Phylogeny ofCurrent phylogenies of the intracellular bacteria belonging to the genus Wolbachia identify six major clades (A-F), termed 'supergroups', but the branching order of these supergroups remains unresolved. Supergroups A, B and E include most of the wolbachiae found thus far in arthropods, while supergroups C and D include most of those found in filarial nematodes. Members of supergroup F have been found in arthropods (i.e. termites), and have previously been detected in the nematode Mansonella ozzardi, a causative agent of human filariasis. To resolve the phylogenetic positions of Wolbachia from Mansonella spp., and other novel strains from the flea Ctenocephalides felis and the filarial nematode Dipetalonema gracile, the authors generated new DNA sequences of the Wolbachia genes encoding citrate synthase (gltA), heat-shock protein 60 (groEL), and the cell division protein ftsZ. Phylogenetic analysis confirmed the designation of Wolbachia from Mansonella spp. as a member of the F supergroup. In addition, it was found that divergent lineages from Dip. gracile and Cte. felis lack any clear affiliation with known supergroups, indicating further genetic diversity within the Wolbachia genus. Finally, although the data generated did not permit clear resolution of the root of the global Wolbachia tree, the results suggest that the transfer of Wolbachia spp. from arthropods to nematodes (or vice versa) probably occurred more than once.
INTRODUCTIONThe genus Wolbachia encompasses obligately intracellular bacteria that are cytoplasmically transmitted in arthropods and filarial nematodes (Werren, 1997;Bandi et al., 2001;Bordenstein et al., 2003). Unlike most other obligately intracellular bacteria, the genus Wolbachia forms a monophyletic clade comprising both mutualistic and parasitic lineages that showcase the diversity of symbiotic associations. In arthropods, Wolbachia spp. are commonly known as reproductive parasites, since they distort host reproductive strategies to selfishly enhance their maternal transmission into the next generation (Werren, 1997). Wolbachia spp. have a remarkably high prevalence in some arthropod groups (Jeyaprakash & Hoy, 2000). In contrast, the Wolbachia lineages infecting filarial nematodes are beneficial 3Present address: Dipartimento di Biotecnologie e Bioscienze, Università degli Studi di Milano Bicocca, Milano, Italy.4Present address: Department of Biology, University of California, 900 University Avenue, Riverside, CA, USA.The GenBank/EMBL/DDBJ accession numbers for the nucleotide sequences reported in this paper are listed in Table 1.The phylogeny inferred from the concatenated dataset (gltA, groEL, ftsZ), rooted with two outgroup species (Anaplasma marginale and Ehrlichia ruminantium), is shown in Supplementary Fig. S1, available with the online version of this paper.Abbreviations: BI, Bayesian inference; ML, maximum-likelihood; TBR, tree bisection and reconnection.