Printed in Great Britain © Company of Biologists Limited igSo TEMPERATURE-SENSITIVE PERIODS OF MUTATIONS AFFECTING CELL DIVISION IN TETRAHYMENA THERMOPHILA

Temperature-sensitive periods were determined by application of temperature shifts and shocks to 3 temperature-sensitive cell division arrest (cda) mutants of Tetrahymena thermophila. A restrictive temperature, 36 °C, was found at which all 3 mutants are fully penetrant, yet other physiological effects are minimal. At this temperature, the temperature-sensitive period of cdaCl is a unique 5-min period in mid-division, that of cdaAi is a similarly brief period situated about 0-5 h prior to cell division, while the temperature-sensitive period of cdaHi is 20 to 30 min long and immediately precedes cell division. These periods either coincide with (cdaCi, cdaHi) or immediately precede (cdaAi) the onset of phenotypic abnormality at the restrictive temperature. Brief exposure to 36 °C during the temperature-sensitive period in any of these mutants brings about irreversible arrest of division furrows in progress or preparation. Mutant cells suffering such arrest can, however, divide again at a permissive temperature by forming new furrows at different sites