SYNOPSIS. Dimorphic populations of Tetrahymena patula L‐FF result when this organism is grown axenically in Loefer's medium supplemented with aqueous lettuce extract. The two different cell types in these cultures are identified as “macrostomes”and “microstomes.” Isolation of microstomes and macrostomes from the dimorphic cultures has shown that both cell types reproduce by binary fission in this medium, and further that each cell type will occasionally give rise to the other under these conditions. Isolation experiments have also shown that both microstomes and macrostomes from the dimorphic cultures are capable of giving rise to the reproductive cyst. The cells which are liberated from the cyst have small mouths but are best referred to as “tomites,” in order to distinguish them from the self‐perpetuating microstomes described above. The relatively small, rapidly swimming tomites will either transform into macrostomes or reproduce as microstomes. The life histoy of T. patula thus consists of three cell types as follows, each capable of giving rise to the other two: a macrostome , a microstome , and a tomite. Silver preparations of the dimorphic cultures have shown that oral replacement takes place. This consists of resorption of the oral apparatus and its replacement by differentiation of new mouth parts from an anarchic field of kinetosomes which arises immediately posterior to the old oral apparatus. It is suggested that this process is involved in the macrostome‐microstome interconversions. Growth curves were determined for T. patula feeding on a variety of substrates. These data indicate that this species is extremely well adapted as a carnivore. It grows very poorly on bacteria, but will grow well in axenic culture if aqueous lettuce extract is provided. The lettuce extract greatly reduces the generation time and increases the maximum cell density. Loss of the micronucleus in stain L‐FF is reported and discussed. The loss may be an indication that this strain has entered the senile phase of the Maupasian life cycle.
SYNOPSIS. The division delay responses of Tetrahymena (“set‐back” curves) which result from heat shocks of the type used in synchronizing studies have been examined in logarithmic and synchronized T. pyriformis GL. The curves indicate an increasing sensitivity to heat shocks with increasing cell age, a sudden sharp increase just prior to the “transition point” and a loss of sensitivity at the transition point. A correlated study of morphogenesis in the synchronous generation suggests that the sudden increase in thermal sensitivity before the transition point is due to a dedifferentiation and redifferentiation of the developing oral primordium which can be induced at this time. The results also suggest that the “stabilization” of the primordium which follows the transition point may occur at more than one developmental stage, i.e., is not stage‐specific. It is suggested that stabilization may therefore be due to conditions lying outside the primordium.
ABSTRACT A previously identified Tetrahymena thermophila actin gene (C. G. Cupples and R. E. Pearlman, Proc. Natl. Acad. Sci. USA 83: 5160-5164, 1986), here called ACT1 , was disrupted by insertion of a neo3 cassette. Cells in which all expressed copies of this gene were disrupted exhibited intermittent and extremely slow motility and severely curtailed phagocytic uptake. Transformation of these cells with inducible genetic constructs that contained a normal ACT1 gene restored motility. Use of an epitope-tagged construct permitted visualization of Act1p in the isolated axonemes of these rescued cells. In ACT1Δ mutant cells, ultrastructural abnormalities of outer doublet microtubules were present in some of the axonemes. Nonetheless, these cells were still able to assemble cilia after deciliation. The nearly paralyzed ACT1Δ cells completed cleavage furrowing normally, but the presumptive daughter cells often failed to separate from one another and later became reintegrated. Clonal analysis revealed that the cell cycle length of the ACT1Δ cells was approximately double that of wild-type controls. Clones could nonetheless be maintained for up to 15 successive fissions, suggesting that the ACT1 gene is not essential for cell viability or growth. Examination of the cell cortex with monoclonal antibodies revealed that whereas elongation of ciliary rows and formation of oral structures were normal, the ciliary rows of reintegrated daughter cells became laterally displaced and sometimes rejoined indiscriminately across the former division furrow. We conclude that Act1p is required in Tetrahymena thermophila primarily for normal ciliary motility and for phagocytosis and secondarily for the final separation of daughter cells.
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