Insight into the Evolution of Magnetotaxis in Magnetospirillum spp., Based on mam Gene Phylogeny

Vibrioid- to helical-shaped magnetotactic bacteria phylogenetically related to the genusMagnetospirillum were isolated in axenic cultures from a number of freshwater and brackish environments located in the southwestern United States. Based on 16S rRNA gene sequences, most of the new isolates represent newMagnetospirillum species or new strains of knownMagnetospirillum species, while one isolate appears to represent a new genus basal toMagnetospirillum . Partial sequences of conservedmam genes, genes reported to be involved in the magnetosome and magnetosome chain formation, and form II of the ribulose-1,5-bisphosphate carboxylase/oxygenase gene (cbbM ) were determined in the new isolates and compared. ThecbbM gene was chosen for comparison because it is not involved in magnetosome synthesis; it is highly conserved and is present in all but possibly one of the genomes of the magnetospirilla and the new isolates. Phylogenies based on 16S rRNA,cbbM , andmam gene sequences were reasonably congruent, indicating that the genes involved in magnetotaxis were acquired by a common ancestor of theMagnetospirillum clade. However, in one case, magnetosome genes might have been acquired through horizontal gene transfer. Our results also extend the known diversity of theMagnetospirillum group and show that they are widespread in freshwater environments.