Morgens David W, Cavalcanti Andre R O
Biology Department, Pomona College, 175W 6(th) Street, Claremont, CA 91711, USA.
Biology Department, Pomona College, 175W 6(th) Street, Claremont, CA 91711, USA.
Protist. 2015 May;166(2):224-33. doi: 10.1016/j.protis.2015.03.002. Epub 2015 Mar 12.
Over time and repeated asexual divisions, many ciliate species display the characteristics of senescence, reduced fecundity and increased mortality. Their only path to recovery is sexual conjugation or autogamy. While more traditional models of cellular aging have been proposed, one of the most accepted explanations relies on the faulty mechanism by which ciliates duplicate their somatic nucleus, a process referred to as amitosis. Amitosis involves the random segregation of chromosomes with no consideration for homology. Over subsequent divisions, chromosome copy numbers will fluctuate until an entire chromosome is lost, resulting in death. Via simulations of this process, we find that senescence and death via chromosome loss is not the only possible result of amitosis. Random chromosome loss is less damaging to populations than previously thought, and strict adherence to the model predicts that Paramecium tetraurelia would not senesce. A combination of the reciprocal nature of amitosis and lethal selection against low-copy number chromosomes is responsible for this startling prediction. Additionally, our results provide an alternate explanation to recent evidence for selection on chromosome copy number in Tetrahymena thermophila and peculiar patterns of senescence in Tetrahymena pyriformis.
随着时间的推移和反复的无性分裂，许多纤毛虫物种表现出衰老的特征，繁殖力下降，死亡率增加。它们恢复的唯一途径是有性结合或自体受精。虽然已经提出了更传统的细胞衰老模型，但最被接受的解释之一依赖于纤毛虫复制其体细胞核的错误机制，这一过程称为无丝分裂。无丝分裂涉及染色体的随机分离，不考虑同源性。在随后的分裂过程中，染色体拷贝数会波动，直到整条染色体丢失，导致细胞死亡。通过对这一过程的模拟，我们发现通过染色体丢失导致的衰老和死亡并不是无丝分裂的唯一可能结果。随机染色体丢失对种群的损害比以前认为的要小，严格按照该模型预测，四膜虫不会衰老。无丝分裂的相互性质与针对低拷贝数染色体的致死选择相结合，导致了这一惊人的预测。此外，我们的结果为最近关于嗜热四膜虫染色体拷贝数选择和梨形四膜虫特殊衰老模式的证据提供了另一种解释。
