Viral Interactions and Pathogenesis during Multiple Viral Infections in Agaricus bisporus.

Viral interactions during multiple viral infections were examined in Agaricus bisporus cultures harboring 9 viruses (comprising 18 distinct viral RNAs) by statistically analyzing their relative abundance in fruitbodies. Four clusters of viral RNA were identified that suggested synergism and coreplication. Pairwise correlations revealed negative and positive correlations between clusters, indicating further synergisms and an antagonism involving a group containing a putative hypovirus and four nonhost ORFan RNAs (RNAs with no similarity to known sequences) possibly acting as defective interfering RNAs. The disease phenotype was observed in 10 to 15% of the fruitbodies apparently randomly located among asymptomatic fruitbodies. The degree of symptom expression consistently correlated with the levels of the multipartite virus AbV16. Diseased fruitbodies contained very high levels of AbV16 and AbV6 RNA2; these levels were orders of magnitude higher than those in asymptomatic tissues and were shown statistically to be discretely higher populations of abundance, indicating an exponential shift in the replicative capacity of the virus. High levels of AbV16 replication were specific to the fruitbody and not found in the underlying mycelium. There appeared to be a stochastic element occurring in these viral interactions, as observed in the distribution of diseased symptoms across a culture, differences in variance between experiments, and a number of additional viruses undergoing the step-jump in levels between experiments. Possible mechanisms for these multiple and simultaneous viral interactions in single culture are discussed in relation to known virus-host regulatory mechanisms for viral replication and whether additional factors could be considered to account for the 1,000-fold increase in AbV16 and AbV6 RNA2 levels.IMPORTANCE How viruses interact in a multiple-virus infection was examined by quantifying the levels of 18 viral RNAs in fruiting cultures of the agriculturally cultivated fungus Agaricus bisporus and statistically analyzing and modeling their abundance. Synergistic, antagonistic, and neutral interactions occurred simultaneously in cultures. The viral RNAs were grouped into four clusters, each displaying similar relative abundance, and between clusters, further interactions were found with positive, negative, or no correlations. Mushroom fruitbodies showing disease symptoms were distributed apparently randomly across the culture. These symptoms were associated with the presence of viral RNAs from two different clusters at very high levels, 1,000-fold higher than asymptomatic fruitbodies. The role of viral interaction together with stochastic factors and the regulation of host antiviral defenses in pathogenesis are discussed.

Brown mushroom symptom expression following infection of an Agaricus bisporus crop with MVX associated dsRNAs.

Mushroom Virus X (MVX) is associated with a range of symptoms observed in mushroom crops. The most prominent symptom in Ireland is the occurrence of 'brown' or 'off-white' mushrooms in white strain crops. The browning symptoms are associated with the presence of four low molecular weight dsRNAs: MVX(0.6), MVX(0.8), MVX(1.8) and MVX(2.0), however viral dsRNAs also occur in non-symptomatic mushrooms. Three virus-infected mushroom cultures containing MVX(1.8) and MVX(2.0) were used to infect experimental crops at different rates and at different times in the crop cycle to test the effect on symptom expression. Mushroom colour was measured by chromometer, and the ΔE value calculated. RT-PCR was used to test for the presence of MVX(1.8) dsRNA in harvested mushrooms. Results indicate that following infection, browning symptom expression is variable both within and between crops. Control mushrooms from 1st and 2nd flush had ΔE values of 7-12, with most being <10. In contrast, 1st flush mushrooms from virus infected treatments had ΔE values of 6-25, with most being >10 while 2nd flush mushrooms had ΔE values similar to controls. Only mushrooms with ΔE > 15 appeared visibly brown or off colour. The transient and inconsistent nature of MVX-associated browning symptoms is discussed.