2008 年 41 巻 1 号 p. 15-19
Each symbiotic Chlorella sp. of the ciliate Paramecium bursaria is enclosed in a perialgal vacuole derived from the host digestive vacuole, thereby the alga is protected from digestion by the host lysosomal fusion. Algae-free paramecia and isolated symbiotic algae from algae-bearing cells still have an ability to live each other. Algae-free cells can be easily reinfected with the algae isolated from algae-bearing cells by ingestion into digestive vacuoles. The symbiotic associations of these eukaryotic cells are excellent models for studying the evolution of eukaryotic cells through the secondary endosymbiosis between protozoa and algae. However, the detailed algal infection process had been unclear. Using pulse-label of the alga-free paramecia with the isolated symbiotic algae and chase method, we clarified the timing of the algal infection process to the algae-free paramecia. Furthermore, four important cytological events needed to establish endosymbiosis were found: (1) some algae show resistance to the host lysosomal enzymes in the host digestive vacuole, (2) algal escape from the host digestive vacuole by budding of the digestive vacuole membrane, (3) differentiation of the host digestive vacuoles wrapping the green algae into the perialgal vacuole membrane to protect from lysosomal fusion, and (4) an attachment of the green algae wrapped by the perialgal membrane to beneath the host cell membrane. This algal infection process is different from those so far known infection processes in other symbiotic and parasitic organisms.