The ciliate Paramecium tetraurelia exhibits two major developmental pathways - the vegetative cell cycle and the sexual pathway. The latter manifests itself in two forms -autogamy (self- fertilization) and conjugation (cross-fertilization) between cells of complementary mating types. In the normal life history young cells are immature for autogamy, but enter conjugation readily. Autogamy first occurs normally at about 20 fissions of age and conjugation disappears by 25. This study documents and analyzes the two major phenomena underlying this unusual life history. It shows how their interaction produces the observed pattern of immaturity, maturity, and senescence (Fig. 1). There are two principal findings of this study. First that commitment to autogamy leads to rapid loss of mating reactivity and second, that there are different starvation thresholds for initiation of mating reactivity and autogamy. The starvation threshold for initiation of mating reactivity is constant, while that for initiation of autogamy decreases progressively as clonal age increases. During the immature period for autogamy the lag between onset of mating reactivity and induction of autogamy decreases with increasing clonal age. The progressive decrease in the starvation threshold required for induction of autogamy brings about the end of the mature period for conjugation. As autogamy is initiated at progressively earlier stages in the growth of a culture, fewer and fewer cells reach the level of starvation required for initiation of mating reactivity prior to induction of autogamy. When the threshold for induction of autogamy becomes so low that no cells develop mating reactivity prior to entering autogamy, the period of maturity for conjugation is over and autogamy becomes the sole sexual process for the remainder of the life history.
