title: 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 single-stranded 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.
  
  
  OBJECTIVE
  Mycoviruses 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.
authors.affiliation: State Key Laboratory of Agricultural Microbiology, The Provincial Key Lab of Plant Pathology of Hubei Province, College of Plant Science and Technology, Huazhong Agricultural University, Wuhan, People's Republic of China College of Forestry and Horticulture, Hubei University for Nationalities, Enshi, People's Republic of China.
authors.forename: Minghong
authors.initials: M
authors.lastname: Wang
authors.affiliation: State Key Laboratory of Agricultural Microbiology, The Provincial Key Lab of Plant Pathology of Hubei Province, College of Plant Science and Technology, Huazhong Agricultural University, Wuhan, People's Republic of China.
authors.forename: Yong
authors.initials: Y
authors.lastname: Wang
authors.affiliation: State Key Laboratory of Agricultural Microbiology, The Provincial Key Lab of Plant Pathology of Hubei Province, College of Plant Science and Technology, Huazhong Agricultural University, Wuhan, People's Republic of China.
authors.forename: Xiangzhong
authors.initials: X
authors.lastname: Sun
authors.affiliation: State Key Laboratory of Agricultural Microbiology, The Provincial Key Lab of Plant Pathology of Hubei Province, College of Plant Science and Technology, Huazhong Agricultural University, Wuhan, People's Republic of China.
authors.forename: Jiasen
authors.initials: J
authors.lastname: Cheng
authors.affiliation: State Key Laboratory of Agricultural Microbiology, The Provincial Key Lab of Plant Pathology of Hubei Province, College of Plant Science and Technology, Huazhong Agricultural University, Wuhan, People's Republic of China.
authors.forename: Yanping
authors.initials: Y
authors.lastname: Fu
authors.affiliation: NWAFU-PU Joint Research Center, State Key Laboratory of Crop Stress Biology for Arid Areas, College of Plant Protection, Northwest A&F University, Yangling, Shanxi Province, People's Republic of China.
authors.forename: Huiquan
authors.initials: H
authors.lastname: Liu
authors.affiliation: State Key Laboratory of Agricultural Microbiology, The Provincial Key Lab of Plant Pathology of Hubei Province, College of Plant Science and Technology, Huazhong Agricultural University, Wuhan, People's Republic of China.
authors.forename: Daohong
authors.initials: D
authors.lastname: Jiang
authors.affiliation: Department of Plant Pathology, University of Kentucky, Lexington, Kentucky, USA.
authors.forename: Said A
authors.initials: SA
authors.lastname: Ghabrial
authors.affiliation: State Key Laboratory of Agricultural Microbiology, The Provincial Key Lab of Plant Pathology of Hubei Province, College of Plant Science and Technology, Huazhong Agricultural University, Wuhan, People's Republic of China jiataoxie@mail.hzau.edu.cn.
authors.forename: Jiatao
authors.initials: J
authors.lastname: Xie
mesh_terms: D001203:Ascomycota / Q000821:virology*; D001483:Base Sequence; D016000:Cluster Analysis; D018076:DNA, Complementary / Q000235:genetics; D022761:Gene Transfer, Horizontal / Q000235:genetics*; D008854:Microscopy, Electron; D008969:Molecular Sequence Data; D009690:Nucleic Acid Conformation; D016366:Open Reading Frames / Q000235:genetics; D010802:Phylogeny; D012328:RNA Viruses / Q000235:genetics*; D012330:RNA, Double-Stranded / Q000235:genetics; D016415:Sequence Alignment; D017422:Sequence Analysis, DNA
chemical_list: D018076:DNA, Complementary; D012330:RNA, Double-Stranded