridae family can no longer be divided based on host affiliation.

Molecular and Ecological Studies of a Virus Family (Iridoviridae) Infecting Invertebrates and Ectothermic Vertebrates.

Research involving viruses within the family Iridoviridae (generically designated iridovirids to distinguish members of the family Iridoviridae from members of the genus Iridovirus) has markedly increased in recent years [...].

A voting mechanism-based linear epitope prediction system for the host-specific Iridoviridae family.

BACKGROUND: The Iridoviridae family is categorized into five genera and clustered into two subfamilies: Alphairidovirinae includes Lymphocystivirus, Ranavirus (GIV), and Megalocystivirus (TGIV), which infect vertebrate hosts and Betairidovirinae includes Iridovirus and Chloriridovirus, which infect invertebrate hosts. Clustered Iridoviridae subfamilies possess host-specific characteristics, which can be considered as exclusive features for in-silico prediction of effective epitopes for vaccine development. A voting mechanism-based linear epitope (LE) prediction system was applied to identify and endorse LE candidates with a minimum length requirement for each clustered subfamily RESULTS: The experimental results showed that four conserved epitopes among the Iridovirideae family, one exclusive epitope for invertebrate subfamily and two exclusive epitopes for vertebrate family were predicted. These predicted LE candidates were further validated by ELISA assays for evaluating the strength of antigenicity and cross antigenicity. The conserved LEs for Iridoviridae family reflected high antigenicity responses for the two subfamilies, while exclusive LEs reflected high antigenicity responses only for the host-specific subfamily CONCLUSIONS: Host-specific characteristics are important features and constraints for effective epitope prediction. Our proposed voting mechanism based system provides a novel approach for in silico LE prediction prior to vaccine development, and it is especially powerful for analyzing antigen sequences with exclusive features between two clustered groups.

Phylogenetic analysis of novel posaviruses detected in feces of Japanese pigs with posaviruses and posa-like viruses of vertebrates and invertebrates.

Posaviruses and posa-like viruses are unclassified viruses with sequence similarity to viruses of the order Picornavirales. They have been reported in various vertebrates and invertebrates. We identified 11 posavirus-like sequences in porcine feces and performed phylogenic analysis. Previously reported Japanese posaviruses and those identified in this study clustered with posavirus 1, 4, and 7 and husavirus 1, while five viruses branched into three independent lineages, tentatively named posavirus 10, 11, and 12. Interestingly, posaviruses, except for posavirus 8 and 9, husaviruses, and rasaviruses, formed a cluster consisting of viruses only from pigs, humans, and rats, while posavirus 8 and 9, fisavirus, and basaviruses clustered with posa-like viruses from invertebrates.

Relationship Between G-Quadruplex Sequence Composition in Viruses and Their Hosts.

A subset of guanine-rich nucleic acid sequences has the potential to fold into G-quadruplex (G4) secondary structures, which are functionally important for several biological processes, including genome stability and regulation of gene expression. Putative quadruplex sequences (PQSs) G3+N1-7G3+N1-7G3+N1-7G3+ are widely found in eukaryotic and prokaryotic genomes, but the base composition of the N1-7 loops is biased across species. Since the viruses partially hijack their hosts' cellular machinery for proliferation, we examined the PQS motif size, loop length, and nucleotide compositions of 7370 viral genome assemblies and compared viral and host PQS motifs. We studied seven viral taxa infecting five distant eukaryotic hosts and created a resource providing a comprehensive view of the viral quadruplex motifs. Overall, short-looped PQSs are predominant and with a similar composition across viral taxonomic groups, albeit subtle trends emerge upon classification by hosts. Specifically, there is a higher frequency of pyrimidine loops in viruses infecting animals irrespective of the viruses' genome type. This observation is confirmed by an in-depth analysis of the Herpesviridae family of viruses, which showed a distinctive accumulation of thermally stable C-looped quadruplexes in viruses infecting high-order vertebrates. The occurrence of viral C-looped G4s, which carry binding sites for host transcription factors, as well as the high prevalence of viral TTA-looped G4s, which are identical to vertebrate telomeric motifs, provide concrete examples of how PQSs may help viruses impinge upon, and benefit from, host functions. More generally, these observations suggest a co-evolution of virus and host PQSs, thus underscoring the potential functional significance of G4s.

Expanding the RNA Virosphere by Unbiased Metagenomics.

Although viruses comprise the most abundant genetic material in the biosphere, to date only several thousand virus species have been formally defined. Such a limited perspective on virus diversity has in part arisen because viruses were traditionally considered only as etiologic agents of overt disease in humans or economically important species and were often difficult to identify using cell culture. This view has dramatically changed with the rise of metagenomics, which is transforming virus discovery and revealing a remarkable diversity of viruses sampled from diverse cellular organisms. These newly discovered viruses help fill major gaps in the evolutionary history of viruses, revealing a near continuum of diversity among genera, families, and even orders of RNA viruses. Herein, we review some of the recent advances in our understanding of the RNA virosphere that have stemmed from metagenomics, note future directions, and highlight some of the remaining challenges to this rapidly developing field.