Paramecium as a Model Organism for Studies on Primary and Secondary Endosymbioses

Endosymbiosis is a driving force in eukaryotic cell evolution. This phenomenon has occurred several times and has yielded a wide diversity of eukaryotic cells. Despite the importance of endosymbiosis, however, molecular mechanisms for its induction between different microorganisms are not so well known. To elucidate these mechanisms, experiments for synchronous induction of the endosymbiosis by symbionts isolated from the symbiont-bearing host cells and the symbiont-free host cells are indispensable. Also, the infection process needs to be easily observable under a microscope. In many endosymbiotic communities, however, both the endosymbionts and the symbiont-free host cells have already lost the ability to survive and grow independently. Consequently, re-induction of the endosymbiosis was difficult. We have developed optimum experimental conditions for the induction of primary and secondary endosymbiosis using the ciliate Paramecium and their endosymbionts.