Mechanisms for establishing primary and secondary endosymbiosis in Paramecium

Primary (eukaryote and procaryote) and secondary (eukaryote and eukaryote) endosymbioses are driving forces in eukaryotic cell evolution. These phenomena are still contributing to acquire new cell structures and functions. To understand mechanisms for establishment of each endosymbiosis, experiments that can induce endosymbiosis synchronously by mixing symbionts isolated from symbiont-bearing host cells and symbiont-free host cells are indispensable. Recent progress on endosymbiosis using Paramecium and their endonuclear symbiotic bacteria Holospora or symbiotic green alga Chlorella has been remarkable, providing excellent opportunities for elucidating host-symbiont interactions. These organisms are now becoming model organisms to know the mechanisms for establishing primary and secondary endosymbioses. Based on experiments of many researchers, we introduce how these endosymbionts escape from the host lysosomal fusion, how they migrate in the host cytoplasm to localize specific locations within the host, how their species specificity and strain specificity of the host cells are controlled, how their life cycles are controlled, how they escape from the host cell to infect more young host cell, how they affect the host viability and gene expression, what kind of substances are needed in these phenomena, and what changes had been induced in the symbiont and the host genomes.