The expression of sexual activity in Paramecium caudatum is repressed for about 50 fissions after conjugation. The ability of the macronucleus to affect the expression of sexual activity according to the age of clonal development was investigated using the macronuclear fusion-reorganization method. When a mature macronucleus was transplanted into an immature cell and fused with a macronucleus in the immature cell, the clones derived from the recipient showed sexual immaturity. In a reverse experiment, an immature macronucleus was transplanted into a mature cell, and the clones also showed sexual immaturity. The ability of the macronucleus to transform mature cells to immature cells was clonal age-dependent. The characteristics of the immature-mature hybrid macronucleus indicate that the immature macronucleus is dominant over the mature macronucleus with respect to the ability to express sexual activity. On the other hand, in cells of the early immaturity period, the micronucleus, known as the germ nucleus, shows the ability to undergo meiosis and eventually to produce progeny under control of the mature macronucleus. The expression of sexual activity is thought to be governed by the clonal age of the macronucleus and not by the clonal age of the micronucleus or cytoplasm. The macronucleus seems to determine the ability to express sexual activity by counting post-conjugation divisions and keeping track of the age of the clone.

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