FIG. 1.

TSA treatment of conjugating cells at 3 h after mixing causes cells to arrest in metaphase of meiosis I. The developmental stages of conjugation previously described in references 22 and 39 are schematically represented as follows. Lanes: 1, cell pairing; 2, crescent stage (prophase of meiosis I); 3, metaphase of meiosis I; 4, end of meiosis II; 5, prezygotic mitosis of one of the four haploid nuclei; 6, karyogamy; 7, macronuclear development I, which is distinguished by the central location of the parental macronucleus, the anterior location of the new macronuclei, and the posterior location of the new micronuclei; 8, macronuclear development II, in which the parental macronucleus condenses and paired cells separate; 9, macronuclear development III, which begins when the parental macronucleus has been resorbed; 10, the final new macronucleus stage, in which one of the two micronuclei is eliminated and the new macronuclei have undergone DNA amplification. The percentage of cells in each cytological stage was determined after fixing and staining with DAPI. At least 200 pairs were scored for each time point by fluorescence microscopy. About 95% of the cells paired within the first hour after mixing. The white rectangles show the untreated cells. The black rectangles show the outcome for cells treated with TSA at 3 h after mixing for 2 h (until 5 h after mixing) and for 4 h (until 7 h after mixing).