Physical Proximity May Promote Lateral Acquisition of Bacterial Symbionts in Vesicomyid Clams

<div><p>Vesicomyid clams harbor intracellular sulfur-oxidizing bacteria that are predominantly maternally inherited and co-speciate with their hosts. Genome recombination and the occurrence of non-parental strains were recently demonstrated in symbionts. However, mechanisms favoring such events remain to be identified. In this study, we investigated symbionts in two phylogenetically distant vesicomyid species, <i>Christineconcha regab</i> and <i>Laubiericoncha chuni</i>, which sometimes co-occur at a cold-seep site in the Gulf of Guinea. We showed that each of the two species harbored a single dominant bacterial symbiont strain. However, for both vesicomyid species, the symbiont from the other species was occasionally detected in the gills using fluorescence <i>in situ</i> hybridization and gene sequences analyses based on six symbiont marker genes. Symbiont strains co-occurred within a single host only at sites where both host species were found; whereas one single symbiont strain was detected in <i>C. regab</i> specimens from a site where no <i>L. chuni</i> individuals had been observed. These results suggest that physical proximity favored the acquisition of non-parental symbiont strains in Vesicomyidae. Over evolutionary time, this could potentially lead to genetic exchanges among symbiont species and eventually symbiont displacement. Symbiont densities estimated using 3D fluorescence <i>in situ</i> hybridization varied among host species and sites, suggesting flexibility in the association despite the fact that a similar type of metabolism is expected in all symbionts.</p>