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

在几个谱系中的真核生物执行多个分裂细胞周期，在这个周期中，细胞首先生长到许多倍大，然后在不干预生长的情况下分裂几次。为了解释细胞分裂的数量是如何调节的，我研究了不同蓝藻的细胞分裂阈值大小，即通过二次分裂增殖的蓝藻、分别形成四个和四到八个子细胞的蓝藻和Galdieria。我的结果表明，它们的细胞分裂阈值细胞大小与连续细胞分裂的数量相关。在蓝藻和蓝藻中，细胞分裂的一个细胞大小检查点(S)存在于G1期晚期。当CDKA被抑制在G1期并被条件CDKA耗尽而异常放大时，CDKA恢复后，它经历了两次或两次以上的连续细胞分裂，没有干预生长，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。为了进一步评估细胞大小是如何在G1期被监测并决定细胞分裂的承诺，我研究了G1期细胞周期蛋白在氰基二磷酸裂殖体中的表达模式。在芽殖酵母中，G1细胞周期蛋白水平被认为与细胞大小有关，并调节承诺和G1/S转换。研究发现，在氰基二胞菌中，G1期细胞周期蛋白确实随着细胞的生长(大小)而积聚，且G1期细胞周期蛋白的过表达加速了G1/S的转变。这些结果共同表明，G1期细胞周期蛋白水平可能决定了这些藻类以及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.