ABSTRACT
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 "â´" cells which separate after conjugation about 2.5 h earlier than normal cells and do not undergo postzygotic nuclear divisions. In the "â´" crosses (the crosses of the "â´" × 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 "â´" 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.
ABSTRACT
The sensitivity of Paramecium tetraurelia (=P. aurelia syngen 4) cells to pulse treatments with various doses of Actinomycin D (AMD) was estimated by comparing the generation times of treated and untreated sister cells. It was found that the delay of division in treated cells depended on the concentration of AMD, on their "age" at the time of the pulse treatment, and on their individual sensitivity. Sensitivity of Paramecium to AMD changes during the cell cycle in a predictable way. About 3 1/2 hr before the normally expected cell fission (total generation time approximately 5 1/2 hr) there is a decrease of sensitivity. Thereafter, the cell enters a new stage with a progressive increase of sensitivity. This 2nd phase ends at the "transition point" (approximately 2 hr before cell division), when sensitivity drops abruptly. The division process itself may be altered and slowed down by high concentrations of AMD, even if the drug is applied after the transition point, but this process can never be completely annulled. The impairment of the division mechanism may lead to morphologic anomalies in the offspring. Resorption of oral anlagen in P. tetraurelia probably never occurs during the cell cycle after AMD treatment. The reason for individual variability of the cells, mechanisms controlling development, and the question of an obligate sequence of gene action in each cell cycle are discussed.
