My research focuses primarily on the mechanisms underlying the establishment of primary and secondary endosymbiosis in ciliated protists of the genus Paramecium. Endosymbiosis has played a fundamental role in the evolution of eukaryotic cells. Recent studies on algal evolution have revealed that endosymbiotic events have occurred multiple times during evolution, generating a remarkable diversity of eukaryotic lineages. Despite its evolutionary importance, however, the molecular mechanisms responsible for the establishment and maintenance of endosymbiosis between different microorganisms remain poorly understood.

To elucidate these mechanisms, experimental reestablishment of endosymbiosis using symbionts isolated from symbiont-bearing host cells and symbiont-free host cells is indispensable. In many endosymbiotic systems, however, both the endosymbionts and the aposymbiotic host cells have lost the ability to survive independently.

Species of the ciliated protist Paramecium provide excellent model systems for reestablishment experiments because they frequently harbor prokaryotic and/or eukaryotic endosymbionts. Although most bacterial endosymbionts of Paramecium cannot grow outside host cells because of genome reduction, endonuclear symbiotic bacteria of the genus Holospora retain infectivity for several days after isolation from host cells. While Holospora symbionts confer various stress resistances on the host, they are not essential for host survival. Therefore, endosymbiosis between symbiont-free Paramecium cells and isolated Holospora cells can readily be reestablished simply by mixing them and allowing phagocytosis by the host cells.

Among Paramecium species, only P. bursaria and P. chlorelligerum harbor symbiotic green algae in their cytoplasm. Importantly, both the host and algal symbionts retain the ability to grow independently, enabling experimental reestablishment of endosymbiosis by simple mixing of the two partners. Thus, interactions between Paramecium and Holospora, as well as between P. bursaria or P. chlorelligerum and symbiotic green algae, provide powerful experimental systems for investigating not only the mechanisms underlying the establishment of primary symbiosis, but also secondary symbiosis processes associated with eukaryotic cell evolution.

My research addresses the following questions:

• How do symbionts invade the host cytoplasm?
• How do symbionts avoid digestion by host lysosomal enzymes?
• How do symbionts synchronize their growth with host cell division?
• How and when does horizontal gene transfer from symbionts to the host genome occur?
• What benefits do host cells gain from endosymbiosis that enable adaptation to new environments and expansion of ecological niches?
• What phenotypic changes are induced in both host and symbiont following establishment of endosymbiosis?
• Are these phenotypic changes reversible depending on the presence or absence of the symbiont?
• Do these phenotypic changes contribute to environmental adaptation?

      In addition, my other research interests include (1) the use of Paramecium   to combat global warming and (2) the effects of environmental radiation on Paramecium.

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