Clonal death associated with the number of fissions in Paramecium caudatum

Contrary to an earlier report suggesting clonal immortality in Paramecium caudatum, the clones of the same species in this study terminated inevitably with a maximal life-span of 658 fissions and showed, prior to clonal death, decreased fission rate, increased probability of the appearance of non-dividing or dead cells after cell division, and increased frequency of morphological abnormality and of division asynchrony. Clonal senescence and death after a limited number of fissions was reproducible even if subclones derived from the original clone of a known fission age were examined again after a lag of 468 days. These results indicate that clones of P. caudatum are mortal and that they use fissions, not days, to measure life-span. Possible causes for the discrepancies between the earlier report and the present one are discussed.