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Bipartite network analysis of the archaeal virosphere: evolutionary connections between viruses and capsid-less mobile elements

Abstract : Archaea and particularly hyperthermophilic crenarchaea are hosts to many unusual viruses with diverse virion shapes and distinct gene compositions. As is typical of viruses in general, there are no universal genes in the archaeal virosphere. Therefore, to obtain a comprehensive picture of the evolutionary relationships between viruses, network analysis methods are more productive than traditional phylogenetic approaches. Here we present a comprehensive comparative analysis of genomes and proteomes from all currently known taxonomically classified and unclassified, cultivated and uncultivated archaeal viruses. We constructed a bipartite network of archaeal viruses that includes two classes of nodes, the genomes and gene families that connect them. Dissection of this network using formal community detection methods reveals strong modularity with 10 distinct modules and 3 putative supermodules. However, compared to the previously analyzed similar networks of eukaryotic and bacterial viruses, the archaeal virus network is sparsely connected. With the exception of the tailed viruses related to the bacteriophages of the order Caudovirales and the families Turriviridae and Sphaerolipoviridae that are linked to a distinct supermodule of eukaryotic viruses, there are few connector genes shared by different archaeal virus modules. In contrast, most of these modules include, in addition to viruses, capsid-less mobile elements, emphasizing tight evolutionary connections between the two types of entities in archaea. The relative contributions of distinct evolutionary origins, in particular from non-viral elements, and insufficient sampling to the sparsity of the archaeal virus network remain to be determined by further exploration of the archaeal virosphere. IMPORTANCE: Viruses infecting archaea are among the most mysterious denizens of the virosphere. Many of these viruses display no genetic or even morphological relationship to viruses of bacteria and eukaryotes, raising questions regarding their origins and position in the global virosphere. Analysis of 5740 protein sequences from 116 genomes allowed dissection of the archaeal virus network and showed that most groups of the archaeal viruses are evolutionarily connected to capsid-less mobile genetic elements, including various plasmids and transposons. This finding could reflect actual independent origins of the distinct groups of archaeal viruses from different non-viral elements, providing important insights into the emergence and evolution of the archaeal virome.
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Jaime Iranzo, Eugene V. Koonin, David Prangishvili, Mart Krupovic. Bipartite network analysis of the archaeal virosphere: evolutionary connections between viruses and capsid-less mobile elements. Journal of Virology, American Society for Microbiology, 2016, 90 (24), pp.11043-11055. ⟨10.1128/JVI.01622-16⟩. ⟨pasteur-01375582⟩

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