J.Cell Sci. 5.365-372(1969) 365 Printed in Great Britain STUDIES ON THE AUTONOMY OF PELLICULAR DNA IN PARAMECIUM JOAN SMITH-SONNEBORN AND W.PLAUT Zoology Research Building, University of Wisconsin, Madison, Wisconsin, U.S.A. SUMMARY This autoradiographic study was designed to elucidate the relationship between the macro- nucleus and pellicular DNA in Paramecium. The capacity of the cell to synthesize pellicular DNA in the absence of the macronucleus was established by demonstrating the incorporation of tritiated thymidine into DNase-sensitive material in the pellicles of amacronucleate cells. Moreover, using a technique which leads to selective labelling of the macronucleus in normal paramecia, we have looked for evidence of transfer of labelled DNA from the macronucleus to the pellicle with time. Finding none, we conclude that labelled pellicular DNA is not of macro- nuclear origin, and that labelled pellicular DNA synthesis is not directly dependent on the presence of the macronucleus. INTRODUCTION The presence of DNA in the pellicle of Paramecium has been demonstrated in our previous study (Smith-Sonneborn & Plaut, 1967). This paper addresses itself to the questions of origin and autonomy of pellicular DNA. Specifically, we are asking whether pellicular DNA can be synthesized in the absence of the macronucleus and whether there is detectable migration of DNA to the pellicle from the macronucleus in its presence. The data obtained lead us to the conclusion that little, if any, of pellicular DNA is of immediate macronuclear origin, and that the presence of the macronucleus is not essential for its synthesis. EXPERIMENTAL DESIGNS The capacity of paramecia to incorporate tritiated thymidine into the pellicle in the absence of a macronucleus was investigated using cells bearing the 'Amac' gene (Nobili, 1961). In its presence the distribution of the macronucleus during division is irregular so that a certain percentage of the fissionts will lack a macronucleus. The inability of such cells to divide was used to identify the amacronucleates. Migration of DNA from macronucleus to pellicle was studied through the use of labelled Escherichia colt cells as the source of labelled DNA precursor (Berger & Kimball, 1964). Unlike exogenously supplied tritiated thymidine, which leads to labelling of DNA in both macronucleus and pellicle, this method results in heavily labelled nuclei and unlabelled pellicles. Such selectively labelled cells were permitted to undergo several fissions in the absence of additional labelled precursors and the pellicles were examined for the appearance of labelled DNA.