An Analysis Employing Temperature-Sensitive Mutants

Utilization of temperature-sensit ive mutants of Tetrahymena pyriformis affected in cell division or developmental pathway selection has permit ted elucidation of causal dependencies interrelating micronuclear and macronuclear replication and division, oral development , and cytokinesis. In those mutants in which cell division is specifically blocked at restrictive temperatures , micronuclear division proceeds with somewhat accelerated periodicity but maintains normal coupling to predivision oral development . Macronuclear division is almost totally suppressed in an early acting mutant (mol a) that prevents formation of the fission zone, and is variably affected in other mutants (such as mo3) that allow the fission zone to form but arrest constriction. However , macronuclear D N A synthesis can proceed for about four cycles in the nondividing mutant cells. A second class of mutants (psm) undergoes a switch of developmental pathway such that cells fail to enter division but instead repeatedly carry out an unusual type of oral replacement while growing in nutrient medium at the restrictive temperature . Under these circumstances no nuclei divide, yet macronuclear D N A accumulation continues. These results suggest that (a) macronuclear division is stringently affected by restriction of cell division, (b) micronuclear division and replication can continue in cells that are undergoing the type of oral development that is characteristic of division cycles, and (c) macronuclear D N A synthesis can continue in growing cells regardless of their developmental status. The observed relationships among events are consistent with the further suggestion that the cell cycle in this organism may consist of separate clusters of events, with a varying degree of coupling among clusters. A minimal model of the Tetrahymena cell cycle that takes these phenomena into account is suggested. One way to uncover causal relations between successive or simultaneous cellular developmental processes is to interfere selectively with one process and then observe the effects on the others. Microsurgical dissection provides a paradigm for this experimental logic: for example, Tartar (61) was able to demonstrate conclusively that the oral development of a ciliate is not necessary for the cell division that follows it, by simply cutting out the developing oral apparatus of Stentor and then observing that fission proceeds nonetheless. However, where cells are small and not readily operable, and/or the processes under study are pervasive physiological 242 THE .JOURNAL OF CELL BIOLOGY" VOLUME 71, 1976pages 242-260 on A ril 0, 2017 D ow nladed fom Published October 1, 1976