Regionalized cell proliferation in the symbiont-bearing gill of the hydrothermal vent musselBathymodiolus azoricus

Deep-sea musselsBathymodiolusspp. harbor high densities of chemosynthetic bacterial symbionts located within their gill epithelial cells. Compared to non-symbiotic coastal mussel relatives of similar size,Bathymodiolusgills are considerably larger, a feature often considered an adaptation to symbiosis because it is related to the presence of intracellular bacteria in epithelial cells located in the lateral zone. In order to document the mechanisms underlying these sizes differences, this study compares gill cell proliferation patterns inBathymodiolus azoricusandMytilus edulisusing microscopy-based approaches. We used incubation experiments with a synthetic nucleotide (5-ethynyl 2 '-deoxyuridine, EdU), detectable throughout novel cell divisions, and phosphohistone H3 immunolabeling, a marker of mitosis. The results revealed proliferation areas in the ciliated zone and in the bacteria-loaded bacteriocytes located close to the frontal zone of gill filaments, swept by the incurrent sea-waterflow, and also in the dorsal region of gills inB. azoricus. Cell proliferation seems far less intensive inM. edulis. This study overall suggests high cell turnover and fast tissue dynamics in symbiont-bearing mussels.