Characterization of a Novel Megabirnavirus from Sclerotinia sclerotiorum Reveals Horizontal Gene Transfer from Single-Stranded RNA Virus to Double-Stranded RNA Virus

Abstract

Mycoviruses have been detected in all major groups of filamentous fungi, and their study represents an important branch of virology. Here, we characterized a novel double-stranded RNA (dsRNA) mycovirus, Sclerotinia sclerotiorum megabirnavirus 1 (SsMBV1), in an apparently hypovirulent strain (SX466) of Sclerotinia sclerotiorum. Two similarly sized dsRNA segments (L1-and L2-dsRNA), the genome of SsMBV1, are packaged in rigid spherical particles purified from strain SX466. The full-length cDNA sequence of L1-dsRNA/SsMBV1 comprises two large open reading frames (ORF1 and ORF2), which encode a putative coat protein and an RNA-dependent RNA polymerase (RdRp), respectively. Phylogenetic analysis of the RdRp domain clearly indicates that SsMBV1 is related to Rosellinia necatrix megabirnavirus 1 (RnMBV1). L2-dsRNA/SsMBV1 comprises two nonoverlapping ORFs (ORFA and ORFB) encoding two hypothetical proteins with unknown functions. The 5=-terminal regions of L1-and L2-dsRNA/SsMBV1 share strictly conserved sequences and form stable stem-loop structures. Although L2-dsRNA/SsMBV1 is dispensable for replication, genome packaging, and pathogenicity of SsMBV1, it enhances transcript accumulation of L1-dsRNA/SsMBV1 and stability of virus-like particles (VLPs). Interestingly, a conserved papain-like protease domain similar to a multifunctional protein (p29) of Cryphonectria hypovirus 1 was detected in the ORFA-encoded protein of L2-dsRNA/SsMBV1. Phylogenetic analysis based on the protease domain suggests that horizontal gene transfer may have occurred from a singlestranded RNA (ssRNA) virus (hypovirus) to a dsRNA virus, SsMBV1. Our results reveal that SsMBV1 has a slight impact on the fundamental biological characteristics of its host regardless of the presence or absence of L2-dsRNA/SsMBV1. IMPORTANCEMycoviruses are widespread in all major fungal groups, and they possess diverse genomes of mostly ssRNA and dsRNA and, recently, circular ssDNA. Here, we have characterized a novel dsRNA virus (Sclerotinia sclerotiorum megabirnavirus 1 [SsMBV1]) that was isolated from an apparently hypovirulent strain, SX466, of Sclerotinia sclerotiorum. Although SsMBV1 is phylogenetically related to RnMBV1, SsMBV1 is markedly distinct from other reported megabirnaviruses with two features of VLPs and conserved domains. Our results convincingly showed that SsMBV1 is viable in the absence of L2-dsRNA/SsMBV1 (a potential large satellite-like RNA or genuine genomic virus component). More interestingly, we detected a conserved papain-like protease domain that commonly exists in ssRNA viruses, including members of the families Potyviridae and Hypoviridae. Phylogenetic analysis based on the protease domain suggests that horizontal gene transfer might have occurred from an ssRNA virus to a dsRNA virus, which may provide new insights into the evolutionary history of dsRNA and ssRNA viruses. M ycoviruses (or fungal viruses) with RNA or, rarely, DNA genomes are ubiquitous in plant-pathogenic fungi (1, 2). Based on genome nature, all known RNA mycoviruses...

Novel, diverse RNA viruses from Mediterranean isolates of the phytopathogenic fungus, Rosellinia necatrix: insights into evolutionary biology of fungal viruses

“…A). All field isolates of megabirnaviruses carry two genomic dsRNA segments (Chiba et al ., ; Nerva et al ., ; Sasaki et al ., ; Wang et al ., ), except for a megabirna‐related dsRNA virus (Rhizoctonia solani RNA virus HN008), whose genome is assumed to be an undivided dsRNA (Zhong et al ., ). Only dsRNA 1 of RnMBV3 was sequenced and no contigs were obtained from dsRNA 2.…”

“…This homologous region of PgLV1, except for the 2A‐like peptide motif, does not show any sequence similarity to other related hypoviruses such as FpHV1. These strongly suggest HGT between a (+)ssRNA virus (RnHV1) and a dsRNA virus (phlegiviruses), as in the case for the Phytoreo_S7 domain (Liu et al ., ; Hisano et al ., ) and papain‐like cysteine protease (Wang et al ., ). Likewise, the well‐conserved 2A‐like motif of yadokariviruses (Fig.…”

Hypovirulence of Sclerotium rolfsii Caused by Associated RNA Mycovirus

“…Moreover, because they are the primary inoculum source for disease epidemiology, this deficiency might be harmful for pathogen spread but favor the control of this pathogen. Interactions between some hypovirulence-associated viruses and their hosts have contributed to the establishment of some host-mycovirus systems, by which we can better understand the molecular basis of fungal biology, especially for fungal pathogenesis (Xie and Jiang, 2014; Wang et al, 2015). These host-mycovirus systems, including Cryphonectria parasitica-hypovirus (Nuss, 1996, 2011; Dawe and Nuss, 2001, 2013; Hillman and Suzuki, 2004; Milgroom and Cortesi, 2004; Nuss, 2005; Pearson et al, 2009), Helmintosporium victoriae-HvV190S (Li H. et al, 2011; Dunn et al, 2013; Ghabrial et al, 2013), Sclerotinia sclerotiorum-mycovirus (Li et al, 2008), Rosellinia necatrix-mycovirus (Salaipeth et al, 2013; Yaegashi et al, 2013), and Fusarium graminearum-mycovirus (Kwon et al, 2009; Cho et al, 2012), have been established and thoroughly elucidated, providing plenty of references to study fungal pathogenicity (Xie and Jiang, 2014).…”

“…Due to their potential use in biological control, studying the phenomenon of mycovirus-mediated hypovirulence is attractive for numerous researchers. In addition, the hypovirulence-associated mycoviruses, which have divergent molecular characteristics, will also enhance our understanding of virus diversity and evolution and contribute to the establishment of host-mycovirus systems (i.e., Cryphonectria parasitica-hypovirus, Helmintosporium victoriae-HvV190S, Sclerotinia sclerotiorum-mycovirus, Rosellinia necatrix-mycovirus, and Fusarium graminearum- mycovirus), which are beneficial for studying virus-host interactions including fungal and viral pathogenesis at the molecular level (Xie and Jiang, 2014; Wang et al, 2015). …”

Fungal DNA virus infects a mycophagous insect and utilizes it as a transmission vector

“…Moreover, mycoviruses could potentially be explored to control human fungal diseases (6). Mycoviruses are also very important toward an understanding of the diversity, evolution, and ecology of viruses; they are often found to be phylogenetically related to plant and animal viruses (7,8), and some newly discovered mycoviruses showed linkages between dsRNA and ssRNA viruses (9)(10)(11). Chiba and Suzuki found that one mycovirus could interfere with the replication of an unrelated mycovirus (12).…”