Takagi Y, Nobuoka T, Doi M
Department of Biology, Nara Women's University, Japan.
J Cell Sci. 1987 Aug;88 ( Pt 1):129-38. doi: 10.1242/jcs.88.1.129.
Paramecia cells, like human diploid cells cultured in vitro, provide a useful model system for understanding the mechanism that limits division potential. The reported maxima of the clonal lifespan of Paramecium tetraurelia fall into two ranges: from 220 to 258 fissions and from 310 to 325 fissions. We found that neither the selection of vigorous lines nor the cryptic occurrence of autogamy offers a plausible explanation for the much longer lifespans in the latter range. We found the sporadic occurrence of very long clonal lifespans, such as 330 fissions, without selection and autogamy. Selection, which was evaluated by using different methods to maintain lines, had little effect on the extension of the maximal clonal lifespan, whereas it did have a marked effect on the extension of the mean clonal lifespan. Autogamy, which was checked with two closely linked marker genes, was frequent, but only during the period when lines were terminating. Statistical analysis on the mean clonal lifespans of two groups of subclones cultured at 25 and 20 degrees C or with rich and poor nutrition showed that the clonal lifespan that was fission-related under favourable conditions tended to dissociate from fissions under less favourable conditions. We discuss mechanisms that determine the clonal lifespan, programmed events contributing to the maximum clonal lifespan and random events contributing to the mean clonal lifespan.
草履虫细胞与体外培养的人类二倍体细胞一样，为理解限制分裂潜能的机制提供了一个有用的模型系统。报道的四膜虫无性系寿命最大值分为两个范围：220至258次分裂以及310至325次分裂。我们发现，无论是选择活力强的品系还是隐性自配的发生，都无法对后一范围内长得多的寿命给出合理的解释。我们发现，在没有选择和自配的情况下，偶尔会出现非常长的无性系寿命，比如330次分裂。通过使用不同方法维持品系来评估的选择，对最大无性系寿命的延长影响很小，而对平均无性系寿命的延长有显著影响。用两个紧密连锁的标记基因进行检测发现，自配很频繁，但只发生在品系即将结束的时期。对在25摄氏度和20摄氏度下或在营养丰富和营养贫乏条件下培养的两组亚克隆的平均无性系寿命进行统计分析表明，在有利条件下与分裂相关的无性系寿命往往与不太有利条件下的分裂情况脱节。我们讨论了决定无性系寿命的机制、对最大无性系寿命有贡献的程序性事件以及对平均无性系寿命有贡献的随机事件。
