Nuclear differentiation and transitional cellular phenotypes in the life cycle of Paramecium

Abstract

The life cycle of Paramecium aurelia includes successive stages of sexual immaturity, maturity and senescence reflecting a series of cellular phenotypes which are heritable for brief periods; in the course of vegetative reproduction new cell types are differentiated and the clone has achieved the next life cycle stage. Conjugation transforms a sexually mature cell into a sexually immature exconjugant. The duration of the period of sexual immaturity is shown to be controlled by the action of at least one genetic locus and by the past history of the parental clone. Experimental techniques broadly analogous to nuclear transplantations in amphibia and other microorganisms permit an analysis of the role of the macronucleus in the differentiation of new life cycle stages; the data are consistent with the hypothesis that the macronucleus undergoes age-correlated changes and that these are ultimately responsible for the reappearance of mature cells in an exconjugant clone. Since the parental generation is found to influence the duration of immaturity of its progeny, the prezygotic macronucleus is presumed to control, in some unknown manner, the development of the postzygotic macronucleus. The hypothesis of progressive macronuclear differentiation and internuclear communication appears to account in a satisfactory way for other life cycle phenomena.