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

Abstract

Endosymbiotic associations with more than 1 bacterial phylotype are rare among 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, and 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 larger symbiont belongs to the gamma subclass of the Proteobacteria and clusters with other previously described chemoautotrophic endosymbionts. The smaller symbiont represents a novel phylotype within the alpha subclass of the Proteobacteria. This is distinctly different from all other chemoautotrophic hosts with symbiotic bacteria which belong to either the gamma or epsilon Proteobacteria. In addition, a third bacterial morphotype as well as a third unique phylotype belonging to the spirochetes was discovered in these hosts. Such a phylogenetically diverse assemblage of endosymbiotic bacteria is not known from other marine invertebrates.