Secrets and mysteries of primary symbiosis establishment

Plants (algae and land plants) can trace their origins to symbiotic relationships between cyanobacteria-like photosynthetic bacteria and host cells. This process known as primary endosymbiosis gave rise to red algae, green plants, and glaucophyte algae. A number of hypotheses have been put forth to explain how the primary endosymbiosis was established and how the endosymbiont DNA was transferred horizontally into the host genome. This review focuses on these hypotheses concerning primary endosymbiosis and introduces the characteristics and rationale of each hypothesis. Recent advances in molecular biology and genome technology for a more sophisticated understanding of the mechanisms of primary endosymbiosis will also be discussed.