Division of Symbiosis and Cell Evolution / Miyagishima Group

Chloroplasts arose from a cyanobacterial endosymbiont, which introduced photosynthesis into eukaryotes. It is widely believed that synchronization of division in the eukaryotic host cell and in the endosymbiont was critical for the host cell to maintain the endosymbiont/chloroplast permanently. However, it is unclear how the division of the endosymbiont (the chloroplast) and host cell became synchronized. Using the unicellular red alga Cyanidioschyzon merolae, we show that the host cell enters into the metaphase only when chloroplast division has commenced. A similar phenomenon also was observed in the glaucophyte alga Cyanophora paradoxa. It thus seems likely that the acquisition of the cell-cycle checkpoint of chloroplast division played an important role in the establishment of the chloroplast in ancient algae.

The interactive synchronization of division in the eukaryotic host cell and the chloroplast in algae. The host cell restricts the onset of chloroplast division to the S phase by S-phase–specific expression of nucleus-encoded chloroplast-division proteins. The formation of the competent chloroplast-division machinery and constriction of the chloroplast division site lift the prophase arrest so that the host cell enters into the metaphase only when chloroplast division progresses.