THE INFLUENCE OF TACHYTELICALLY (RAPIDLY) EVOLVING SEQUENCES ON THE TOPOLOGY OF PHYLOGENETIC TREES - INTRAFAMILY RELATIONSHIPS AND THE PHYLOGENETIC POSITION OF PLANCTOMYCETACEAE AS REVEALED BY COMPARATIVE-ANALYSIS OF 16S RIBOSOMAL-RNA SEQUENCES

The 16S rDNA of six members of four genera of the family Planctomycetaceae have been analyzed and compared with representatives of all major phyla of the domain Bacteria. When the individual sequences are tested separately, species of Planctomyces and Pirellula cluster with Chlamydia psittaci, while Gemmata obscuriglobus and Isosphaera pallida constitute independant and much more ancient lines of descent. All planctomycetes however cluster together when their sequences are compared in the same analysis with those of reference sequences. Then, their positon in the dendrogram of relationships mirrors the presence of an ancient phylum, following the Thermotoga line of descent. This unusual behaviour in data analysis, together with common, derived signature nucleotides in the sequences of planctomycetes and Chlamydia psittaci, the lower than average binary similarity values for planctomycetes with members of other phyla, and the unique phenotypic and genotypic properties, support the hypothesis (Woese, 1987) that planctomycetes are rapidly evolving bacteria.