[Timing of macronuclear DNA synthesis in carnivorous forms of Tetrahymena paravorax].

The macronuclear DNA synthesis in well-fed carnivorous forms of Tetrahymena paravorax has been investigated with autoradiographic experiments. The macrostome cell type of this ciliate is induced by stomatin and fed on living prey (T. pyriformis). The DNA synthesis is re-initiated when the macrostomes become active predators; its duration (approximately 5,5 h) corresponds to a series of four additional rounds of replication. The DNA content is therefore elevated to the 32 G1 level in cells which attain their maximum size before the first division occurs. The majority of daughter cells of the first generation do not perform the oral replacement and subsequently can not ingest prey. They undergo a short-duration division cycle without any apparent growth and without an intervening macronuclear S phase. This entails that the mean DNA content of daughter cells of the second generation is a quarter of the maximum value. More than 85 per cent of these cells get a larger oral apparatus and can feed on prey. The next wave of DNA synthesis (approximately 2,75 h) corresponds to two rounds of replication which re-establish the 32 G1 level. These results suggest that the normal reproduction cycle of carnivorous forms of T. paravorax displays a sequence of two successive DNA replication rounds followed by two successive division with variations of the DNA content between the 32 G1 and 8 G1 levels.