Abstract
Heterobasidion species are highly destructive basidiomycetous conifer pathogens of the Boreal forest region. Earlier studies have revealed dsRNA virus infections of families Curvulaviridae and Partitiviridae in Heterobasidion strains, and small RNA deep sequencing has also identified infections of Mitoviridae members in these fungi. In this study, the virome of Heterobasidion parviporum was examined for the first time by RNA-Seq using total RNA depleted of rRNA. This method successfully revealed new viruses representing two established (+)ssRNA virus families not found earlier in Heterobasidion: Narnaviridae and Botourmiaviridae. In addition, we identified the presence of a recently described virus group tentatively named “ambiviruses” in H. parviporum. The H. parviporum isolates included in the study originated from experimental forest sites located within 0.7 km range from each other, and a population analysis including 43 isolates was conducted at one of the experimental plots to establish the prevalence of the newly identified viruses in clonally spreading H. parviporum individuals. Our results indicate that viral infections are considerably more diverse and common among Heterobasidion isolates than known earlier and include ssRNA viruses with high prevalence and interspecies variation.
Highlights
The existence of viral infections in fungi was first discovered by Hollings (1962) who described virus-associated disease symptoms in the cultivated mushroom Agaricus bisporus
The hosts of mycoviruses were detected with RT-PCR, and DNA of virus positive H. parviporum isolates was used to confirm the novel viruses were not integrated to the genome of the host
The RNA sequencing (RNA-Seq) approach used in the present study successfully revealed members of all three virus families known to infect Heterobasidion species: Partitiviridae, Mitoviridae (HetMV3 represented by two ssRNA Viruses of Heterobasidion new strains and one variant), and Curvulaviridae and viruses representing groups not detected in isolates of Heterobasidion earlier: a putative narna-like virus, an ourmia-like virus, and four ambi-like viruses
Summary
The existence of viral infections in fungi was first discovered by Hollings (1962) who described virus-associated disease symptoms in the cultivated mushroom Agaricus bisporus. High-throughput sequencing (HTS) studies have recently revolutionized our understanding on the virosphere and revealed numerous novel virus clades that await classification by the International Committee on Taxonomy of Viruses (ICTV). Among these are many new fungal narna-like viruses (Chiapello et al, 2020, 2021; Sutela et al, 2020; Chiba et al, 2021; Jia et al, 2021) and the unclassified virus group tentatively named “ambiviruses” recently detected in asco- and basidiomycetous fungi (Sutela et al, 2020; Forgia et al, 2021; Linnakoski et al, 2021). The accumulating sequence information has led to the recognition of new host taxa for viruses, for example, plants as hosts for mitoviruses (Nibert et al, 2018) and fungi as hosts of ourmia-like viruses ( classified in the viral family Botourmiaviridae in order Ourlivirales), and the re-classification of many fungal viruses, for example, the division of family Narnaviridae into Mitoviridae and Narnaviridae
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