Paramecium cells with endosymbionts.
Left, P. caudatum with symbiotic bacteria Holospora obtusa in the macronucleus (Photograph, M. Fujishima).
Right, P. bursaria with symbiotic algae Chlorella variabilis in the cytoplasm (Kodama and Fujishima, Endosymbionts in Paramecium, pp.31-55, Springer, 2009).
Both endosymbionts can be isolated from homogenates of their host cells by Percoll continuous gradient centrifugation for Holospora and sucrose discontinuous centrifugation for Chlorella cells, and can induce reestablishment of endosymbiosis by mixing the isolated symbionts with the symbiont-free host cells. We clarified infection processes of Holospora and Chlorella cells to their host Paramecium cells as shown in schematic diagrams described below.