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  1. Virus–host interactions evolve along a symbiosis continuum from antagonism to mutualism. Long-term associations between virus and host, such as those in chronic infection, will select for traits that drive the interaction towards mutualism, especially when susceptible hosts are rare in the population. Virus–host mutualism has been demonstrated in thermophilic archaeal populations where Sulfolobus spindle-shaped viruses (SSVs) provide a competitive advantage to their host Sulfolobus islandicus by producing a toxin that kills uninfected strains. Here, we determine the genetic basis of this killing phenotype by identifying highly transcribed genes in cells that are chronically infected with a diversity of SSVs. We demonstrate that these genes alone confer growth inhibition by being expressed in uninfected cells via a Sulfolobus expression plasmid. Challenge of chronically infected strains with vector-expressed toxins revealed a nested network of cross-toxicity among divergent SSVs, with both broad and specific toxin efficacies. This suggests that competition between viruses and/or their hosts could maintain toxin diversity. We propose that competitive interactions among chronic viruses to promote their host fitness form the basis of virus–host mutualism. This article is part of the theme issue ‘The secret lives of microbial mobile genetic elements’. 
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  2. ABSTRACT Theory, simulation, and experimental evolution demonstrate that diversified CRISPR-Cas immunity to lytic viruses can lead to stochastic virus extinction due to a limited number of susceptible hosts available to each potential new protospacer escape mutation. Under such conditions, theory predicts that to evade extinction, viruses evolve toward decreased virulence and promote vertical transmission and persistence in infected hosts. To better understand the evolution of host-virus interactions in microbial populations with active CRISPR-Cas immunity, we studied the interaction between CRISPR-immune Sulfolobus islandicus cells and immune-deficient strains that are infected by the chronic virus SSV9. We demonstrate that Sulfolobus islandicus cells infected with SSV9, and with other related SSVs, kill uninfected, immune strains through an antagonistic mechanism that is a protein and is independent of infectious virus. Cells that are infected with SSV9 are protected from killing and persist in the population. We hypothesize that this infection acts as a form of mutualism between the host and the virus by removing competitors in the population and ensuring continued vertical transmission of the virus within populations with diversified CRISPR-Cas immunity. IMPORTANCE Multiple studies, especially those focusing on the role of lytic viruses in key model systems, have shown the importance of viruses in shaping microbial populations. However, it has become increasingly clear that viruses with a long host-virus interaction, such as those with a chronic lifestyle, can be important drivers of evolution and have large impacts on host ecology. In this work, we describe one such interaction with the acidic crenarchaeon Sulfolobus islandicus and its chronic virus Sulfolobus spindle-shaped virus 9. Our work expands the view in which this symbiosis between host and virus evolved, describing a killing phenotype which we hypothesize has evolved in part due to the high prevalence and diversity of CRISPR-Cas immunity seen in natural populations. We explore the implications of this phenotype in population dynamics and host ecology, as well as the implications of mutualism between this virus-host pair. 
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  3. Abstract

    Characterizing the molecular interactions of viruses in natural microbial populations offers insights into virus–host dynamics in complex ecosystems. We identify the resistance ofSulfolobus islandicustoSulfolobusspindle‐shaped virus (SSV9) conferred by chromosomal deletions of pilin genes,pilA1andpilA2that are individually able to complement resistance. Mutants with deletions of bothpilA1andpilA2or the prepilin peptidase, PibD, show the reduction in the number of pilins observed in TEM and reduced surface adherence but still adsorb SSV9. The proteinaceous outer S‐layer proteins, SlaA and SlaB, are not required for adsorption nor infection demonstrating that the S‐layer is not the primary receptor for SSV9 surface binding. Strains lacking both pilins are resistant to a broad panel of SSVs as well as a panel of unrelatedS. islandicusrod‐shaped viruses (SIRVs). Unlike SSV9, we show thatpilA1orpilA2is required for SIRV8 adsorption. In sequencedSulfolobusstrains from around the globe, one copy of eachpilA1andpilA2is maintained and show codon‐level diversification, demonstrating their importance in nature. By characterizing the molecular interactions at the initiation of infection betweenS. islandicusand two different types of viruses we hope to increase the understanding of virus–host interactions in the archaeal domain.

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