Very little is known about fungal viruses, and to date no studies have characterized natural communities of fungi and their viruses. Some fungal viruses exert strong effects on their fungal hosts (positive or negative), or on other symbionts of the infected fungi. These effects include hypovirulence (virus-infected fungi are less pathogenic to plants) or hypervirulence in pathogenic fungi, and heat-tolerance in an endophytic fungus. Most studies of fungal viruses focus on viruses of economically important fungi such as human or plant pathogens. To determine the prevalence and diversity of fungal viruses in a natural setting, we surveyed virus diversity in a community of fungi associated with one species of the parasitic plant dodder (Cuscuta cuspidata) and its most frequently infected host plant, western ragweed (Ambrosia psilostachya), in two years and at four sites on the Tall Grass Prairie Preserve in Pawhuska, OK. We extracted double-stranded RNA (dsRNA) from 245 representative fungal isolates, followed by agarose gel electrophoresis. Then we conducted reverse-transcription Polymerase Chain Reaction (rt-PCR) on samples showing banding patterns consistent with dsRNA viruses, using random primers, followed by cloning, and obtained partial sequences.

Results/Conclusions

At least 21 genera of fungi were isolated from surface-sterilized tissues of dodder and parasitized ragweed. These fungi included Alternaria alternata, Stemphylium solani, Cladosporium sp., Leptosphaeria spp., Curvularia spp., Phoma spp., and Cercospora spp., which may be latent saprotrophs, latent pathogens, or endophytes. We found band patterns consistent with viruses in 13-14% of samples. Viruses represented the Endornaviridae (4), Totiviridae (6), Astroviridae (1), Chrysoviridae (2), Hypoviridae (1), Narnaviridae (2), Partitiviridae (1), and some unknowns (13), as determined from BLASTx searches against GenBank. Because we have recovered few representatives of any particular virus type among multiple fungal isolates, and species-sampling curves for both fungi and their viruses increase steadily, the diversity of fungal viruses may be very high. However, the four endornaviruses may represent the same (or closely related) species, as some of their partial sequences align with each other. In addition, one Totivirus was found in both years of the study, one in each of two sites. Thus, fungal viruses may represent a frontier of cryptic biodiversity with the potential to alter the fitness and ecology of their fungal hosts and associated plants.