Proof of Concept of the Yadokari Nature: a Capsidless Replicase-Encoding but Replication-Dependent Positive-Sense Single-Stranded RNA Virus Hosted by an Unrelated Double-Stranded RNA Virus

New clades of viruses infecting the obligatory biotroph Bremia lactucae representing distinct evolutionary trajectory for viruses infecting oomycetes

Recent advances in high throughput sequencing (HTS) approaches allowed a broad exploration of viromes from different fungal hosts, unveiling a great diversity of mycoviruses with interesting evolutionary features. The word mycovirus historically applies also to viruses infecting oomycetes but most studies are on viruses infecting fungi, with less mycoviruses found and characterized in oomycetes, particularly in the obligatory biotrophs. We, here, describe the first virome associated to Bremia lactucae, the causal agent of lettuce downy mildew, which is an important biotrophic pathogen for lettuce production and a model system for the molecular aspects of the plant-oomycetes interactions. Among the identified viruses, we could detect (1) two new negative sense ssRNA viruses related to the yueviruses, (2) the first example of permuted RdRp in a virus infecting fungi/oomycetes, (3) a new group of bipartite dsRNA viruses showing evidence of recent bi-segmentation and concomitantly, a possible duplication event bringing a bipartite genome to tripartite, (4) a first representative of a clade of viruses with evidence of recombination between distantly related viruses, (5) a new open reading frame (ORF)an virus encoding for an RdRp with low homology to known RNA viruses, and (6) a new virus, belonging to riboviria but not conserved enough to provide a conclusive phylogenetic placement that shows evidence of a recombination event between a kitrinoviricota-like and a pisuviricota-like sequence. The results obtained show a great diversity of viruses and evolutionary mechanisms previously unreported for oomycetes-infecting viruses, supporting the existence of a large diversity of oomycetes-specific viral clades ancestral of many fungal and insect virus clades.

Continued mycovirus discovery expanding our understanding of virus lifestyles, symptom expression, and host defense

High-throughput sequencing technologies have greatly expanded the RNA virome in general and have led to an exponential increase in new fungal viruses, also known as mycoviruses. Mycoviruses are omnipresent in fungi and usually induce symptomless infections. Some mycoviruses infecting fungi pathogenic to plants, insects, and mammals are known to modify host virulence positively and negatively and attract particular interests. In addition, fungal viruses continue to provide intriguing research materials and themes that lead to discoveries of peculiar viruses as infectious entities and insights into virus evolution and diversity. In this review, we outline the diversity and neolifestyle of recently discovered fungal RNA viruses, and phenotypic alterations induced by them. Furthermore, we discuss recent advances in research regarding the fungal antiviral defense and viral counterdefense, which are closely associated with host phenotype alterations. We hope that this article will enhance understanding of the interesting and growing fungal virology field.

Exploration of the yadokari/yadonushi nature of YkV3 and RnMBV3 in the original host and a model filamentous fungus

The yadokari/yadonushi nature is a recently discovered virus lifestyle; "yadokari" refers to the ability of capsidless positive-sense (+) RNA viruses (yadokariviruses) to utilize the capsids of phylogenetically distant double-stranded RNA (dsRNA) viruses possibly as the replication site, while "yadonushi" refers to the ability of dsRNA viruses to provide capsids to yadokariviruses. This virus-virus interaction, however, has been only studied with limited pathosystems. Here, we established a new study model with a capsidless (+)RNA yadokarivirus YkV3 (family Yadokariviridae) and its capsid donor RnMBV3 (family Megabirnaviridae) in the original host fungus Rosellinia necatrix and a model filamentous fungal host Cryphonectria parasitica. YkV3 has a simple genome structure with one open reading frame of 4305 nucleotides encoding a single polyprotein with an RNA-dependent RNA polymerase and a 2A-like self-cleavage peptide domain. Reverse genetics of YkV3 in R. necatrix showed that YkV3 tolerates a nucleotide substitution in the extreme 5'-terminus. The insertion of two termination codons immediately downstream of the 2A-like cleavage site abolished YkV3 viability, suggesting the importance of the C-terminal portion of the polyprotein of unknown function. Transfection of RnMBV3 and YkV3 into an RNA silencing-deficient mutant Δdcl2 of C. parasitica showed the replication competency of both viruses. Comparison between the wild-type and Δdcl2 strains of C. parasitica in virus accumulation suggested that RnMBV3 and YkV3 are susceptible to RNA silencing in C. parasitica. Taken together, we have established a platform to further explore the yadokari/yadonushi nature using genetically manipulable host fungal and virus strains.

Mycovirus-induced hypovirulence in notorious fungi Sclerotinia: a comprehensive review

Members of the genus Sclerotinia are notorious plant pathogens with a diverse host range that includes many important crops. A huge number of mycoviruses have been identified in this genus; some of these viruses are reported to have a hypovirulent effect on the fitness of their fungal hosts. These mycoviruses are important to researchers from a biocontrol perspective which was first implemented against fungal diseases in 1990. In this review, we have presented the data of all hypovirulent mycoviruses infecting Sclerotinia sclerotiorum isolates. The data of hypovirulent mycoviruses ranges from 1992 to 2023. Currently, mycoviruses belonging to 17 different families, including (+) ssRNA, (-ssRNA), dsRNA, and ssDNA viruses, have been reported from this genus. Advances in studies had shown a changed expression of certain host genes (responsible for cell cycle regulation, DNA replication, repair pathways, ubiquitin proteolysis, gene silencing, methylation, pathogenesis-related, sclerotial development, carbohydrate metabolism, and oxalic acid biosynthesis) during the course of mycoviral infection, which were termed differentially expressed genes (DEGs). Together, research on fungal viruses and hypovirulence in Sclerotinia species can deepen our understanding of the cellular processes that affect how virulence manifests in these phytopathogenic fungi and increase the potential of mycoviruses as a distinct mode of biological control. Furthermore, the gathered data can also be used for in-silico analysis, which includes finding the signature sites [e.g., hypovirus papain-like protease (HPP) domain, "CCHH" motif, specific stem-loop structures, p29 motif as in CHV1, A-rich sequence, CA-rich sequences as in MoV1, GCU motif as in RnMBV1, Core motifs in hypovirus-associated RNA elements (HAREs) as in CHV1] that are possibly responsible for hypovirulence in mycoviruses.

The good, the bad and the cryptic: The multifaceted roles of mycoviruses and their potential applications for a sustainable agriculture

Mycoviruses are natural inhabitants of fungi and have been identified in almost all fungal taxonomic groups. Mycoviruses that infect phytopathogenic fungi are now becoming a hot research area due to their potential for the biocontrol of important plant pathogens. But, before considering a mycovirus for biocontrol, we should be fully aware of the effects it induces in a fungal host and its interactions with other viruses, fungal strains and even the host plants. Mycoviral infections are generally associated with different effects, ranging from hypovirulence to hypervirulence, but they can often be cryptic (latent infections). The cryptic lifestyle has been associated to many mycoviruses, but thanks to growing knowledge we are now aware that it is often associated to axenic conditions while the real effects can be observed only in nature. Other mycoviruses either promote (hypervirulence) or (hypovirulence) fungal pathogenicity by a strong impact on the fungal physiology or by blocking the production of toxins or effectors. Finally, indirect effects of mycoviral infections can also be provided to the plant that hosts the fungal isolate, highlighting not only their potential as direct biocontrol agents but also as priming agents for plant resilience to biotic and abiotic stresses. This review provides a broad overview of mycoviral interactions both with their hosts and with other mycoviruses, highlighting the most interesting examples. In contrast to what has been observed to date, we believe that the collective availability of these data will not only improve our understanding of mycoviruses, but also increase our confidence in considering them as alternative measures against fungal diseases to improve the sustainable production of food and feed commodities.

A Capsid Protein Fragment of a Fusagra-like Virus Found in Carica papaya Latex Interacts with the 50S Ribosomal Protein L17

Papaya sticky disease is caused by the association of a fusagra-like and an umbra-like virus, named papaya meleira virus (PMeV) and papaya meleira virus 2 (PMeV2), respectively. Both viral genomes are encapsidated in particles formed by the PMeV ORF1 product, which has the potential to encode a protein with 1563 amino acids (aa). However, the structural components of the viral capsid are unknown. To characterize the structural proteins of PMeV and PMeV2, virions were purified from Carica papaya latex. SDS-PAGE analysis of purified virus revealed two major proteins of ~40 kDa and ~55 kDa. Amino-terminal sequencing of the ~55 kDa protein and LC-MS/MS of purified virions indicated that this protein starts at aa 263 of the deduced ORF1 product as a result of either degradation or proteolytic processing. A yeast two-hybrid assay was used to identify Arabidopsis proteins interacting with two PMeV ORF1 product fragments (aa 321-670 and 961-1200). The 50S ribosomal protein L17 (AtRPL17) was identified as potentially associated with modulated translation-related proteins. In plant cells, AtRPL17 co-localized and interacted with the PMeV ORF1 fragments. These findings support the hypothesis that the interaction between PMeV/PMeV2 structural proteins and RPL17 is important for virus-host interactions.

ICTV Virus Taxonomy Profile: Yadokariviridae 2023

The family Yadokariviridae, with the genera Alphayadokarivirus and Betayadokarivirus, includes capsidless non-segmented positive-sense (+) RNA viruses that hijack capsids from phylogenetically distant double-stranded RNA viruses. Yadokarivirids likely replicate inside the hijacked heterocapsids using their own RNA-directed RNA polymerase, mimicking dsRNA viruses despite their phylogenetic placement in a (+) RNA virus lineage. Yadokarivirids can have negative or positive impacts on their host fungi, through interactions with the capsid donor dsRNA viruses. This is a summary of the International Committee on Taxonomy of Viruses (ICTV) report on the family Yadokariviridae, which is available at ictv.global/report/yadokariviridae.

Cellular and Natural Viral Engineering in Cognition-Based Evolution

Neo-Darwinism conceptualizes evolution as the continuous succession of predominately random genetic variations disciplined by natural selection. In that frame, the primary interaction between cells and the virome is relegated to host-parasite dynamics governed by selective influences. Cognition-Based Evolution regards biological and evolutionary development as a reciprocating cognition-based informational interactome for the protection of self-referential cells. To sustain cellular homeorhesis, cognitive cells collaborate to assess the validity of ambiguous biological information. That collective interaction involves coordinate measurement, communication, and active deployment of resources as Natural Cellular Engineering. These coordinated activities drive multicellularity, biological development, and evolutionary change. The virome participates as the vital intercessory among the cellular domains to ensure their shared permanent perpetuation. The interactions between the virome and the cellular domains represent active virocellular cross-communications for the continual exchange of resources. Modular genetic transfers between viruses and cells carry bioactive potentials. Those exchanges are deployed as nonrandom flexible tools among the domains in their continuous confrontation with environmental stresses. This alternative framework fundamentally shifts our perspective on viral-cellular interactions, strengthening established principles of viral symbiogenesis. Pathogenesis can now be properly appraised as one expression of a range of outcomes between cells and viruses within a larger conceptual framework of Natural Viral Engineering as a co-engineering participant with cells. It is proposed that Natural Viral Engineering should be viewed as a co-existent facet of Natural Cellular Engineering within Cognition-Based Evolution.

Virion-Associated Nucleic Acid-Based Metagenomics: A Decade of Advances in Molecular Characterization of Plant Viruses

Over the last decade, viral metagenomic studies have resulted in the discovery of thousands of previously unknown viruses. These studies are likely to play a pivotal role in obtaining an accurate and robust understanding of how viruses affect the stability and productivity of ecosystems. Among the metagenomics-based approaches that have been developed since the beginning of the 21st century, shotgun metagenomics applied specifically to virion-associated nucleic acids (VANA) has been used to disentangle the diversity of the viral world. We summarize herein the results of 24 VANA-based studies, focusing on plant and insect samples conducted over the last decade (2010 to 2020). Collectively, viruses from 85 different families were reliably detected in these studies, including capsidless RNA viruses that replicate in fungi, oomycetes, and plants. Finally, strengths and weaknesses of the VANA approach are summarized and perspectives of applications in detection, epidemiological surveillance, environmental monitoring, and ecology of plant viruses are provided. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Novel and diverse mycoviruses co-infecting a single strain of the phytopathogenic fungus Alternaria dianthicola

Alternaria dianthicola is a pathogenic fungus that causes serious leaf or flower blight on some medicinal plants worldwide. In this study, multiple dsRNA bands in the range of 1.2-10 kbp were found in a Alternaria dianthus strain HNSZ-1, and eleven full-length cDNA sequences of these dsRNA were obtained by high-throughput sequencing, RT-PCR detection and conventional Sanger sequencing. Homology search and phylogenetic analyses indicated that the strain HNSZ-1 was infected by at least nine mycoviruses. Among the nine, five viruses were confirmed to represent novel viruses in the families Hypoviridae, Totiviridae, Mymonaviridae and a provisional family Ambiguiviridae. Virus elimination and horizontal transmission indicated that the (-) ssRNA virus, AdNSRV1, might be associated with the slow growth and irregular colony phenotype of the host fungus. As far as we know, this is the first report for virome characterization of A. dianthus, which might provide important insights for screening of mycovirus for biological control and for studying of the interactions between viruses or viruses and their host.

Three-Layered Complex Interactions among Capsidless (+)ssRNA Yadokariviruses, dsRNA Viruses, and a Fungus

We have previously discovered a virus neo-lifestyle exhibited by a capsidless positive-sense (+), single-stranded (ss) RNA virus YkV1 (family Yadokariviridae) and an unrelated double-stranded (ds) RNA virus YnV1 (proposed family "Yadonushiviridae") in a phytopathogenic ascomycete, Rosellinia necatrix. YkV1 has been proposed to replicate in the capsid provided by YnV1 as if it were a dsRNA virus and enhance YnV1 replication in return. Recently, viruses related to YkV1 (yadokariviruses) have been isolated from diverse ascomycetous fungi. However, it remains obscure whether such viruses generally show the YkV1-like lifestyle. Here, we identified partner viruses for three distinct yadokariviruses, YkV3, YkV4a, and YkV4b, isolated from R. necatrix that were coinfected with multiple dsRNA viruses phylogenetically distantly related to YnV1. We first established transformants of R. necatrix carrying single yadokarivirus cDNAs and fused them with infectants by single partner candidate dsRNA viruses. Consequently, YkV3 and YkV4s replicated only in the presence of RnMBV3 (family Megabirnaviridae) and RnMTV1 (proposed family "Megatotiviridae"), respectively. The partners were mutually interchangeable between the two YkV4 strains and three RnMTV1 strains but not between other combinations involving YkV1 or YkV3. In contrast to YkV1 enhancing YnV1 accumulation, YkV4s reduced RnMTV1 accumulation to different degrees according to strains. Interestingly, YkV4 rescued the host R. necatrix from impaired growth induced by RnMTV1. YkV3 exerted no apparent effect on its partner (RnMBV3) or host fungus. Overall, we revealed that while yadokariviruses generally require partner dsRNA viruses for replication, each yadokarivirus partners with a different dsRNA virus species in the three diverse families and shows a distinct symbiotic relation in a fungus. IMPORTANCE A capsidless (+)ssRNA virus YkV1 (family Yadokariviridae) highjacks the capsid of an unrelated dsRNA virus YnV1 (proposed family "Yadonushiviridae") in a phytopathogenic ascomycete, while YkV1 trans-enhances YnV1 replication. Herein, we identified the dsRNA virus partners of three yadokariviruses (YkV3, YkV4a, and YkV4b) with genome organization different from YkV1 as being different from YnV1 at the suborder level. Their partners were mutually interchangeable between the two YkV4 strains and three strains of the partner virus RnMTV1 (proposed family "Megatotiviridae") but not between other combinations involving YkV1 or YkV3. Unlike YkV1, YkV4s reduced RnMTV1 accumulation and rescued the host fungus from impaired growth induced by RnMTV1. YkV3 exerted no apparent effect on its partner (RnMBV3, family Megabirnaviridae) or host fungus. These revealed that while each yadokarivirus has a species-specific partnership with a dsRNA virus, yadokariviruses collectively partner extremely diverse dsRNA viruses and show three-layered complex mutualistic/antagonistic interactions in a fungus.

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.

Mycovirus Hunting Revealed the Presence of Diverse Viruses in a Single Isolate of the Phytopathogenic Fungus Diplodia seriata From Pakistan

Diplodia seriata in the family Botryosphaeriaceae is a cosmopolitan phytopathogenic fungus and is responsible for causing cankers, fruit rot and leaf spots on economically important plants. In this study, we characterized the virome of a single Pakistani strain (L3) of D. seriata. Several viral-like contig sequences were obtained via a previously conducted next-generation sequencing analysis. Multiple infection of the L3 strain by eight RNA mycoviruses was confirmed through RT-PCR using total RNA samples extracted from this strain; the entire genomes were determined via Sanger sequencing of RT-PCR and RACE clones. A BLAST search and phylogenetic analyses indicated that these eight mycoviruses belong to seven different viral families. Four identified mycoviruses belong to double-stranded RNA viral families, including Polymycoviridae, Chrysoviridae, Totiviridae and Partitiviridae, and the remaining four identified mycoviruses belong to single-stranded RNA viral families, i.e., Botourmiaviridae, and two previously proposed families “Ambiguiviridae” and “Splipalmiviridae”. Of the eight, five mycoviruses appear to represent new virus species. A morphological comparison of L3 and partially cured strain L3ht1 suggested that one or more of the three viruses belonging to Polymycoviridae, “Splipalmiviridae” and “Ambiguiviridae” are involved in the irregular colony phenotype of L3. To our knowledge, this is the first report of diverse virome characterization from D. seriata.

A Capsidless Virus Is trans-Encapsidated by a Bisegmented Botybirnavirus

RNA viruses usually have linear genomes and are encapsidated by their own capsids. Here, we newly identified four mycoviruses and two previously reported mycoviruses (a fungal reovirus and a botybirnavirus) in the hypovirulent strain SCH941 of Sclerotinia sclerotiorum. One of the newly discovered mycoviruses, Sclerotinia sclerotiorum yadokarivirus 1 (SsYkV1), with a nonsegmented positive-sense single-stranded RNA (+ssRNA) genome, was molecularly characterized. SsYkV1 is 5,256 nucleotides (nt) in length, excluding the poly(A) structure, and has a large open reading frame that putatively encodes a polyprotein with the RNA-dependent RNA polymerase (RdRp) domain and a 2A-like motif. SsYkV1 was phylogenetically positioned into the family Yadokariviridae and was most closely related to Rosellinia necatrix yadokarivirus 2 (RnYkV2), with 40.55% identity (78% coverage). Although SsYkV1 does not encode its own capsid protein, the RNA and RdRp of SsYkV1 are trans-encapsidated in virions of Sclerotinia sclerotiorum botybirnavirus 3 (SsBV3), a bisegmented double-stranded RNA (dsRNA) mycovirus within the genus Botybirnavirus. In this way, SsYkV1 likely replicates inside the heterocapsid comprised of the SsBV3 capsid protein, like a dsRNA virus. SsYkV1 has a limited impact on the biological features of S. sclerotiorum. This study represents an example of a yadokarivirus trans-encapsidated by an unrelated dsRNA virus, which greatly deepens our knowledge and understanding of the unique life cycles of RNA viruses. IMPORTANCE RNA viruses typically encase their linear genomes in their own capsids. However, a capsidless +ssRNA virus (RnYkV1) highjacks the capsid of a nonsegmented dsRNA virus for the trans-encapsidation of its own RNA and RdRp. RnYkV1 belongs to the family Yadokariviridae, which already contains more than a dozen mycoviruses. However, it is unknown whether other yadokariviruses except RnYkV1 are also hosted by a heterocapsid, although dsRNA viruses with capsid proteins were detected in fungi harboring yadokarivirus. It is noteworthy that almost all presumed partner dsRNA viruses of yadokariviruses belong to the order Ghabrivirales (most probably a totivirus or toti-like virus). Here, we found a capsidless +ssRNA mycovirus, SsYkV1, from hypovirulent strain SCH941 of S. sclerotiorum, and the RNA and RdRp of this mycovirus are trans-encapsidated in virions of a bisegmented dsRNA virus within the free-floating genus Botybirnavirus. Our results greatly expand our knowledge of the unique life cycles of RNA viruses.