2008 年 41 巻 1 号 p. 27-31
It is widely believed that all known plastids originated from a single primary endosymbiosis in which a cyanobacterium was engulfed and retained by a heterotrophic protist. However, there is an interesting organism called Paulinella chromatophora that may change this widely accepted view.
P. chromatophora, a cercozoan protist, is a fresh water testate amoeba that contains two cyanobacterium-like structures called “cyanelles” in the cell. Past researches have failed to cultivate the cyanelles separately from the host cells and demonstrated that the cyanelles divided within the host cells and were handed over to daughter cells. In recent studies, it has been revealed that the cyanelle of P. chromatophora does not share a common ancestor with known plastids but originated from a cyanobacterium that belongs to the Synechococcus/Prochlorococcus lineage.
These situation led the idea that P. chromatophora represent the second example of the primary endosymbiosis that is in progress. Further study on the symbiotic relationship between the cyanelles and the host seen in this organism would provide important insight for the mechanism of primary plastid acquisition.