Non-segmented RNA virus.

Recently, dsRNA viruses have been identified from various fungi that encode two ORFs within the single-segment genome, where the RdRP sequences have more similarity to the RdRPs of partitiviruses (multi-segment genomes) than totiviruses (mono-segment genomes). The virus Beauveria bassiana non-segmented dsRNA virus 1 (BbNRV1), assembled using the RNA-seq data from a transcriptome experiment of the highly virulent isolate B. bassiana ARSEF 8028 [9], groups with these previously identified viruses (Fig 2). A second virus from this group was also identified in this analysis: Colletotrichum higginsianum non-segmented RNA virus 1 (ChNRV1); this virus served as an internal control for the virus discovery pipeline as it was previously identified via a similar approach from two different Colletotrichum higginsianum RNA-seq datasets [8].

BbNRV1 and ChNRV1, together with four previously identified viruses, form a strongly supported group that is separate from other characterized fungal viruses (Fig 2). It has been noted by multiple researchers that these fungal viruses share similarities with a family of plant viruses, the Amalgaviridae, specifically the mono-segmented genome structure and partitivirus-like RdRP sequence. Therefore, a phylogenetic analysis of the RdRP protein sequence was performed which demonstrates that, while the mono-segmented genome structure is shared, this new clade of fungal viruses is also distinct from the plant amalgaviruses (Fig 3A). Indeed, the amalgavirus group of plant viruses branches off before the BbNRV1-related and Partitivirus clades. A percent identity matrix generated with RdRP and CP coding sequences further demonstrates a separation between this new group and existing virus families (Fig 3B). For the RdRP pairwise comparisons, 50% identity easily differentiates the three clades from each other. Additionally, the CP sequence comparisons demonstrate shared similarities within the three clades and less than 20% similarity is shared between the clades.

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Fig 3. Analyses of identified novel non-segmented RNA viral genomes in relation to Amalgaviridae and Partitiviridae.

(A) Phylogram of the ORF containing the RdRP motif for members of the new mycovirus group along with select partitiviruses, totiviruses and amalgaviruses; sequence from Penicillium chrysogenum virus (PcCV) serves as the outgroup. Full names and accession numbers for protein sequences used in the alignment are in S2 Table. Scale bar on the phylogenetic tree represents 1.0 amino acid substitutions per site and numbers at the nodes indicate bootstrap support over 70% (1000 replicates). Diagrams illustrating the genome structure and predicted ORFs are present for each group of viruses. (B) Percent identity matrix, generated by Clustal-Omega 2.1, of new mycoviral family, partitiviruses, and amalgaviruses. The top half of the matrix, in the blue scale, is the percent identity of the RDRP protein coding sequence and the bottom half of the matrix, in the green scale, is the percent identity of the putative CP. For sake of clarity, the 100% identity along the diagonal has been removed and outlined stars are used to note junction point for the row and column of the viruses identified. Only RdRP values ≥50% and CP values ≥30% are indicated. (C) Density plot of read counts per nucleotide across the viral genome for BbNRV1. Graphical depiction of the genome structure, ORFs and predicted motifs are on along the top and a heatmap of normalized read counts is on the bottom. Read counts are normalized to a 0 – 1 scale, where the maximum read count for a given genome equals 1. Total reads mapped to the virus genome and average read coverage are noted. The red star indicates the virus identified in this study. RdRP: RNA-dependent RNA Polymerase; CP: Coat Protein; ORF: Open Reading Frame.

https://doi.org/10.1371/journal.pone.0219207.g003

The genome structure of viruses within this group consists of two predicted ORFs surrounded by variable length UTRs (Fig 3C). The 5’UTR lengths fall into two distinct ranges: less than 40 nt for Penicillium janczewskii Beauveria bassiana-like virus 1 (PjBblV1) [4], ChNRV1 [8], and UvHNND1 [33], or between 260 to 320 nt for the three viruses isolated from B. bassiana [34,35] and Alternaria longipes dsRNA virus 1 (AlV1-HN28) [36]. The protein encoded by ORF1 is predicted to be from 314 to 394 aa and in a +1 frame relative to the protein encoded by ORF2. The intergenic sequence lengths are generally less than 100 nt, with the exception of AlV1-HN28. ORF2 nucleotide sequences range from 1578 to 1773 nt, and encoded proteins, which encode an RdRP domain, range in size from 525 to 590 aa in length. A slippery site sequence identified between ORF1 and ORF2 of ChNRV1 suggested that an ORF1-ORF2 fusion protein may be made via a -1 ribosomal frameshift; indeed, trypsin digest followed by mass spectrometry of a protein near the predicted weight of the fusion protein returned signatures for both the ORF1 and ORF2 protein sequences [8]. Numerous attempts to isolate virus particles definitely produced by a virus within this group remain unsuccessful [4,8,34].

Due to the shared genome structure and partitivirus-like RdRP protein sequence, early viruses from this group were tentatively grouped with the plant Amalgaviridae family. However, with the identification of additional genome sequences, a more in depth phylogenetic analysis demonstrates the mycoviruses described here are separate. A new genus, “Unirnavirus”, has recently been suggested to reflect the single, unified genome segment that contains both ORFs [35]. As percent identity is a common criteria for species demarcation, the pairwise comparisons here provide possible thresholds for determining new species within this group of viruses; excluding the three viruses isolated from different strains of B. bassiana, a threshold of 75% identity in the RdRP protein sequence and 70% in the ORF1 coding sequence would be appropriate criteria to distinguish between the viruses isolated from unique fungal hosts.

Alternaviridae.

Initially, a single segment encoding a single gene with an RdRP domain was identified from an Aspergillus heteromorphus sequencing sample (PRJNA250969; unpublished). BLAST alignment to ‘nr’ identified five related RdRP mycoviral sequences from viruses with multi-segmented genomes. Fusarium graminearum alternavirus 1 (FgAV1) includes at least one additional genomic segment while Alternaria alternata virus 1 (AaV1) [37] and Aspergillus foetidus dsRNA mycovirus (AfdsV) [38] each contain a total of four genomic segments. Using the additional sequences from these related viruses, two more contigs were identified from the Aspergillus heteromorphus Trinity assembly. Thus, the AheAV1 genome contains at least three genomic segments, the largest of which encodes the RdRP enzyme and the two smaller segments that encode proteins of unknown function (Table 1). Recently it has been proposed that a new family be established, “Alternaviridae” [38] we support the formation of this family, and the requisite genus (Alternavirus) and propose that AheAV1 is also a member of this new mycoviral family.

Previously it was observed that members of this clade group together, but separate from other families [37,38]. A phylogenetic analysis using the RdRP coding sequence demonstrates that the group containing AheAV1 forms a well-supported clade that appears related to the Chrysoviridae and Totiviridae families (Fig 2). Members of the Totiviridae family have single-segment genomes containing two ORFs, while members of the Chrysoviridae have multi-segment genomes. While the RdRP segment sizes are similar between alternaviruses and chrysoviruses (~3,500 bp), the remaining two to three segments are hundreds of base pairs shorter than those of chrysoviruses. Further, the proteins of unknown function have no predicted domains or shared sequence homology outside of this group of viruses. The three viral RdRP sequences isolated from different species of Aspergillus, including AheAV1, are most closely related; similarly, the RdRP sequences from the two viruses from Fusarium species are most related (Fig 2). Indeed, a pairwise percent identity comparison of both the RdRP and HP1 protein sequences demonstrates the very high identity among the viruses from the two Fusarium and also from the two Aspergillus viruses (Fig 4A). This analysis also illustrates a clear delineation between alternaviruses and chrysoviruses.

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Fig 4. Analyses of Aspergillus heteromorphus alternavirus 1 (AheAV1), a member of the recently proposed family Alternaviridae.

(A) Percent identity matrix, generated by Clustal-Omega 2.1, of alternaviruses and members of the Chrysoviridae family. The top half of the matrix, in the blue scale, is the percent identity of the RdRP protein coding sequence (from the RNA1 segment) and the bottom half of the matrix, in the green scale, is the percent identity of the second largest genomic fragment, encoding GP1. For sake of clarity, the 100% identity along the diagonal has been removed and outlined stars are used to note junction point for the row and column of the virus identified. Values ≥20% identity are noted. (B) Density plot of read counts per nucleotide across the viral genomic segments of AheAV1. Graphical depiction of the genome structure, predicted ORFs and motifs for each segment are on top and a heatmap of normalized read counts is below each diagram. Read counts are normalized to a 0 – 1 scale, where the maximum read count for a given genome equals 1. Total reads mapped to the virus genome and average read coverage are noted. (C) Alignment of 5’UTR sequences of the three genomic segments of AheAV1. Conserved nucleotides are highlighted in blue. (D) Alignment of RdRP domains of Alternaviruses found in (A). The eight conserved RdRP motifs of dsRNA polymerases are noted along the top of the alignment and the ADD triad, found only in Alternavirus sequences, is highlighted in green. The number of amino acids not shown in alignment are noted in parentheses. Viruses identified in this study are indicated with a red star. RdRP: RNA-dependent RNA Polymerase; GP1: Gene Product 1.

https://doi.org/10.1371/journal.pone.0219207.g004

A common feature to viral genomes with multiple segments is the presence of a conserved sequence within the 5’ UTRs of each fragment. Sequence conservation was described within the UTRs of the other putative alternaviruses, including AfV [38] and AaV1 [37]. Alignment of the 5’UTR sequences of the three genomic segments of AheAV1 demonstrates sequence conservation (Fig 4C). Alignment of the source RNA-seq reads to the viral genome segments reveals a high amount of total reads, with over four million reads mapped across the three segments (Fig 4B). This represents nearly 25% of the reads used for the analysis (reads that did not align to the fungal genome) and over 5% of the total reads from the sample. In an effort to identify a possible fourth genomic fragment for AheAV1, both the 5’UTR conserved sequence and the high read count were used as criteria to further query the Trinity assembled contigs however no additional contigs were identified.

An alignment of the RdRP protein sequences from the five alternavirus genomes reveals the presence of the expected eight conserved domains found within viral RdRP proteins (Fig 4D). Certain differences are shared among these viruses that are unique when compared to known viral sequences. Specifically, the triad within domain VI has an alanine (ADD) instead of the nearly universally conserved glycine (GDD).

Taken together, a number of criteria identify a new Alternavirus: multiple genome segments, where the largest encodes an RdRP protein, and the presence of an alanine within motif VI of the RdRP protein sequence. Classification of a new species within the genus Alternavirus remains to be established. To date, the distinguishing factors between viruses within this genus are the fungal host species and the number of genomic segments. While the fungal host species and number of genomic segments differ between AheAV1 and AfdsV, the percent identity between the amino acid sequences from RNA1 and RNA2 is high, at 93% and 91% respectively. At this junction, it is not possible to determine if AheAV1 is a new species within the genus Alternavirus.

Totiviridae.

Ten of the viral sequences identified in this study are similar to other known members of the Totiviridae family, nine within the genus Victorivirus and one within the genus Totivirus.

Totivirus.

Hortaea werneckii totivirus 1 (HwTV1), the first viral sequence described for this fungal species to our knowledge (PRJNA356640; [18]), is a member of the Totivirus genus (Fig 2). Species demarcation within Totivirus requires biological characterization of the virus within a host cell; however, isolation of a virus from a distinct host species and less than 50% sequence identity of the RdRP at the protein level is a proxy for identifying a separate species [39]. HwTV1 was identified from a new fungal host, H. werneckii, and shares less than 44% sequence similarity to the most closely related known totiviruses (S1A Fig) thus satisfying the proxy criteria for a new virus species. The RdRP protein sequence of HwTV1 shares the greatest similarity to two totiviruses identified within the fungus Xanthophyllomyces dendrorhous, XdV-L1-A and XdV-L1-B [40], along with the invertebrate virus Wuhan insect virus 26 (WiV26), isolated from RNA-sequencing data of a mixed sample of flea and ants isolated in Hubei, China [41].

A common feature to members of the Totivirus genus is the presence of two partially overlapping open reading frames that produce a fusion protein via a -1 frameshift event with the 5’ ORF encoding the major capsid protein and the 3’ ORF encoding an RdRP. Characteristics of a frameshift site are (1) a heptanucleotide sequence ‘slippery site’ with the sequence X XXY YYZ and (2) a downstream RNA pseudoknot [42]. As expected, the two open reading frames in HwTV1 are frame 3 (CP) and frame 2 (RdRP). A canonical slippery site, G GGU UUU, is present upstream of the ORF1 stop codon at position 1,940 – 1,946 nt, and two predicted pseudoknots are present immediately 3’ of the slippery site (S1B Fig). A conserved domain search within the encoded proteins reveals the presence of the major coat protein (L-A virus) domain in ORF1, in addition to the RdRP domain in ORF2 (S1C Fig) A total of 12,385 reads align to the HwTV1 genome, resulting in an average coverage of 341x, with slightly higher coverage observed across the 5’ half of the genome (S1C Fig).

Victorivirus.

The remaining nine viruses in the Totiviridae family were found within two branches of the Victorivirus genus. The prototype virus of this genus, Helminthosporium victoriae virus 190S (HvV190S) [43], is present within one clade along with Colletotrichum eremochloae totivirus 1 (CeTV1) (PRJNA262412; unpublished), Colletotrichum navitas totivirus 1 (CnTV1) (PRJNA262225; unpublished), and Penicillium digitatum totivirus 2 (PdTV2) (PRJNA254400; [23]). A second clade contains Colletotrichum caudatum totivirus 1 (CcTV1) (PRJNA262373; unpublished), Colletotrichum zoysiae totivirus 1 (CzTV1) (PRJNA262216; unpublished), Penicillium digitatum totivirus 1-KH8 (PdTV1-KH8) (PRJNA254400; [23]), Thelebolus microsporus totivirus 1 (TmTV1) (PRJNA372886; unpublished), Tolypocladium ophioglossoides totivirus 1 (ToTV1) (PRJNA292830; [26]), and Aspergillus homomorphus totivirus 1 (AhoTV1) (PRJNA250984; unpublished) along with other victoriviruses (Fig 2).

Alignment of the RNA-seq reads to the viral genomes reveals a wide range of average coverage values: from 8x to 526x (S2C and S2D Fig). A peak in coverage observed in CcTV1 and CzTV1 immediately precedes an octanucleotide sequence (AGGGUUCC) which has been observed in the 5’UTR of some other victoriviruses including Alternaria arborescens victorivirus 1 (AaVV1) [44] and Epichoë festucae virus 1 (EfV1) [45]. The peak in coverage of CnTV1 immediately precedes an octanucleotide sequence slightly different than previously reported (CGGCCUCC). A similar peak in coverage was not observed for AhoTV1, which also contains the AAGGGUUCC octamer sequence within the 5’UTR, although this viral sequence had the lowest average coverage. The genome structure diagrams drawn above the coverage heat maps highlight features of the viruses that are further described below.

Unlike the frame-shifting observed in totiviruses, the RdRP of Victoriviruses is expressed as a separate protein from the coat protein [46] via a termination-reinitiation strategy [47]. The AUG start codon either overlaps the stop codon of the upstream of ORF1 or slightly precedes it, and is nearly always found in the -1 frame relative to ORF1 [43]. The tetranucleotide sequence, AUGA, is observed for PdTV2, PdTV3, CzTV1, and AhoTV1. For CnTV1, TmTV1, and ToTV1, the last nucleotide of the ORF1 stop codon overlaps the ORF2 start codon, creating the pentamer sequences UGAUG, UAAUG, and UAAUG respectively. In each of these seven instances, there is the expected -1 frameshift between ORF1 and ORF2. For CeTV1 and CcTV1, a two nucleotide spacer is observed between the start codon of ORF2 and the stop codon of ORF1 (AUGCAUGA and AUGAAUAA respectively) that is similar to the sequences observed in SsRV1 and SsRV2 (AUGAAUAA and AUGAGUAA respectively) [48]. Thus ORF2 of CeTV1 and CcTV1, as well as SsRV1 and SsRV2, is found in the +1 frame relative to ORF1.

A characteristic of the coat protein sequences of victoriviruses is an abundance of alanine, glycine, and proline (A,G,P) residues, particularly towards the C-terminal end. An analysis of between the CP protein sequences from victorivirus species and totivirus species illustrates a generally elevated percentage of these three amino acids across the entire CP coding sequence of victoriviruses in comparison to totiviruses, with a notable increase observed in the two bins closest to the C-terminus of the protein (S2B Fig). The same pattern is also observed for all nine proposed victorivirus sequences identified in this study.

These analyses demonstrate that these nine sequences share conserved characteristics found in members of the genus Victorivirus. Criteria for species demarcation within this genus are fungal host species and percent identity of the RdRP and CP amino acid sequences [39], with a cutoff of 60% identity for both. The CP and RdRP sequences of CnTV1, TmTV1, and ToTV1 satisfy both the sequence-based criteria of < 60% identity and unique fungal host criteria to be characterized as new species (S2A Fig).

For a growing number of recently published sequences with strong phylogenetic ties to victoriviruses, the cutoff of 60% identity for the RdRP and/or CP is not satisfied, only the criteria of a differing fungal host (S2A Fig). For five of the six other Victoriviruses identified in this study, a 60% pairwise identity would not distinguish between viral sequences, only the host-based criteria. The sixth virus, PdTV1-KH8, shares 97% (RdRP) and 98% (CP) identity with the already published sequence for Penicillium digitatum virus 1 (PdV1) [49] thus neither the sequence-based nor host-based criteria are met. Notably, PdTV2 and PdTV1-KH8 were both identified from the same data sample, however they share only 33.5% identity. The coincident infection of P. dignitatum KH8 by two victoriviruses is not unprecedented: Sphaeropsis sapinea RNA virus 1 and Sphaeropsis sapinea RNA virus 2 were isolated from the same host and share less than 60% similarity [48].

Clearly a precise sequence-based cut-off to distinguish between mycoviral species has not yet been identified. Until further biological studies to query the host range of these similar viruses can be performed, we would recommend more stringent values for pairwise percent identities for species demarcation within the Victorivirus genus: 80% for CP sequences and 90% for RdRP sequences in addition to the identity of the fungal host. Following this recommendation, the eight viruses, CnTV1, TdTV2, CeTV1, CcTV1, CzTV1, AhoTV1, TmTV1, and ToTV1, would be identified as new viral species within the Victorivius genus. To our knowledge, this is the first mycovirus described for C. navitas, C. eremochloae, C. zoysiae, C. caudatum, Thelebolus microsporus, and Tolypocladium ophioglossoides.

Partitiviridae.

Five genera have been assigned to the family Partitiviridae, along with fifteen viruses unassigned to a genus [50]. Genome structure of this family requires two distinct genome segments, each containing a single ORF: dsRNA1 and dsRNA2. The RdRP is found on dsRNA1 while the CP is found on dsRNA2, and highly conserved sequences can be found in the 5’UTR of both viral genome segments [50]. To date, viruses isolated from fungi have been classified into one of three of the Partitiviridae genera: Alphapartitivirus, Betapartitiviruse, and Gammapartitivirus. In this study, sixteen viruses were identified that clearly group among the known partitiviruses (Fig 2).

Alphapartitivirus.

Five viruses identified in this study phylogenetically group within the Alphapartitivirus genus: Clohesyomyces aquaticus partitivirus 1 (CaPV1) (PRJNA372822; [11]), Grosmannia clavigera partitivirus 1 (GcPV1) (PRJNA184372; [17]), Drechslerella stenobrocha partitivirus 1 (DsPV1) (PRJNA236481[13]), Gaeumannomyces tritici partitivirus 1 and 2 (GtPV1, GtPV2) (PRJNA268052; [16]). To our knowledge, the first three are the first viruses identified in these fungal species while the two partitiviruses in G. tritici are the first partitiviruses from this fungus. As with other members of this genus, the genomic segment encoding the RdRP is approximately 1.9 to 2.0 kb while the genomic segment encoding the CP is between 1.7 and 1.9 kb (Table 1). Alignment of the RNA-seq reads to the viral genome sequences reveals average coverage per nucleotide ranging from 48x to 28,101x (S3B Fig). In general, the two dsRNA segments do not appear to have similar coverage per nucleotide across the segments, and in all but one instance, the dsRNA2 segment has the higher average coverage. For the two viruses isolated from G. tritici, 23x more reads align to GtPV1 than GtPV2 (S3B Fig).

Multiple sequence alignments of the 5’UTR regions of the two RNA genomic sequences for each identified viral genome revealed highly conserved stretches of nucleotides specific to each virus (S3C Fig). As dsRNA satellite sequences have been previously described for the alphapartitivirus Cherry chlorotic rusty spot associated partitivirus [51] the conserved sequences present in dsRNA1 and dsRNA2 were used to query the remaining Trinity contigs, however no putative satellite RNAs were identified.

The criteria for species demarcation within the genus Alphapartitivirus are ≤90% identity in the RdRP protein sequence and ≤80% identity in the CP protein sequence. All five viral genomes identified in this study satisfy these criteria and as such are new species of the genus Alphapartitivirus (S3A Fig).

Betapartitivirus.

Two virus genomes were identified as members of the genus Betapartitivirus: Trichoderma citrinoviride partitivirus 1 (TcPV1) (PRJNA304029; [28]) and Fusarium poae partitivirus 1-2516 (FpPV1-2516) (PRJNA319914; [15]). To our knowledge, this is the first partitivirus identified from T. citrinoviride while FpPV1-2516 is the third instance of a species of betapartitivirus described to date from the fungus F. poae.

The RdRP coding sequence from FpPV1-2516 shares 99.9% and 97.9% identity with FpV1 (F. poae strain A11) and FpV1 (F. poae strain 240374) respectively (S4A Fig). Similarly, the CP sequence of FpPV1-2516 shares 99.8% identity with both of the FpV1-CP sequences. Despite the high sequence similarity observed, these three viruses appear to originate from different locales. FpV1-240374 was published in 2016 following deep RNA-sequencing of the fungal strain F. poae MAFF 240374, a fungus isolated from a Triticum aestivum spikelet in Japan in 1991 [52]. FpV1(strain A11) was identified in 1998 from the strain F. poae A-11 isolated from wheat grain in Hungary [53]. Finally, FpPV1-2516, identified in this study, was found in the RNA-seq data from F. poae strain 2516, a fungus isolated in Belgium from wheat [15]. While FpPV1-2516 is not a unique viral species, it does have a unique genome sequence and unique history from the other two known viruses and as such, has been deposited at Genbank (Table 1). A pairwise percent identity analysis of TcPV1 with other betapartitiviruses reveals that this is a unique sequence since identity values for both the RdRP and CP sequences are below the established thresholds for species demarcation of 90% identity and 80% respectively (S4A Fig) [50].

A sequence alignment of the 5’UTR sequences of the two genomic segments of TcPV1 reveals stretches of highly conserved nucleotides, characteristic of partitiviral genomes (S4C Fig). Additionally, the two genomic segments are both 2.3 kb (Table 1), which is within the described range for betapartitiviruses of 2.1 to 2.4 kb [50]. As observed with the majority of the alphapartitiviruses described above, the average coverage per nucleotide for the genomic segments of FpPV1-2516 and TcPV1 was higher for the dsRNA2 segment that encodes the CP than the RdRP segment (S4B Fig).

Gammapartitivirus.

Five viruses identified in this analysis grouped with members of the genus Gammapartitivirus (Fig 2): Penicillium digitatum partitivirus 1-HSF6 (PdPV1-HSF6) (PRJNA352307; unpublished) and Pseudogymnoascus destructans partitivirus 1-20631 (PdPV1-20631) (PRJNA66121; unpublished) along with Magnaporthe grisea partitivirus 1 (MgPV1) (PRJNA269089; [19]), Thelebolus microsporus partitivirus 1 (TmPV1) (PRJNA372886; unpublished), and Phyllosticta citriasiana partitivirus 1 (PcPV1) (PRJNA250394; unpublished). The latter three viruses are, to our knowledge, the first mycoviruses described for these fungal species and, as described below, satisfy the criteria to be identified as new viral species.

The dsRNA1 genomic segment encoding the RdRP for all five viruses is approximately 1.7 kb while the dsRNA2 genomic segment encoding the CP is between 1.5 and 1.6 bp (Table 1) both within the expected range of other known members of the genus Gammapartitivirus of 1.6 to 1.8 kb for dsRNA1 and 1.4 to 1.6 kb for dsRNA2 [50]. A multiple sequence alignment analysis of the 5’UTR sequences from the two genomic segments of each virus revealed stretches of conserved nucleotides within four of the five viruses; PcPV1 was the exception, however the 5’UTR sequence of dsRNA1 is only 17 nt (S5C Fig). Attempts to extend this sequence by manually querying the RNA-seq data were unsuccessful.

Total reads aligned and average coverage per nucleotide were calculated following alignment of the RNA-seq reads to the viral genome sequences. In general, the average coverage per nucleotide for dsRNA1 is similar to that of dsRNA2 (S5B Fig), a departure from the alignment results for the alphapartitiviruses and betapartitiviruses described above.

A pairwise percent identity analysis compared the RdRP coding sequences and the CP coding sequences for the five viruses to other sequences within the genus Gammapartitivirus (S5A Fig). The current criteria for species demarcation is 90% for the RdRP sequence and 80% for the CP sequence [50]. Genic and protein sequences from PdPV1-20631 share 100% identity with the virus PdPVpa from Pseudogymnoascus destructans 80251, although there are differences observed in the length of the 5’ and 3’ UTR sequences. The RdRP and CP protein sequences of PdPV1-HSF6, isolated from Penicillium digitatum, shares 95% identity with the proteins from Penicillium stoloniferum virus S (PsVS). Due to the high sequence similarity and the relative close relationship between the two fungal hosts from the Penicillium genus, this virus sequence does not meet the criteria to qualify as a new viral species. Nonetheless, these viral sequences have been deposited in GenBank due to the differences within the nucleotide sequences from published viral genomes and the identity of the host fungal strain. The RdRP and CP protein sequences from MgPV1, PcPV1, and TmPV1 do not share significant identity with known gammaparitivirus sequences and are isolated from new fungal hosts. As such, these three viruses are characterized as new viral species, belonging to the genus Gammapartitivirus.

New genus: Epsilonpartitivirus.

Two viral genomes, Colletotrichum eremochloae partitivirus 1 (CePV1) (PRJNA262412; unpublished) and Penicillium aurantiogriseum partiti-like virus (PaPlV) (PRJNA369604; [12]), were identified that are phylogenetically related to an emerging group of partitiviruses that are separate from, but related to, the genus Gammapartitivirus (Fig 2); this group has been tentatively named Epsilonpartitivirus [12]. Identification of the PaPlV sequences was expected as this sample was RNA-seq data from Cryphonectria parasitica infected with PaPlV, previously isolated from P. aurantiogriseum by the same researchers [12]. To distinguish this viral genome from that isolated from the original host, the virus has been designated PaPV1-cp. Also, the slight modification of the virus name, from partiti-like to partitivirus, reflects a recent improved understanding that this is a partitivirus due to the identification of a RNA segment encoding a putative CP [12].

Members of this proposed genus have been identified from both fungal and invertebrate hosts, although currently the viral sequences form two sister clades separated by kingdom (Fig 2). Genome sizes of the dsRNA segments of putative epsilonpartitiviruses are from 1.7 to 1.9 kb for dsRNA1 and 1.5 to 1.8 kb for dsRNA2 (Table 1). Using a pairwise comparison analysis, a value of 85% identity for the RdRP protein sequence and 75% for the CP sequence would currently be appropriately stringent for species demarcation of viruses isolated from different fungal hosts (Fig 5A).

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Fig 5. Analyses of identified viral genomes belonging to the two proposed genera, Epsilonpartitivirus and Zetapartitivirus, within the Partitiviridae family.

(A) Percent identity matrix, generated by Clustal-Omega 2.1, between members of the two new groups and gammapartitiviruses, the most closely related, ICTV-recognized genus. The top half of the matrix, in the blue scale, is the percent identity of RdRP protein sequences and the bottom half of the matrix, in the green scale, is the percent identity of CP protein sequences. For sake of clarity, the 100% identity along the diagonal has been removed and outlined stars are used to note junction point for the row and column of the viruses identified. Two cutoff values were applied and as such only percent identities ≥29% among RdRP sequences and ≥17% among CP sequences are indicated. Virus names in italics refer to viruses isolated from insect, not fungal, hosts. Bolded virus names are member species recognized by ICTV. (B and E) Density plot of read counts per nucleotide across the viral genome segments for the two epsilonparitiviruses (B) and two zetapartitiviruses (E). Graphical depiction of the genome structure, predicted ORFs, and RdRP motif are along the top and a heatmap of normalized read counts is below. Read counts are normalized to a 0 – 1 scale, where the maximum read count for a given genome equals 1; heatmap scale found in (E) applies also to (B). Total reads mapped to the virus genome and average read coverage are noted for each virus. (C) Alignment of the 5’ UTR sequences from the two genomic segments of the new species of epsilonpartitivirus, CePV1. Conserved nucleotides are highlighted in blue and numbers in parentheses indicate nucleotides not shown. (D) Alignment of the six conserved motifs within the RdRP protein sequence of all four proposed zetapartitiviruses. Amino acids highlighted in blue are only common among zetapartitiviruses while residues in grey are shared with gammapartitiviruses. (F and G) The 5’UTR (F) and 3’UTR (G) sequences from both genomic segments for all four zetapartitiviruses were aligned using MAFFT and a graphical representation of the most highly conserved sequences found is depicted as a sequence logo, generated using WebLogo version 3, whereby the height of a nucleotide indicates the sequence conservation at that position (http://weblogo.threeplusone.com/). A consensus sequence for each UTR is indicated below in grey. Red stars indicate viruses identified in this study. PV: partitivirus; RdRP: RNA-dependent RNA Polymerase; CP: Coat Protein.

https://doi.org/10.1371/journal.pone.0219207.g005

Observed here, sequences isolated from the PaPlV1-infected C. parasitica sequencing sample contain a number of SNPs in comparison to the source PaPlV sequence; these changes may reflect an adaptive response by the virus to a new fungal host [12]. A positive effect on fungal growth is observed whereby the virus confers a positive effect on fungal growth of C. parasitica in the presence of 2% salts that has not yet been confirmed in the original host P. aurantiogriseum. Read coverage along the viral genome segments isolated from C. parasitica is on average low, 18x for dsRNA1 and 5x for dsRNA1 (Fig 5B), similar to the published report [12].

Both of the viral genome segments of CePV1 share the greatest percent identity with PaPlV1, but below the threshold suggested above, at 84% and 73% for the RdRP and CP respectively (Fig 5A). Similar to other partitiviruses, a multiple sequence alignment of the 5’UTR sequences of the two genome segments of CePV1 reveals regions of sequence conservation (Fig 5C).

Initially a mycoviral sequence was isolated from a Colletotrichum navitas sequencing sample (PRJNA262223; unpublished); however, other than small variances within the 5’ and 3’ UTRs, the nucleotide sequence is identical to that of CePV1. It is possible that two different, closely related, fungi are infected with the same virus, however as noted above with respect to the PaPlV1-infected C. parasitica, SNPs can quickly appear within a viral genome sequence when in a new host. Therefore further analyses were performed to determine the likelihood of a CePV1-like sequence hosted by C. navitas. First it was determined that, after controlling for the unaligned reads pool size, 2.6x more viral reads were present in the C. eremochloae sample than the C. navitas sample. Second, to identify possible cross-contamination between the two species within the sequencing data, non-virus-derived reads were reciprocally aligned to the other fungal genome. Thus reads from C. navitas were aligned to the C. eremochloae genome, and vice versa. Here we observed that 5% of reads from C. eremochloae aligned to the C. navitas genome while 65% of reads in the C. navitas pool aligned to the C. eremochloae genome. Evaluating both of these results, we propose that the likely host of this viral sequence is C. eremochloae and the identification of the same sequence within the C. navitas sample is likely due to cross contamination from the C. eremochloae sample. Despite the possible ambiguity in fungal host for this viral sequence, we recommend its inclusion in the recently suggested genus Epsilonpartitivirus as a new viral species.

New genus: Zetapartitivirus.

The final two viruses identified within the Partitiviridae branch of the phylogenetic tree (Fig 2) are Penicillium brasilianum partitivirus 1 (PbPV1) (PRJEB7514; [22]) and Delitschia confertaspora partitivirus 1 (DcPV1) (PRJNA250740; unpublished). Both viruses, the first to be described in these fungal species, phylogenetically group with Alternaria alternata partitivirus 1 (AaPV1) [54] and Botryosphaeria dothidea partitivirus 1 (BdPV1) [55], a distinct clade within the Partitiviridae that is related to, but separate from, the genus Gammapartitivirus (Fig 2).

These four viruses share many sequence-based similarities. A 13-nt highly conserved sequence (TCACAAYATYAYA) is present in the 5’UTR sequence of both dsRNA genome segments from all four viruses (Fig 5F). Additionally, a 10-nt conserved sequence (TYTCATTARR) is found within the 3’UTR sequence of both dsRNA segments of all four viruses (Fig 5G). While these four viruses contain the six conserved motifs common to RdRP proteins from other members of the Partitiviridae, including the GDD triad in motif VI, there are distinct differences that are shared that distinguish these viruses from the existing genera (Fig 5D). Further, the range of lengths for the RdRP protein, from 569 to 572 aa, and CP protein, from 453 to 501 aa (Table 1), are unique ranges among the currently described genera [50]. A pairwise percent identity analysis of the RdRP and CP sequences from the four viruses suggests a threshold for species demarcation at 90% for the RdRP sequence and 80% for the CP sequence, similar to other genera within the family (Fig 5A).

Alignment of the RNA-seq reads to the viral genomes demonstrates that dsRNA2 has a higher average coverage per nucleotide for both viruses, with approximately 2x more coverage observed between the segments of PbPV1 and 7x between the segments of DcPV1 (Fig 5E).

We would propose that DcPV1 and PbPV1, along with AaPV1 and BdPV1 belong to a new genus within the family Partitivirus, called Zetapartitivirus. The genomic segment and protein sequence lengths are consistent and unique within the Partitivirus family, and the RdRP protein sequences form a distinct clade that is separate from the closest genus, Gammapartitivirus (Fig 2).

Chrysoviridae.

To date, nine species have been classified as belonging to the Chrysoviridae family of dsRNA viruses that infect either ascomycota or basidiomycota fungi [56]. Members of this family have multipartite genomes, consisting of three to five segments. Here we identify two viral genomes with sequence similarity to Chrysoviruses: Beauveria bassiana chrysovirus 1 (BbCV1) (PRJNA306902; [10]) and Penicillium raistrickii chrysovirus 1 (PrCV1) (PRJNA250734; unpublished) (Fig 2).

Phylogenetically, chrysoviruses are grouped into two distinct clusters, where cluster 1 contains members with either three or four segmented genomes, while cluster 2 contains viruses with either four or five segments [56]. The two viral sequences identified in this study, BbCV1 and PrCV1, belong with cluster 1 chrysoviruses; both genomes are comprised of four genomic segments and the RdRP sequences phylogenetically group with the ICTV-recognized members within cluster 1, Isaria javanica chrysovirus 1 (IjCV1) [57] and Penicillium chrysogenum virus (PcCV1) [58] (Fig 2). Further, as detailed below, we recommend that these are both new viral species within the genus Chrysovirus.

To our knowledge, these are the first chrysoviruses identified for both of these fungal hosts, and indeed the first virus from P. raistrickii described, thus satisfying the host-based criteria for species demarcation. Following identification of the RdRP-encoding contig, the identity of the other three segments was determined via a BLASTP search using protein sequences of known chrysoviruses against the predicted protein sequences of the Trinity contigs. The combined length of dsRNAs for both BbCV1 and PrCV1 are 12.46 and 12.56 kb respectively (Table 1), within the observed range of other chrysoviruses (11.5 to 12.8 kb) [56]. In general, chrysovirus genome segments are numbered by size, however segment 4 of PrCV1 is slightly longer than segment 3 (Table 1), similar to segment 4 of Amasya cherry disease associated chrysovirus (AcdaCV) which is also larger than its respective segment 3 [59]. A BLASTP analysis confirms that segment 4 of PrCV1 best matches the segment 4 of Penicillium chrysogenum virus and thus is numbered accordingly. An alignment of the RNA-seq reads to the viral genome segments reveals uniform average coverage per nucleotide for the four genomic segment of PrCV1 (from 15x to 32x) and slightly more variable average coverage of BbCV1 segments, ranging from 10x to 72x (S6D Fig). The dsRNA genomic segment lengths therefore also satisfy the criteria for species demarcation within the genus Chrysovirus.

The 5’UTR of chrysoviruses is generally between 140 and 400 nt in length and contains two regions of high sequence conservation, Box 1 and Box 2 [56], although a number of recently characterized viruses have shorter 5’UTRs, including Cryphonectria nitschkei chrysovirus 1 (CnCV1) which has 82 nt 5’UTR for segment 1 [60] and AcdaCV which has 5’UTR lengths of 87, 96, 95, and 106 bp [59]. With the exception of the 5’UTR of BbCV1-segment 1 (106 nt), the 5’UTR of all segments discovered for BbCV1 and PrCV1 fall within the published range (S6B and S6C Fig). A highly conserved region is present within the area corresponding to “Box 1” within the 5’UTR of the four genome segments of BbCV1 (S6B Fig, top panel) and the expected “CAA” sequence is also observed in all sequences (S6B Fig, bottom panel). Similarly both conserved regions are observed within the 5’UTR of PrCV1 (S6C Fig). Thus the size and composition of the 5’UTR sequences of BbCV1 and PrCV1 also satisfy the criteria for chrysovirus classification.

The final criteria considered for species demarcation is the percent identity of the RdRP and CP protein sequences relative to other members within the genus, where the thresholds are 70% and 53% respectively [56]. BbCV1 RdRP and CP share 83% and 69% identity with the related proteins of IjCV1, while PrCV1 RdRP and CP protein sequences share 88% and 79% identity with those of Penicillium chrysogenum virus (PcCV1) (S6A Fig). Since BbCV1 was isolated from a new fungal host, we propose classifying it as a new species of chrysovirus, while the relative relatedness between the hosts P. raistrickii and P. chrysogenum suggest that PrCV1 is not a new species of chrysovirus. Further identification of additional viruses plus biological characterization of host specificity will provide the needed insights to properly classify viruses within this family.

Endornaviridae.

Members of the Endornaviridae family of viruses have long, single segment, dsRNA genomes, generally encoding a single polyprotein. In addition to the RdRP domain and helicase domain (Viral helicase 1 or DEADx helicase) found in all endornaviruses, viral genomes may also encode a viral methyltransferase domain and/or a glycosyltransferase domain. Recently a two-genus split was proposed [61] where the common characteristics of clade I (Alphaendornavirus) include a plant-host, larger genomes (ranging from 13.8 to 17.5 kb), the presence of a UGT domain and a site-specific nick towards the 5’end of the genome sequence [62], while members of clade II (Betaendornavirus) have fungal-hosts and generally have genome sizes ranging from 9.5 to 11.4 kb, encode a viral methyltransferase domain but not a UGT domain, and lack the site-specific nick [61].

Here four new virus sequences were identified that group with known endornaviruses (Fig 6A), Gyromitra esculenta endornavirus 1 (GeEV1) (PRJNA372840; unpublished) and Morchella importuna endornavirus 1, 2, and 3 (MiEV1, MiEV2, MiEV3) (PRJNA372858; unpublished). These are the first viruses, to our knowledge, identified from both G. esculenta and the morel mushroom, M. importuna.

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Fig 6. Phylograms for three different families of (+)ssRNA viruses.

RdRP motifs from the Endornaviridae family (A), Narnaviridae and Ourmiaviruses (B), and Hypoviridae (C) family were used to construct maximum likelihood phylogenetic trees generated with RaxML, using the LG+G+I amino acid substitution model. Full names of viruses and sequences can be found in S2 Table, including the outgroup, Potato Virus Y (PVY). Bolded names are ICTV-recognized member species while italicized names are related, but unclassified viruses. Full names and accession numbers for protein sequences used in the alignment are in S2 Table. Scale bar on the phylogenetic tree represents 1.0 amino acid substitutions per site and numbers at the nodes indicate bootstrap support over 70% (1000 replicates). Viruses identified in this study are noted with a red star.

https://doi.org/10.1371/journal.pone.0219207.g006

Alphaendornavirus.

A phylogenetic analysis of the RdRP motif from related endornaviruses and the viruses identified in this study reveals MiEV2 and GeEV1 form a well-supported clade with other fungal endornaviruses, including Ceratobasidium endornavirus A (CEVA) [63], and Rhizoctonia solani endornavirus 2 (RsEV2) [64], that is related to, but separate from alphaendornaviruses isolated from plant hosts, including the type species Oryza sativa endornavirus (OsEV) [65] (Fig 6A). The genome sizes of MiEV2 and GeEV1 are 15.3 and 14.5 kb respectively, well within the observed range of other Alphaendornaviruses (Table 1). Additionally, a conserved domain search within the protein coding sequences determined both viruses encode a viral helicase and an RdRP domain, but not a viral methyltransferase domain, nor a UGT domain (S7B Fig). This combination of protein domains appears to be shared among mycoviruses and not plant-hosted endornaviruses.

GeEV1 has a predicted AUG at position 5, resulting in a shorter 5’ UTR sequence than observed in other endornaviruses. The predicted start codon for MiEV2 however is at position 476, resulting in an unexpectedly long 5’UTR; as the entire sequence upstream of this codon is in-frame with the coding sequence, it is possible that this genome sequence is partially truncated and is missing the true AUG sequence. As manual analysis of the RNA-seq data did not result in an extension of the 5’ region of MiEV2, we recommend further analysis of the biological material to confirm the 5’ region of this virus. An alignment of RNA-seq reads to the two viral genomes reveals uniform coverage across both genome sequences, with a total of 4977 reads and 8171 reads respectively (S7B Fig).

The two criteria for species demarcation within the genus Alphaendornavirus are difference in host and an overall nucleotide sequence identity below 75%. Both viruses described here satisfy these criteria. Indeed, nucleotide identity among Endornaviruses is generally below 50%. An analysis of the protein sequences of the RdRP motif and the viral helicase motif reveal similar results, as sequences are between 39% and 67% identical for the RdRP sequences and 26% to 51% identical among the helicase sequences (S7A Fig). Together these analyses support the recognition of GeEV1 and MiEV2 as new species within the Endornaviridae family, and the genus Alphaendornavirus in particular.

Betaendornavirus.

The two other endornaviruses identified were MiEV1 and MiEV3, which group with the betaendornaviruses. MiEV3, in particular, forms a strong phylogenetic relationship with other characterized betaendornaviruses, while MiEV1 is the outermost member of this clade (Fig 6A). Unexpectedly, the genome sizes of both viruses are more than 25% larger than other betaendornaviruses; MiEV1 has a genome size of 16.5 kb while MiEV3 has a genome size of 14.3 kb (Table 1). However, the motifs within the protein sequences are as expected, as both MiEV1 and MiEV3 contain a viral methyltransferase domain, helicase domain(s), and an RdRP domain (S7D Fig). Specifically MiEV1 and MiEV3 both encode a DEADx helicase domain and a viral helicase 1 domain, similar to the type species Sclerotinia sclerotiorum endornavirus-1 (SsEV1) (NC_021706.1). Alignment of the RNA-seq reads to the viral genomes reveals an average coverage per nucleotide of 29x and 51x for MiEV1 and MiEV3 respectively (S7D Fig), and coverage appears uniform across both genomes.

Similar to the alphaendornaviruses, nucleotide identity is generally below 50%, with the exception of SsEV1 and Sclerotinia sclerotiorum endornavirus 2 strain DIL-1 (SsEV2) which share 82% nucleotide identity. Further analysis of the protein sequences from the RdRP motifs and viral methyltransferase motifs determined that a percent identity threshold for either motif of 75% would be appropriate for species demarcation (S7C Fig). In particular, both MiEV1 and MiEV3 are well below both a nucleotide-based or a protein-motif based threshold. Thus, despite the same fungal host, the uniqueness of the viral genomes sequence for both viruses suggests that both MiEV1 and MiEV3 are new species of the family Endornaviridae, and belong within the genus Betaendornavirus.

Tobamo-like virus.

A virus sequence was identified in an Acidomyces richmondensis sample (PRJNA250470; unpublished) with a single segment genome sequence, 10,291 nt in length, that encoded four predicted ORFs (Table 1). The gene product from ORF2 contains the RdRP domain (Fig 7B) and a BLAST-P search of the ‘nr’ database with this sequence identified two related, full length mycoviruses, Macrophomina phaseolina tobamo-like virus 1 (MpTlV1) [5] and Podosphaera prunicola tobamo-like virus 1 (PpTlV1) [66]. As such, this newly identified virus has been named Acidomyces richmondensis tobamo-like virus 1 (ArTlV1).

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Fig 7. Analyses of Acidomyces richmondensis tobamo-like virus 1 (ArTlV1) and related viruses.

(A) Phylogram of the RdRP-motif for ArTlV1, related mycoviruses, and select members of the genus Tobamovirus and family Bromoviridae; sequence from Rubella Virus (RV) serves as the outgroup. Bolded names are ICTV-recognized member species. Full names and accession numbers for protein sequences used are in S2 Table. Numbers at the nodes indicate bootstrap support values ≥65% (1000 replicates). Scale bar on the phylogenetic tree represents 1.0 amino acid substitutions per site. Diagrammatic genome structures are to the right of the phylogram. (B) Density plot of read counts per nucleotide across the ArTlV1 viral genome. Graphical depiction of the genome structure, predicted ORFs, and encoded motifs are along the top and a heatmap of normalized read counts is on the bottom. Reads are normalized to a 0–1 scale, where the maximum read count equals 1. Total reads mapped to the virus genome and average read coverage are noted. (C) Percent identity matrix, generated by Clustal-Omega 2.1. The top half of the matrix, in the blue scale, is the percent identity of the RdRP motif sequence while and the bottom half of the matrix, in the green scale, is the percent identity of the residues from Viral Helicase motif. For sake of clarity, the 100% identity along the diagonal has been removed and outlined stars are used to note junction point for the row and column of ArTlV1. Cutoff values of 44% and 40% were applied to the RdRP and Helicase matrices respectively. Viruses identified in this study are noted with a red star. VMt: Viral Methyltransferase; VHel: Viral Helicase; RdRP: RNA-dependent RNA Polymerase; DH: DEXDc Helicase; ORF: Open Reading Frame.

https://doi.org/10.1371/journal.pone.0219207.g007

For all three mycoviral sequences the first and largest ORF contains a viral methyltransferase and viral helicase domain and ORF2 encodes the RdRP domain (Fig 7B). ORF3 and ORF4 in MpTlV1 have been putatively identified as encoding a movement protein and coat protein respectively [5].

A phylogenetic analysis of the RdRP motif from the three mycoviruses compared to select members of the families Virgaviridae, Bromoviridae and genus Idaeovirus demonstrated that ArTlV1 is more closely related to the two previously identified mycoviruses than known plant viruses (Fig 7A). Further, the group of mycoviruses form a sister clade to the Virgaviridae viruses. Of the viruses currently characterized from this family, only members of the genus Tobamovirus have a non-segmented genome similar to ArTlV1 and relatives; other members have multipartite genomes [67].

To further characterize this emerging group of mycoviruses, a percent identity analysis was undertaken, comparing the amino acid sequences from the RdRP and viral helicase motifs to members of the Virgaviridae family. The RdRP motif sequences among the mycoviruses share 56 to 63% identity with one another, but less than 44% identity with Virgaviridae viruses (Fig 7C). Further, sequences from the viral helicase domain were 59 to 62% identical among mycoviruses, and less than 40% shared identity outside of this clade. Due to these values, it appears that all three viruses are unique viral sequences.

Finally, the RNA-seq data was aligned to the ArTlV1 genome sequence, which revealed uniform coverage along the length of the genome, averaging 1,167 reads per nucleotide (Fig 7B).

To date all viruses of the Virgaviridae family infect a plant host, although many members are transmitted by nematodes or plasmodiophorids, a group of parasitic microscopic organisms unrelated to fungi or oomycetes [67]. Tobacco mosaic virus (TMV), the best characterized Tobamovirus, can infect, replicate, and persist in the ascomycota fungi Colletotrichum acutatum [68]. Further, a virus from the Bromoviridae family, Cucumber mosaic virus (CMV), was shown to naturally infect the phytopathogenic fungi Rhizoctonia solani and as well as replicate within Valsa mali but not C. parasitica, and F. graminearum [69]. Due to the close contact between phytopathogenic fungi and plants a change in host range of a tobamo-like virus is plausible. Interestingly, the A. richomondensis strain analyzed in this study was isolated from acid mine drainage biofilms [70] and is not known to be phytopathogenic. Acid mine drainage biofilms are complex communities comprised of different fungi along with bacteria, archaea and bacteriophages [71,72]. The role of the mycovirus identified in this study in such an extreme and heterogeneous environment remains to be determined.

Ambiguiviridae.

Four viral genome sequences were identified that share similarity to unclassified (+)ssRNA mycoviruses: Trichoderma harzianum ssRNA virus 1 (ThAV1) (PRJNA216008; [29]), Setosphaeria turcica ssRNA virus 1 (StAV1) (PRJNA250530; unpublished), Verticillium longisporum ssRNA virus 1 (VlAV1) (PRJNA308558; unpublished), and Periconia macrospinosa ssRNA virus 1 (PmAV1) (PRJNA262386; [24]). To our knowledge, this is the first viral sequence described for the later three fungal species, and the first ssRNA virus isolated from T. harzianum.

Six other viruses appear related to these four newly identified viruses: Diaporthe RNA virus (DRV), Magnaporthe oryzae RNA virus (MoRV), Soybean leaf-associated ssRNA virus 1 and 2 (SlaRV1, SlaRV2), Verticillium dahliae RNA virus (VdRV), and Sclerotinia sclerotiorum umbra-like virus 1 (SsUlV1) [5,6,73–75]. The RdRP protein sequence of this group of mycoviruses shares some sequence similarity to RdRPs of recently described (+)ssRNA viruses from insects, as well as members of the plant-infecting family Tombusviridae. Phylogenetic analysis demonstrates the fungal viruses form a well-supported clade that is separate from both the insect and plant viruses (Fig 8A). A pairwise identity analysis of the RdRP motif region from the viruses analyzed in the phylogenetic tree reveals a range of identities between the fungal viruses, from 36% to 63%, while only sharing 25% to 33% identity with the insect viruses and 26% to 32% identity with the members of the Tombusviridae (Fig 8D). A similar analysis using the ORF1 protein coding sequence, from only the mycoviral genomes, demonstrates 18 to 38% identity among sequences (Fig 8D).

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Fig 8. Analyses of four newly identified (+)ssRNA viral genomes and viruses belonging to the proposed family Ambiguiviridae.

(A) Phylogram of the RdRP-motif for four new viruses and related viruses along with select members of the family Tombusviridae and members of an unnamed group of (+)ssRNA insect viruses; sequence from Tobacco Mosaic Virus (TMV) serves as the outgroup. Bolded names are ICTV-recognized member species while italicized names are recognized, but unclassified viruses. Full names and accession numbers for protein sequences used are in S2 Table. Numbers at the nodes indicate bootstrap support values ≥60% (1000 replicates). Scale bar on the phylogenetic tree represents 1.0 amino acid substitutions per site. Diagrammatic genome structures and predicted ORFs for each clade of viruses is to the right of the phylogram. (B) Density plot of read counts per nucleotide across four viral genomes. Graphical depiction of the genome structure, predicted ORFs, encoded motifs and the amber stop codon (TAG) for each genome are along the top and a heatmap of normalized read counts is on the bottom. Reads are normalized to a 0–1 scale, where the maximum read count for a given genome equals 1. Total reads mapped to the virus genome and average read coverage are noted. (C) Multiple sequence alignment of the RdRP motifs of members of the Ambiguiviridae family. Motifs A-E found in (+)ssRNA viral RdRP sequences are indicated along the top of the alignment. Conserved residues that differ from canonical (+)ssRNA RdRP sequences, including the unusual GDN triad found in Motif C, are highlighted in blue. Numbers in parentheses are residues not shown. (D) Percent identity matrix, generated by Clustal-Omega 2.1, of mycoviruses that comprise the proposed family Ambiguiviridae. The top half of the matrix, in the blue scale, is the percent identity of the RdRP motif sequence and the bottom half of the matrix, in the green scale, is the percent identity of the ORF1 amino acid sequence. For sake of clarity, the 100% identity along the diagonal has been removed and outlined stars are used to note junction point for the row and column of the viruses identified. All RdRP motif sequences share ≥35% identity and ORF1 protein sequences share ≥15% identity. Viruses identified in this study are noted with a red star. RdRP: RNA-dependent RNA Polymerase; ORF: Open Reading Frame; HP: Hypothetical Protein.

https://doi.org/10.1371/journal.pone.0219207.g008

Together these ten fungal viruses are characterized by a single segment genome, 2.6 kb to 4.5 kb in length, that encodes two ORFs, where the second ORF contains the RdRP domain (Table 1). The amber stop codon ‘UAG’, found at the end of the first ORF, has been suggested to result in readthrough to produce a fusion protein with the downstream RdRP [75]. All members of this group of viruses contain the amber stop codon, and both ORF1 and ORF2 are found in the same frame, a requirement for readthrough. Additionally, all 10 viruses share a number of key differences in residues commonly conserved within (+)ssRNA viruses. For instance, the second aspartic acid residue in motif A, a feature of (+)ssRNA viruses [76], is instead a glutamic acid (Fig 8C). Additionally, these 10 viruses have a GDN triad in motif C versus the canonical GDD found in (+)ssRNA viruses (Fig 8C). This triad has previously been observed within some negative-strand (-)ssRNA viruses [76], although within (+)ssRNA viruses, modification of the GDD to GDN has repeatedly demonstrated a detrimental effect on enzymatic activity [77–79]. Finally, within motif D, a positively-charged lysine, common to nearly all RdRP sequences [76,80], is a nonpolar proline or valine residue (Fig 8C).

The 5’UTRs range in size from 283 bp to 889 bp in length for all the mycoviruses in this group, with the exception of VdRV1 and VlAV1, the two viruses isolated from species of Verticillium; these 5’UTRs are 11 nt and 54 nt respectively. Similarly, the 3’UTR sequences range in length from 372 bp to 1279 bp, while those of VdRV1 and VlAV1 are 182 nt and 29 nt respectively (Fig 8C). Average coverage per nucleotide across the viral genomes of the four identified viruses is fairly uniform across the genome with ranges from 15x for VlAV1 to 734x for PmAV1 (Fig 8B).

The characteristics of these four identified viruses and their closest mycoviral relatives supports the creation of a new family of viruses within the (+)ssRNA class. Initially the host of DRV was identified as D. ambigua, although subsequent analyses determined the host was instead D. perjuncta [81]; the name has been updated by ICTV, but remains uncorrected in other databases. We propose the establishment of the family Ambiguiviridae that contains DRV and other recently discovered viruses related to it, including the four described in this study. The proposed name references the unusual nature of the GDN triad within the RdRP motif of these (+)ssRNA viruses and the ambiguity related to the host of DRV, the first virus described for this group.

Yadokariviridae.

Samples Aspergillus homomorphus (PRJNA250984; unpublished) and Penicillium digitatum (PRJNA352307; unpublished), which yielded a totivirus (AhoTV1) and partitivirus (PdPV1) respectively, each also contained a second, unrelated contig containing an RdRP motif. BLAST-X analysis of these nucleotide sequences determined they were both related to a group of unclassified, (+)ssRNA viruses, that includes four other mycoviruses. As described below, these two viruses are new members of a recently proposed family, Yadokariviridae [82].

Aspergillus homomorphus yadokarivirus 1 (AhoYV1) and Penicillium digitatum yadokarivirus 1 (PdYV1), along with other members of this group, are characterized by having single segment genomes that encode a single predicted ORF, the RdRP (Table 1). Genome sizes range from 3,144 nt (PdYV1) to 5,089 (Yado-kari virus 1, YkV1). The length of the RdRP protein sequence is predicted to range from 962 aa for Aspergillus foetidus slow virus 2 (AfSV2) [83] to 1,430 aa for YkV1 [84]. The lengths of the 5’UTRs are all greater than 500 nt, except for PdYV1. Attempts to extend the sequence of the 5’UTR of PdYV1 using the RNA-seq data were not successful; however, as the average coverage per nucleotide for PdYV1 was low, just 23x (Fig 9C), additional experiments with the fungal source material may be fruitful. Lengths of the 3’UTRs for all six viruses are also largely similar, between 200 to 400 nt, except for YkV1 which has a 3’UTR of 1,223 nt. A genome segment belonging to a Yadokarivirus that encodes a coat protein has not been identified. Alignment of the RNA-seq reads to the genomes of AhoYV1 and PdYV1 reveals an average coverage per nucleotide of 151x and 23x respectively (Fig 9B and 9C).

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Fig 9. Analysis of Aspergillus homomorphus yadokarivirus 1 (AhoYV1) and Penicillium digitatum yadokarivirus 1 (PdYV1) viral genome sequences, members of the proposed family Yadokariviridae.

(A) A phylogram of the RdRP coding sequence from members of the Yadokariviridae family and select (+)ssRNA viruses from the families Caliciviridae, Fusariviridae, and Hypoviridae; sequence from Potato Virus Y (PVY) serves as the outgroup. Virus names have been abbreviated; full names and accession numbers for protein sequences used in the alignment are in S2 Table. Phylogenetic tree was generated with RAxML, using the LG+G+I amino acid substitution model. Scale bar represents 1.0 amino acid substitutions per site and numbers at the nodes indicate bootstrap support over 70% (1000 replicates). To the right of the phylogram are representative diagrams of the genome structures for each family. (B and C) Density plot of read counts per nucleotide across the viral genome for (B) AhoYV1 and (C) PdYV1. Graphical depiction of the genome structure, the predicted single ORF and the RdRP motif are along the top and a heatmap of normalized read counts is on the bottom. Read counts are normalized to a 0–1 scale, where the maximum read count for a given genome equals 1. Legend for (C) is the same as the legend found in (B). Total reads mapped to the virus genome and average read coverage are noted for each virus. (D) Percent identity matrix generated by Clustal-Omega 2.1 using the RdRP motif sequences from the viruses analyzed in (A). For the sake of clarity, only the top half of the matrix is included and the 100% values along the diagonal have been removed. Percent identities greater than 25% are indicated. Viruses identified in this study are noted with a red star. RdRP: RNA-dependent RNA Polymerase; ORF: Open Reading Frame.

https://doi.org/10.1371/journal.pone.0219207.g009

A phylogenetic analysis of the RdRP protein sequence reveals a well-supported clade that is most closely related to the Caliciviridae and separate from other known mycoviral families including Hypoviridae and fusariviruses (Fig 9A). AhoYV1 in particular is most closely related to AfSV2; both viruses were isolated from a member of the genus Aspergillus [83].

As percent identity between virus RdRP sequences is a common criteria for defining new groups of viruses, the RdRP motif sequences from the viruses in Fig 9A were analyzed to identify a potential threshold value. Indeed a threshold between 26 to 35% identity easily classifies these viruses together, but separate from other known (+)ssRNA viruses (Fig 9D). An additional threshold of 95% would to distinguish each sequence evaluated here as a unique viral species; however, as the pairwise percent identity between AhoYV1 and AfSV2 of 94% is somewhat higher than is generally observed for unique species, a lower threshold may be more appropriate. Identification of additional sequences belonging to this group of viruses will further refine these sequence-based characteristics.

Another feature common to these viruses is the presence of at least one additional virus within the fungal host. Co-incident infection is not an uncommon occurrence in fungi; however, it has been demonstrated for two yadokariviruses, AfSV2 and YkV1, that isolated virus particles contain a second virus, related to Totiviruses [83,84]. Further, Zhang and colleagues demonstrated that YkV1 is trans-encapsidated with the capsid protein from the other virus, YnV1 [84]. As caliciviruses and totiviruses share a distant but strongly supported ancestry following diversification of the picorna-like superfamily [85], and both AfSV2 and YkV1 are present within virions encapsidated by totivirus capsid proteins, it is intriguing to speculate about the mechanisms at play, and the requirements for such an interaction. A totivirus was also identified in the A. homomorphus sample (AhoTV1, discussed above), however only a partitivirus (PdPV1-HSF6) was identified along with PdYV1. For the remaining two members of this group of viruses, it is not possible to know the identity of the putative coat-protein donor, as PaFlV1 was found to co-infect P. aurantiogriseum along with a fusarivirus, totivirus, unclassified virus, and partitivirus [4] and FpMyV2 was identified in a fungal strain that contained 13 other viruses from a variety of families [52].

Further study of the fungi that host a member of this group of viruses will answer questions regarding the relationship between the capsid-less virus and the capsid-donor, and may provide an interesting system for studying the evolution of a capsid-less virus. Due to the distinct phylogenetic relationship between these six identified and known (+)ssRNA viruses, along with the unique lifestyle, we support the recommendations to create a new virus family, named Yadokariviridae after YkV1, where yadokari, is the Japanese word for hermit crab [82,86].

Narnaviridae and Ourmia-like virus.

Members of the family Narnaviridae have single molecule, positive-strand RNA genomes ranging in length from 2.3 to 2.9 kb and encode an RdRP as the only polypeptide [87]. There are currently two genera within this family: Narnavirus and Mitovirus, where mitoviruses are characterized as viruses that replicate within the mitochondria of filamentous fungal hosts [87]. Indeed, a non-interrupted ORF is only predicted in these viruses when using the mitochondrial codon usage where tryptophan is encoded by UGA [88]. Here we describe five new viruses with similarity to members of the genus Mitovirus from RNA-seq datasets from four fungal hosts: Setosphaeria turcica mitovirus 1 (StMV1) (PRJNA250530; unpublished), Colletotrichum falcatum mitovirus 1 (CfMV1) (PRJNA272832; [89]), Loramyces juncicola mitovirus 1 (LjMV1) (PRJNA372853; unpublished), and Ophiocordyceps sinensis mitovirus 1 and 2 (OsMV1, OsMV2) (PRJNA292632; [21]).

The five mitoviruses have genome sizes within the expected range (Table 1), and a single ORF is present only when using the mitochondrial codon table. The percent of A+U in the coding strand for these five viruses ranges from 53% to 70%; characteristically, mitoviruses have 62 to 73% A+U [87], although the percent observed in recently identified viruses is as low as 53% [5]. The predicted protein within each genome contains a Mitovir_RNA_pol domain, with a length ranging from 652 to 716 aa. Alignment of the RNA-seq reads to the viral genomes demonstrates end to end coverage for each virus, with average coverage per nucleotide ranging from 51x for OsMV1 to 540x for LjMV1 (S8D Fig).

A phylogenetic analysis of the RdRP coding sequence demonstrates strong support for all five newly identified viruses among members of the genus Mitovirus (Fig 7B). Criteria for species demarcation within the genus Mitovirus is not yet full defined, however, viruses with greater than 90% identity within the RdRP protein sequences are considered the same species [42]. Here we observe that StMV1, CfMV1, OsMV1, OsMV2, and LjMV1 have less than 52% identity to any currently available mitovirus sequence (S8B Fig). Taken together, the above characteristics demonstrate that these five viruses are new species within the genus Mitovirus.

Ourmiaviruses are also positive-strand RNA viruses, composed of three genome segments that encode an RdRP, movement protein, and coat protein, isolated from plant hosts [41,90]. Despite usage of standard eukaryotic codons, phylogenetically the RdRP protein sequence is most closely related to that of Narnavirus [41]. Recently viruses have been identified from fungal hosts with sequence similarity to that of the plant-hosted ourmiaviruses. One such virus was identified in this study: Aspergillus neoniger ourmia-like virus 1 (AnOlV1) (PRJNA250996; unpublished). The single segment, 2.7 kb genome encodes a single polyprotein that contains an RdRP motif (Table 1). Alignment of the RNA-seq data to the genome revealed an average coverage of 1007x (S8C Fig).

A phylogenetic analysis of this virus, along with related ourmia-like viruses from other fungi, plant-hosted ourmiaviruses, and mitoviruses illustrates a well-supported clade containing AnOlV1 along with Phomopsis longicolla RNA virus 1 (PlRV1) [91] and Botrytis ourmia-like virus HAZ2-3 (BolV) [92] (Fig 7B). A percent identity analysis of the fungal and plant viruses found in Fig 7B demonstrates a percent identity of approximately 50% among the recognized Ourmiaviruses, while the fungal-hosted viruses shared from 17 to 37% identity (S8A Fig), with AnOlV1 and PlRV1 the most similar. As there is limited sequence similarity, and AnOlV1 was identified from a new fungal host, it would appear that this is a new species within this yet-to-be fully characterized viral genus. Identification of additional Ourmiaviruses and related mycoviruses will shed further light on the specific relationship between these plant and fungal viruses.

Hypoviridae.

The Hypoviridae family consists of positive sense, single-stranded RNA viruses that are approximately 9.1 to 12.7 kb in length, containing either one long ORF or two ORFs [93]. Further, these viruses are capsidless and use a host-derived, trans-Golgi network for genome replication [94,95]. Infection by Hypoviruses can often result in reduced fungal virulence, a hallmark symptom of early members of the Hypoviridae family.

Hypoviruses identified from four transcriptomic datasets are described here: Sclerotinia homoeocarpa hypovirus 1 (ShHV1) (PRJNA167556; [25]), Trichoderma asperellum hypovirus 1 (TaHV1) (PRJNA261111; [27]), Setosphaeria turcica hypovirus 1 (StHV1) (PRJNA250530; unpublished), and Fusarium graminearum hypovirus 1 (FgHV1) (PRJNA263651; [14]). The virus FgHV1 was expected, as the sequencing experiment featured F. graminearum strain HN10 infected with FgHV1 [14]; to our knowledge, this is the first mycovirus identified from S. turcica and T. asperellum, and the first hypovirus from S. homoeocarpa.

There are currently four species officially recognized in the Hypoviridae family: Cryphonectria hypovirus 1, 2, 3, and 4 (CHV1, CHV2, CHV3, and CHV4), although several other related viruses have recently been identified from eight other fungal species. Of the currently recognized hypoviruses two clades form that appear to be distantly related, whereby CHV1 and CHV2 are most closely related and CHV3 and CHV4 are together in a separate grouping. To this end, two new genera have been proposed, Alphahypovirus [96] and Betahypovirus [97], where CHV1 and CHV2 are classified in the former and CHV3 and CHV4 in the later [93]. A phylogenetic analysis using the RdRP protein sequence of the four viruses identified here, along with known and related hypoviruses, revealed that FgHV1, ShHFV1, and TaHV1 groups with the Alphahypoviruses while StHV1 was found with the Betahypoviruses (Fig 7C). A further analysis of these viral sequences, described below within the context of the two recently proposed genera, determined that ShHV1, StHV1, and TaHV1 are new species within the Hypovirus genus.

Alphahypovirus.

Initially, members of Alphahypovirus were characterized as having two ORFs, where the second ORF contained protease, RdRP, and Helicase domains. This included CHV1 [98], CHV2 [99], FgHV1 [96], and Rosellinia necatrix hypovirus 2 (RnHV2) [100]. However, identification of additional viruses has revealed members with a single, large ORF, including Fusarium langsethiae hypovirus 1 (FlHV1) [101], Fusarium graminearum hypovirus 2 (FgHV2) [96], Fusarium poae hypovirus 1 (FpHV1) [52], and Rosellinia necatrix hypovirus 1 (RnHV1) [100]. Independent of the number of ORFs, alphahypoviruses have large genomes, with lengths ranging from 12.5 to 14.4 kb. Similarly, newly identified viruses ShHV1 and TaHV1 have genome lengths of 12.4 and 14.2 kb respectively, and single-ORF genome structures (S9A Fig, Table 1).

For a new species to be classified, criteria generally include host species and percent identity with other identified species. Official criteria do not yet exist for the family Hypoviridae, and separate criteria for the two proposed genera may be appropriate given the lack of sequence homology between the two groups. Previously it has been observed that CHV1 and CHV2, while both isolated from Cryphonectria parasitica, demonstrate only ~67% percent identity within the RdRP amino acid sequence. Expanding this analysis to all viruses within the genus Alphahypovirus reveals a maximal identity of 90%, between FgHV2 and FpHV1 (S9B Fig); this excludes the 99% identity observed between the two sequences of FgHV1 which are known to be the same species of virus [14]. Interestingly, there is a cluster of similar hypoviruses from three different Fusarium species, FlHV1, FpHV1, and FgHV2, which excludes the fourth Fusarium hypovirus, FgHV1. An appropriate threshold for evaluating new species of Alphahypovirus remains to be determined; however as the maximum shared identity with existing references sequences is less than 51% for ShHV1 and 31% for TaHV1, and both viruses were identified from unique fungal hosts, these appear to be new species within the family Hypovirus.

A final analysis of the three alphahypoviruses identified from this study was the alignment of RNA-seq reads to the viral genomes. Total reads that aligned range from 7,913 for ShHV1 to 323,433 for FgHV1, and in all three instances, the peak coverage is closest to the 3’end of the genome (S9A Fig).

Betahypovirus.

Viruses in the Betahypovirus genus generally encode one ORF that contains protease, RdRp, Helicase, and UGT domains [97] and tend to have shorter genomes than the Alphahypoviruses, at approximately 9 to 11 kb in length. These same characteristics are also observed in StHV1, which has a length of 9,163 bp and a polyprotein predicted to encode a papain-like cysteine protease domain along with a UGT, peptidase-C97, RdRP, and Helicase-HA2 domain (S9C Fig, Table 1). Additionally, the 5’end of the 5’UTR sequence has high sequence identity across viral genomes from this genus; a multiple sequence alignment of this region from StHV1 and related Betahypoviruses reveals similarly high conservation within the StHV1 genome, including a 15-nt sequence that is present in all viruses analyzed (CTGGTTTATACTCTG) (S9D Fig). Characterization of the StHV1 sequence also included alignment of the RNA-seq reads to the viral genome, resulting in a total of 376,89 reads aligned, and an average coverage per nucleotide of 617x (S9C Fig).

A percent identity analysis of the RdRP amino acid sequences of viruses within the genus Betahypovirus reveals a maximal identity of 68%, between PlHV and CHV3 (S9B Fig) while StHV1 shares 46% to 54% identity with other viruses. As such, a threshold of 70% identity within the RdRP amino acid is suggested as a criteria to distinguish between different species of Betahypoviruses.

Fusariviridae.

Members of the proposed family Fusariviridae have mono-segmented, plus strand, single-stranded RNA genomes that encode two to four ORFs. ORF1 is the largest and contains the RdRP and Helicase motifs, while the subsequent and second largest ORF encodes an unknown protein, often containing an SMC motif. One to two additional small proteins have been predicted at the 3’end of some virus genomes. Here we have identified seven new viruses that group with fusariviruses.

Aspergillus ellipticus fusarivirus 1 (AeFV1) (PRJNA250911; unpublished), Morchella importuna fusarivirus 1 (MiFV1) (PRJNA372858; unpublished), Neurospora discreta fusarivirus 1 (NdFV1) (PRJNA257829; [20]), and Rutstroemia firma fusarivirus 1 (RfFV1) (PRJNA372878; unpublished) are to our knowledge, the first viruses to be identified in these species. Sclerotinia homoeocarpa fusarivirus 1 (ShFV1) (PRJNA167556; [25]) and Gaeumannomyces tritici fusarivirus 1 (GtFV1) (PRJNA268052; [16]) are the first fusariviruses identified in these fungi, and this is the first full-length description of Zymoseptoria tritici fusarivirus 1 (ZtFV1) (PRJNA237967; [30–32]PRJEB8798; [99]) (PRJNA253135; [[30–32]) (PRJNA179083; unpublished).

The genome of MiFV1 encodes three predicted ORFs, where the first contains both the RdRP and helicase motifs, the second contains no predicted domains, and the third ORF contains an SMC (structural maintenance of chromosomes) domain (S10C Fig). The genomic structure of the other six viruses identified in this study encode two predicted ORFs, again with the first encoding RdRP and helicase motifs (S10C Fig). ORF2 of ZtFV1 contains a predicted Spc7 domain (found in kinetochore proteins), a domain also found in Rosellinia necatrix fusarivirus 1 (RnFV1) [102]. Within ORF2 of AeFV1, GtFV1, NdFV1 and RfFV1 is a predicted SMC domain, while no functional domain is predicted within ORF2 of ShFV1. A previous phylogenetic analysis determined that the closest relatives to the viral SMCs are the proteins from the eukaryotic SMC5 and SMC6 subfamily and that because the SMC domain is widely present throughout members of the Fusariviridae family, a common ancestral origin is probable [103].

A phylogenetic analysis of the RdRP amino acid sequence of these seven viruses and twelve other published fusariviruses reveals two strongly supported clusters (S10A Fig). “Group 1” contains 14 viruses while “Group 2” contains the remaining five fusariviruses. The genome structures of the two clusters suggests some general patterns; all members of Group 1 have a predicted functional domain within the second largest ORF, either an SMC or an Spc7 domain as described above. Although, FgV1 encodes three ORFs, unlike related viruses that encode just two. Conversely, viruses within Group 2 generally do not have a functional domain predicted within ORF2, the exception is MiFV1 which contains an SMC domain. Further, three ORFs are predicted for four of the five members, with ShFV1 being the exception. Identification of additional viral genomes that belong to this family will help to elucidate the support for these separate clusters and may reveal additional distinguishing characteristics.

As Fusariviridae is not yet an officially recognized family of viruses, specific characteristics for distinguishing new viral species or genera have yet to be established. In addition to phylogenetic analyses and identity of fungal host species, percent identity across the protein coding sequence of the RdRP-containing ORF is frequently used as a classifier. To date, fusariviruses previously characterized have all been isolated from unique fungal species, including two different species of Fusarium, and none share greater than 57% identity with one another (S10B Fig). Viruses identified in this study are also from unique fungal species, including ShFV1 which was isolated from a different species than Sclerotinia sclerotiorum fusarivirus 1 (SsFV1) [104]. Using the data available to date, two criteria are recommended to qualify viruses as new species: a unique fungal host and a percent identity of the RdRP sequence ≤60% in comparison to existing fungal sequences.

The ZtFV1 sequence described here was identified within the SRA sample PRJNA179083, however there are three other RNA-seq experiments available for this organism from three other research groups. All four sequencing samples contain the same fusarivirus; indeed a full length sequence was isolated from samples from both the Max Planck Institute for Terrestrial Microbiology (PRJNA237967; [31]) and Rothamsted Research (PRJEB8798; [30]). The sample from the Institut national de la recherche agronomique (INRA) (PRJNA253135; [32]) yielded a contig covering 76% of the full length, while a bowtie2 alignment of reads from this sample to the full-length sequence demonstrated reads across the full length sequence. Indeed, the average read coverage per nucleotide using reads from each of the four samples was high, ranging from 3548x to 15278x (S10C Fig). As the length of 5’ poly(G) and 3’ poly(A) sequences differed slightly among the four samples, the most parsimonious version was selected and deposited into GenBank (Table 1). Other than these differences, no SNPs were observed among the four samples. While the only information related to when these samples were sequenced is the date the data was made public the range in dates, between November 2012 and June 2015, suggests that the viral genome sequence may have remained unchanged across multiple years. An interesting follow-up experiment would be a re-sequencing of one of these strains by the same lab to observe the viral genome sequence following multiple years of culturing within the fungal host.

MiRV1.

In addition to the three endornaviruses and one fusarivirus described above, a fifth viral sequence was identified in the M. importuna sample (PRJNA372858; unpublished). The single-segment sequence was determined to be 10,132 nt in length, with a single predicted ORF of 3,295 aa (Table 1). A conserved domain search revealed a viral methyltransferase domain and a viral helicase domain in addition to the RdRP_2 motif similar to positive-strand RNA viruses (S11C Fig). A BLAST-P search with the predicted protein sequence returned a single full length mycovirus sequence, Sclerotinia sclerotiorum RNA virus L (SsRV-L) [105], in addition to matches to Hepatitis Virus E and Cordoba virus. Other top hits include partial sequences corresponding to the RdRP domain of Rhizoctonia solani RNA virus 1, 2, and 3 (RsRV-1,RsRV-2, RsRV-3) [106]. Both MiRV1 and SsRV-L contain a single ORF with methyltransferase, helicase and RdRP domains, and neither contain peptidase motifs [105].

Previous phylogenetic analyses of SsRV-L and the three viruses from R. solani demonstrated a relationship between these viruses and members of the alpha-like virus superfamily, specifically viruses from the Hepeviridae and Alphatetraviridae families [105,106]. Endornaviridae, whose members infect both fungi and plants, are also a family of alpha-like viruses [107], but appear to be more distantly related to this emerging group of mycoviruses. Expanding the phylogenetic analysis to include MiRV1 confirms that MiRV1 is a member of this new clade of mycoviruses (S11A Fig). Further, a multiple sequence alignment of the RdRP motif sequences demonstrated the presence of the eight conserved RdRP motifs common among (+)ssRNA viruses, along with certain residues within these motifs were shared only among the mycoviruses that group with MiRV1 (S11B Fig). The low percent identity between MiRV1 and the related mycoviruses indicates that this is a new species of virus (S11D Fig). Identification of additional related viral sequences will help elucidate the relationship of MiRV1 and the rest of the Alpha-like viruses.

Unknown comovirus-like sequences.

An RdRP-containing contig with a top match to Turnip Ringspot Virus (TRV) was identified in sequencing samples from Colletotrichum tofieldiae (PRJNA287627; [108]) and Zymoseptoria brevis (PRJNA277175; [109]). Further inquiry also identified the second genomic segment of TRV, which encodes the movement and coat proteins, from both the C. tofieldiae and Z. brevis Trinity contigs. The coding sequences of RNA1 and RNA2 are 98.5% and 99.7% identical between the two isolates suggesting a close relationship between the sequences, despite the unrelated hosts.

As Turnip Ringspot Virus (genus: Comovirus; family: Secoviridae) can infect Brassicaceae [110], and a fungal host has not yet been described for viruses from this family, we postulate that within the C. tofieldiae sample, this sequence could be due to cross contamination from Arabidopsis. This BioProject included both media-grown fungi and an infection series of Arabidopsis thaliana; the specific SRA record used in this analysis was from a media-grown sample. Further analysis of the infection series determined that the viral sequence is identifiable in 19 of 24 (79%) of the mock-infected A. thaliana samples, 19 of 24 (79%) of the A. thaliana plus C. tofieldiae samples, and two of three fungus-only samples. An explanation for the uneven distribution of the viral sequences among these samples remains to be determined. Conversely, the Z. brevis sample was part of a media-grown, fungus-only transcriptomic experiment and as such, a putative plant-based source of the TRV-like sequences is not readily apparent. Interestingly, while the Z. brevis sample is from Christian-Albrechts University of Kiel (Kiel, Germany) and the C. tofieldiae sample is from the Max Planck Institute for Plant Breeding Research (Kohn, Germany), both were sequenced at the Max Planck Genome Center (Cologne, Germany) near the same time and on the same machine based on the identifiers in the sequencing headers. However, cross contamination between the samples is unlikely as it would be expected the observed viral sequences would be 100% identical which is not the case. At this junction, it is not possible to confirm the host or source of the viral genome sequences identified in the C. tofieldiae and Z. brevis samples, nor determine if these represent a new group of mycoviruses that are related to Comoviruses.

Sankey diagram.

The online tool SankeyMATIC [125] was used generate the Sankey diagram in Fig 1. Data tables used as input are available through the FigShare FileSet (DOI: 10.6084/m9.figshare.7476359).

Pseudoknot structures.

The online tool DotKnot was used to predict pseudoknots in certain viral genome sequences [126,127]. PseudoViewer 3.0 was used for visualization of the predicted pseudoknots [128].

Read distribution plots.

Distribution of reads along the viral genome segment(s) was visualized by first aligning the reads to the viral genome using bowtie2, then calculating the hits per nucleotide along the viral sequence(s) using the custom Perl script plotRegion_RNAseq_hitsPerNT.pl. The resulting output table was the input for an R script to create hits per nucleotide heat maps for each viral segment. The R code for each plot, along with the input data table, is available through the FigShare FileSet (DOI: 10.6084/m9.figshare.7476359).

Phylogenetic analysis.

To identify related sequences for phylogenetic analysis, the predicted amino acid sequence for the RdRP was used to query the ‘nr’ database at NCBI. Protein sequences of the top hits, generally with e-values approaching or equal to 0.0, were downloaded. Partial sequences were excluded. Known viral sequences that appeared as the top hit for multiple viruses identified in this study were deduplicated. The complete list of Accession IDs used for phylogenetic analysis is available in S2 Table.

To build the trees, amino acid sequences were aligned with MAFFT [129], and resultant alignments were degapped and trimmed using the tool trimAL [130] with a gap threshold of 0.9 which allowed for a gap in up to 10% of the sequences at a given position. For seven of the nine phylogenetic trees, the number of alignment sites that remained after trimming ranged between 300 and 400 amino acids; a slightly smaller range was observed for the MiRV1 tree (242 to 252 amino acids), and a larger range of alignment sites remained for the Hypoviridae tree (1292 to 1592 amino acids). The unaligned, untrimmed fasta files and the aligned, trimmed fasta files are available through the FigShare FileSet (DOI: 10.6084/m9.figshare.7476359). The later files were used as input for the creation of maximum likelihood phylogenetic trees using RAxML version 8.2.4 [131] and the LG+G+I amino acid substitution model. RAxML tree files with branch lengths were viewed in Dendroscope version 3.5.7 [132] and exported as PDF for figure preparation; both the RAxML and the PDF files are available within the FigShare FileSet (DOI: 10.6084/m9.figshare.7476359).

Percent identity matrices.

Percent identities among protein coding sequences, either full length or motif-specific, were determined via multiple sequence alignment using Clustal Omega [133]. In instances where two different proteins were evaluated and then combined into one matrix, such as the RdRP protein and the Coat Protein, first the RdRP protein sequences were analyzed; the order of the output in this percent identity matrix was then applied to the second set of protein sequences to be analyzed and the option “Order: Input” was selected in Clustal-Omega. Percent identity matrices were converted to heat map plots using a custom R script. This script along with the input matrices are available through the FigShare FileSet (DOI: 10.6084/m9.figshare.7476359).