(A-D) Zygotene-stage meiocytes of WT (A, C) and the spo11-1-Δ03 mutant (B, D) showing TUNEL (A, B), γH2AX signals (C, D) in green and DAPI stained chromatin in red. Most mutant cells exhibited no signal, but ~10% of cells exhibited a few signals (arrows). (E-H) Meiocytes at zygotene and pachytene stages from WT and spo11-1-Δ03 hybridized with 5S rDNA (green, arrows) and telomere probes (white signals in E and G) demonstrate impaired homologous pairing in the spo11-1 mutant, although telomeres cluster normally. (I-L) Meiocytes at diakinesis and metaphase I from WT and spo11-1-Δ03 hybridized with 5S rDNA (green, arrows) and 180bp-knob probes (white signals) showing that normal crossover formation is revoked in spo11-1 meiocytes. Note that a bivalent detected in Fig 3L represents non-homologous chromosome association. All scale bars represent 5 μm.