![]() Host factors rather than viral titre or order of infection appear to have the biggest influence on the equilibrium levels of introduced viral strains to Citrus trees 15. At the viral population level, Syller 14 described different synergistic and antagonistic multiple viral interactions in plants leading to various outcomes, such as pathogenesis, viral evolution and the spatial separation of viruses among plant tissues. Horizontal gene transfer has been inferred between positive-sense RNA viruses from different families in the fungal pathogen, Sclerotinia sclerotiorum 13. Multiple virus infections involve numerous interactions among the viruses themselves, the host and the environment, resulting in changes at both the molecular and population levels that can lead to a transition in life-styles from persistent to acute, resulting in a disease phenotype 12. Furthermore fungal growth, which is characterised by cytoplasmic exchange in the vegetative and sexual phases, promotes the accumulation of multiple viruses which then develop coexistence strategies within an individual hypha, colony or hyphal network. Initial infections are therefore likely to be rare and ancient events which, once established, lead to long-term co-existence between virus and host in a non-lethal persistent life-style. For fungi, viral infection occurs predominantly via cytoplasmic exchange following cell-to-cell contact and initial infections from free non-cellular viruses have been difficult to demonstrate with only one example, a DNA virus, of fungal infection 8, 11. Multiple viral infections are often asymptomatic representing persistent static life-styles which appear to be benign, symbiotic or possibly beneficial to the host. In fungi, infection has been associated with a large numbers of distinct RNAs 8: 26 RNAs have been identified in the fungus Beauveria bassiana with 11 in a single isolate 9, and 26 RNAs found in the cultivated mushroom Agaricus bisporus with 16 in a single isolate 10. While in Homo sapiens infection with single viruses is mostly associated with a single pathogenic outcome, there is evidence that multiple viral infections can change disease symptoms 5, 6, 7. Seven RNA viruses have been sequenced from the grapevine, Vitis vinifera, associated with the disorder Syrrah/Shiraz decline 1, 2 and multiple virus infections have been described in insects such as the honey bee 3, 4. Multiple viral infections occur in many Eukaryotic organisms, animals, plants and fungi. As the non-cellular transmission of mycoviruses is rare, the founding infections may be ancient, preserved in wild Agaricus populations, which act as reservoirs for subsequent cell-to-cell infection when host populations are expanded massively through fungiculture. ![]() The viral composition of growing cultures was dynamic, with evidence of gains and losses depending on the environment and included new hypothetical viruses when compared with the current transcriptome and EST databases. Large numbers of viral RNAs were detected in multiple Agaricus samples up to 24 in samples symptomatic for disease and 8–17 in asymptomatic samples, suggesting adaptive strategies for co-existence. Multiple infections of Agaricus may represent a diverse, dynamic and interactive viral ecosystem with sequence variability ranging over 2 orders of magnitude and evidence of recombination, horizontal gene transfer and variable fragment numbers. ![]() The viruses, all positive sense single-stranded, were classified into diverse orders/families. Two viruses were multipartite with component RNAs showing correlative abundances and common 3′ motifs. Thirty unique non-host RNAs were sequenced in the cultivated fungus, Agaricus bisporus, comprising 18 viruses each encoding an RdRp domain with an additional 8 ORFans (non-host RNAs with no similarity to known sequences). ![]()
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