Behavior of germinal micronuclei under control of the somatic macronucleus during conjugation in Paramecium caudatum

In conjugating pairs of Paramecium caudatum, the micronuclear events occur synchronously in both members of the pair. To find out whether micronuclear behavior is controlled by the somatic macronucleus or by the germinal micronucleus, and whether or not synchronization of micronuclear behavior is due to intercellular communication between conjugating cells, the behavior of the micronucleus was examined after removal of the macronuclei from either or both cells of a mating pair at various stages of conjugation. When macronuclei were removed from both cells of a pair, micronuclear development was arrested 1 to 1.5 hr after macronuclear removal. When the macronucleus of a micronucleate cell mating with an amicronucleate cell was removed later than 3 to 3.5 hr of conjugation, that is, an early stage of meiotic prophase of the micronucleus, micronuclear events occurred normally in the operated cell. These results suggest that most micronuclear events are under the control of the macronucleus and that the gene products provided by the macronucleus are transferable between mating cells. One such product is required for induction of micronuclear division and is provided just before metaphase of the first meiotic division of the micronucleus. This factor is effective at a lower concentration in the cytoplasm and/or is more transferable between mating cells than the factors required for other stages. This factor, which seems to be present at least until the stage of micronuclear disintegration, is able to induce repeated micronuclear division as long as it remains active. The factor can act on a micronucleus which has not passed through a meiotic prophase. Moreover, the results suggest the existence of a second factor which is provided by the macronucleus after the first meiotic division that inhibits further micronuclear division.