Molecular characterization of a novel single-stranded RNA virus, ChRV1, isolated from the plant-pathogenic fungus Colletotrichum higginsianum

Characterization of a novel hypovirus isolated from the phytopathogenic fungus Colletotrichum camelliae

Some members of genus Colletotrichum are important plant pathogens. Here, we report a novel positive single-stranded RNA virus, Colletotrichum camelliae hypovirus 1 (CcHV1), from strain GXNN11-2 of Colletotrichum camelliae. The complete genome of CcHV1 is 9907 nucleotides (nt) in length and contains a single large open reading frame (ORF) from nt 352 to 9006. This ORF encodes a polyprotein with four conserved domains, namely UDP-glycosyltransferase, RNA-dependent RNA polymerase (RdRp), peptidase, and DEAD-like helicase. The CcHV1 polyprotein shares the highest similarity with Fusarium concentricum hypovirus 1. Phylogenetic analysis indicated that CcHV1 clustered with members of the genus Betahypovirus within the family Hypoviridae. This is the first report of a hypovirus in a member of the genus Colletotrichum.

A novel narnavirus isolated from Colletotrichum curcumae strain 780-2T

The complete genome of a novel mycovirus, Colletotrichum curcumae narnavirus 1 (CcNV1), derived from the phytopathogenic fungus Colletotrichum curcumae strain 780-2T, was sequenced and analyzed. The full sequence of CcNV1 is 3,374 nucleotides in length and contains a single large open reading frame (ORF) encoding an RNA-dependent RNA polymerase (RdRp) of 1,087 amino acids with a molecular mass of 124.2 kDa that shares the closest similarity with that of Monilinia narnavirus H (53.02% identity). RdRp phylogeny analysis showed that CcNV1 is a new member of the proposed genus "Betanarnavirus" within the family Narnaviridae. This is the first report of a novel narnavirus infecting the phytopathogenic fungus C. curcumae, the causal agent of leaf blight of Curcuma wenyujin.

Mycoviruses as a part of the global virome: Diversity, evolutionary links and lifestyle

Knowledge of mycovirus diversity, evolution, horizontal gene transfer and shared ancestry with viruses infecting distantly related hosts, such as plants and arthropods, has increased vastly during the last few years due to advances in the high throughput sequencing methodologies. This also has enabled the discovery of novel mycoviruses with previously unknown genome types, mainly new positive and negative single-stranded RNA mycoviruses ((+) ssRNA and (-) ssRNA) and single-stranded DNA mycoviruses (ssDNA), and has increased our knowledge of double-stranded RNA mycoviruses (dsRNA), which in the past were thought to be the most common viruses infecting fungi. Fungi and oomycetes (Stramenopila) share similar lifestyles and also have similar viromes. Hypothesis about the origin and cross-kingdom transmission events of viruses have been raised and are supported by phylogenetic analysis and by the discovery of natural exchange of viruses between different hosts during virus-fungus coinfection in planta. In this review we make a compilation of the current information on the genome organization, diversity and taxonomy of mycoviruses, discussing their possible origins. Our focus is in recent findings suggesting the expansion of the host range of many viral taxa previously considered to be exclusively fungal, but we also address factors affecting virus transmissibility and coexistence in single fungal or oomycete isolates, as well as the development of synthetic mycoviruses and their use in investigating mycovirus replication cycles and pathogenicity.

Mycovirus Diversity and Evolution Revealed/Inferred from Recent Studies

High-throughput virome analyses with various fungi, from cultured or uncultured sources, have led to the discovery of diverse viruses with unique genome structures and even neo-lifestyles. Examples in the former category include splipalmiviruses and ambiviruses. Splipalmiviruses, related to yeast narnaviruses, have multiple positive-sense (+) single-stranded (ss) RNA genomic segments that separately encode the RNA-dependent RNA polymerase motifs, the hallmark of RNA viruses (members of the kingdom Orthornavirae). Ambiviruses appear to have an undivided ssRNA genome of 3∼5 kb with two large open reading frames (ORFs) separated by intergenic regions. Another narna-like virus group has two fully overlapping ORFs on both strands of a genomic segment that span more than 90% of the genome size. New virus lifestyles exhibited by mycoviruses include the yado-kari/yado-nushi nature characterized by the partnership between the (+)ssRNA yadokarivirus and an unrelated dsRNA virus (donor of the capsid for the former) and the hadaka nature of capsidless 10-11 segmented (+)ssRNA accessible by RNase in infected mycelial homogenates. Furthermore, dsRNA polymycoviruses with phylogenetic affinity to (+)ssRNA animal caliciviruses have been shown to be infectious as dsRNA-protein complexes or deproteinized naked dsRNA. Many previous phylogenetic gaps have been filled by recently discovered fungal and other viruses, which haveprovided interesting evolutionary insights. Phylogenetic analyses and the discovery of natural and experimental cross-kingdom infections suggest that horizontal virus transfer may have occurred and continue to occur between fungi and other kingdoms.

Infection of Nigrospora nonsegmented RNA Virus 1 Has Important Biological Impacts on a Fungal Host

Nigrospora nonsegmented RNA virus 1 (NoNRV1) has been reported previously in the fungus Nigrospora oryzae, but its biological effects on its host are unknown. In this work, we isolated a strain 9-1 of N. oryzae from a chrysanthemum leaf and identified NoNRV1 infection in the isolated strain. The genome sequence of NoNRV1 identified here is highly homologous to that of the isolate HN-21 of NoNRV1 previously reported; thus, we tentatively designated the newly identified NoNRV1 as NoNRV1-ZJ. Drug treatment with Ribavirin successfully removed NoNRV1-ZJ from the strain 9-1, which provided us with an ideal control to determine the biological impacts of NoNRV1 infection on host fungi. By comparing the virus-carrying (9-1) and virus-cured (9-1C) strains, our results indicated that infection with NoNRV1 promoted the pigmentation of the host cells, while it had no discernable effects on host growth on potato dextrose agar plates when subjected to osmotic or oxidative stress. Interestingly, we observed inhibitory impacts of virus infection on the thermotolerance of N. oryzae and the pathogenicity of the host fungus in cotton leaves. Collectively, our work provides clear evidence of the biological relevance of NoNRV1 infection in N. oryzae, including pigmentation, hypovirulence, and thermotolerance.

Molecular characterization of a novel penoulivirus from the phytopathogenic fungus Colletotrichum camelliae

Colletotrichum camelliae is a widespread filamentous phytopathogenic fungus. In this study, a novel mycovirus designated as "Colletotrichum camelliae botourmiavirus 1" (CcBV1) was isolated from strain ZJQT11 of C. camelliae, and its complete genome sequence was determined. CcBV1 has a genome of 2,506 nucleotides and contains a large open reading frame (ORF) that encodes an RNA-dependent RNA polymerase (RdRp) with 672 amino acids and a predicted molecular mass of 75.23 kDa. A BLASTp search showed that RdRp encoded by CcBV1 is closely related to that of Pyricularia oryzae ourmia-like virus 1 with 73.22% identity. Phylogenetic analysis indicated that CcBV1 clustered in the penoulivirus clade within the family Botourmiaviridae. To the best of our knowledge, this is the first report of a penoulivirus in C. camelliae.