Mimiviridae Family Research Articles

Pacmanvirus isolated from the Lost City hydrothermal field extends the concept of transpoviron beyond the family Mimiviridae.

The microbial sampling of submarine hydrothermal vents remains challenging, with even fewer studies focused on viruses. Here we report the first isolation of a eukaryotic virus from the Lost City hydrothermal field, by co-culture with the laboratory host Acanthamoeba castellanii. This virus, named pacmanvirus lostcity, is closely related to previously isolated pacmanviruses (strains A23 and S19), clustering in a divergent clade within the long-established family Asfarviridae. Its icosahedral particles are 200nm in diameter, with an electron-dense core surrounded by an inner membrane. Its genome of 395 708bp (33% G + C) is predicted to encode 473 proteins. However, besides these standard properties, pacmanvirus lostcity was found associated with a new type of selfish genetic element, 7kb in length, whose architecture and gene content are reminiscent of those of transpovirons, hitherto specific to the family Mimiviridae. Like previously described transpovirons, this element propagates as an episome within its host virus particles and exhibits partial recombination with its genome. In addition, an unrelated 2kb long episome was also associated with pacmanvirus lostcity. Together, the transpoviron and the 2kb episome might participate to exchanges between pacmanviruses and other large DNA virus families. It remains to be elucidated if the presence of these mobile genetic elements is restricted to pacmanviruses or was simply overlooked in other members of the Asfarviridae.

Transition metals and oxidation reactions trigger stargate opening during the initial stages of the replicative cycle of the giant Tupanvirus.

Tupanviruses, members of the family Mimiviridae, infect phagocytic cells. Particle uncoating begins inside the phagosome, with capsid opening via the stargate. The mechanism through which this opening takes place is unknown. Once phagocytized, metal ion flux control and ROS are induced to inactivate foreign particles, including viruses. Here, we studied the effect of iron ions, copper ions, and H2O2 on Tupanvirus particles. Such treatments induced stargate opening in vitro, as observed by different microscopy techniques. Metal-treated viruses were found to be non-infectious, leading to the hypothesis that stargate opening likely resulted in the release of the viral seed, which is required for infection initiation. To the best of our knowledge, this is the first description of a giant virus capsid morphological change induced by transition metals and H2O2, which may be important to describe new virulence factors and capsid uncoating mechanisms.

Functional redundancy revealed by the deletion of the mimivirus GMC-oxidoreductase genes.

The mimivirus 1.2 Mb genome was shown to be organized into a nucleocapsid-like genomic fiber encased in the nucleoid compartment inside the icosahedral capsid. The genomic fiber protein shell is composed of a mixture of two GMC-oxidoreductase paralogs, one of them being the main component of the glycosylated layer of fibrils at the surface of the virion. In this study, we determined the effect of the deletion of each of the corresponding genes on the genomic fiber and the layer of surface fibrils. First, we deleted the GMC-oxidoreductase, the most abundant in the genomic fiber, and determined its structure and composition in the mutant. As expected, it was composed of the second GMC-oxidoreductase and contained 5- and 6-start helices similar to the wild-type fiber. This result led us to propose a model explaining their coexistence. Then we deleted the GMC-oxidoreductase, the most abundant in the layer of fibrils, to analyze its protein composition in the mutant. Second, we showed that the fitness of single mutants and the double mutant were not decreased compared with the wild-type viruses under laboratory conditions. Third, we determined that deleting the GMC-oxidoreductase genes did not impact the glycosylation or the glycan composition of the layer of surface fibrils, despite modifying their protein composition. Because the glycosylation machinery and glycan composition of members of different clades are different, we expanded the analysis of the protein composition of the layer of fibrils to members of the B and C clades and showed that it was different among the three clades and even among isolates within the same clade. Taken together, the results obtained on two distinct central processes (genome packaging and virion coating) illustrate an unexpected functional redundancy in members of the family Mimiviridae, suggesting this may be the major evolutionary force behind their giant genomes.

Cytopathic effects in Mimivirus infection: understanding the kinetics of virus-cell interaction.

Giant viruses have brought new insights into different aspects of virus-cell interactions. The resulting cytopathic effects from these interactions are one of the main aspects of infection assessment in a laboratory routine, mainly reflecting on the morphological features of an infected cell. In this work, we follow the entire kinetics of the cytopathic effect in cells infected by viruses of the Mimiviridae family, spatiotemporally quantifying typical features such as cell roundness, loss of motility, decrease in cell area and cell lysis. Infections by Acanthamoeba polyphaga mimivirus (APMV), Tupanvirus (TPV) and M4 were carried out at multiplicity of infection (MOI) 1 and MOI 10 in Acanthamoeba castellanii. Monitoring of infections was carried out using time lapse microscopy for up to 72 hours. The images were analyzed using ImageJ software. The data obtained indicate that APMV is the slowest virus in inducing the cytopathic effects of rounding, decrease in cell area, mobility and cell lysis. However, it is the only virus whose MOI increase accelerates the lysis process of infected cells. In turn, TPV and M4 rapidly induce morphological and behavioral changes. Our results indicate that mimiviruses induce different temporal responses within the host cell and that it is possible to use these kinetic data to facilitate the understanding of infection by these viruses.

Giant DNA Viruses Infecting Unicellular Protists

Giant DNA Viruses Infecting Unicellular Protists

The giant mimivirus 1.2 Mb genome is elegantly organized into a 30-nm diameter helical protein shield.

Mimivirus is the prototype of the Mimiviridae family of giant dsDNA viruses. Little is known about the organization of the 1.2 Mb genome inside the membrane-limited nucleoid filling the ~0.5 µm icosahedral capsids. Cryo-electron microscopy, cryo-electron tomography, and proteomics revealed that it is encased into a ~30-nm diameter helical protein shell surprisingly composed of two GMC-type oxidoreductases, which also form the glycosylated fibrils decorating the capsid. The genome is arranged in 5- or 6-start left-handed super-helices, with each DNA-strand lining the central channel. This luminal channel of the nucleoprotein fiber is wide enough to accommodate oxidative stress proteins and RNA polymerase subunits identified by proteomics. Such elegant supramolecular organization would represent a remarkable evolutionary strategy for packaging and protecting the genome, in a state ready for immediate transcription upon unwinding in the host cytoplasm. The parsimonious use of the same protein in two unrelated substructures of the virion is unexpected for a giant virus with thousand genes at its disposal.

Isolation and Identification of a Large Green Alga Virus (Chlorella Virus XW01) of Mimiviridae and Its Virophage (Chlorella Virus Virophage SW01) by Using Unicellular Green Algal Cultures.

Virophages are a group of small double-stranded DNA viruses that infect protist hosts and parasitize the viral factory of host giant/large viruses to propagate. Here, we discover a novel cell-virus-virophage (CVv) tripartite interaction system by using unicellular micro-green algae (Chlorella sp.) as eukaryotic hosts for the first time. Viral particles, resembling known virophages and large alga viruses, are detected in culture supernatants and inside algal cells. Complete genomic sequences of the virophage (Chlorella virus virophage SW01 [CVv-SW01]; 24,744 bp) and large virus (Chlorella virus XW01 [CV-XW01]; 407,612 bp) are obtained from the cocultures. Both genomic and phylogenetic analyses show that CVv-SW01 is closely related to virophages previously found in Dishui Lake. CV-XW01 shares the greatest number of homologous genes (n = 82) with Cafeteria roenbergensis virus (CroV) and phylogenetically represents the closest relative to CroV. This is the first report of a large green alga virus being affiliated with a heterotrophic zooplankton-infecting Cafeteriavirus of the family Mimiviridae. Moreover, the codon usage preferences of CV-XW01 and CVv-SW01 are highly similar to those of CroV and its virophage Mavirus, respectively. The discovery of such a novel CVv system with the green alga Chlorella sp. as the single cellular eukaryotic host paves a way to further investigate the potential interaction mechanism of CVv and its significance in the ecology of green algae and the evolution of large/giant viruses and their parasitic viruses. IMPORTANCE Parasitic virophages are small unicellular eukaryotic dsDNA viruses that rely on the viral factories of coinfecting giant/large dsDNA viruses for propagation. Presently, the identified eukaryotic hosts of isolated virophages were restricted to a free-living amoeba, Acanthamoeba polyphaga, and a widespread marine heterotrophic flagellate, Cafeteria roenbergensis. In this study, we successfully discovered and identified a novel tripartite interaction system comprised of a micro-green alga (Chlorella sp.), Mimiviridae large green alga virus, and virophage at the coculture level, with Chlorella sp. as the eukaryotic host, based on combination analysis of infection, morphotype, genome, and phylogeny. The large green alga virus CV-XW01 represents the closest relative to the Mimiviridae giant virus Cafeteria roenbergensis virus, host virus of the virophage Mavirus, as well as a novel large virus of Mimiviridae that infects a non-protozoan protist host. The virophage CVv-SW01 highly resembles Mavirus in its codon usage frequency and preference, although they are phylogenetically distantly related. These findings give novel insights into the diversity of large/giant viruses and their virophages.

Virophages: association with human diseases and their predicted role as virus killers.

The fascinating discovery of the first giant virus, Acanthamoeba polyphaga mimivirus (APMV), belonging to the family Mimiviridae in 2008, and its associated virophage, Sputnik, have left the world of microbiology awestruck. To date, about 18 virophages have been isolated from different environmental sources. With their unique feature of resisting host cell infection and lysis by giant viruses, analogous to bacteriophage, they have been assigned under the family Lavidaviridae. Genome of T-27, icosahedral-shaped, non-enveloped virophages, consist of dsDNA encoding four proteins, namely, major capsid protein, minor capsid protein, ATPase and cysteine protease, which are essential in the formation and assembly of new virophage particles during replication. A few virophage genomes have been observed to contain additional sequences like PolB, ZnR and S3H. Another interesting characteristic of virophage is that Mimivirus lineage A is immune to infection by the Zamilon virophage through a phenomenon termed MIMIVIRE, resembling the CRISPR-Cas mechanism in bacteria. Based on the fact that giant viruses have been found in clinical samples of hospital-acquired pneumonia and rheumatoid arthritis patients, virophages have opened a novel era in the search for cures of various diseases. This article aims to study the prospective role of virophages in the future of human therapeutics.

Marseilleviruses: An Update in 2021.

The family Marseilleviridae was the second family of giant viruses that was described in 2013, after the family Mimiviridae. Marseillevirus marseillevirus, isolated in 2007 by coculture on Acanthamoeba polyphaga, is the prototype member of this family. Afterward, the worldwide distribution of marseilleviruses was revealed through their isolation from samples of various types and sources. Thus, 62 were isolated from environmental water, one from soil, one from a dipteran, one from mussels, and two from asymptomatic humans, which led to the description of 67 marseillevirus isolates, including 21 by the IHU Méditerranée Infection in France. Recently, five marseillevirus genomes were assembled from deep sea sediment in Norway. Isolated marseilleviruses have ≈250 nm long icosahedral capsids and 348–404 kilobase long mosaic genomes that encode 386–545 predicted proteins. Comparative genomic analyses indicate that the family Marseilleviridae includes five lineages and possesses a pangenome composed of 3,082 clusters of genes. The detection of marseilleviruses in both symptomatic and asymptomatic humans in stool, blood, and lymph nodes, and an up-to-30-day persistence of marseillevirus in rats and mice, raise questions concerning their possible clinical significance that are still under investigation.

A persistent giant algal virus, with a unique morphology, encodes an unprecedented number of genes involved in energy metabolism.

Viruses have long been viewed as entities possessing extremely limited metabolic capacities. Over the last decade, however, this view has been challenged, as metabolic genes have been identified in viruses possessing large genomes and virions-the synthesis of which is energetically demanding. Here, we unveil peculiar phenotypic and genomic features of Prymnesium kappa virus RF01 (PkV RF01), a giant virus of the Mimiviridae family. We found that this virus encodes an unprecedented number of proteins involved in energy metabolism, such as all four succinate dehydrogenase (SDH) subunits (A-D) as well as key enzymes in the β-oxidation pathway. The SDHA gene was transcribed upon infection, indicating that the viral SDH is actively used by the virus- potentially to modulate its host's energy metabolism. We detected orthologous SDHA and SDHB genes in numerous genome fragments from uncultivated marine Mimiviridae viruses, which suggests that the viral SDH is widespread in oceans. PkV RF01 was less virulent compared with other cultured prymnesioviruses, a phenomenon possibly linked to the metabolic capacity of this virus and suggestive of relatively long co-evolution with its hosts. It also has a unique morphology, compared to other characterized viruses in the Mimiviridae family. Finally, we found that PkV RF01 is the only alga-infecting Mimiviridae virus encoding two aminoacyl-tRNA synthetases and enzymes corresponding to an entire base-excision repair pathway, as seen in heterotroph-infecting Mimiviridae These Mimiviridae encoded-enzymes were found to be monophyletic and branching at the root of the eukaryotic tree of life. This placement suggests that the last common ancestor of Mimiviridae was endowed with a large, complex genome prior to the divergence of known extant eukaryotes.IMPORTANCE Viruses on Earth are tremendously diverse in terms of morphology, functionality, and genomic composition. Over the last decade, the conceptual gap separating viruses and cellular life has tightened because of the detection of metabolic genes in viral genomes that express complex virus phenotypes upon infection. Here, we describe Prymnesium kappa virus RF01, a large alga-infecting virus with a unique morphology, an atypical infection profile, and an unprecedented number of genes involved in energy metabolism (such as the tricarboxylic (TCA) cycle and the β-oxidation pathway). Moreover, we show that the gene corresponding to one of these enzymes (the succinate dehydrogenase subunit A) is transcribed during infection and is widespread among marine viruses. This discovery provides evidence that a virus has the potential to actively regulate energy metabolism with its own gene.

Investigation of starry sturgeon Acipencer stellatus in the Black Sea and Sea of Azov for AcIV-E infection

Specific sturgeon nucleocytoplasmic large DNA viruses (sNCLDV) infect several species of the Acipenseridae family. sNCLDV have previously been referred to as unclassified members of the family Iridoviridae. They have recently been moved from Iridoviridae to Mimiviridae family. One of these viruses, Acipenser iridovirus-European (AcIV-E), is present in farms across Europe where it has occasionally caused mild to severe losses in sturgeons. In this study, we provide data on AcIV-E search in a susceptible species such as starry sturgeon (Acipencer stellatus) from the Black sea and Sea of Azov. In 2020, starry sturgeons were sampled in Odessa region of the Black Sea and Zaporizhzhia region of the Sea of Azov. None of the sampled fish demonstrated pathological signs of disease and were healthy upon visual observation all individuals. Total DNA was extracted and submitted to a generic PCR targeting different fragments of major capsid protein (MCP) gene of AcIV-E. No virus-specific products were obtained in any starry sturgeon sample from both seas, while the expected products of MCP gene fragments as well as the fragments of beta-actin gene in control reactions were successfully amplified using fish and viral DNA of AcIV-E isolated from Siberian sturgeon in 2018–2019. Although AcIV-E is difficult to identify in asymptomatic wild fish, but it probably can play a role in starry sturgeon pathology across the Black Sea basin. A high risk of virus spread to wild populations is possible since this virus was identified in sturgeon farms upstream the Dnipro river. Therefore, AcIV-E, even in the presence of other pathogens, should be studied in sturgeon hatcheries as well as in possible vectors of disease, but the huge area of inspection could be a challenge.

First genetic characterization of sturgeon mimiviruses in Ukraine.

A group of pathogenic nucleocytoplasmic large DNA viruses (NCLDVs) related to the Mimiviridae family infect farmed sturgeons across Europe, causing mild-to-severe losses. One of these viruses, Acipenser iridovirus-European (AcIV-E), was identified in six sturgeon species. During the 2018-2019 period, nine sick Siberian (A.baerii) and Russian (A.gueldenstaedtii) sturgeons were sampled in Ukrainian farms and tested for the presence of AcIV-E using real-time PCR. The presence of AcIV-E was confirmed in some samples. High-resolution melting (HRM) assay and Sanger sequencing demonstrated the presence in three farms of two alleles of the major capsid protein (MCP) gene, called var1 and var2. Five samples carried both var1 and var2 at varying ratios, and the sixth sample was infected with only var1. These results constitute the first detection of AcIV-E in Ukraine and the first detection of a sample carrying only var1. The full-length sequences of the MCP genes confirmed the existence of two genetic lineages of AcIV-E, tentatively named V1 and V2, each displaying multiple substitutions in the MCP gene. Some of the MCP sequences showed a genetic relationship to both V1 and V2 lineages, depending on the fragment examined. Most likely, these sequences resulted from recombination events.

Isolation and genomic characterization of a new mimivirus of lineage B from a Brazilian river.

Since its discovery, the first identified giant virus associated with amoebae, Acanthamoeba polyphaga mimivirus (APMV), has been rigorously studied to understand the structural and genomic complexity of this virus. In this work, we report the isolation and genomic characterization of a new mimivirus of lineage B, named "Borely moumouvirus". This new virus exhibits a structure and replicative cycle similar to those of other members of the family Mimiviridae. The genome of the new isolate is a linear double-strand DNA molecule of ~1.0 Mb, containing over 900 open reading frames. Genome annotation highlighted different translation system components encoded in the DNA of Borely moumouvirus, including aminoacyl-tRNA synthetases, translation factors, and tRNA molecules, in a distribution similar to that in other lineage B mimiviruses. Pan-genome analysis indicated an increase in the genetic arsenal of this group of viruses, showing that the family Mimiviridae is still expanding. Furthermore, phylogenetic analysis has shown that Borely moumouvirus is closely related to moumouvirus australiensis. This is the first mimivirus lineage B isolated from Brazilian territory to be characterized. Further prospecting studies are necessary for us to better understand the diversity of these viruses so a better classification system can be established.

Giant Mimiviridae CsCl Purification Protocol.

While different giant viruses' purification protocols are available, they are not fully described and they use sucrose gradient that does not reach an equilibrium. Here, we report a protocol for the purification of members of the Mimiviridae family virions resulting from Acanthamoeaba castellanii infections. Viruses are harvested after cell lysis and purified through a high density CsCl gradient to optimize the isolation of the virus from the cell debris or other potential contaminants. Due to the large size of the virion capsids, reaching half a micrometer diameter, the quality of the process can be monitored by light microscopy. The resulting purified particles can then be used to perform new infections, DNA extraction, structural studies, sugar composition analyses, sub-compartment characterization or proteomic experiments.

A55 Molecular systematics of sturgeon nucleocytoplasmic large DNA viruses

Namao virus (NV) is a sturgeon nucleocytoplasmic large DNA virus (sNCLDV) that can cause a lethal disease of the integumentary system in lake sturgeon Acipenser fulvescens. As a group, the sNCLDV have not been assigned to any currently recognized taxonomic family of viruses. In this study, a dataset of NV DNA sequences was generated and assembled as two non-overlapping contigs of 306 and 448 base pairs (bp) and then used to conduct a comprehensive systematics analysis using Bayesian phylogenetic inference for NV, other sNCLDV, and representative members of six families of the NCLDV superfamily. The phylogeny of NV was reconstructed using protein homologues encoded by nine nucleocytoplasmic virus orthologous genes (NCVOGs): NCVOG0022—mcp, NCVOG0038—DNA polymerase B elongation subunit, NCVOG0076—VV A18-type helicase, NCVOG0249—VV A32-type ATPase, NCVOG0262—AL2 VLTF3-like transcription factor, NCVOG0271—RNA polymerase II subunit II, NCVOG0274—RNA polymerase II subunit I, NCVOG0276—ribonucleotide reductase small subunit, and NCVOG1117—mRNA capping enzyme. The accuracy of our phylogenetic method was evaluated using a combination of Bayesian statistical analysis and congruence analysis. Stable tree topologies were obtained with datasets differing in target molecule(s), sequence length, and taxa. Congruent topologies were obtained in phylogenies constructed using individual protein datasets and when four proteins were used in a concatenated approach. The major capsid protein phylogeny indicated that ten representative sNCLDV form a monophyletic group comprised of four lineages within a polyphyletic Mimi-Phycodnaviridae group of taxa. Overall, the analyses revealed that Namao virus is a member of the Mimiviridae family with strong and consistent support for a clade containing NV and CroV as sister taxa.

Role of the R349 Gene and Its Repeats in the MIMIVIRE Defense System.

MIMIVIRE is a defense system described in lineage A Mimivirus (Mimiviridae family) that mediates resistance against Zamilon virophage. It is composed of putative helicase and nuclease associated with a gene of unknown function called R349, which contains four 15 bp repeats homologous to the virophage sequence. In a previous study, the silencing of such genes restored virophage susceptibility. Moreover, the CRISPR Cas-4 like activity of the nuclease has recently been characterized. In this study, a recently isolated Mimivirus of lineage A with R349 gene lacking 3 of 4 repeats was demonstrated to be susceptible to Zamilon. To reinforce the importance of the R349 gene in the MIMIVIRE system, we developed and presented, for the first time to our knowledge, a protocol for Mimivirus genomic editing. By knocking out R349 gene in a Mimivirus lineage A, we observed the replication of Zamilon, indicating that this gene is critical in the resistance against this specific group of virophages.

Guarani Virophage, a New Sputnik-Like Isolate From a Brazilian Lake.

Virophages are critical regulators of viral population dynamics and potential actors in the stability of the microbial networks. These small biological entities predate the replicative cycle of giant viruses, such as the members of the Mimiviridae family or their distant relatives, which produce within the cytoplasm of their host cells a viral factory harboring a complex biochemistry propitious to the growth of the smaller parasites. In this paper, we describe the isolation and the characterization of a new virophage, the eighth, that we named Guarani. We observed that Guarani exhibits a late replication cycle compared to its giant virus host. In addition, like all Sputnik strains, Guarani is able to infect the three lineages A, B and C of the Mimiviridae family, and affects the replication and the infectivity of its host virus. In terms of genetic content, Guarani has a 18,967 bp long double-stranded DNA genome encoding 22 predicted genes very similar to Sputnik genes, except for ORF19 and ORF12. The former is more related to Zamilon while the latter seems to be novel. The architecture of the Guarani genome is closely related to Sputnik and Zamilon strains, suggesting a common origin for all these virophages.

Distribution of SNSs in Mimivirus Genomes and the Classification of Mimiviruses Isolated from Japan.

Mimiviruses have been detected in various habitats. Analyses of single nucleotide substitutions (SNSs) have revealed that SNSs are mainly localized on both ends of the mimivirus genome, and mimivirus lineage A has been split into three genotype groups; therefore, mimiviruses may be classified into lineages and genotype groups based on SNSs. We isolated 9 mimiviruses from Japan and analyzed SNSs. These isolates were classified as lineage A genotype group type 2, suggesting that the local diversity of members of the family Mimiviridae isolated from Acanthamoeba spp. is lower than that of giant viruses from other families isolated in Japan.

The Complex Nature of Tupanviruses.

The discovery of giant viruses revealed a new level of complexity in the virosphere, raising important questions about the diversity, ecology, and evolution of these viruses. The family Mimiviridae was the first group of amoebal giant viruses to be discovered (by Bernard La Scola and Didier Raoult team), containing viruses with structural and genetic features that challenged many concepts of classic virology. The tupanviruses are among the newest members of this family and exhibit structural, biological, and genetic features never previously observed in other giant viruses. The complexity of these viruses has put us one step forward toward the comprehension of giant virus biology and evolution, but also has raised important questions that still need to be addressed. In this chapter, we tell the history behind the discovery of one of the most complex viruses isolated to date, highlighting the unique features exhibited by tupanviruses, and discuss how these giant viruses have contributed to redefining limits for the virosphere.

"Tupanvirus", a new genus in the family Mimiviridae.

The genus "Tupanvirus" is a new proposed taxon to be included in the family Mimiviridae. The two known tupanvirus isolates were isolated from soda lake and oceanic sediments samples collected in Brazil and were named "tupanvirus soda lake" and "tupanvirus deep ocean", respectively. These viruses exhibit similarities to amoeba-infecting mimiviruses, but there are also several differences that place them in a separate group within the family Mimiviridae. Their virions have a mean size of 1.2µm, which include a mimivirus-like capsid and a large cylindrical tail, both covered by fibrils. The linear double-stranded DNA genomes of up to 1,516,267 base pairs encode over 1,200 genes, among which ~ 30% have no homologs in any database, including in other mimivirus genomes. Compared to other mimiviruses, tupanviruses exhibit a broader host range and cause a cytotoxic effect in host and non-host organisms, a phenotype that is not observed for other mimiviruses. Remarkably, these viruses possess the most complete gene set related to the protein synthesis process, including 20 aminoacyl-tRNA synthetases, 67-70 tRNAs, many translation factors, and genes involved in maturation and modification of tRNA and mRNA, among others. Moreover, diverse phylogenomic analyses put tupanviruses in a distinct group within the family Mimiviridae. In light of the set of different features observed for these giant viruses, we propose establishment of a new genus to allow proper classification of two known tupanviruses and possibly many more similar viruses yet to be characterized.