Figure 3

Histone modifications during conjugation. Nuclear events induced at the onset of sexual reproduction, when two Stylonychia cells of different mating types conjugate. Left: The old macronucleus is fragmented into a variable number of fragments (p), which partly persist in subsequent stages of differentiating cells, but are eventually degraded. Meiosis of diploid micronuclei (m) leads to haploid intermediates. Middle: One haploid micronucleus in each cell undergoes a postmeiotic mitosis (m*); remaining micronuclei (m) are degraded. One of the resulting haploid nuclei becomes a migratory micronucleus, and these nuclei are exchanged among the conjugating cells. Right: The migratory micronuclei fuse with their stationary counterparts to build a diploid synkaryotic micronucleus (m). This synkaryon divides mitotically, and one of the resulting nuclei becomes a new micronucleus. The other differentiates into a new macronucleus. In situ antibody staining (color code as described in Figure 2): Similar to the macronucleus of vegetative cells, several histone modifications are detected in the fragmented parental macronucleus (p): H3K9ac (A), H3K14ac (B), H3K4me1 (C), H3K4me3 (D), and H3S28p (J). H3K36me3 is lost from the micronucleus at this stage (E), whereas H3K27me3 (I), and H3S28p (J) persist. H3K27me1 (H) is transiently detected in micronuclei (m*) exclusively at this developmental stage.