CHARACTERIZATION OF CHEMOAUTOTROPHIC BACTERIAL SYMBIONTS IN A GLUTLESS MARINE WORM (OLIGOCHAETA, ANNELIDA) BY PHYLOGENETIC 16S RIBOSOMAL-RNA SEQUENCE-ANALYSIS IN IN-SITU HYBRIDIZATION

The phylogenetic relationships of chemoautotrophic endosymbionts in th e gutless marine oligochaete Inanidrilus leukodermatus to chemoautotro phic ecto- and endosymbionts from other host phyla and to free-living bacteria were determined by comparative 16S rRNA sequence analysis. Fl uorescent in situ hybridizations confirmed that the 16S rRNA sequence obtained from these worms originated from the symbionts. The symbiont sequence is unique to I. leukodermatus. In phylogenetic trees inferred by both distance and parsimony methods, the oligochaete symbiont is p eripherally associated with one of two clusters of chemoautotrophic sy mbionts that belong to the gamma subdivision of the Proteobacteria. Th e endosymbionts of this oligochaete form a monophyletic group with che moautotrophic ectosymbionts of a marine nematode. The oligochaete and nematode symbionts are very closely related, although their hosts belo ng to separate, unrelated animal phyla. Thus, cospeciation between the nematode and oligochaete hosts and their symbionts could not have occ urred. Instead, the similar geographic locations and habitats of the h osts may have influenced the establishment of these symbioses.