Cell cycle analysis in a multinucleate green alga, Boergesenia forbesii (Siphonocladales, Chlorophyta)*

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SUMMARYThe cell cycle (nuclear division cycle) of a multinucleate green alga, Boergesenia forbesii (Harvey) Feldmann was studied using microspectrophotometry and BrdU incorporation techniques. Mitosis was observed frequently 1‐4 h after the beginning of the light period, on a 16:8 h LD cycle at 25°C. Mitotic nuclei formed discrete patches. Other nuclei remained in the G1 period. The DNA synthetic phase (S phase) was estimated to last about 12 h from microspectrophotometric study using aphidicolin inhibition just before the S phase and release from it. The G2 period was estimated to be about 2 h, because a labeled prophase nucleus could be detected when the samples were labeled with BrdU continuously over 3 h. The incorporation pattern of BrdU changed through the S phase nucleus. In early S phase, BrdU staining was detected as many dots in the entire nucleus, while in late S phase, it was detected as several discrete regions along the nuclear membrane. Almost all nuclei in B. forbesii were in the G1 stage after nuclear division, and the nuclei in several patches of the cell simultaneously initiated DNA synthesis. Once the nuclei entered into S phase, these nuclei continued into G2 and mitosis. In other words, the cell cycle regulation of entrance into S phase from G1 is an important factor in the growth and morphogenesis in B. forbesii.