Significantly low level of small RNA accumulation derived from an encapsidated mycovirus with dsRNA genome

The role of RNA silencing as an antiviral defence has been well elucidated in plants and invertebrates, but not in filamentous fungi. We have previously determined the complete genome sequence of Magnaporthe oryzae virus 2 (MoV2), a dsRNA virus that infects the rice blast fungus Magnaporthe oryzae. In this study, we detected small interfering RNAs (siRNAs) from both positive- and negative-strand MoV2 viral RNA, suggesting that the RNA silencing machinery in M. oryzae functions against the mycovirus. Cloning and characterisation of MoV2 siRNAs indicated that, in MoV2, the ratio of virus-derived siRNAs to total small RNA is significantly lower than that in either plant viruses or Cryphonectria hypovirus 1 (CHV1), another mycovirus. Nevertheless, any MoV2-encoded proteins did not exhibit RNA silencing suppressor activity in both the plant and fungal systems. Our study suggests the existence of a novel viral strategy employed to evade host RNA silencing.