Bacteriophages are known to display a broad range of host spectra, typically infecting a small set of related bacterial species. The transfer of bacteriophages between more distant clades of bacteria has often been postulated, but remains mostly unaddressed. In this work we leverage the sequencing of novel cluster of phages infecting Streptomyces bacteria and the availability of large numbers of complete phage genomes in public repositories to address this question. Using phylogenetic and comparative genomics methods, we show that several clusters of Actinobacteria-infecting phages are more closely related between them, and with a small group of Firmicutes phages, than with any other Actinobacteriophage lineage. These data indicate that this heterogeneous group of phages shares a common ancestor with well-defined genome structure. Analysis of genomic %GC content shows that these Actinobacteriophages are poorly adapted to their Actinobacteria hosts, suggesting that this phage lineage originated in an ancestor of the Firmicutes, adapted to high %GC content members of this phylum and later migrated to the Actinobacteria.

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