Phylogenetic diversity of bacterial endosymbionts in the gutless marine oligochete Olavius loisae (Annelida)

Endosymbiotic associations with more than 1 bacterial phylotype are rare am ong chemoautotrophic hosts. In gutless marine oligochetes 2 morphotypes of bacterial endosymbionts occur just below the cuticle between extensions of the epidermal cells. Using phylogenetic analysis, in situ hybridization, an d denaturing gradient gel electrophoresis based on 16S ribosomal RNA genes. it is shown that in the gutless oligochete Olavius loisae, the 2 bacterial morphotypes correspond to 2 species of diverse phylogenetic origin. The la rger symbiont belongs to the gamma subclass of the Proteobacteria and clust ers with other previously described chemoautotrophic endosymbionts. The sma ller symbiont represents a novel phylotype within the alpha subclass of the Proteobacteria. This is distinctly different from all other chemoautotroph ic hosts with symbiotic bacteria which belong to either the gamma or epsilo n Proteobacteria. In addition, a third bacterial morphotype as well as a th ird unique phylotype belonging to the spirochetes was discovered in these h osts. Such a phylogenetically diverse assemblage of endosymbiotic bacteria is not known from other marine invertebrates.