Mating Type Determination in Variety 8, Tetrahymena Pyriformis.

WO major kinds of mechanisms controlling the expression of hereditary Tmating type specificities have been described among the ciliates. In Euplotes patella (KIMBALL 1942; POWERS 1943; SONNEBORN 1947), Euplotes uannus ( HECKMANN 1961 j and Paramecium bursaria ( JENNINGS 1942; SIEGEL and LARISON 1960; SIEGEL 1963) , mating types are inherited as if uniquely controlled by simple Mendelian differences. In Paramecium aurelia ( SONNEBORN 1947) and variety 1 of Tetrahymena pyriformis (NANNEY and CAUGHEY 1953; NANNEY, CAUGHEY and TEFANKJIAN 1955), by contrast, only an array of mating type potentialities is under genetic control. Which mating type is actually expressed by any given offspring depends not only on the set of potentialities it inherits, but also on the function of a mechanism which ultimately prevents the expression of all but one of the potentialities. The selection of different phenotypes in different lines with the same array of potentialities represents a case of cellular differentiation. The ability to cross differentiated cells, so far restricted to the single-celled organisms, has made possible a further clarification of the process involved. In P. aurelia, SONNEBORN (see 1947 review) has demonstrated that the cellular organelle which controls the expression of mating types is the macronucleus (somatic or vegetative nucleus). The micronucleus (germinal or sexual nucleus) serves as the reservoir of the array of genetic potentialities for the macronucleus of the next sexual generation. Thus, the process which, at every generation, limits the number of expressed potentialities is accompanied by a (macro-) nuclear differentiation. Furthermore, a given differentiated state is inherited throughout the life cycle of the macronucleus, i.e. until the next sexual reorganization, when a new macronucleus is differentiated de novo from the germ line. Mechanisms which perpetuate phenotypic differences in the absence of genotypic differences, have been designated as “epigenetic” (NANNEY 1958a, b) . The object of the present study was to analyze genetically the mechanism of mating type determination in variety 8, T . pyriformis, and its relationship to the