Infection of Paramecium bursaria by Symbiotic Chlorella Species

Abstract

Abstract Paramecium bursaria and endosymbiotic Chlorella species retain their ability to grow independently, but can reestablish endosymbiosis by mixing. Infection is induced through the host’s digestive vacuoles (DVs). Acidosomal and lysosomal fusions to the DVs begin at 0.5 and 2–3 min after mixing, respectively. Pulse-labeling of ­algae-free paramecia with isolated symbiotic algae for 1.5 min and chasing for various times shows that some algae acquire temporal resistance to lysosomal enzymes in the DVs. They begin to escape from the DVs by budding of the vacuole membrane at 30 min after mixing. Then each small vacuole enclosing a green alga differentiates to a perialgal vacuole (PV), which gives protection from the host lysosomal fusion, and which translocates beneath the host cell surface. Algal cell division in the PV begins at about 24 h after mixing. Infection experiments with infection-capable and infection-incapable algae indicate that the infectivity is based on their ability to localize beneath the host surface after escaping from DVs. Algal proteins synthesized during photosynthesis serve some important functions to prevent expansion of the PV and to attach under the host surface, and to protect the PV from host lysosomal fusion. Although molecular mechanisms for these phenomena remain to be elucidated, accumulated evidence suggests that the symbiotic Chlorella sp. have a very long evolutionary history. This chapter mainly addresses studies of DV differentiation, the infection process of the algae, characteristics of the PV membrane, and related phenomena.KeywordsAlgal CellVacuole MembraneSymbiotic AlgaHost Cell SurfaceChlorella SpeciesThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.