The phylogenetic relationships among the species of and were studied by comparison of their 16S rDNA sequences. The analysis of almost complete sequences confirmed the early evolutionary divergence of the freshwater and marine species of reported previously [Stahl, D. A., Key, R., Flesher, B. & Smit, J. (1992). J Bacteriol 174, 2193-2198]. The freshwater species formed two distinct clusters. One cluster contained the species and and are very closely related (sequence identity 99.8%). The second cluster was not exclusive and contained the species and , as well as and . The marine species and were very closely related, with a sequence identity of 99.7%. These two species were most closely but distantly related to the marine hyphal/budding bacteria and , which formed a deep phylogenetic line with Rhodobacter and Rhodobacter capsulatus. is unrelated to the other species of and belongs to the alpha-4 subclass of the , forming a distinct cluster with and . The taxonomic implications of the polyphyletic nature of the genus and the absence of a type culture for the type species of the genus, , are discussed.


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