真核藻類を用いた多分裂細胞周期の進化メカニズムの解明

真核生物在几个谱系中进行多次分裂细胞周期，细胞首先生长到许多倍的大小，然后在没有中间生长的情况下分裂几次。为了说明细胞分裂的数量是如何被调节的，我研究了不同的蓝藻红藻中细胞分裂的阈值细胞大小，即通过双重裂变增殖的cyanidialean red algae，即分别形成4个和4到8个子细胞的Cyanidium和Galdieria。结果表明，它们的细胞分裂阈值大小与细胞连续分裂的次数有关。在Cyanidioschyzon和Cyanidium中，细胞分裂的细胞大小检查点存在于G1期晚期。当Cyanidioschyzon在G1期被抑制并因条件CDKA耗竭而异常增大时，在CDKA恢复后，它会连续进行两次或两次以上的细胞分裂，而不干预生长，这表明细胞分裂的阈值大小是决定细胞分裂数量的主要因素。为了进一步评估在G1期如何监测细胞大小并决定细胞分裂的承诺，我检查了G1周期蛋白在Cyanidioschyzon中的表达模式。在出芽酵母中，G1周期蛋白水平与细胞大小成正比，并调节细胞的承诺和G1/S转变。我发现在Cyanidioschyzon中，G1 cyclin确实随着细胞的生长（大小）而积累，并且G1 cyclin过表达加速了G1/S转变。这些结果共同表明，G1周期蛋白水平可能决定了这些藻类和其他真核生物中连续细胞分裂的数量。

Eukaryotes in several lineages perform multiple fission cell cycle in which cells first grow to manyfold in size, then divide several times without intervening growth. To address how the number of cell divisions being regulated, I had examined threshold cell sizes for cell division in different cyanidialean red algae, namely Cyanidioschyzon which proliferates by binary fission, Cyanidium and Galdieria which form four and four to eight daughter cells respectively. My result showed that their threshold cell size for cell division correlate with number of successive cell divisions. In Cyanidioschyzon and Cyanidium, a cell size checkpoint for cell division(s) exists in late G1 phase. When Cyanidioschyzon was arrested in G1 phase and abnormally enlarged by conditional CDKA depletion, it underwent two or more successive cell divisions without intervening growth after CDKA recovered, suggesting that the threshold size for cell division is a major factor in determining the number of cell divisions. To further assess how cell size is monitored in G1 phase and decides for commitment to cell division, I examined expression pattern of G1 cyclin in Cyanidioschyzon. In budding yeast, G1 cyclin level was suggested to scale with cell size and regulate commitment and G1/S transition. I found that in Cyanidioschyzon, the G1 cyclin did accumulates with cell growth (size), and G1 cyclin overexpression accelerated G1/S transition. These results collectively suggest that G1 cyclin levels likely determine number of successive cell divisions in these algae and possibly in other eukaryotes.