Diversity, characteristics and potential for biological control of Sclerotinia sclerotiorum mycoviruses in New Zealand
Mycoviruses present a potential means of controlling plant pathogenic fungi, such as Sclerotinia sclerotiorum, that avoid the drawbacks associated with fungicide-based control, such as resistance and environmental impact. Mycoviruses have been used to reduce the virulence of fungal pathogens, such as using hypoviruses to control the chestnut blight fungus Cryphonectria parasitica, and there are increasing reports of hypovirulence-mediating mycoviruses in other fungi, including S. sclerotiorum. Of the New Zealand S. sclerotiorum isolates tested, 73% contained double-stranded RNAs (dsRNAs), indicative of virus presence, some of which had a hypovirulent phenotype. Data from Illumina sequencing showed that mycoviruses in S. sclerotiorum are more diverse than was apparent from dsRNA profiles. Twelve viral genomes were fully sequenced most of which were novel, increasing the total number of S. sclerotiorum mycoviruses from eight to 20. Mitoviruses were the most prevalent with ten genomes representing five novel species and a previously characterised mitovirus. Studies on an isolate harbouring three mitoviruses, demonstrated that they are horizontally transmitted via anastomosis and vertically transmitted through ascospores, with associated reduced virulence and malformed mitochondria of the host. Curing experiments revealed that Sclerotinia sclerotiorum mitovirus 3 (SsMV3/NZ1) played a significant role in the expression of the hypovirulent phenotype. Other viruses detected and molecularly and phylogenetically characterised included an endornavirus assigned the name Sclerotinia sclerotiorum endornavirus 1 (SsEV1/11691) and a putative novel member of Hypoviridae, potentially representing a new genus and assigned the name Sclerotinia sclerotiorum hypovirus 2 (SsHV2/5472). Phylogenetic analysis based on the amino acid sequences of the endornaviral methyletransferase (MTR), helicase (Hel) and RNA dependent RNApolymerase (RdRp) domains revealed that endornaviruses clustered into two distinct groups, one of which includes members from ascomycetous fungi having shorter genomes than those of the second group and possibly lacking a coding strand nick. This observation suggests the classification of Endornaviridae into two genera, Alphaendornavirus and Betaendornavirus. The genome of the hypovirus SsHV2 is the longest among hypoviruses with unique combination of characteristics, including some typically associated with alphahypoviruses and some from betahypoviruses, suggesting the role of recombination in the evolution of the Hypoviridae.
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