[Analysis of phylogenetic criteria for estimation of the rank of taxa in methane-oxidizing bacteria].

To determine a possibility of application of phylogenetic criteria for estimating the taxa rank, the intra- and interspecies, as well as intergeneric relatedness of methanotrophs on the basis of 16S rRNA gene sequences was estimated. We used sequences of 16S rRNA genes of the studied isolates of obligate methanotrophs which have been deposited in UCM (Ukrainian Collection of Microorganisms), and of type strains of other obligate methanotrophs species (from GenBank database). It is shown, that the levels of interspecies and intergeneric relatedness in different families of methanotrophs are not identical, and therefore they can be used for differentiation of taxa only within one family. The carried out analysis has shown, that it is necessary to reconsider taxonomic position: (1) of two phenotypically similar species of Methylomonas (M. aurantiaca and M. fodinarum), similarity of 16S rRNA genes which is 99.4%, similarity of their total DNA--up to 80% that rather testifies to strain differences, than to species differences; (2) of species Methylomicrobium agile and M album which are phylogenetically more related to genus Methylobacter (97% of affinity), than Methylomicrobium (94% of affinity); (3) of genera of the family Beijerinckiaceae (Methylocella and Methylocapsa), and also genera of the family Methylocystaceae (Methylosinus and Methylocystis), whereas high level of relatedness (97% and more) of these bacteria with other methanotrophic genera (within one family) practically corresponds to a range of relatedness of species (within some genera) in the family Methylococcaceae. When determining phylogenetic criteria which can characterize the ranks of taxa, it was revealed, that the levels of interspecies relatedness of methanotrophic genera of the families Methylocystaceae and Beijerinckiaceae (97.8-99.1% and 97.8%, accordingly) considerably exceed the level of genera formation in the family Methylococcaceae (94.0-98.2%) and, moreover, approach the value of intraspecies relatedness of the family Methylococcaceae (97.5-99.3%). Coefficients of intraspecies relatedness of methanotrophs of the families Methylocystaceae and Beijerinckiaceae are sometimes equal to interspecies relatedness. Hence, taxa of various rank can have the identical level of genes divergence. Thus, methanotrophic taxons of the families Methylocystaceae and Beijerinckiaceae have not demonstrated precise phylogenetic criteria which could correspond both to the species rank, or the genus rank. At the same time, the criteria, being adequate to the rank of certain taxa, are revealed in methanotrophs of the family Methylococcaceae. The level of genotypic relatedness of strains of the same species is in the range of 97.5-99.3%, species of the same genus--94-98%, the highest levels of relatedness between genera of this family are 90-96%.