The phylogenetic relationships of Caulobacter, Asticcacaulis and Brevundimonas species and their taxonomic implications

The phylogenetic relationships among the species of Caulobacter, Asticcacau lis and Brevundimonas were studied by comparison of their 16S rDNA sequence s. The analysis of almost complete sequences confirmed the early evolutiona ry divergence of the freshwater and marine species of Caulobacter reported previously [Stahl, D. A., Key, R,, Flesher, B, & Smit, J. (1992), J Bacteri ol 174, 2193-2198]. The freshwater species formed two distinct clusters. On e cluster contained the species Caulobacter bacteroides, Caulobacter cresce ntus, Caulobacter fusiformis and Caulobacter henricii. C, bacteroides and C , fusiformis are very closely related (sequence identity 99.8%). The second cluster was not exclusive and contained the species Caulobacter intermediu s, Caulobacter subvibrioides and Caulobacter variabilis, as well as Brevund imonas diminuta and Brevundimonas vesicularis, The marine species Caulobact er halobacteroides and Caulobacter maris were very closely related, with a sequence identity of 99.7%, These two species were most closely but distant ly related to the marine hyphal/budding bacteria Hyphomonas jannaschiana an d Hirschia baltica, which formed a deep phylogenetic line with Rhodobacter sphaeroides and Rhodobacter capsulatus, Caulobacter leidyia is unrelated to the other species of Caulobacter and belongs to the alpha-4 subclass of th e Proteobacteria, forming a distinct cluster with Asticcacaulis excentricus and Asticcacaulis biprosthecium, The taxonomic implications of the polyphy letic nature of the genus Caulobacter and the absence of a type culture for the type species of the genus, Caulobacter vibrioides, are discussed.