Formal Comment to Pettengill: The Time to Most Recent Common Ancestor Does Not (Usually) Approximate the Date of Divergence

In 2013 Zhou et al. concluded that Salmonella enterica serovar Agona represents a genetically monomorphic lineage of recent ancestry, whose most recent common ancestor existed in 1932, or earlier. The Abstract stated 'Agona consists of three lineages with minimal mutational diversity: only 846 single nucleotide polymorphisms (SNPs) have accumulated in the non-repetitive, core genome since Agona evolved in 1932 and subsequently underwent a major population expansion in the 1960s.' These conclusions have now been criticized by Pettengill, who claims that the evolutionary models used to date Agona may not have been appropriate, the dating estimates were inaccurate, and the age of emergence of Agona should have been qualified by an upper limit reflecting the date of its divergence from an outgroup, serovar Soerenga. We dispute these claims. Firstly, Pettengill's analysis of Agona is not justifiable on technical grounds. Secondly, an upper limit for divergence from an outgroup would only be meaningful if the outgroup were closely related to Agona, but close relatives of Agona are yet to be identified. Thirdly, it is not possible to reliably date the time of divergence between Agona and Soerenga. We conclude that Pettengill's criticism is comparable to a tempest in a teapot.