My research focuses mainly on mechanism for establishment of the primary and secondary endosymbiosis in ciliated protist Paramecium species. Endosymbiosis is a primary force in eukaryotic cell evolution. Recent studies of algal evolution show that this phenomenon has occurred several times and has yielded a wide diversity of eukaryotic cells. Despite the importance of this phenomenon, however, molecular mechanisms for induction of endosymbiosis between different microorganisms are not so well known. To elucidate this phenomenon, experiments for reestablishment of the endosymbiosis by symbionts isolated from the symbiont-bearing host cells and the symbiont-free host cells are indispensable. In many endosymbiotic communities, however, both the endosymbionts and the aposymbiotic host cells have already lost the ability to survive and grow independently.

The ciliated protista, Paramecium species, are extremely valuable cells that enable the reestablishment experiments of the endosymbiosis, which frequently bear prokaryotic, eukaryotic, or both endosymbionts in the cell. Although most endosymbiotic bacteria of Paramecium species cannot grow outside the host cell as a result of their reduced genome size, endonuclear symbiotic bacteria Holospora species, even when isolated from the host cells, can maintain their infectivity to new host cell for few days at room temperature. Although the host can acquire various stress resistances by endosymbiosis with Holospora, the symbionts are not indispensable for the host’s survival. Consequently, reestablishment of endosymbiosis between the symbiotic bacteria-free Paramecium and Holospora cells isolated from the host cells can be induced easily through the host phagocytosis by mixing them.

On the other hand, among Paramecium species, only P. bursaria and P. chlorelligerum have the capacity to harbor endosymbiotic green algae in the cytoplasm. Irrespective of mutual relationship between P. bursaria and the symbiotic algae, both cells still retain the ability to grow without the partner, and can easily reestablish endosymbiosis by mixing them. Thus, interactions between Paramecium species and Holospora species, and between P. bursaria or P. chlorelligerum and symbiotic green algae provide excellent opportunities to elucidate not only for control mechanisms for establishment of primary symbiosis, but also for secondary symbiosis leading to eukaryotic cell evolution.

My research is focused on

• How does the symbiont invade the host cytoplasm?

• How can the symbiont avoid digestion by the host's lysosomal enzymes?

• How can the symbiont grow synchronously with the host cell?

• How and when is the horizontal gene transfer from symbiont to the host genome induced?

• What benefit does the host cell receive by endosymbiosis to maintain endosymbiosis and to adapt to new environments for expanding the range that the hosts can live in? 

• What changes in the symbiont and host phenotype are induced by the establishment of the endosymbiosis? Are the phenotipic changes reversible, depending on the presence or absence of the symbiont? Are the phenotypic changes effective for 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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