Timing of meiotic events in the wild type and in mutants.
(A) Cell mating (conjugation) begins immediately after the mixing of starved cells of different mating types. Each cell possesses a polyploid somatic nucleus (the macronucleus–MAC) and a diploid germline nucleus (the micronucleus–MIC), the latter of which undergoes meiosis. Programmed meiotic DSBs occur within 2 h after mixing and trigger elongation of the MIC . Meioses in the two conjugating cells progress fairly synchronously. About 3.5 h after mixing, the MIC is fully elongated to about twice the length of the cell. During elongation, the pairing of homologous loci increases . After that, the MIC gradually contracts and enters a stage resembling the diplonema of canonical meiosis, which is characterized by the formation of distinct chromatin threads. About 4.5 h after mixing, five bivalents appear in the wild type, arrange in a metaphase I plate, and are separated in a closed first meiotic division. (B) In the wild type, Dmc1 appears soon after MIC elongation begins and disappears at the onset of diplonema. At this time point, BrdU is incorporated, indicating recombinational repair synthesis . Mcmd1 and Pamd1 first appear in the elongating MIC and disappear at the onset of diplonema. (C, D) In mcmd1Δ (C) and pamd1Δ (D) cells, Dmc1 appears normally but has completely disappeared by the time the micronucleus has fully elongated. At this time point, the first foci of incorporated BrdU appear.