Symbiosis involves interactions between genetically different biological species and plays a key role in evolution by allowing organisms to gain new functions. Intracellular symbioses between chemoautotrophic bacteria and marine invertebrates dominate the fauna at deep-sea hydrothermal vents and seeps. The host invertebrates are nutritionally dependent on the intracellular chemoautotrophic bacteria. However, the detailed mechanisms of their symbiosis remain unknown. Reductive genome evolution has occurred in vertically transmitted intracellular symbionts. The detailed process of reductive genome evolution is also unknown. To understand the mechanisms of intracellular symbiosis and evolution,the Symbiosis and Evolution Research Team is investigating interactions between host invertebrates and symbionts and the reductive genome evolution of symbionts using methods from molecular biology, cellular biology, and biochemistry.