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A summary of this ICTV Report chapter has been published as an ICTV Virus Taxonomy Profile article in the Journal of General Virology, and should be cited when referencing this online chapter as follows:
Dàohóng Jiāng (姜道宏), María A. Ayllón, Shin-Yi L. Marzano, and the ICTV Report Consortium, 2019, ICTV Virus Taxonomy Profile: Mymonaviridae, Journal of General Virology, (In Press)
Members of the family Mymonaviridae produce enveloped filamentous virions containing a single molecule of linear, negative-sense RNA of ≈10 kb. The family currently includes a single genus, Sclerotimonavirus. Mymonaviruses usually infect filamentous fungi, and one virus, Sclerotinia sclerotiorum negative-stranded RNA virus 1, induces hypovirulence in the fungal host.
Table 1.Mymonaviridae. Characteristics of members of the family Mymonaviridae
Sclerotinia sclerotiorum negative-stranded RNA virus 1 [KJ186782], species Sclerotinia sclerotimonavirus, genus Sclerotimonavirus
Enveloped, filamentous virions 25–50 nm in diameter and ≈1,000 nm in length
Single molecule of linear, negative-sense RNA of ≈10 kb
Ribonucleoprotein (RNP) complexes containing anti-genomic RNA serve as templates for synthesis of nascent RNP complexes containing genomic RNA
The viral RNA-directed RNA polymerase binds the encapsidated genome at the leader region, and then sequentially transcribes each gene by recognizing start and stop signals flanking viral genes. This produces subgenomic RNAs that serve as mRNAs
Realm Riboviria, Phylum Negarnaviricota, subphylum Haploviricotina, class Monjiviricetes, order Mononegavirales, family Mymonaviridae, one genus Sclerotimonavirus with 7 species assigned
Virions are filamentous, 25–50 nm in diameter, ≈1,000 nm in length, and may be enveloped by a membrane (Figure 1A.Mymonaviridae). The outer surface of virions does not appear to be covered with spikes. Nucleocapsids released from virions are single, left-handed, helical structures that, when tightly coiled, have a diameter of 20–22 nm and a length of 200–2,000 nm (Figure 1B.Mymonaviridae). Nucleocapsids consist of polymerized nucleoprotein (NP) monomers (Figure 1C.Mymonaviridae).
Figure 1.Mymonaviridae. Morphology and structure of virions and nucleoprotein-RNA complexes (RNPs). (A) Filamentous, possibly enveloped virions (marked by white triangles) and ribonucleoprotein (RNP) complexes. (B) Purified tight or loose coils of RNP complexes. (C) Rings that constitute the coils and nucleoprotein (NP) monomers.
Virions contain a single molecule of a linear, negative-sense RNA genome of ≈10 kb (Table 2.Mymonaviridae). The genome lacks a poly(A) tail at the 3′-terminus and is uncapped at the 5′-terminus. The two termini are not complementary in sequence. Defective viral RNA genomes with incomplete 5′- and 3′-termini can be found in virion preparations.
Table 2.Mymonaviridae. Mymomnavirus genome sequences
Available sequence (nt)
Mymonaviruses express at least 6 proteins (Figure 2A.Mymonaviridae). The nucleocapsid possibly contains two nucleoproteins (NP) with different molecular masses, ≈43 kDa and ≈41 kDa, or protein p41 may be a degraded product of p43. The nucleoproteins encapsidate the virus genome. The RNA-dependent RNA polymerase (L) mediates virus genome replication and transcription. The functions of the remaining 4 proteins are unclear.
The mymonavirus genome is predicted to have six major non-overlapping open reading frames (ORFs I–VI) that are arranged linearly (Figure 2A.Mymonaviridae). These ORFs are separated by non-coding intergenic regions containing highly conserved gene-junction sequences (Figure 2B.Mymonaviridae) and are expressed as individual transcription units (Figure 2C.Mymonaviridae). ORFs II–VI are located in the +2 reading frame, whereas ORF I is located in the +1 reading frame. ORF II encodes NP, and ORF V encodes L.
Mymonaviruses are believed to replicate in the fungal cytoplasm, but their replication strategy is not well studied. RNP complexes can be used directly as templates for replication and transcription. Replication usually occurs within RNP complexes and requires L to synthesize full-length positive-sense antigenomes that serve as templates for the synthesis of negative-sense progeny genomes.
Figure 2.Mymonaviridae. Genome organization and characteristics of Sclerotinia sclerotiorum negative-stranded RNA virus 1 (SsNSRV-1). (A) Genome length and organization. Boxes indicate position and length of each ORF, which are labelled with Roman numerals, except for two ORFs, NP (II) and L (V), which encode the nucleoprotein (NP) and RNA-dependent RNA polymerase (L). (B) Alignment of the putative gene-junction sequences between ORFs in a 3′-to-5′orientation. Conserved sequences [3′-(A/U)(U/A/C)UAUU(U/A)AA(U/G)AAAACUUAGG(A/U)(G/U)-5′] are highlighted in blue. Different shades indicate levels of conservation with the darkest colour indicating the highest conservation. (C) Deduced transcription map based on 5′- and 3′-RACE (modified from (Liu et al., 2014)).
Mymonaviruses have been characterized in three phytopathogenic fungi. Sclerotinia sclerotiorum negative-stranded RNA virus 1 (SsNSRV-1) was isolated from a Chinese strain AH98 of Sclerotinia sclerotiorum (Liu et al., 2014), Sclerotinia sclerotiorum negative-stranded RNA virus 2 (SsNSRV-2), Sclerotinia sclerotiorum negative-stranded RNA virus 3 (SsNSRV-3), and Sclerotinia sclerotiorum negative-stranded RNA virus 4 (SsNSRV-4) were isolated from S. sclerotiorum strains collected from USA (Marzano et al., 2016), and Sclerotinia sclerotiorum negative-stranded RNA virus 7 (SsNSRV-7) was detected in an Australian strain of S. sclerotiorum (Mu et al., 2017). Fusarium graminearum negative-stranded RNA virus 1 (FgNSRV-1) infects its natural host Fusarium graminearum and was collected in China (Wang et al., 2018). Botrytis cinerea mymonavirus 1 (BcMyV1), an unclassified mymonavirus, was isolated from the phytopathogenic fungus Botrytis cinerea (Hao et al., 2018). Four mymonaviruses including soybean leaf-associated negative-stranded RNA virus 1 (SLaNSRV-1), soybean leaf-associated negative-stranded RNA virus 2 (SLaNSRV-2), soybean leaf-associated negative-stranded RNA virus 3 (SLaNSRV-3) and soybean leaf-associated negative-stranded RNA virus 4 (SLaNSRV-4) were also discovered in soybean phyllosphere phytobiomes (Marzano and Domier 2016) and Hubei rhabdo-like virus 4 (HbRLV-4) was detected from invertebrate arthropod (Shi et al., 2016).
SsNSRV-1 has been identified as a hypovirulence factor in S. sclerotiorum (Liu et al., 2014). SsNSRV-1 infection results in hypovirulence phenotypes, including slow growth, loss of the ability to produce sclerotia, and loss of pathogenicity on rapeseed. The virus can be transmitted horizontally through hyphal fusion. Transmission through sexual spores (ascospores) and asexual spores (conidia) are not known to occur.
The family currently includes only a single genus.
Mymonaviridae: from Myco and Mononegavirales.
Phylogenetic relationships across the family have been established from maximum-likelihood trees generated from the amino acid sequences of the L core domain of mymonaviruses and other selected viruses (Figure 3.Mymonaviridae). Based on phylogenetic analysis, mymonaviruses are closely related to bornaviruses (Mononegavirales: Bornaviridae) and nyamiviruses (Mononegavirales: Nyamiviridae).
Figure 3.Mymonaviridae. Maximum-likelihood phylogenetic tree constructed based on the core RNA-directed RNA polymerase motifs of mymonaviruses and the selected mononegaviruses from the families Filoviridae, Pneumoviridae, Paramyxoviridae, Bornaviridae, Nyamiviridae, and Rhabdoviridae. The position of SsNSRV-1 is indicated with bold text. Numbers at the nodes indicate bootstrap values over 50% (1,000 replicates). Evolutionary analyses were conducted in MEGA7 (Kumar et al., 2016). This phylogenetic tree and corresponding sequence alignment are available to download from the Resources page.
Mymonaviruses are clustered with bornaviruses and nyaviruses, but they form a separate subclade that is different from these two groups of mononegaviruses. Compared with Borna disease virus, Nyamanini virus, and Midway virus, it differs largely in genome size and gene organization.
Since only one genus (Sclerotimonavirus) is currently recognized in the family Mymonaviridae, the family description above corresponds to the genus description. For clarity, the additional information that can be found on the genus page is also presented below.
Members of different species in the genus may be distinguished on the basis of nucleoprotein sequence differences (30% amino acid difference cut-off) and coding-complete/complete genome sequence differences (30% nucleotide difference cut-off).
Sclerotimonavirus: from Sclerotinia (the genus of a fungal host), and Mononegavirales.
Botrytis cinerea mymonavirus 1
Sclerotinia sclerotiorum negative-stranded RNA virus 7
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