chloroplast, cyanobacteria, lipids, DNA polymerase,
The origin of plastids has been studied in the framework of endosymbiogenesis from the first proposal by Mereschkowsky in 1905, and the enhanced proposal by Sagan (Margulis) in 1967. Recent works, however, revealed that the situation might not be as simple as that supposed to be. Many traits that were considered as the evidence for the continuity of plastids and cyanobacteria turned out to represent, in fact, discontinuity between them. Oxygenic photosynthesis is the major trait common in both cyanobacteria and chloroplasts, but various other common traits could be results of convergent or canalized evolution constrained by the oxygenic photosynthesis. We consider that the complete replacement of the synthetic pathway of galactolipids is an example for this constrained evolution. Discontinuity also includes the plastid replication system, which is borrowed from the mitochondrial system. Our interests are also extended to the enigmatic peptidoglycan and the presumptive localization of oil body in the chloroplast.