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.