Effects of mutagens on the clonal lifespan of Paramecium tetraurelia

Abstract There has been interest in the phenomenon that a cell cannot undergo unlimited reproduction under adequate conditions and undergoes senescence. In holotrichous ciliates, Paramecium has a limit of vegetative reproduction without sexual reproduction but Tetrahymena does not always have a limited lifespan. Comparing the two species woudl increase our knowledge of the mechanism of cellular clonal aging. We previously showed that mutations induced by X-rays shorten clonal lifespan. In this study, we examined whether mutagens shorten the clonal lifespan of Paramecium tetraurelia. P. tetraurelia was exposed to the alkylating agent N -methyl- N ′-nitro- N -nitrosoguanidine (MNNG), 0.045 mg/ml, for 30 min. The animal was exposed to MNNG 6 times in total while young (under 80 divisions from the start of a clonal life cycle) or 4 times during the senescent stage. MNNG shortened the clonal lifespan as expressed by the decrease in fission number from 186 ± 55 (4 cell lines) to 136 ± 21 (6 cell lines) with the first two treatments but with further exposures the lifespan increased to 182 ± 15 (5 cell lines). MNNG had no effect when administered at the older age. Exposure of P. tetraurelia to 4-nitroquinoline- N -oxide at 0.021 mg/ml twice for 12 and 15 min at the younger age reduced the mean clonal lifespan from 143 ± 28 to 125 ± 21 and the maximum lifespan from 263 ± 33 to 175 ± 25.