Molecular evidence for a close relative of the arthropod endosymbiont Wolbachia in a filarial worm

While data are accumulating on intracellular bacterial symbioses in arthropods and vertebrates, remarkably little is known about the occurrence and biological implications of these associations in nematodes. The filarial worm Dirofilaria immitis is one of the few nematodes for which intracellular bacteria have been described. D. immitis is a dog pathogen, and is transmitted to this definitive vertebrate host by an intermediate mosquito host. Present data indicates that the intracellular bacteria of D. immitis are transovarially transmitted to offspring. However, these intracellular bacteria have not yet been cultured, and no data have been published on their actual distribution in D. immitis populations, on their positioning in the overall scheme of eubacterial evolution, or on their relationships with the intracellular bacteria of vertebrates and arthropods. This latter issue appears quite intriguing if we consider the widespread diffusion of rickettsiae and rickettsia-like bacteria in both vertebrates and arthropods, and the ability of these intracellular microorganisms to spread among taxonomically distant hosts by horizontal transmission. Analysis of the genes encoding the small subunit ribosomal RNA (16S rDNA) is a useful approach for investigating the phylogeny of uncultured bacteria. We report a 16S rDNA-based phylogeny for the endosymbiont of D. immitis, an rDNA-based method for its detection by polymerase chain reaction (PCR).