Co-infection by two distinct totivirus-like double-stranded RNA elements in Chalara elegans (Thielaviopsis basicola)

Metagenomic of Liver Tissue Identified at Least Two Genera of Totivirus-like Viruses in Molossus molossus Bats

The Totiviridae family of viruses has a unique genome consisting of double-stranded RNA with two open reading frames that encode the capsid protein (Cap) and the RNA-dependent RNA polymerase (RdRpol). Most virions in this family are isometric in shape, approximately 40 nm in diameter, and lack an envelope. There are five genera within this family, including Totivirus, Victorivirus, Giardiavirus, Leishmaniavirus, and Trichomonasvirus. While Totivirus and Victorivirus primarily infect fungi, Giardiavirus, Leishmaniavirus, and Trichomonasvirus infect diverse hosts, including protists, insects, and vertebrates. Recently, new totivirus-like species have been discovered in fish and plant hosts, and through metagenomic analysis, a novel totivirus-like virus (named Tianjin totivirus) has been isolated from bat guano. Interestingly, Tianjin totivirus causes cytopathic effects in insect cells but cannot grow in mammalian cells, suggesting that it infects insects consumed by insectivorous bats. In this study, we used next-generation sequencing and identified totivirus-like viruses in liver tissue from Molossus molossus bats in the Amazon region of Brazil. Comparative phylogenetic analysis based on the RNA-dependent RNA polymerase region revealed that the viruses identified in Molossus bats belong to two distinct phylogenetic clades, possibly comprising different genera within the Totiviridae family. Notably, the mean similarity between the Tianjin totivirus and the totiviruses identified in Molossus bats is less than 18%. These findings suggest that the diversity of totiviruses in bats is more extensive than previously recognized and highlight the potential for bats to serve as reservoirs for novel toti-like viruses.

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.

Fungal Viruses Unveiled: A Comprehensive Review of Mycoviruses

Mycoviruses (viruses of fungi) are ubiquitous throughout the fungal kingdom and are currently classified into 23 viral families and the genus botybirnavirus by the International Committee on the Taxonomy of Viruses (ICTV). The primary focus of mycoviral research has been on mycoviruses that infect plant pathogenic fungi, due to the ability of some to reduce the virulence of their host and thus act as potential biocontrol against these fungi. However, mycoviruses lack extracellular transmission mechanisms and rely on intercellular transmission through the hyphal anastomosis, which impedes successful transmission between different fungal strains. This review provides a comprehensive overview of mycoviruses, including their origins, host range, taxonomic classification into families, effects on their fungal counterparts, and the techniques employed in their discovery. The application of mycoviruses as biocontrol agents of plant pathogenic fungi is also discussed.

Characterization of Two Novel Toti-Like Viruses Co-infecting the Atlantic Blue Crab, Callinectes sapidus, in Its Northern Range of the United States

The advancement of high throughput sequencing has greatly facilitated the exploration of viruses that infect marine hosts. For example, a number of putative virus genomes belonging to the Totiviridae family have been described in crustacean hosts. However, there has been no characterization of the most newly discovered putative viruses beyond description of their genomes. In this study, two novel double-stranded RNA (dsRNA) virus genomes were discovered in the Atlantic blue crab (Callinectes sapidus) and further investigated. Sequencing of both virus genomes revealed that they each encode RNA dependent RNA polymerase proteins (RdRps) with similarities to toti-like viruses. The viruses were tentatively named Callinectes sapidus toti-like virus 1 (CsTLV1) and Callinectes sapidus toti-like virus 2 (CsTLV2). Both genomes have typical elements required for −1 ribosomal frameshifting, which may induce the expression of an encoded ORF1–ORF2 (gag-pol) fusion protein. Phylogenetic analyses of CsTLV1 and CsTLV2 RdRp amino acid sequences suggested that they are members of two new genera in the family Totiviridae. The CsTLV1 and CsTLV2 genomes were detected in muscle, gill, and hepatopancreas of blue crabs by real-time reverse transcription quantitative PCR (RT-qPCR). The presence of ~40 nm totivirus-like viral particles in all three tissues was verified by transmission electron microscopy, and pathology associated with CsTLV1 and CsTLV2 infections were observed by histology. PCR assays showed the prevalence and geographic range of these viruses, to be restricted to the northeast United States sites sampled. The two virus genomes co-occurred in almost all cases, with the CsTLV2 genome being found on its own in 8.5% cases, and the CsTLV1 genome not yet found on its own. To our knowledge, this is the first report of toti-like viruses in C. sapidus. The information reported here provides the knowledge and tools to investigate transmission and potential pathogenicity of these viruses.

Population Structure of Double-Stranded RNA Mycoviruses That Infect the Rice Blast Fungus Magnaporthe oryzae in Japan

Various viruses infect Magnaporthe oryzae (syn. Pyricularia oryzae), which is a well-studied fungus that causes rice blast disease. Most research has focused on the discovery of new viruses and the hypovirulence-associated traits conferred by them. Therefore, the diversity and prevalence of viruses in wild fungal populations have not been explored. We conducted a comprehensive screening of M. oryzae mycoviruses from various regions in Japan using double-stranded RNA (dsRNA) electrophoresis and RT-PCR assays. We detected three mycoviruses, Magnaporthe oryzae virus 2 (MoV2), Magnaporthe oryzae chrysovirus 1 (MoCV1), and Magnaporthe oryzae partitivirus 1 (MoPV1), among 127 of the 194 M. oryzae strains screened. The most prevalent virus was MoPV1 (58.8%), which often co-infected in a single fungal strain together with MoV2 or MoCV1. MoV2 and MoCV1 were found in 22.7 and 10.8% of strains, respectively, and they were usually distributed in different regions so that mixed-infection with these two mycoviruses was extremely rare. The predominance of MoPV1 in M. oryzae is supported by significant negative values from neutrality tests, which indicate that the population size of MoPV1 tends to increase. Population genetic analyses revealed high nucleotide diversity and the presence of phylogenetically diverse subpopulations among the MoV2 isolates. This was not the case for MoPV1. Furthermore, studies of a virus-cured M. oryzae strain revealed that MoV2 does not cause any abnormalities or symptoms in its host. However, a leaf sheath inoculation assay showed that its presence slightly increased the speed of mycelial growth, compared with virus-free mycelia. These results demonstrate that M. oryzae in Japan harbors diverse dsRNA mycovirus communities with wide variations in their population structures among different viruses.

Carvalho R, C. Pacheco LG, P. d. Almeida JP, M. d. A. Belitardo EM, S. Pinheiro C, S. Campos G, R. G. R. Aguiar E. High-Quality Resolution of the Outbreak-Related Zika Virus Genome and Discovery of New Viruses Using Ion Torrent-Based Metatranscriptomics

Arboviruses, including the Zika virus, have recently emerged as one of the most important threats to human health. The use of metagenomics-based approaches has already proven valuable to aid surveillance of arboviral infections, and the ability to reconstruct complete viral genomes from metatranscriptomics data is key to the development of new control strategies for these diseases. Herein, we used RNA-based metatranscriptomics associated with Ion Torrent deep sequencing to allow for the high-quality reconstitution of an outbreak-related Zika virus (ZIKV) genome (10,739 nt), with extended 5'-UTR and 3'-UTR regions, using a newly-implemented bioinformatics approach. Besides allowing for the assembly of one of the largest complete ZIKV genomes to date, our strategy also yielded high-quality complete genomes of two arthropod-infecting viruses co-infecting C6/36 cell lines, namely: Alphamesonivirus 1 strain Salvador (20,194 nt) and Aedes albopictus totivirus-like (4618 nt); the latter likely represents a new viral species. Altogether, our results demonstrate that our bioinformatics approach associated with Ion Torrent sequencing allows for the high-quality reconstruction of known and unknown viral genomes, overcoming the main limitation of RNA deep sequencing for virus identification.

The alkalophilic fungus Sodiomyces alkalinus hosts beta- and gammapartitiviruses together with a new fusarivirus

Mixed infection by three dsRNA viruses, a novel betapartitivirus, a gammapartitivirus, and a novel fusarivirus, has been identified in four isolates of the obligate alkalophilic fungus Sodiomyces alkalinus. The first, Sodiomyces alkalinus partitivirus 1 (SaPV1), is placed within the genus Betapartitivirus and is related to Ustilaginoidea virens partitivirus 2. The taxonomic position of the second virus is less clear as it shares high (85%) amino acid sequence identity but significantly low (77%) nucleotide sequence identity of the capsid protein with Colletotrichum truncatum partitivirus 1. The third, the novel Sodiomyces alkalinus fusarivirus 1 (SaFV1), is related to Fusarium poae fusarivirus 1. All the viruses show efficient vertical transmission through asexual and sexual spores. These novel coexisting viruses do not evoke apparent phenotypic alteration to their fungal host. This is the first description of a viral infection in an alkalophilic fungus.

Taxonomy, biogeography and importance of Heterobasidion viruses

The genus Heterobasidion consists of several species of necrotrophic and saprotrophic fungi, and includes some of the most detrimental organisms in boreal conifer forests. These fungi host a widespread and diverse mycovirus community composed of more than 16 species of Partitiviridae, a species of Narnaviridae and one taxonomically unassigned virus related to the Curvularia thermal tolerance virus. These viruses are able to cross species borders, co-infect single host strains and cause phenotypic changes in their hosts. The abundance of viruses increases over time in Heterobasidion infection centers, and they are targeted by fungal RNA interference. Long-term field studies are essential for obtaining a comprehensive view of virus effects in the nature.

Viruses accumulate in aging infection centers of a fungal forest pathogen

Fungal viruses (mycoviruses) with RNA genomes are believed to lack extracellular infective particles. These viruses are transmitted laterally among fungal strains through mycelial anastomoses or vertically via their infected spores, but little is known regarding their prevalence and patterns of dispersal under natural conditions. Here, we examined, in detail, the spatial and temporal changes in a mycovirus community and its host fungus Heterobasidion parviporum, the most devastating fungal pathogen of conifers in the Boreal forest region. During the 7-year sampling period, viruses accumulated in clonal host individuals as a result of indigenous viruses spreading within and between clones as well as novel strains arriving via airborne spores. Viral community changes produced pockets of heterogeneity within large H. parviporum clones. The appearance of novel viral infections in aging clones indicated that transient cell-to-cell contacts between Heterobasidion strains are likely to occur more frequently than what was inferred from genotypic analyses. Intraspecific variation was low among the three partitivirus species at the study site, whereas the unassigned viral species HetRV6 was highly polymorphic. The accumulation of point mutations during persistent infections resulted in viral diversification, that is, the presence of nearly identical viral sequence variants within single clones. Our results also suggest that co-infections by distantly related viral species are more stable than those between conspecific strains, and mutual exclusion may play a role in determining mycoviral communities.

Virus community dynamics in the conifer pathogenic fungus Heterobasidion parviporum following an artificial introduction of a partitivirus

Viruses infecting the conifer pathogenic fungus Heterobasidion annosum sensu lato are intracellular and spread via anastomosis contacts. In the laboratory, these viruses transmit readily even between somatically incompatible isolates, but their dispersal capacity in natural conditions has not been previously studied. We introduced a mycovirus to a heavily diseased forest site by inoculating Norway spruce stumps with heartrot decay using a mycelial suspension of Heterobasidion parviporum strain RT3.49C hosting the partitivirus strain HetRV4-pa1. The Heterobasidion population at the sample plot was screened for mycoviruses prior to and after the inoculation. Based on sequence analysis, the resident H. parviporum strains harbored six different strains of the virus species Heterobasidion RNA virus 6 (HetRV6) and one strain of HetRV4 prior to the inoculation. After three growth seasons, the inoculated H. parviporum host strain was not detected, but the introduced virus had infected two resident H. parviporum genets. The presence of a preexisting HetRV6 infection did not hinder spread of the introduced partitivirus but resulted in coinfections instead. The resident HetRV6 virus population seemed to be highly stable during the incubation period, while the single indigenous HetRV4 infection was not detected after the inoculation. In laboratory infection experiments, the introduced virus could be transmitted successfully into all of the resident H. parviporum genets. This study shows for the first time transmission of a Heterobasidion virus between somatically incompatible hosts in natural conditions.

Molecular characterization of totiviruses in Xanthophyllomyces dendrorhous

Background

Occurrence of extrachromosomal dsRNA elements has been described in the red-yeast Xanthophyllomyces dendrorhous, with numbers and sizes that are highly variable among strains with different geographical origin. The studies concerning to the encapsidation in viral-like particles and dsRNA-curing have suggested that some dsRNAs are helper viruses, while others are satellite viruses. However, the nucleotide sequences and functions of these dsRNAs are still unknown. In this work, the nucleotide sequences of four dsRNAs of the strain UCD 67–385 of X. dendrorhous were determined, and their identities and genome structures are proposed. Based on this molecular data, the dsRNAs of different strains of X. dendrorhous were analyzed.

Results

The complete sequences of L1, L2, S1 and S2 dsRNAs of X. dendrorhous UCD 67–385 were determined, finding two sequences for L1 dsRNA (L1A and L1B). Several ORFs were uncovered in both S1 and S2 dsRNAs, but no homologies were found for any of them when compared to the database. Instead, two ORFs were identified in each L1A, L1B and L2 dsRNAs, whose deduced amino acid sequences were homologous with a major capsid protein (5’-ORF) and a RNA-dependent RNA polymerase (3’-ORF) belonging to the Totiviridae family. The genome structures of these dsRNAs are characteristic of Totiviruses, with two overlapped ORFs (the 3’-ORF in the −1 frame with respect to the 5’-ORF), with a slippery site and a pseudoknot in the overlapped regions. These structures are essential for the synthesis of the viral polymerase as a fusion protein with the viral capsid protein through −1 ribosomal frameshifting. In the RNase protection analysis, all the dsRNAs in the four analyzed X. dendrorhous strains were protected from enzymatic digestion. The RT-PCR analysis revealed that, similar to strain UCD 67–385, the L1A and L1B dsRNAs coexist in the strains VKM Y-2059, UCD 67–202 and VKM Y-2786. Furthermore, determinations of the relative amounts of L1 dsRNAs using two-step RT-qPCR revealed a 40-fold increment of the ratio L1A/L1B in the S2 dsRNA-cured strain compared to its parental strain.

Conclusions

Three totiviruses, named as XdV-L1A, XdV-L1B and XdV-L2, were identified in the strain UCD 67–385 of X. dendrorhous. The viruses XdV-L1A and XdV-L1B were also found in other three X. dendrorhous strains. Our results suggest that the smaller dsRNAs (named XdRm-S1 and XdRm-S2) of strain UCD 67–385 are satellite viruses, and particularly that XdRm-S2 is a satellite of XdV-L1A.

Cardiomyopathy syndrome of atlantic salmon (Salmo salar L.) is caused by a double-stranded RNA virus of the Totiviridae family

Cardiomyopathy syndrome (CMS) of farmed and wild Atlantic salmon (Salmo salar L.) is a disease of yet unknown etiology characterized by a necrotizing myocarditis involving the atrium and the spongious part of the heart ventricle. Here, we report the identification of a double-stranded RNA virus likely belonging to the family Totiviridae as the causative agent of the disease. The proposed name of the virus is piscine myocarditis virus (PMCV). On the basis of the RNA-dependent RNA polymerase (RdRp) sequence, PMCV grouped with Giardia lamblia virus and infectious myonecrosis virus of penaeid shrimp. The genome size of PMCV is 6,688 bp, with three open reading frames (ORFs). ORF1 likely encodes the major capsid protein, while ORF2 encodes the RdRp, possibly expressed as a fusion protein with the ORF1 product. ORF3 seems to be translated as a separate protein not described for any previous members of the family Totiviridae. Following experimental challenge with cell culture-grown virus, histopathological changes are observed in heart tissue by 6 weeks postchallenge (p.c.), with peak severity by 9 weeks p.c. Viral genome levels detected by real-time reverse transcription (RT)-PCR peak earlier at 6 to 7 weeks p.c. The virus genome is detected by in situ hybridization in degenerate cardiomyocytes from clinical cases of CMS. Virus genome levels in the hearts from clinical field cases correlate well with the severity of histopathological changes in heart tissue. The identification of the causative agent for CMS is important for improved disease surveillance and disease control and will serve as a basis for vaccine development against the disease.

Identification and molecular characterization of a new nonsegmented double-stranded RNA virus isolated from Culex mosquitoes in Japan

Two infectious agents were isolated from Culex species mosquitoes in Japan and were identified as distinct strains of a new RNA virus by a method for sequence-independent amplification of viral nucleic acids. The virus designated Omono River virus (OMRV) replicated in mosquito cells in which it produced a severe cytopathic effect. Icosahedral virus particles of approximately 40 nm in diameter were detected in the cytoplasm of infected cells. The OMRV genome was observed to consist of a nonsegmented, 7.6-kb double-stranded RNA (dsRNA) and contain two overlapping open reading frames (ORFs), namely ORF1 and ORF2. ORF1 was found to encode a putative dsRNA-binding protein, a major capsid protein, and other putative proteins, which might be generated by co- and/or post-translational processing of the ORF1 polyprotein precursor, and ORF2 was found to encode a putative RNA-dependent RNA polymerase (RdRp), which could be translated as a fusion with the ORF1 product by a -1 ribosomal frameshift. Phylogenetic analysis based on RdRp revealed that OMRV is closely related to penaeid shrimp infectious myonecrosis virus and Drosophila totivirus, which are tentatively assigned to the family Totiviridae. These results indicated that OMRV is a new member of the family of nonsegmented dsRNA viruses infecting arthropod hosts, but not fungal or protozoan hosts.

Mycoviruses of filamentous fungi and their relevance to plant pathology

Mycoviruses (fungal viruses) are reviewed with emphasis on plant pathogenic fungi. Based on the presence of virus-like particles and unencapsidated dsRNAs, mycoviruses are common in all major fungal groups. Over 80 mycovirus species have been officially recognized from ten virus families, but a paucity of nucleic acid sequence data makes assignment of many reported mycoviruses difficult. Although most of the particle types recognized to date are isometric, a variety of morphologies have been found and, additionally, many apparently unencapsidated dsRNAs have been reported. Until recently, most characterized mycoviruses have dsRNA genomes, but ssRNA mycoviruses now constitute about one-third of the total. Two hypotheses for the origin of mycoviruses of plant pathogens are discussed: the first that they are of unknown but ancient origin and have coevolved along with their hosts, the second that they have relatively recently moved from a fungal plant host into the fungus. Although mycoviruses are typically readily transmitted through asexual spores, transmission through sexual spores varies with the host fungus. Evidence for natural horizontal transmission has been found. Typically, mycoviruses are apparently symptomless (cryptic) but beneficial effects on the host fungus have been reported. Of more practical interest to plant pathologists are those viruses that confer a hypovirulent phenotype, and the scope for using such viruses as biocontrol agents is reviewed. New tools are being developed based on host genome studies that will help to address the intellectual challenge of understanding the fungal-virus interactions and the practical challenge of manipulating this relationship to develop novel biocontrol agents for important plant pathogens.

The nucleotide sequence and genome organization of Magnaporthe oryzae virus 1

Magnaporthe oryzae virus 1 (MoV1) found in Magnaporthe oryzae, the pathogenic fungus responsible for rice blast, is a small icosahedral virus with a nonsegmented double-stranded RNA genome. The viral genome has two open reading frames (ORF 1 and 2). The deduced amino acid sequences of both ORF 1 and ORF 2 show a significant similarity to those of capsid protein and RdRp, respectively, of members of the family Totiviridae. Both a comparison of genome organization and phylogenic analysis have indicated that MoV1 is closely related to some of the totiviruses that infect filamentous fungi. These results suggest that MoV1 belongs to the family Totiviridae.

Survey and expression analysis of five new chitin synthase genes in the biotrophic rust fungus Puccinia graminis

We have isolated and characterised the first set of chitin synthase genes from a rust fungus, a large group of economically highly important, obligately biotrophic plant pathogens. Puccinia graminis was used as a model organism for the rust fungi which are not well investigated on the molecular level today. One of the major structural components of most fungal cell walls is the chitin polymer which is synthesised by a family of enzymes called chitin synthases. In P. graminis, we have isolated five new chitin synthase genes from four different classes, chsII, chsIIIa, chsIIIb, chsIV, and chsV. The genes contain a high number of introns, unusual for other known fungal chitin synthases. The dinucleic stage of the fungus seems to contain two slightly different genes or alleles for four isoforms. One isoform, chsIIIa, seems to be expressed only in the youngest stages of fungal growth. Analysis of the derived proteins shows that together with other basidiomycete CHS, the pgtCHS form separate subgroups in the phylogenetic tree. This set of five rust chitin synthase genes, with some unusual features compared to known fungal chitin synthases, allows new insights into chitin synthase classification, and may help in the development of novel functional fungicides.

Diversity, complexity and transmission of double-stranded RNA elements in Chalara elegans (synanam. Thielaviopsis basicola)

Double-stranded (ds) RNA banding patterns were determined in 21 wild-type strains of the soilborne plant pathogen Chalara elegans originating from different geographic regions worldwide. Five strains, each with a unique dsRNA pattern, were selected for cDNA cloning, northern blot analysis and dsRNA transmission experiments. Four strains contained multiple (up to 6) dsRNA elements (2.0kbp to 12kbp in size) and one strain contained a single 2.8kbp fragment. These five strains were distinguished from one another by their unique RAPD-PCR patterns. Seven partial cDNA clones were derived from the predominant 2.8, 5.3, and 12kbp dsRNA elements. Nucleotide sequence analysis and northern blot hybridizations revealed a high degree of genetic dissimilarity among the different molecular-size dsRNA elements, even those found within a single strain. Four clones from the 5.3kbp dsRNA fragment showed a 23-43 % amino acid identity to either the coat protein or RNA-dependent RNA polymerase regions of viruses in the Totiviridae. One clone from the 2.8kbp dsRNA fragment had a 55-57 % amino acid identity to the RdRp region of viruses in the Narnaviridae. Two clones from the 12kbp dsRNA fragment showed no significant homology to any known virus group. Colonies derived from 100 single-conidia isolates of C. elegans strains with the 2.8, 5.3 and 12kbp elements all contained the corresponding dsRNA element, indicating that dsRNA transmission through conidia was highly efficient, regardless of molecular size. However, transmission of dsRNA between the mycelium of strains of C. elegans could not be achieved in this study. Genetically unique strains carrying diverse dsRNA elements appear to have evolved within populations of C. elegans. Based on our findings, there are at least 3 groups of viruses present in C. elegans.

Molecular and Biological Characterization of a Mitovirus in Chalara elegans (Thielaviopsis basicola)

ABSTRACT A 2.8-kb double-stranded RNA (dsRNA) element in strain BK18 of Chalara elegans originally isolated from cotton soil in California was characterized by obtaining a full-length cDNA sequence (2,896 nucleotides in length) from a series of overlapping clones. Sequence analysis revealed the presence of one large open reading frame (ORF I) using the mitochondrial genetic code, with 20 to 34% amino acid identity to the ORF I of other previously reported fungal mitochondrial RNA viruses. The ORF I encoded a putative protein of 705 amino acids and contained the conserved motif characteristic of RNA-dependent RNA polymerases. Purification of mitochondria from strain BK18 confirmed the co-localization of this dsRNA, and northern blot hybridization with a strand-specific probe revealed the (+) single-stranded nature. This Chalara elegans mitovirus (CeMV) is designated as a new member of the genus Mitovirus of the family Narnaviridae. Using dsRNA-specific primers, the ORF I region (positions 427 to 2544) was obtained from an additional 2.8-kb dsRNA element in strain HA2 originating from carrot roots in the Netherlands. Both ORFs had 98% homology at the nucleotide and amino acid levels. CeMV was also found to be present in five additional strains of C. elegans from different geographic locations worldwide, and a 97 to 100% nucleotide sequence identity was observed within a 300-bp region of ORF I in these strains. To determine the biological effects of CeMV on C. elegans, attempts to cure strain BK18 of the dsRNA were made. Sequential transfers of mycelium at 35 to 37 degrees C yielded a colony which lacked the 2.8-kb dsRNA when visualized on agarose gels and also in northern blot hybridization analysis. However, reverse transcription-polymerase chain reaction with specific primer sets revealed a band, indicating that dsRNA replication had been significantly repressed (latent). The wild type and latently infected strains were compared for colony morphology, growth rate, melanin production, various enzymatic assays (polyphenoloxidase, laccase, tyrosinase, and esterase), and virulence on carrot roots. Colony morphology on V8 agar was comparable between the two strains, while growth rate, melanin production, and virulence were enhanced in the latently infected strain. There were no detectable differences in enzymatic activity. Transmission electron microscopy of hyphae of the wild type and latently infected strains revealed differences in the number and size of the mitochondria, which were enhanced in the latently infected strain. Our results show that CeMV is a new member of the genus Mitovirus with some disruptive effects on its fungal host and is present in C. elegans strains from different locations worldwide.