Cryphonectria hypovirus 3, a virus species in the family hypoviridae with a single open reading frame

Abstract

Isolate Grand Haven (GH) 2 is a naturally occurring isolate of the chestnut blight fungus, Cryphonectria parasitica, that is greatly reduced in virulence due to the presence of a double-stranded RNA virus. Unlike many other virus-infected, hypovirulent isolates, GH2 is not substantially reduced in pigmentation, conidiation, or laccase expression compared to its virus-free counterpart. The dsRNA genome of the GH2 virus was cloned, sequenced, and compared to hypovirulence-associated viruses of the family Hypoviridae. GH2 dsRNA is considerably smaller than previously characterized members of the family, 9.8 kb compared to 12.5-12.7 kb for other members. The genome organization of GH2 dsRNA reflected the substantial difference in genome size. Like other members of the family, one strand contained a poly(A)(+) tail at the 3' end and a long sequence with several minicistrons at the 5' end of the same strand. Only a single open reading frame (ORF) of 8622 nucleotides was predicted from deduced translations of the poly(A)(+)-containing strand, however. This contrasts with the two-ORF structures of previously characterized members. Analysis of the deduced ORF of GH2 dsRNA revealed putative proteinase, RNA polymerase, and helicase domains similar to those previously identified in confirmed members of the virus family Hypoviridae. GH2 dsRNA was more distantly related to Cryphonectria hypovirus (CHV) 1-EP713 and CHV2-NB58 than the latter two were to each other but has features in common with each of those viruses. We propose that the GH2 virus be included in this taxon as a member of the genus Hypovirus, representing a strain of a new species, CHV3.