DNA RELATEDNESS BETWEEN XENORHABDUS SPP (ENTEROBACTERIACEAE), SYMBIOTIC BACTERIA OF ENTOMOPATHOGENIC NEMATODES, AND A PROPOSAL TO TRANSFER XENORHABDUS-LUMINESCENS TO A NEW GENUS, PHOTORHABDUS GEN-NOV

The levels of DNA relatedness for a broad sample of Xenorhabdus strain s isolated from different species of entomopathogenic nematodes (Stein ernematidae and Heterorhabditidae) and from different geographical sou rces were estimated by the hydroxyapatite method. The level of DNA-DNA relatedness for the two phases of each isolate tested was not signifi cantly different from 100%, demonstrating unequivocally that the phase variation demonstrated by all Xenorhabdus spp. is not due to contamin ation. The isolates of the described Xenorhabdus species coalesced int o different DNA relatedness groups, confirming that Xenorhabdus nemato philus, Xenorhabdus bovienii, Xenorhabdus poinarii, and Xenorhabdus be ddingii, defined on the basis of phenotypic differences, are valid spe cies. The symbiont of Steinemema intermedia also coalesced with the X. bovienii isolates. This was the only symbiont of seven recently descr ibed and unamed Steinernema spp. (including Steinernema ritteri, Stein emema rara, and Steinernema anomali) that formed a group with any of t he previously described Xenorhabdus species; new species descriptions are required to accommodate the other taxa, but too few isolates were available to allow satisfactory descriptions of them. The DNA relatedn ess data also showed that the bacteria currently classified as Xenorha bdus luminescens are significantly different from all other Xenorhabdu s strains. These data strongly support indications from previous studi es of phenotypic characteristics, cellular fatty acids, and DNA relate dness that X. luminescens should be classified as a separate genus. A new genus, Photorhabdus, with an amended description of the type speci es, Photorhabdus luminescens, is proposed.