Mean macronuclear DNA contents are variable in the ciliate Tetrahymena.

Using cytophotometry the mean macronuclear DNA content of the ciliate Tetrahymena was found to be variable in two cell lines examined. While environmental changes (different culture fluids, temperature shifts) influence the mean DNA content of at least one of the two cell lines, spontaneous changes of the means are observed in both lines. The mean DNA contents of cultures maintained permanently in defined medium at various temperatures suggest the existence of a lower (7.5 pg) and an upper (13.6 pg) limit of the mean macronuclear DNA contents. Greatly differing means, either below or above those limits, were only occasionally found in cultures shifted to different temperatures for periods of less than 3 days. Eventually, the G2 mean macronuclear DNA contents of these cultures differed greatly from the doubled G1 mean value (maximum: mean G2 value equalling four times the G1 mean). Analysis of the maximum and minimum G1 DNA contents found in any culture indicates no absolute macronuclear DNA content triggering the regulation of macronuclear DNA content. There is no stringent correlation between the average cell volume and the average macronuclear DNA content. The data are discussed with reference to reported mechanisms of regulation of macronuclear DNA content.