Polymorphism in Tetrahymena vorax *

Two types of polymorphism, differing greatly in stability characteristics, have been recognized in Tetrahymena vorax strain V2. The basic life history involves three stages: a microstome, a macrostome, and a cyst. The appearance of each of these stages is dependent upon conditions of substrate availability, i.e. , macrostomes persist in the presence of suitable ciliate prey, microstomes persist in the presence of bacteria or organic nutrient solutions, and cysts appear under conditions of starvation. Superimposed upon the basic life history is a second level of polymorphism: three markedly different types of sublines have been obtained from a single parental clone. No intermediate types have been observed. When grown under identical controlled conditions, the three types of sublines differ significantly in cell and nuclear size, meridian number, growth properties, competence to form cysts and macrostomes, and serotype. Each subline of a given type is subject to transformation into one or both of the other types. These transformations are usually spontaneous events with very low frequencies, although certain transformations may be induced. Once a transformation has occurred, the new phenotype is inherited by subsequent generations of cells for an indefinite period in a way which is independent of environmental influences. The available information suggests that the three types of sublines are not the result of gene mutations, but rather represent alternate epigenetic stable states of a single cellular system.