Fluorescence cytophotometry was used to examine quantitative variation and regulation of macronuclear DNA content in 44 clones representing 13 species in 3 species-complexes of the genus Tetrahymena. Mean DNA amounts for G2 macronuclei generally ranged from 20–50 pg, with extreme means of 10 and 196 pg. Both intra- and interspecific variation was usually significant at the 5% level, and in some instances DNA amounts for the same clone in repeated experiments were significant different. Nevertheless, intraclonal ranges both within and among species frequently showed considerable overlap and, in the context of all the ciliates, the range of means is small. Calculations suggest that a biologically more meaningful measure, gene dosage, is also evolutionarily conserved. Additional evolutionary constraints are found in the regulation of macronuclear DNA content. Analysis of G1 and G2 intraclonal variances shows that in all species the variance added by regular unequal macronuclear division is removed by modification of cell-cycle events according to Model II regulation. In Model II, macronuclei with a small amount of DNA undergo an additional S phase before nuclear and cell division, whereas macronuclei with a large amount of DNA omit an S phase. Chromatin extrusion is also a regular feature of macronuclear division in most species, but its role in regulation is unclear. Extrusion regulates downward the mean amount of DNA but may actually contribute to unequal division and therefore add rather than remove variance.

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