Causal relations among cell cycle processes in Tetrahymena pyriformis. An analysis employing temperature-sensitive mutants.

Abstract

Utilization of temperature-sensitive mutants of Tetrahymena pyriformis affected in cell division or developmental pathway selection has permitted 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 DNA 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 DNA 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 DNA 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

What Do Genic Mutations Tell Us about the Structural Patterning of a Complex Single-Celled Organism?

“…cdaA-1 is perhaps the most extensively studied mutation of this set, with 20 publications (15,17,18,19,20,48,49,52,54,56,59,60,81,83,86,87,119,120,121,156). cdaA-1 is a 100%-penetrant temperature-sensitive mutation, located on chromosome 4R (15; E. Hamilton, personal communication), that at 39°C permits an OP to be initiated and develop at its normal position but prevents all other known structural (49) (Fig.…”

“…Micronuclear division, which accompanies oral development (Fig. 3), occurred normally in cdaA-1 cells at a restrictive temperature, but macronuclear division, which normally occurs after the fission zone forms, failed to take place under these conditions (54); periodic DNA synthesis continued within the undivided macronucleus (20,54). The cdaA-1 cells grew, went through multiple cell cycles and additional rounds of oral development (81, 86) without dividing, and eventually became gigantic, irregular monsters (49).…”

Heat synchronization of cell division in Tetrahymena thermophila and a mutant, NP1, with a temperature sensitive defect for oral development

“…In addition, it provides a tool for screening for mutants super-or insensitive to heat shocks. Such mutants may provide insight into the control mechanisms for oral development, nuclear division and cell division (8,9,17,28,30,35). growing at 37~ suggest that oral development is blocked in stage 2 (21, unpublished results).…”

Abnormal Micronuclear Telomeres Lead to an Unusual Cell Cycle Checkpoint and Defects in Tetrahymena Oral Morphogenesis

“…Waves of morphogenetic activity then organize this field into a set of uniquely sculpted membranelles, creating the cell's oral apparatus. There appear to be four parallel programs involving the nuclei and oral apparatus that are coordinated as cells divide: (i) they replicate their micronucleus by mitosis (28,29), (ii) they replicate their macronucleus by amitotic fission (14,15,36), (iii) they synthesize and assemble a gallery of cortical organelles associated with the oral apparatus (9, 10), and (iv) they undergo cytokinesis in a fashion coordinated with the completion of the other three programs (2,11,13,35,37).…”