Hidden diversity of double-stranded DNA phages in symbiotic Rhizobium species.

In this study, we addressed the extent of diversification of phages associated with nitrogen-fixing symbiotic Rhizobium species. Despite the ecological and economic importance of the Rhizobium genus, little is known about the diversity of the associated phages. A thorough assessment of viral diversity requires investigating both lytic phages and prophages harboured in diverse Rhizobium genomes. Protein-sharing networks identified 56 viral clusters (VCs) among a set of 425 isolated phages and predicted prophages. The VCs formed by phages had more proteins in common and a higher degree of synteny, and they group together in clades in the associated phylogenetic tree. By contrast, the VCs of prophages showed significant genetic variation and gene loss, with selective pressure on the remaining genes. Some VCs were found in various Rhizobium species and geographical locations, suggesting that they have wide host ranges. Our results indicate that the VCs represent distinct taxonomic units, probably representing taxa equivalent to genera or even species. The finding of previously undescribed phage taxa indicates the need for further exploration of the diversity of phages associated with Rhizobium species. This article is part of the theme issue 'The secret lives of microbial mobile genetic elements'.