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We investigated the macronuclear DNA genomes of several marine and fresh‐water ciliates. The marine forms studied were: Uronema nigricans, Parauronema virginianum, Parauronema acutum, and two strains of Miamiensis avidus; the fresh‐water ciliates included: Tetrahymena pyriformis, Paramecium octaurelia, and P. caudatum. The organisms were cultured axenically and the DNA extracted from isolated and purified macronuclear preparations. Reassociation rate constants of purified DNA preparations used to calculate kinetic complexity were determined both optically and by hydroxyapatite chromatography. Analytical complexity was determined chemically. Ciliate macronuclear DNA appeared to reassociate as a single unique sequence, except for a small fraction (4% of the total DNA) that was repetitive and renatured rapidly. Values for the kinetic complexities of macronuclear DNA in these forms varied over a relatively narrow range, from 1.5 to 3.8 times 1010 daltons, and were only 7–15x larger than that of the bacterium Escherichia coli. On the other hand, values for analytical complexities of macronuclear DNA of marine and fresh‐water ciliates varied over two orders of magnitude and were related to the size of the animals. It is suggested that ploidy levels of macronuclear DNA in these ciliates may represent a functionally permanent amplification of the genome.

A simple method is described for plating and cloning ciliates and other protozoa, based on a principle differing from that traditionally used for plating and cloning bacteria and other microorganisms. This procedure, referred to as te silicone-oil-plating-procedure (SOPP), involves vortexing small volumes of culture medium containing protozoa with larger volumes of a non-toxic silicone oil and plating the resulting unstable emulsion in small plastic petri plates. Discrete microdroplets of culture medium form containing protozoa entrapped and immobilized between the hydrophobic surfaces of teh plastic petri dish and the oil. Protozoa, isolated by this method grow, divide, and multiply to form clones. The procedure may be used for plating and cloning protozoa in bacterized and axenic culture. Variations of te basic method may be applied to isolating protozoa from the wild, washing protozoa to remove microorganisms, screening for potential mutants, and for replica plating.