title: Forever competent: deep-sea bivalves are colonized by their chemosynthetic symbionts throughout their lifetime.
abstract: Symbiotic bivalves at hydrothermal vents and cold seeps host chemosynthetic bacteria intracellularly in gill cells. In bivalves, the gills grow continuously throughout their lifetime by forming new filaments. We examined how newly developed gill tissues are colonized in bivalves with horizontal and vertical symbiont transmission (Bathymodiolus mussels versus a vesicoymid clam) using fluorescence in situ hybridization and transmission electron microscopy. Symbiont colonization was similar in mussels and clams and was independent of the transmission modes. Symbionts were absent in the growth zones of the gills, indicating that symbionts colonize newly formed gill filaments de novo after they are formed and that gill colonization is a continuous process throughout the host's lifetime. Symbiont abundance and distribution suggested that colonization is shaped by the developmental stage of host cells. Self-infection, in which new gill cells are colonized by symbionts from ontogenetically older gill tissues, may also play a role. In mussels, symbiont infection led to changes in gill cell structure similar to those described from other epithelial cells infected by intracellular pathogens, such as the loss of microvilli. A better understanding of the factors that affect symbiont colonization of bivalve gills could provide new insights into interactions between intracellular bacteria and epithelial tissues.
authors.affiliation: Department of Symbiosis, Max Planck Institute for Marine Microbiology, Celsiusstrasse 1, Bremen, 28359, Germany.
authors.forename: Cecilia
authors.initials: C
authors.lastname: Wentrup
authors.forename: Annelie
authors.initials: A
authors.lastname: Wendeberg
authors.forename: Mario
authors.initials: M
authors.lastname: Schimak
authors.forename: Christian
authors.initials: C
authors.lastname: Borowski
authors.forename: Nicole
authors.initials: N
authors.lastname: Dubilier
mesh_terms: D000818:Animals; D001419:Bacteria / Q000254:growth & development*; D049872:Bivalvia / Q000254:growth & development / Q000378:metabolism / Q000382:microbiology* / Q000648:ultrastructure; D005880:Gills / Q000254:growth & development / Q000378:metabolism / Q000382:microbiology* / Q000648:ultrastructure; D017404:In Situ Hybridization, Fluorescence; D046529:Microscopy, Electron, Transmission; D018809:Proliferating Cell Nuclear Antigen / Q000032:analysis; D013559:Symbiosis
chemical_list: D018809:Proliferating Cell Nuclear Antigen