NUCLEO-CYTOPLASMIC INTERACTION DURING CONJUGATION IN TETRAHYMENA x DAVID L. NANNEY 2 Department of Zoology, University of Michigan, Ann Arbor, Michif/nn Studies on the ciliated Protozoa, particularly Paramecium aurelia (Sonneborn, 1950b, 1951), have provided much information on the roles of the nucleus and the cytoplasm in cellular differentiation. One important observation in these studies (Sonneborn, 1947) is that the cytoplasm may control the kind of macronucleus de-veloped in a cell; specifically, the cytoplasm may determine whether a new macronucleus will differentiate so as to control one or the other of the possible mating types. It has been suggested (Nanney, 1953) that the cytoplasm in these cells has been determined by the kind of macronucleus previously occupying the cell. This does not detract from the importance of the cytoplasm in cellular heredity, but emphasizes the importance of nucleo-cytoplasmic interactions. Related to this problem of what determines the kind of macronucleus to develop is the problem of what determines whether a particular nucleus will differentiate into a macronucleus. In many ciliates the fertilization nucleus pro-duced at nuclear reorganization divides twice to produce four presumably identical nuclei : two of these differentiate as macronuclei and two as micronuclei. Long ago Maupas (1889) suggested that this difference in the development of nuclei was due to localized differences in the cytoplasm surrounding the nuclei at a critical time in their development. Maupas based this suggestion on observations made on a group of ciliates, including particularly Colpidimn, Leucophrys and Glaucoma. He observed that in these organisms the fertilization nucleus divided twice and that the spindles for the second post-zygotic division were oriented in such a fashion that two of the four division products in each cell were placed at the extreme anterior end of the cell and two were placed at the extreme posterior end of the cell. Those which were placed at the anterior end of the cell were observed to enlarge and become the new macronuclei while those at the posterior end remained small and be-came the new micronuclei. Here was a clear visible correlation between the location of a nucleus in the cytoplasm and its subsequent development. Although this correlation strongly suggested cytoplasmic control of nuclear development, other interpretations were possible and were not excluded. A number of observations similar to those of Maupas have been made on a variety of organisms since Maupas' time and recently Sonneborn (1951) has directed attention to the nuclear events at conjugation in Paramecimn where several additional instances of apparent cytoplasmic control of nuclear activity are 1 This work was supported by a Faculty Research Grant, No. 270, from the Horace H. Rackham School of Graduate Studies of the University of Michigan. 2 The author wishes to express his appreciation to Prof. A. M. Elliott, who provided the strains of Tetrahymena used in the present study. He is also grateful to Prof. T. M. Sonneborn and Dr. Ruth V. Dippell for many helpful criticisms in the preparation of the manuscript. 133
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