Chloroplast division checkpoint in eukaryotic algae

Significance 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 algaCyanidioschyzon 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 algaCyanophora 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.