Analysis of spounaviruses as a case study for the overdue reclassification of tailed phages

Tailed bacteriophages are the most abundant and diverse viruses in the world, with genome sizes ranging from 10 kbp to over 500 kbp. Yet, due to historical reasons, all this diversity is confined to a single virus order - Caudovirales, composed of just four families: Myoviridae, Siphoviridae, Podoviridae, and the newly created Ackermannviridae family. In recent years this morphology-based classification scheme has started to crumble under the constant flood of phage sequences, revealing that tailed phages are even more genetically diverse than once thought. This prompted us, the Bacterial and Archaeal Viruses Subcommittee of the International Committee on Taxonomy of Viruses (ICTV) to consider overall reorganization of phage taxonomy. In this study, we used a wide range of complementary methods – including comparative genomics, core genome analysis, and marker gene phylogenetics – to show that the group of Bacillus phage SPO1-related viruses previously classified into the Spounavirinae ...