Population structure of a novel putative mycovirus infecting the conifer root-rot fungus Heterobasidion annosum sensu lato

Abstract

We describe a novel putative mycovirus infecting the conifer root-rot fungus Heterobasidion annosum sensu lato. This virus, designated as Heterobasidion RNA virus 6 (HetRV6), is taxonomically distant from all previously known viruses of Heterobasidion species, but somewhat related to the Curvularia thermal tolerance virus and the Fusarium graminearum virus 4. Based on a population analysis including 35 virus strains from Heterobasidion abietinum, Heterobasidion parviporum, Heterobasidion annosum sensu stricto and Heterobasidion occidentale, HetRV6 showed a considerable degree of geographical and host-related differentiation. The North American and Eurasian virus populations were clearly separated. In Eurasia, we observed cases of discrepancy between virus and host taxonomy, suggesting interspecies virus transfer. HetRV6 was also successfully transmitted between the three European species H. abietinum, H. annosum and H. parviporum. Based on growth rate tests on agar plates and spruce stem pieces, HetRV6 seemed to be cryptic or slightly mutualistic to its host.

Viruses accumulate in aging infection centers of a fungal forest pathogen

“…In this investigation, we show that Heterobasidion isolates at a forest site with a long history of root and butt rot had a viral infection rate of 48-67%, which was considerably higher than the typical 15-17% that was observed in earlier studies (Ihrmark, 2001;Vainio et al, 2012) and suggests that viruses accumulate over time in Heterobasidion clones. Our 7-year follow-up work also revealed the appearance of novel viral infections in aging clones.…”

“…Four distinct viral species occurred at the sample plot: three putative members of Partitiviridae and the yet-to-be-assigned HetRV6 (Vainio et al, 2012). We found no indications of additional viral strains being present at the study plot, as the dsRNA patterns shown by CF11 chromatography agreed with the RT-PCR results and all dsRNA-positive isolates could be associated with one (or more) of these four viral species by RT-PCR.…”

“…To distinguish specific HetRV6 strains from minor sequence variants, we performed a phylogenetic analysis that included published sequences of HetRV6 from distant locations in its global distribution (Vainio et al, 2012). Based on Neighbor-Joining (NJ) analysis (Supplementary Figure S1), eight distinct clusters of HetRV6 sequences (corresponding to viral strains) were recovered from the study site.…”

“…The unassigned species Heterobasidion RNA virus 6 (HetRV6) is responsible for approximately 70% of dsRNA infections in European strains of Heterobasidion (Vainio et al, 2012) and shares a relatively high polymerase similarity with the Curvularia thermal tolerance virus (Márquez et al, 2007). All other Heterobasidion viruses described to date are members of Partitiviridae (Ihrmark, 2001;Vainio et al, 2010Vainio et al, , 2011aVainio et al, , b, 2013b.…”

“…These viruses also spread vertically via basidiospores and conidia (Ihrmark et al, 2002(Ihrmark et al, , 2004. Most Heterobasidion viruses do not cause drastic phenotypic changes in their host, but some may reduce the germination of basidiospores (Ihrmark et al, 2004) or affect the growth of the host fungus (Vainio et al, 2010(Vainio et al, , 2012. Notably, a single virus strain may have beneficial, commensal or detrimental effects on a single host isolate, depending on environmental and ecological conditions (Hyder et al, 2013).…”