According to
ICTV's taxon history: •
Lambda phage appeared in the first report of 1971, unassigned to order, family, or subfamily. • Genus
Lambda phage group created in the second report (1975). To it were also added three phages: PA2, phiD328, and phi80 (none of which are in 2024
Lambdavirus). • Assigned to family
Siphoviridae in 1984 upon the family's creation. • Renamed to
Lambda-like phages in sixth report (1995). By this point phi80 and PA2 were still in the genus. HK022,
HK97, phiD328 were abolished. • Siphoviridae moved to newly created order
Caudovirales in 1998. • Renamed to
Lambda-like viruses in ICTV's seventh report in 1999. • In 2012, the genus was renamed again, to
Lambdalikevirus. In 2018 HK022 was moved to
Hendrixvirus by rename of the genus. In 2019 HK022 was moved to its own genus
Shamshuipovirus. • PA2 was abolished at an unknown date and re-admitted in 2016 in the genus
Tl2011virus. In 2018 it was moved to
Oslovirus by rename of the genus. • phiD328 and phi80 are not found anywhere in the 2024 ICTV taxonomy. Given the above history, the latest document that defines the boundary of the genus is the 2017 document that created
Hk97virus. Unfortunately the document does not specifically address the inclusion criteria for
Lambdavirus, only the evidence for their new genus being monophyletic and reasonably similar with each other. The only part that shows the similarity among the members of
Lambdavirus was in figure 2, a Gegenees BLASTN analysis using window of 100 bp and a slide of 50 bp. The lowest Gegeenes similarity among members of the group was 62.5%. The general guideline from ICTV's Bacterial and Archaeal Viruses Subcommitee in 2017 was 50% nucleotide similarity. The term
lambdoid phage remains commonly used. It originally consisted of an operational definition of "phages that can recombine with lambda and form a functional, recombinant phage". The term was slightly expanded in the genomic era to mean phages that have a similar-enough functional gene order and at least some patch of homology so that, if
hypothetically recombined with lambda, it would form a functional phage with all required genes. The possibility of recombination does not necessarily imply a recent common ancestor and the apparently recombinant character of P22 adds to the problem. Phages that have been called lambdoid include members as diverse as
Lederbergvirus P22,
Byrnievirus HK97, Salmonella phage ES18, Salmonella phage Gifsy-2, and
Traversvirus tv933W. ==References==