| Summary: | Chemoautolithotrophic bacteria thriving in deep-sea hydrothermal vents and cold seeps play an important role as primary producers in these remote, extreme ecosystems, and some of them have evolved symbiotic relationships with diverse marine invertebrate animals. These symbiotic bacteria harness energy through oxidation of reduced gases such as hydrogen sulfide, methane, hydrogen emissions, and eventually, their organic materials are transferred to their host invertebrates. Using symbiont-specific, multi-loci Next Generation Sequencing (NGS) data, geographical population genetics for symbiotic bacteria of Bathymodiolinae vent mussels were examined, with samples from nine vent localities. This is one of the most dominant and widespread taxa in global chemosynthetic environments on the sea floor, along the East Pacific Rise, Galápagos Rift, and the Pacific-Antarctic Ridge. The hope of this study is to gain insights into the evolution of bacteria in space and time. Particularly, the symbiotic bacteria of these Bathymodiolinae species have free-living and intracellular symbiotic stages, which allowed examination of their population dynamics and evolution in benthic environments. Per the results, it appears that two sister parapatric Bathymodiolinae species harbor the same symbiont species, thiotrophic Gamma-proteobacteria, regarding16SrRNA encoding gene, but completely divergent lineages living in the north and south of the Easter Microplate regarding six protein-coding genes. Such divergence of bacteria seems to be initiated by orogeny of the Easter Microplate dated 2.5–5.3 million years ago, and equivalent to synonymous substitution rates of 0.77–1.62% per nucleotide per million years of protein-coding genes.
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