Fluorescence cytophotometry was used to study nuclear DNA content and synthesis patterns during meiosis, fertilization and macronuclear development in the ciliated protozoon, Tetrahymena pyriformis, syngen 1. It was found that cells entered conjugation with a G1 (45C) macronucleus and a G2 (4C) micronucleus. During meiosis the micronucleus was reduced to 4 haploid nuclei, each with a 1C amount of DNA; each meiotic product then replicated to 2C, but only the nucleus next to the attachment membrane in each conjugant divided to form the two 1C gametic nuclei. The gametic nuclei replicated to 2C prior to fertilization; hence there was no S-period in the 4C fertilization nucleus (synkaryon). The first postzygotic division products immediately entered an S-period to become 4C, and at the second postzygotic division, each of the two 4C nuclei in each conjugant divided to form one 2C micronucleus and one 2C macronuclear Anlage. The macronuclear Anlagen began DNA synthesis immediately and were about 8C at the completion of conjugation; the micronuclei did not undergo rapid DNA doubling and measured between 2C and 3C when the conjugants separated. The old macronucleus did not participate in any S-period during conjugation and began to decompose after the second postzygotic division; it contained an average of 24C at the end of conjugation. From this sequence of nuclear divisions a pattern emerges that, unless a general cytoplasmic signal for DNA synthesis is suppressed, DNA synthesis always occurs in micronuclear division products immediately following separation of sister chromatids. Nuclear development continued in the first two cell cycles after conjugation. In exconjugants (the first cycle), macronuclear Anlagen underwent two rounds of DNA synthesis to become 32C and both micronuclei also underwent DNA synthesis. However, prior to the first cell division, one micronucleus and the old macronucleus completely disintegrated, and at the first cell division the remaining 4C micronucleus divided and one macronuclear Anlage was distributed to each resulting caryonide. At the end of the second cell cycle, the dividing macronucleus of each caryonide contained about 128C. These results relate to the question of ploidy of macronuclear subunits. It is argued that the G1 macronucleus contains 22 or 23 diploid subunits, each subunit being a copy of the diploid micronuclear genome. It is suggested that unequal macronuclear division relates to the question of subunit ploidy by playing a role in the phenomenon of macronuclear assortment.
