A Satellite RNA of Ophiostoma novo-ulmi Mitovirus 3a in Hypovirulent Isolates of Sclerotinia homoeocarpa

A special satellite-like RNA of a novel hypovirus from Pestalotiopsis fici broadens the definition of fungal satellite

Satellites associated with plant or animal viruses have been largely detected and characterized, while those from mycoviruses together with their roles remain far less determined. Three dsRNA segments (dsRNA 1 to 3 termed according to their decreasing sizes) were identified in a strain of phytopathogenic fungus Pestalotiopsis fici AH1-1 isolated from a tea leaf. The complete sequences of dsRNAs 1 to 3, with the sizes of 10316, 5511, and 631 bp, were determined by random cloning together with a RACE protocol. Sequence analyses support that dsRNA1 is a genome of a novel hypovirus belonging to genus Alphahypovirus of the family Hypoviridae, tentatively named Pestalotiopsis fici hypovirus 1 (PfHV1); dsRNA2 is a defective RNA (D-RNA) generating from dsRNA1 with septal deletions; and dsRNA3 is the satellite component of PfHV1 since it could be co-precipitated with other dsRNA components in the same sucrose fraction by ultra-centrifuge, suggesting that it is encapsulated together with PfHV1 genomic dsRNAs. Moreover, dsRNA3 shares an identical stretch (170 bp) with dsRNAs 1 and 2 at their 5' termini and the remaining are heterogenous, which is distinct from a typical satellite that generally has very little or no sequence similarity with helper viruses. More importantly, dsRNA3 lacks a substantial open reading frame (ORF) and a poly (A) tail, which is unlike the known satellite RNAs of hypoviruses, as well as unlike those in association with Totiviridae and Partitiviridae since the latters are encapsidated in coat proteins. As up-regulated expression of RNA3, dsRNA1 was significantly down-regulated, suggesting that dsRNA3 negatively regulates the expression of dsRNA1, whereas dsRNAs 1 to 3 have no obvious impact on the biological traits of the host fungus including morphologies and virulence. This study indicates that PfHV1 dsRNA3 is a special type of satellite-like nucleic acid that has substantial sequence homology with the host viral genome without encapsidation in a coat protein, which broadens the definition of fungal satellite.

Two novel mitoviruses from a Canadian isolate of the Dutch elm pathogen Ophiostoma novo-ulmi (93-1224)

Background

Ophiostoma novo-ulmi is the causative agent of Dutch elm disease (DED). It is an ascomycetous filamentous fungus that ranks as the third most devastating fungal pathogen in Canada. The disease front has spread eastward and westward from the epicentre in Ontario and Quebec and is threatening elm populations across the country. Numerous mitigation strategies have been tried to eradicate this pathogen, but success has thus far been limited. An alternative approach might utilize double-stranded RNA (dsRNA) mycoviruses which have been reported to induce hypovirulence in other fungi.

Methods

Using a modified single primer amplification technique (SPAT) in combination with chromosomal walking, we have determined the genome sequence of two RdRp encoding dsRNA viruses from an O. novo-ulmi isolate (93–1224) collected from the disease front in Winnipeg.

Results

We propose that these viruses, which we have named OnuMV1c and OnuMV7 based on sequence similarity to other Ophiostoma mitoviruses, are two new members of the genus Mitovirus in the family Narnaviridae.

Conclusions

The discovery of such dsRNA elements raises the potential for engineering these viruses to include other genetic elements, such as anti-sense or interfering RNAs, to create novel and highly specific biological controls. Naïve fungal hosts could be infected with both the engineered molecule and a helper mitovirus encoding an RdRp which would provide replication capacity for both molecules.

The family narnaviridae: simplest of RNA viruses

Members of the virus family Narnaviridae contain the simplest genomes of any RNA virus, ranging from 2.3 to 3.6 kb and encoding only a single polypeptide that has an RNA-dependent RNA polymerase domain. The family is subdivided into two genera based on subcellular location: members of the genus Narnavirus have been found in the yeast Saccharomyces cerevisiae and in the oomycete Phytophthora infestans and are confined to the cytosol, while members of the genus Mitovirus have been found only in filamentous fungi and are found in mitochondria. None identified thus far encodes a capsid protein; like several other RNA viruses of lower eukaryotes, their genomes are confined within lipid vesicles. As more family members are discovered, their importance as genetic elements is becoming evident. The unique association of the genus Mitovirus with mitochondria renders them potentially valuable tools to study biology of lower eukaryotes.

Occurrence of two different species of mitoviruses in the European race of Gremmeniella abietina var. abietina, both hosted by the genetically unique Spanish population

The genetic structure of the genus Mitovirus community hosted by the European pathogenic conifer fungus Gremmeniella abietina var. abietina was investigated. Gremmeniella abietina is a species complex with a divergent mycovirus community, composed mainly of Totivirus, Partitivirus, and Mitovirus species. In this work, the total doubled-stranded (ds)RNA from 353 isolates from Canada, Finland, Spain, Switzerland, Turkey, and USA was extracted to look for the presence of a ca. 2.5 kb band typical of mitoviruses' genomes. Based on the banding data, 60 partial RNA-dependent RNA polymerase (RdRp) DNA sequences (ca. 500 bp) were amplified with reverse transcription-polymerase chain reaction (RT-PCR) and sequenced. Two distantly related mitovirus groups (species) were observed in the clustering analysis, one of them related to GMV1-1 and the other one related to a new putative species described in this study, GMV2-1. Viruses in these two clusters seemed to be subjected to purifying selection. The cluster with GMV1-1 included viruses observed in the Finnish biotype A and Spanish strains, whereas the cluster including GMV2-1 was composed of viruses of the Finnish biotype B and one from the Spanish population. Thereby, the Spanish population of G. abietina harboured mitovirus strains occurring in both biotype A and B strains, and it is the first one hosting distantly related mycoviruses of a single genus in one population of G. abietina. This may suggest that horizontal transmission of viruses could have occurred between biotype B and the Spanish population.

A novel mycovirus closely related to hypoviruses that infects the plant pathogenic fungus Sclerotinia sclerotiorum

Three dsRNA segments, two similarly sized at 9.5kbp and a third one of approximately 3.6kbp, were extracted from a hypovirulent strain SZ-150 of Sclerotinia sclerotiorum. The complete cDNA sequence of one of the two large dsRNA segment (10398bp, excluding the poly (A) tail) reveals a single ORF that encodes a polyprotein with conserved domains of putative papain-like protease, UDP glucose/sterol glycosyltransferase, RNA-dependent RNA polymerase and viral RNA Helicase. This virus is closely related to Cryphonectria hypovirus (CHV) 3/GH2 and CHV4/SR2 in the family Hypoviridae and designated as Sclerotinia sclerotiorum hypovirus 1 (SsHV1/SZ-150). The satellite-like 3.6kbp dsRNA segment (S-dsRNA) shares high sequence identity with the 5'-UTR of SsHV1/SZ-150. SsHV1/SZ-150 alone is not the primary causal agent for hypovirulence of strain SZ-150 since strains without the S-dsRNA show normal phenotype. This is the first report of a naturally occurring hypovirus that infects a fungus other than Cryphonectria parasitica.

Codivergence of mycoviruses with their hosts

BACKGROUND

The associations between pathogens and their hosts are complex and can result from any combination of evolutionary events such as codivergence, switching, and duplication of the pathogen. Mycoviruses are RNA viruses which infect fungi and for which natural vectors are so far unknown. Thus, lateral transfer might be improbable and codivergence their dominant mode of evolution. Accordingly, mycoviruses are a suitable target for statistical tests of virus-host codivergence, but inference of mycovirus phylogenies might be difficult because of low sequence similarity even within families.

METHODOLOGY

We analyzed here the evolutionary dynamics of all mycovirus families by comparing virus and host phylogenies. Additionally, we assessed the sensitivity of the co-phylogenetic tests to the settings for inferring virus trees from their genome sequences and approximate, taxonomy-based host trees.

CONCLUSIONS

While sequence alignment filtering modes affected branch support, the overall results of the co-phylogenetic tests were significantly influenced only by the number of viruses sampled per family. The trees of the two largest families, Partitiviridae and Totiviridae, were significantly more similar to those of their hosts than expected by chance, and most individual host-virus links had a significant positive impact on the global fit, indicating that codivergence is the dominant mode of virus diversification. However, in this regard mycoviruses did not differ from closely related viruses sampled from non-fungus hosts. The remaining virus families were either dominated by other evolutionary modes or lacked an apparent overall pattern. As this negative result might be caused by insufficient taxon sampling, the most parsimonious hypothesis still is that host-parasite evolution is basically the same in all mycovirus families. This is the first study of mycovirus-host codivergence, and the results shed light not only on how mycovirus biology affects their co-phylogenetic relationships, but also on their presumable host range itself.

A novel virus that infecting hypovirulent strain XG36-1 of plant fungal pathogen Sclerotinia sclerotiorum

BACKGROUND

Sclerotinia sclerotiorum is a notorious plant fungal pathogen which spreads across the world. Hypovirulence is a phenomenon where the virulence of fungal pathogens is decreased, even lost, due to mycovirus infection. The potential of hypoviruses for biological control of the chestnut blight fungus (Cryphonectria parasitica) has attracted much interest, and has led to discovery of new hypovirulent strains in other fungi.

RESULTS

A hypovirulent strain, strain XG36-1, was isolated from a typical lesion on the stem of rapeseed (Brassica napus) caused by Sclerotinia sclerotiorum. Strain XG36-1 grew on PDA very slowly (average 2.5 +/- 0.1 mm/d) with sectoring, and developed abnormal colony morphology with few sclerotia. Unlike health strains (such as wildtype strain XG-13), it was unable to induce lesions on detached leaves of rapeseed. Sclerotia of strain XG36-1 produced apothecia rarely. A sexual progeny test showed that the phenotypes of all 104 sexual progeny were not different from wildtype strain XG-13 which shows normal phenotype of S. sclerotiorum, and protoplast regeneration tests showed that 25.5% of the regenerants of strain XG36-1 were recovered fully. Furthermore, the hypovirulence and its associated traits could be transmitted to XG36-1A34R, a hygromycin-resistance gene labelled sexual progeny of strain XG36-1, by hyphal anastomosis. Transmission electron microscope (TEM) observation showed that the cytoplasm of strain XG36-1 was destroyed and granulated; the membranes of nuclei and mitochondria were disintegrated; and mitochondrial cristae were cavitated. Viral particles (about 40 nm) in hyphae of strain XG36-1, but not in its sexual progeny and wildtype strain XG-13, could be observed with TEM, and several virus-like particles were uniquely enveloped by single layer membrane in the cells of strain XG36-1. Furthermore, the viral particles could be co-transmitted with the hypovirulence traits through hyphal anastomosis.

CONCLUSION

Hypovirulence and its associated traits of strain XG36-1 could be mediated by a fungal virus. Currently, we could not know the characteristic of this virus, but it likely represent a new type of mycovirus in S. sclerotiorum, and possibly in fungi.

Hypovirulence and Double-Stranded RNA in Botrytis cinerea

ABSTRACT Twenty-one strains of Botrytis cinerea isolated from 13 species of plants grown in China were compared for pathogenicity on Brassica napus, mycelial growth on potato dextrose agar, and presence of double-stranded (ds)RNA. The results showed that the strain CanBc-1 was severely debilitated in pathogenicity and mycelial growth, compared with the 20 virulent strains. A dsRNA of approximately 3.0 kb in length was detected in CanBc-1 and 4 hypovirulent single-conidium (SC) isolates of CanBc-1, but was not detected in the 20 virulent strains of B. cinerea and 4 virulent SC isolates of CanBc-1. Results of the horizontal transmission experiment showed that the hypovirulent trait of CanBc-1 was transmissible and the 3.0-kb dsRNA was involved in the transmission of hypovirulence. Analysis of a 920-bp cDNA sequence generated from the 3.0-kb dsRNA of CanBc-1 indicated that the dsRNA element was a mycovirus, designated as B. cinerea debilitation-related virus (BcDRV). Further analyses showed that BcDRV is closely related to Ophiostoma mitovirus 3b infecting O. novo-ulmi, the causal agent of Dutch elm disease. Mitochondria and cytoplasm in hyphal cells of CanBc-1 became degenerated, compared with the virulent isolate CanBc-1c-66 of B cinerea. This is the first report on the occurrence of Mitovirus-associated hypovirulence in B. cinerea.

Characterization of debilitation-associated mycovirus infecting the plant-pathogenic fungus Sclerotinia sclerotiorum

It was previously reported that three dsRNA segments, designated L, M and S, were isolated from Sclerotinia sclerotiorum strain Ep-1PN and that the M dsRNA segment was coincident with hypovirulence and debilitation of the fungal host. Here, the complete nucleotide sequence of the M dsRNA of 5419 nt, excluding the poly(A) tail, was determined. Sequence analysis revealed the occurrence of a single open reading frame (nt 93-5195) encoding a protein with significant similarity to the replicases of the 'alphavirus-like' supergroup of positive-strand RNA viruses. The M dsRNA-encoded putative replicase protein contained the conserved methyl transferase, helicase and RNA-dependent RNA polymerase (RdRp) domains characteristic of the replicases of potex-like plant viruses (flexiviruses) and Botrytis virus F (BVF), a flexuous rod mycovirus infecting the phytopathogenic fungus Botrytis cinerea. Furthermore, convincing evidence is presented showing that ascospore descendents derived from the debilitated strain Ep-1PN were devoid of dsRNA and exhibited normal colony morphology. Moreover, it was demonstrated that the debilitation phenotype was transmitted from the parental debilitated strain to its normal ascospore progeny via hyphal anastomosis. These results suggest that the M dsRNA from strain Ep-1PN is derived from the genomic RNA of a positive-strand RNA virus, which we designated Sclerotinia sclerotiorum debilitation-associated RNA virus (SsDRV). Although phylogenetic analysis of the conserved RdRp motifs verified that SsDRV is closely related to BVF and to the allexiviruses in the family Flexiviridae, SsDRV is distinct from these viruses, mainly based on the lack of coat protein and movement protein.