Analysis of light/dark synchronization of cell-wall-less Chlamydomonas reinhardtii (Chlorophyta) cells by flow cytometry

The degree of synchrony of light/dark-adapted Chlamydomonas reinhardtii cel l-wall-less cells was analysed by flow cytometry. The cultures exhibited th e classical growth curve reported for synchronously dividing cells. However , analysis of the nuclear DNA content revealed a partial asynchrony of cell division. This allowed us to determine that cell division lasts only 1 h 1 5 min. The changes in the distribution of nuclei in G(1) and S phases did n ot exhibit the DNA peaks expected for highly synchronous cells. However, th ese peaks were observed for the G(2) + M nuclei and reflected the number of cell divisions. Modification of culture conditions including temperature, photoperiod, quantum irradiance and culture density affected the degree of synchrony but did not completely abolish the asynchrony. An overlap between the divisions of cells dividing one, two or three times could account for the partial asynchrony observed. In fact, it is possible that cells perform their divisions synchronously but that the timing and/or the duration of t he cell divisions is different for the three kinds of cells. The synchrony seems to be perturbed when the cells move from the G(1) to the S phase. The transduction of the release signal allowing cells to pass through the rest riction point of the G(1) phase, might be too slow and insufficiently homog eneous between the cells to allow perfect synchronization. Finally, if high ly synchronized cells are needed to study Chlamydomonas reinhardtii cell cy cle, other synchronization procedures should be considered.