Chloroplast division without DNA synthesis during the life cycle of the unicellular algaCyanidium caldarium M-8 as revealed by quantitative fluorescence microscopy

Abstract

The behavior and DNA content of the cell and chloroplast nuclei (synonymous with nucleoids; ct-nuclei) during the life cycle have been studied in a synchronized population of cells of the unicellular algaCyanidium caldarium M-8. Cells were examined by epifluorescence microscopy after staining with 4′,6-diamidino-2-phenylindole (DAPI), and by fluorimetry using a video-intensified microscope photon-counting system (VIMPICS). The young cell contains a single petal-like chloroplast, a spherical cell nucleus and several mitochondria, and the cell nucleus and the chloroplast divide in that order just prior to cytokinesis. The chloroplast contains a ring-shaped ct-nucleus which is located at the periphery of the chloroplast during the life cycle. During the first 40 h after the initiation of sychronous cultures, the young cell and its chloroplast increase markedly in size, and the DNA contents per cell nucleus and per ct-nucleus increase approximately two times and 16 times the value in 16-endospore cells, respectively. Four endospore divisions then occur, at intervals of approximately 12 h between 40 h and 90 h, after the initiation of synchronous cultures. The volume of each cell, the volume of each chloroplast, the amount of chloroplast DNA (ct-DNA), and the level of pigmented material in the chloroplast are reduced stepwise after each endospore division until finally, at the 16-endospore stage, they reach approximately 1/16 of the original values for the mother cell. The size of the mitotic spindle also is reduced stepwise as the cell divisions proceed. By contrast, the cell nuclei duplicate their DNA during each endospore division cycle. These results and analysis of other components indicate that the chloroplasts divide into two daughter chloroplasts without any DNA synthesis during four successive cycles of endospore division and also that the DNA content of the chloroplast is intimately related to the volume of the chloroplast and the cell and to the level of pigmented material in the cloroplast, but is not related to the DNA content of the cell nuclei.