How Many Wolbachia Supergroups Exist?

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Abstract
Obligate intracellular bacteria of the genus Wolbachia (Class Alphaproteobacteria, Order Rickettsiales) are currently divided into four taxonomic supergroups on the basis of clustering patterns in ftsZ-based phylogenetic trees (Werren, Zhang, and Guo 1995<$REFLINK> ; Bandi et al. 1998<$REFLINK> ). Supergroups A and B are found only in arthropods, whereas C and D are found only in filarial nematodes. The term supergroup has recently been employed to avoid confusion with designation of more closely related groups based on wsp sequences (Zhou, Rousset, and O'Neill 1998<$REFLINK> ). Wolbachia have generated substantial interest in recent years (Zimmer 2001), primarily because of the effects they have on their arthropod hosts, which include induction of cytoplasmic incompatibility (CI), parthenogenesis, feminization, and male-killing (reviewed in Stouthammer, Breeuwer, and Hurst 1999<$REFLINK> ). Estimation of the phylogenetic relationships within each supergroup has provided useful information about the evolution and biology of these bacteria. The phylogenies of both A and B members have been found to be incongruent with that of their hosts, strongly suggesting horizontal transfer (Werren, Zhang, and Guo 1995<$REFLINK> ). Recently, direct evidence for this phenomenon was found for parasitic wasps sharing a common food source (Huigens et al. 2000). Unlike the case of arthropods, the phylogeny of each nematode Wolbachia supergroup (C and D) appears to match that of their hosts (Casiraghi et al. 2001a<$REFLINK> ), although further gene sequencing studies are required to confirm this. Such phylogenetic congruence would suggest a strictly dependent relationship, and this idea is supported by evidence that removal of Wolbachia using antibiotics has negative effects on the filariae they reside in (Bandi et al. 1999<$REFLINK> ; Langworthy et al. 2000<$REFLINK> ).