Destruction of Chloroplast Nuclei of the Male Gamete by Calcium and Nuclease C In a Cell Model of Chlamydomonas reinhardtii

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Abstract
A cell model was prepared for each mating type cell of Chlamydomonas reinhardtii in order to investigate the molecular mechanism for the maternal inheritance of chloroplast genes. Chloroplast nuclei (cp-nuclei) in the chloroplast were well preserved. Therefore, their disappearance after a change in the external medium could be monitored by high resolution fluorescence microscopy after staining the cells with 4′-6-diamidino-2-phenyl indole. The number of cp-nuclei in the cell model diminished when Ca2+ was added to a cell suspension. Presumably, this ion activated the endogenous nuclease C that remained in the cytoplasm, thus setting up the destruction of chloroplast DNA. Almost all disappeared when Ca2+ was given together with nuclease C. From the kinetics of the disappearance of cp-nuclei after combining the cell model with nuclease C, we found that male gametes begin to lose their cp-nuclei after 20 min, whereas female gametes lose them after 40 min. Furthermore, as a culture aged, the female cp-nuclei became resistant to the destructive attack by nuclease C, but the male cp-nuclei remained sensitive.