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Rosellinia necatrix megabirnavirus 1 (RnMBV1) with a bipartite dsRNA genome (dsRNA1 and dsRNA2) confers hypovirulence to its natural host, the white root rot fungus, and is thus regarded as a potential virocontrol (biocontrol) agent. Each segment has two large ORFs: ORF1 and partially overlapping ORF2 on dsRNA1 encode the major capsid protein (CP) and RNA-dependent RNA polymerase (RdRp), whilst ORF3 and ORF4 on dsRNA2 encode polypeptides with unknown functions. Here, we report the biological and molecular characterization of this virus in the chestnut blight fungus, Cryphonectria parasitica, a filamentous fungus that has been used as a model for mycovirus research. Transfection with purified RnMBV1 particles into an RNA-silencing-defective strain (Δdcl-2) of C. parasitica and subsequent anastomosis with the WT strain (EP155) resulted in stable persistent infection in both host strains. However, accumulation levels in the two strains were different, being ~20-fold higher in Δdcl-2 than in EP155. Intriguingly, whilst RnMBV1 reduced both virulence and growth rate in Δdcl-2, it attenuated virulence without affecting significantly other traits in EP155. Western blot analysis using antiserum against recombinant proteins encoded by either ORF1 or ORF2 demonstrated the presence of a 250 kDa protein in purified virion preparations, suggesting that RdRp is expressed as a CP fusion product via a -1 frameshift. Antiserum against the ORF3-encoded protein allowed the detection of 150, 30 and 23 kDa polypeptides specifically in RnMBV1-infected mycelia. Some properties of an RnMBV1 mutant with genome rearrangements, which occurred after transfection of Δdcl-2 and EP155, were also presented. This study provides an additional example of C. parasitica serving as a versatile, heterologous fungus for exploring virus-host interactions and virus gene expression strategies.

Biological properties and expression strategy of rosellinia necatrix megabirnavirus 1 analysed in an experimental host, Cryphonectria parasitica