Inhibition of oral morphogenesis during conjugation of Tetrahymena thermophila and its resumption after cell separation.

During the early stages of the conjugation of Tetrahymena thermophila, the oral apparatus undergoes partial regression: the deep fiber bundle is resorbed and oral membranelies are partially deciliated. However, further progress of the oral replacement is arrested until cell separation. Only then proliferation of basal bodies for a new oral primordium is unblocked and concomitantly the old oral structures are resorbed. This step is followed by the assembly of new oral membranelies and resorption of the old ones. Thus, the cell separation itself may be the source of an important signal for the cortical morphogenesis. To test this hypothesis we have used micronuclearly defective "✴" cells which separate after conjugation about 2.5 h earlier than normal cells and do not undergo postzygotic nuclear divisions. In the "✴" crosses (the crosses of the "✴" × wt cells) the oral primordium appeared before the cell separation (at the stage of hemikarya) following partial regression of the old oral structures, but only in those pairs in which the separation was already advanced in the separation. Consequently onset of conjugational morphogenesis in the "✴" pairs was correlated with the time of the cell separation, in spite of the differences in the course of nuclear events in the two kinds of pairs.