Chromosoma (Berl.) 40, 127--133 (1973) 9 by Springer-Verlag 1973 DNA Amounts in the Nuclei of Paramecium bursaria* C. A. Cullis** School of Biological Sciences, University of East Anglia, Norwich, England Abstract. DNA amounts in the macronuclei and micronuclei of syngen r of Para- mecium bursaria were determined. There was a wide distribution of values for the amount of DNA in the macronucleus. This distribution remained wide, but moved to lower values as the animals were starved. The micro-nuclear DNA values were constant under all culture conditions, at a value of 7.5 • 10-1~ g per nucleus. This gave an estimate of the haploid genome size, 2.25 • 1012 dMtons, which is approxi- mately ten times that obtained by DNA renaturation studies. Introduction Paramecium bursaria, like other ciliatcd protozoa, has two different types of nuclei, a mieronucleus and a macronuclcus, which differ in their DNA content. The ratio of the DNA amounts in these nuclei varies in different species of Paramecium. Those in the "aurelia" group have small micronuclei with "low" amounts of DNA and a high MAC/MIC DNA ratio, while those in the eaudatum group have large micronuclei with higher amounts of DNA and a low MAC/MIC DNA ratio (Cheissin and Ovchinnikova, 1964). In these studies the DNA values obtained were relative values and not measured against a standard of known DNA content. Thus the genome sizes of the nuclei could not be estimated. In the present study, spcctrophotometric measurements of the DNA were made of Feulgen-stained nuclei and compared to bull sperm nuclei whose DNA content is known. The DNA content of both the macro- nucleus and the micronucleus was measured. Materials and Methods Strains. The stocks of P. bursaria used originated from crosses involving stocks collected from Blackford Pond, Edinburgh, and were kindly donated by Dr. 1~. Born- ford. The stocks have been designated RB3, RBd, PS (Mating types III, IV and VII respectively) and I~B3S (~ating type IIS, which is the same stock as l~B3 but transformed for its mating type from III to II S. Cullis, 1972a). Bull Sperm. As the reference DNA bull sperm, obtained from l~amases II of Nor- folk, was included in the samples prepared for cytophotometric measurements. * This work was supported by a Commonwealth Scholarship to the author. Thanks are due to the Medical Research Council for funds to purchase the Vickers densito- meter. ** Present address see p. 133.