In the past 10 years, the three phage families belonging to the order Caudovirales have grown from 18 genera and 36 species to 14 subfamilies, 204 genera and 873 species. In spite of heroic effort by the Bacterial and Archaeal Viruses Subcommittee (BAVS) of ICTV, a substantial proportion of complete phage genomes in GenBank still have not been assigned to recognized taxa. In the early stages of this classification effort, taxonomic assignments into genera or subfamilies relied heavily on assessment of the percentage of homologous proteins, whereas recently whole genome DNA comparisons have been used. While it would be beneficial to the phage community for BAVS to definitively state, in molecular terms, what constitutes a phage genus, subfamily and family, phage taxonomy is not always straightforward as different viral groups are under different evolutionary pressures. What our work has shown is that a huge diversity exist within “related” phages such as the T4, T7 and SPO1 “superfamilies” of viruses that cannot be logically expressed within the current taxonomy. For example there is as much diversity among the T4-related phages, which are currently grouped into a subfamily (Tevenvirinae), as there is between Herpes-like viruses, which are represented by the order Herpesvirales. Furthermore, a unifying taxonomic structure of what is commonly known as the lambdoid phages is not possible because they fall into the families Siphoviridae (Lambda) and Podoviridae (P22). Recently, the BAVS has employed extensive tools to analyze 93 SPO1-related phages, indicating this group is best described as a separate family.
The possible abolition of the order Caudovirales or the dismantling of the Myoviridae, Podoviridae and Siphoviridae families would mean a paradigm shift in phage classification. Therefore, the BAVS would like to proceed with as much support from the phage community as possible and your input is very much appreciated.