Molecular characterization of a novel mycovirus from binucleate Rhizoctonia AG-A strain A46

The complete genome sequence of a positive-sense single-stranded RNA (+ ssRNA) virus, Rhizoctonia beny-like virus 1 (RBLV1), isolated from binucleate Rhizoctonia AG-A strain A46, was determined. The RBLV1 genome is 10,280 nt in length and contains a short stretch of adenines at the 3' terminus. It contains a single open reading frame (ORF) encoding a 376.30-kDa protein with viral helicase and RNA-dependent RNA polymerase (RdRp) motifs. The encoded protein exhibited the highest sequence similarity to Rhizoctonia cerealis beny-like virus 0928-1 (RcBeLV 0928-1, 45.25%), with a sequence coverage of 63%. Phylogenetic analysis based on ORF protein sequences revealed that RBLV1 is a novel unclassified mycovirus.

Complete nucleotide sequence of a novel mycovirus infecting Rhizoctonia fumigata AG-Ba isolate C-314 Baishi

The complete nucleotide sequence of a novel mycovirus, designated as "Rhizoctonia fumigata bipartite virus 1" (RfBV1), from Rhizoctonia fumigata AG-Ba isolate C-314 Baishi was determined. The genome of RfBV1 is composed of two double-stranded RNAs (dsRNA). dsRNA-1 (2311 bp) contains one open reading frame (ORF), which codes for the putative RNA-dependent RNA polymerase (RdRp) of the virus. dsRNA-2 (1690 bp) contains one ORF, which encodes a putative protein whose function is unknown. Phylogenetic analysis indicated that the RdRp of RfBV1 clustered with several unassigned bipartite viruses belonging to the CThTV-like viruses group, but not the family Amalgaviridae or Partitiviridae. Our study suggests that RfBV1 is a novel mycovirus related to the CThTV-like viruses.

Description, Distribution, and Relevance of Viruses of the Forest Pathogen Gremmeniella abietina

The European race of the ascomycetous species Gremmeniella abietina (Lagerberg) Morelet includes causal agents of shoot blight and stem canker of several conifers in Europe and North America, which are known to host a diverse virome. GaRV6 is the latest and sixth mycovirus species reported within G. abietina. Before its description, one victorivirus and one gammapartitivirus species were described in biotype A, two mitoviruses in both biotypes A and B and a betaendornavirus in biotype B. Possible phenotypic changes produced by mycoviruses on G. abietina mycelial growth have been reported in Spanish mitovirus-free and GaRV6-hosting G. abietina isolates, which had higher growth rates at the optimal temperature of 15 °C, but no other major differences have been observed between partitivirus-like dsRNA and dsRNA-free isolates. In this review, we reappraise the diversity of viruses found in G. abietina so far, and their relevance in clarifying the taxonomy of G. abietina. We also provide evidence for the presence of two new viruses belonging to the families Fusariviridae and Endornaviridae in Spanish isolates.

Vírus que infectam fungos fitopatogênicos

RESUMO: Micovírus são vírus que infectam todos os taxa de fungos. São geralmente crípticos (latentes), mas podem causar pequenas ou imperceptíveis alterações no hospedeiro. Nos fungos fitopatogênicos, os vírus podem interferir com os sintomas e, em alguns casos, reduzir a virulência de seu hospedeiro; por esta razão, são objeto de estudo, por serem um potencial agente de biocontrole e por serem ferramentas importantes para o conhecimento sobre os mecanismos de patogênese de fungos. A presente revisão teve o objetivo de reunir os dados de literatura relacionados aos aspectos gerais da biologia e do comportamento dos micovírus presentes em alguns fungos fitopatogênicos.

A program to compute the soft Robinson-Foulds distance between phylogenetic networks

Background

Over the past two decades, phylogenetic networks have been studied to model reticulate evolutionary events. The relationships among phylogenetic networks, phylogenetic trees and clusters serve as the basis for reconstruction and comparison of phylogenetic networks. To understand these relationships, two problems are raised: the tree containment problem, which asks whether a phylogenetic tree is displayed in a phylogenetic network, and the cluster containment problem, which asks whether a cluster is represented at a node in a phylogenetic network. Both the problems are NP-complete.

Results

A fast exponential-time algorithm for the cluster containment problem on arbitrary networks is developed and implemented in C. The resulting program is further extended into a computer program for fast computation of the Soft Robinson–Foulds distance between phylogenetic networks.

Conclusions

Two computer programs are developed for facilitating reconstruction and validation of phylogenetic network models in evolutionary and comparative genomics. Our simulation tests indicated that they are fast enough for use in practice. Additionally, the distribution of the Soft Robinson–Foulds distance between phylogenetic networks is demonstrated to be unlikely normal by our simulation data.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3500-5) contains supplementary material, which is available to authorized users.

A Resolution of the Static Formulation Question for the Problem of Computing the History Bound

Evolutionary data has been traditionally modeled via phylogenetic trees; however, branching alone cannot model conflicting phylogenetic signals, so networks are used instead. Ancestral recombination graphs (ARGs) are used to model the evolution of incompatible sets of SNP data, allowing each site to mutate only once. The model often aims to minimize the number of recombinations. Similarly, incompatible cluster data can be represented by a reticulation network that minimizes reticulation events. The ARG literature has traditionally been disjoint from the reticulation network literature. By building on results from the reticulation network literature, we resolve an open question of interest to the ARG community. We explicitly prove that the History Bound, a lower bound on the number of recombinations in an ARG for a binary matrix, which was previously only defined procedurally, is equal to the minimum number of reticulation nodes in a network for the corresponding cluster data. To facilitate the proof, we give an algorithm that constructs this network using intermediate values from the procedural History Bound definition. We then develop a top-down algorithm for computing the History Bound, which has the same worst-case runtime as the known dynamic program, and show that it is likely to run faster in typical cases.

A program for verification of phylogenetic network models

MOTIVATION

Genetic material is transferred in a non-reproductive manner across species more frequently than commonly thought, particularly in the bacteria kingdom. On one hand, extant genomes are thus more properly considered as a fusion product of both reproductive and non-reproductive genetic transfers. This has motivated researchers to adopt phylogenetic networks to study genome evolution. On the other hand, a gene's evolution is usually tree-like and has been studied for over half a century. Accordingly, the relationships between phylogenetic trees and networks are the basis for the reconstruction and verification of phylogenetic networks. One important problem in verifying a network model is determining whether or not certain existing phylogenetic trees are displayed in a phylogenetic network. This problem is formally called the tree containment problem. It is NP-complete even for binary phylogenetic networks.

RESULTS

We design an exponential time but efficient method for determining whether or not a phylogenetic tree is displayed in an arbitrary phylogenetic network. It is developed on the basis of the so-called reticulation-visible property of phylogenetic networks.

AVAILABILITY AND IMPLEMENTATION

A C-program is available for download on http://www.math.nus.edu.sg/∼matzlx/tcp_package

CONTACT

matzlx@nus.edu.sg

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

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.

Molecular characterization of a novel mycovirus from Rhizoctonia fumigata AG-Ba isolate C-314 Baishi

The complete genome sequence of a novel dsRNA virus isolated from Rhizoctonia fumigata AG-Ba isolate C-314 Baishi (designated as Rhizoctonia fumigata virus 1, RfV1) was determined. The RfV1 genome was 9,907 bp in length and contained two open reading frames (ORFs). ORF1 potentially coded for a 198.10-kDa protein (P1). P1 shared low but significant amino acid sequence similarity to the putative protein encoded by Lentinula edodes mycovirus (LeV) ORF1. P1 contained a NUDIX domain, which was also present in the putative proteins encoded by the ORF1s of LeV and Phlebiopsis gigantea large virus 1 (PgLV-1). ORF2 potentially coded for a 146.72-kDa protein (P2) that contained the conserved motifs of the RNA-dependent RNA polymerase (RdRp). ORF1 and ORF2 were overlapping, and it was predicted that ORF2 could be translated as a fusion with ORF1 via a ribosomal -1 frameshifting mechanism. Phylogenetic analysis indicated that RfV1 clustered with PgLV-1, LeV and Rosellinia necatrix megabirnavirus 1 (RnMBV1) in a separate clade independent of other virus genera. We propose that RfV1, along with PgLV-1 and LeV, should be grouped into a new viral genus related to the family Megabirnaviridae. This is the first report of the full-length genome sequence of a novel mycovirus isolated from R. fumigata.

Identification, molecular characterization, and evolution of group I introns at the expansion segment D11 of 28S rDNA in Rhizoctonia species

The nuclear ribosomal DNA of Rhizoctonia species is polymorphic in terms of the nucleotide composition and length. Insertions of 349-410 nucleotides in length with characteristics of group I introns were detected at a single insertion point at the expansion segment D11 of 28S rDNA in 12 out of 64 isolates. Eleven corresponded to Rhizoctonia solani (teleomorph: Thanatephorous) and one (AG-Q) to Rhizoctonia spp. (teleomorph: Ceratobasidium). Sequence data showed that all but AG-Q contained conserved DNA catalytic core regions (P, Q, R, and S) essential for selfsplicing. The predicted secondary structure revealed that base-paired helices corresponded to subgroup IC1. Isolates from same anastomosis group and even subgroups within R. solani were variable with regard to possession of introns. Phylogenetic analyses indicated that introns were vertically transmitted. Unfortunately, sequence data from the conserved region from all 64 isolates were not useful for delimiting species. Analyses with IC1 introns at same insertion point, of both Ascomycota and Basidiomycota indicated the possibility of horizontal transfer at this site. The present study uncovered new questions on evolutionary pattern of change of these introns within Rhizoctonia species.

Relationships Between the Pathogen Olpidium virulentus and Viruses Associated with Lettuce Big-Vein Disease

Lettuce big-vein disease (LBVD) is a widespread problem in commercial lettuce producing areas. It was first reported in 1934 in California. In the United States in 2009, it was estimated that 100% of all commercial lettuce production was concentrated in California (in early spring) and Arizona (in winter). The disease also occurs worldwide, particularly in regions in Australia, Europe, Brazil, and New Zealand, and at high altitudes in subtropical regions. LBVD often results in decreased yield from infected plants, mainly in winter growing seasons. Symptoms of this disease are the obvious chlorophyll clearing, which causes the big-vein leaf symptoms from which the disease gets its name, along with crinkled leaves and a reduced head size, which is often oblong in shape. LBVD is associated with a complex of two viruses, Lettuce big-vein associated virus (LBVaV) and Mirafiori lettuce big-vein virus (MLBVV). Motile zoospores of Olpidium virulentus vector LBVaV and MLBVV to the roots of healthy plants and, in the absence of susceptible hosts, its resting spores retain the ability to harbor these viruses for decades in infested soil and in dry roots for 39 months. The ability of viruliferous resting spores to persist in production areas for such extended periods of time, and also the motility of viruliferous zoospores, pose significant difficulties for the management and/or control of the disease.

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.

The double life of Ceratobasidium: orchid mycorrhizal fungi and their potential for biocontrol of Rhizoctonia solani sheath blight of rice

Ceratobasidium includes orchid mycorrhizal symbionts, plant pathogens and biocontrol agents of soilborne plant pathogens. It is not known to what extent members of the first guild also can participate in the others. Ceratobasidium spp. were isolated from roots of Colombian orchids and identified by phylogeny based on nrITS sequences. Phylogenetic grouping of Ceratobasidium spp. isolates corresponded to orchid host substrate (epiphytic vs. terrestrial). Isolates were tested for virulence on rice and for biocontrol of Rhizoctonia solani, causal agent of sheath blight of rice. All Ceratobasidium spp. isolates caused some signs of sheath blight but significantly less than a pathogenic R. solani used as a positive control. When Ceratobasidium spp. isolates were inoculated on rice seedlings 3 d before R. solani, they significantly reduced disease expression compared to controls inoculated with R. solani alone. The use of Ceratobasidium spp. from orchids for biological control is novel, and biodiverse countries such as Colombia are promising places to look for new biocontrol agents.

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 putative partitivirus of the saprotrophic fungus Heterobasidion ecrustosum infects pathogenic species of the Heterobasidion annosum complex

We characterized the bisegmented genome of a putative double-stranded (ds) RNA virus from a Chinese isolate of the fungus Heterobasidion ecrustosum, a member of the Heterobasidion insulare species complex. The larger genomic segment of 1885bp encoded a putative RNA dependent RNA polymerase (RdRp, 585aa), and the smaller one for a putative coat protein of 521aa (1826bp). Phylogenetic analyses suggest that this novel virus species, named as 'Heterobasidion RNA virus 3 from H. ecrustosum, strain 1' (HetRV3-ec1), can be assigned to the family Partitiviridae, being most similar to the Helicobasidium mompa dsRNA mycovirus with RdRp amino acid similarity of 54%. The similarity to known viruses of other Heterobasidion species was notably low (25-39%). The virus could be experimentally transmitted to members of the Heterobasidion annosum complex: the European Heterobasidion abietinum and North American Heterobasidion occidentale, and the original host strain could be cured from the virus by thermal treatment. Microscopical observations showed that hyphae of H. ecrustosum anastomosed occasionally with H. abietinum and H. occidentale, and suggested a possible route for horizontal transmission between these sexually incompatible species. The virus infection seemed to cause variable effects on the growth rate of its fungal hosts, but the results were strongly dependent on fungal strain, growth medium and incubation temperature.