Induction of secondary symbiosis between the ciliate Paramecium and the green alga Chlorella

The ciliate Paramecium bursaria and the green alga Chlorella species can establish endosymbiosis by mixing them through the host digestive vacuoles. Pulse-labeling of alga-free Paramecium cells with isolated symbiotic algae for 1.5 min and chasing for various times showed presence of four cytological phenomena which are indispensable for the algal reinfection. (1) Some algae acquire temporal resistance to lysosomal enzymes in the digestive vacuoles. (2) Not only the resistant green algae but also partially digested brown algae begin to escape from the digestive vacuoles by budding of the vacuole membrane at 30 min after mixing. (3) Then each small vacuole enclosing a green alga differentiates to a perialgal vacuole, which gives protection from the host lysosomal fusion, (4) and the perialgal vacuole translocates and attaches beneath the host cell cortex. Algal cell division in the perialgal vacuole begins at about 24 h after mixing. Algal proteins synthesized during photosynthesis serve some important functions to prevent expansion of the perialgal vacuole and to attach under the host cell cortex, and to protect the perialgal vacuoles from the host lysosomal fusion. The finding of these phenomena for the reestablishment of the endosymbiosis reveals that P. bursaria is an excellent model for studying the infection process of the algae and the evolution of eukaryotic cells through secondary endosymbiosis.