The problem of taxonomic placement of Aciduliprofundum is a good example to demonstrate how one extract information from CVTrees. In the Reysenbach et al. Nature 2006 paper it was taken as the first cultivated member of the DHEV2 (deep-sea hydrothermal euryarchaeate 2) clade based on a maximum-likelihood 16S rRNA tree. Unfortunately, all other 13 members of this clade were represented by 16S rRNA sequences only and no genome data are available so far. The NCBI taxonomy gave an incomplete lineage: Archaea; Euryrchaeota; unclassified Euryarchaeota; missing taxonomic assignment at the rank class and below. In order to make use of CVTree we must touch on the K-issue a little more. The alignment-free comparison of genomes in CVTree is implemented by counting the number of K-peptides in the protein products encoded in a genome followed by subtraction of a random background caused by neutral mutations. The peptide length K looks like a parameter, but it is actually not a parameter. Using a longer K emphasizes species-specificity, while a shorter K takes into account more common features with neighboring species. However, we never adjust K: a fixed K is used for all genomes to construct a tree, but one may construct a series of trees for K = 3, 4, 5, 6, 7, 8, … We have shown repeatedly that R9 K = 5 and 6 lead to best results in the sense of agreement with taxonomy, so usually only a K = 6 tree is given in publications. Let us look at a subtree, i.e., part of a tree, containing the organisms of interest. If the branching order in all trees built for different Ks turns out to be same, it would be a strong support to the branching order. In most cases the branching order varies with K: K = 3 and 4 make sense, K = 5 and 6 yield the best, K = 7 and 8 become slightly worse, etc. For too big a K, even if the closest strains remain grouped together the whole tree may tend to become a star-tree, i.e., every small clade stands in its own and their mutual placements become less meaningful. Therefore, inspection of trees for a range of K-values provides an additional dimension to evaluate the results. For Aciduprofumdum we have a stable pair (<C>Thermococci{18}, <G>Aciduprofumdum{2}) at K=3, 5, 6, 7. At K = 4 we have (<C>Thermococci{18}, (<G>Staphylothermus{2}, <G>Aciduprofundum{2})) In all these cases Thermoplasmata stands farther away from the above pair. However, at K = 8 and 9, when the overall tree picture has been largely distorted, Aciduliprofundum does stand closer to Thermaplasmata. Putting together all the above results we tend to consider the pair (<C>Thermococci{18}, <G>Aciduliprofundum{2}) as reflecting a more probable relation. Confined to the available data for the time being one may assign <G>Aciduliprofundum to <C>Thermococci, e.g., to denote the pair as <C>Thermococci{20}=(<F>Thermococcaceae{18}, <G>Aciduliprofundum{2}) leaving its family unclassified or assign it to a new family. Without further phenotypic and chemotaxonomic evidence it is better not to introduce new taxon names if the present naming scheme is capable to accommodate the leaves without conflict. This was why we wrote “this modification would hold as long as no new facts challenge it”. Anyway, taxonomy has always been a work in progress. One has to be prepared for modifications when new data appear. To make a long story short, we have rewritten the paragraph as: “The Candidatus genus Aciduliprofundum is considered a member of the DHEV2 (deap-sea hydrothermal vent euryarchaeotic 2) phylogenetic cluster. No taxonomic information was given in the original papers [55,56]. The NCBI Taxonomy did not provide definite lineage information for this taxon at the class, order, and family ranks. According to [55] the whole DHEV2 cluster was located close to Thermoplamatales in a maximum-likelihood analysis of 16S rRNA sequences. A similar placement was seen in [54] where a Bayesian tree of the archaeal domain based on concatenation of 57 ribosomal proteins put a lonely Aciduliprofundum next to Thermoplasmata. However, in CVTrees, constructed for all K-values from 3 to 9, Aciduliprofundum juxstaposes with the class Thermococci{18}. An observation in [56] that this organism shares a rare lipid structure with a few species from Thermococcales may hint on its possible association with the latter. If we temporarily presume a lineage <C>Thermococci<O>Unclassified<F>Unclassified<G>Aciduliprofundum… R10 one might have a monophyletic class <C>Thermococci{20}. Since none of the 13 DHEV2 members listed in [55] has a sequenced genome so far, CVTree cannot tell the placement of the DHEV2 cluster as a whole for the time being. It remains an open problem as whether DHEV2 is close to Thermoplasmata or to Thermococci, or a new class is needed to accommodate DHEV2.”
General Comments The topic is relevant for microbial taxonomy.