Unequal distribution of DNA in the macronuclear division of the ciliate Euplotes eurystomus

During asexual fission in the ciliate Euplotes eurystomus, the macronucleus divides amitotically. The macronucleus was found to divide unequally, yielding sister pairs having a mean difference in DNA content of 11.6%. DNA content was determined by the Feulgen reaction using a fluorescent Schiffs reagent, and measuring fluorescence by cytophotometry. Variability in macronuclear DNA content was also examined in randomly-paired non-sister cells, and found to be greater than in sister cells. This greater variability could be due to accumulation of differences over a number of divisions, or to interclonal differences in equality of division. Two categories of non-sister cells were examined: recently divided, and parents constructed by averaging the DNA contents of progeny. Both showed similar variability in quantity of macronuclear DNA. The fact that cells surviving to divide showed no less variability in amount of DNA than cells immediately after division suggests that extremes in amounts of DNA resulting from unequal division are not selected against.