Phylogeny of Wolbachia in filarial nematodes

Intracellular bacteria have been observed in various species of filarial ne matodes (family Onchocercidae). The intracellular bacterium of the canine f ilaria Dirofilaria immitis has been shown to be closely related to Wolbachi a, a rickettsia-like micro-organism that is widespread among arthropods. Ho wever, the relationships between endosymbionts of different filariae, and b etween these and the arthropod wolbachiae, appear not to have been studied. To address these issues we have examined ten species of filarial nematodes for the presence of Wolbachia. For nine species, all samples examined were PCR positive using primers specific for the ftsZ gene of Wobbachia. For on e species, the examined samples were PCR negative. Sequences of the amplifi ed ftsZ gene fragments of filarial wolbachiae fall into two clusters (C and D), which are distinct from the A and B clusters recognized for arthropod wolbachiae. These four lineages (A-D) are related in a star-like phylogeny, with higher nucleotide divergence observed between C and D wolbachiae than that observed between A and B wolbachiae. In addition, within each of the two lineages of filarial wolbachiae, the phylogeny of the symbionts is cons istent with the host phylogeny. Thus, there is no evidence for recent Wolba chia transmission between arthropods and nematodes. Endosymbiont 16S riboso mal DNA sequences from a subset of filarial species support these findings.