ENDOSYMBIOSIS is a primary force in eukaryotic cell evolution. Recent studies of algal evolution have shown that endosymbiosis has occurred several times and has yielded a variety of eukaryotic cells. Despite the importance of this phenomenon, however, molecular mechanisms for the induction of endosymbiosis between different protistsare not well known.Each symbiotic Chlorella species of Paramecium bursaria (Fig. 1) is enclosed in a perialgal vacuole (PV) membrane derived from the host digestive vacuole (DV) membrane. Alga-free paramecia and symbiotic algae are capable of growing independently and paramecia can be reinfected experimentally by mixing them. This phenomenon provides an excellent model for studying cell-to-cell interaction and the evolution of eukaryotic cells through secondary endosymbiosis between different protists.
Fig. 1. Light and fluorescence microscopy of alga-bearing OS1g1N cells. A shows DIC image. B shows fluorescemce microscopy image. Hundread of symbiotic Chlorella spp. are observed in the pramecium cytoplasm (A). Fluorescence of chlorophyll within chloroplat is red colour. Macronucleus (Ma) and micronucleus (Mi) are stained with DAPI (blue). Cy, cytopharynx. (From Kodama and Fujishima, 2010b)
HOWEVER, the detailed algal infection process remains unclear. Using pulse labeling of the alga-free paramecia with the isolatedsymbiotic algae and chase method, we found four necessarycytological events for establishing endosymbiosis (Fig. 2).
Fig. 2. Four important events needed to algal reinfection process. (1)At about 3min after mixing, some algae showresistance to the host lysosomal enzymes in the DVs, even if the digested ones are present. (2) At about 30min after mixing, the alga starts to escape from the DVs as the result ofthe budding of the DV membrane into the cytoplasm. (3) Within 15 min after the escape, the DV membrane enclosing a single green alga differentiates to the PV membrane, which provides protection from lysosomal fusion. (4) The alga localizes at the primary lysosome-less host cell surface by affinity of the PV to unknown structures of the host. At about 24 h after mixing, the alga multiplies by cell division and establishes endosymbiosis. (Updated from Kodama and Fujishima, 2010b)
This algal infection process differs from known infection processes of other symbiotic or parasitic organisms to their hosts. My research topic is clarification of the molecular mechanisms of these 4 events to understand the establishment of Paramecium and Chlorella endosymbiosis.