Mutants of Tetrahymena thermophila with temperature sensitive food vacuole formation : II. Physiological and morphological studies

We studied 13 independently isolated, nitrosoguanidine-induced mutants of Tetrahymena thermophila having heat-sensitive capacity for food vacuole formation. Mutants belonging to the vacA complementation group have defects in the development of a new oral apparatus (OA) and not in phagocytosis per se; OAs formed at 30 °C are functional for at least three cell generations after transfer to 39 °C, while OAs formed at 39 °C are non-functional with regard to phagocytosis. Morphologically, the mutant OAs appear normal under light microscopy. One mutant forms a functional OA and normal contractile vacuole pores (CVP) at 22 °C, but after transfer to 30 °C these organelles disintegrate and vacuole-less cells swell without dividing. Finally, one mutant may be defective in phagocytosis; all daughters formed during the first three cell generations after transfer from 22 °C to 30 °C make food vacuoles, but at a rate which decreases with time. After 24 h at 30 °C food vacuole formation is practically stopped. With one exception, all mutants grow with wild-type generation times at temperatures restrictive for food vacuole formation in growth medium supplemented with high concentrations of iron, copper and folinic acid. The results indicate the feasibility of using a mutant approach to help dissect the developmental mechanisms responsible for the construction of the OA, and to determine the route of entry for compounds required for cell multiplication.