Genomic, proteomic, and phylogenetic analysis of spounaviruses indicates paraphyly of the order Caudovirales

Since the mid-20th century, prokaryotic double-stranded DNA viruses producing tailed particles ("tailed phages") were grouped according to virion tail morphology. In the early 1980s, these viruses were classified into the families Myoviridae , Siphoviridae , and Podoviridae , later included in the order Caudovirales . However, recent massive sequencing of prokaryotic virus genomes revealed that caudovirads are extremely diverse. The official taxonomic framework does not adequately reflect caudovirad evolutionary relationships. Here, we reevaluate the classification of caudovirads using a particularly challenging group of viruses with large dsDNA genomes: SPO1-like viruses associated with the myovirid subfamily Spounavirinae . Our extensive genomic, proteomic, and phylogenetic analyses reveal that some of the currently established caudovirad taxa, especially at the family and subfamily rank, can no longer be supported. Spounavirins alone need to be elevated to family rank and divided into at least five major clades, a first step in an impending massive reorganization of caudovirad taxonomy.