PHYLOGENETIC EVIDENCE FOR THE TAXONOMIC HETEROGENEITY OF PHOTORHABDUS-LUMINESCENS

The sequences of the 16S rRNA gene of 40 strains of bacterial symbiont s isolated from the nematodes Heterorhabditis spp, and seven bacterial symbionts of the nematodes Steinernema spp, which were isolated from different geographical areas, as well as the type strain of Xenorhabdu s japonicus, were determined and compared to each other and to the seq uences of several reference strains of members of the Enterobacteriace ae. The data confirmed the separate status of the two genera of symbio nts of entomopathogenic rhabditid nematodes, The symbionts of Heterorh abditis spp. clustered with the type strain of Photorhabdus luminescen s, while the symbionts of Steinernema spp, grouped with Xenorhabdus sp ecies, X. japonicus clustered with the other Xenorhabdus species. Phyl ogenetic analysis of 15 almost complete 16S ribosomal DNA (rDNA) seque nces of the Heterorhabditis symbionts indicated that there were severa l subclusters. The properties correlated with these subclusters are no t yet apparent, although there may be some geographical and ecological correlations, For example, among the nematode-symbiotic bacteria, the members of subclusters I and III are from southeastern and midwestern North America, respectively, while the members of subclusters II and IV are primarily from Europe and Australia, respectively, The nonsymbi otic strains of P. luminescens form a highly homologous subcluster by themselves. The results of DNA-DNA hybridization studies performed wit h a few selected strains of five of the 16S rDNA subclusters support t he existence of several genospecies within P, luminescens.