Fig. 6. Homologous pairing of a chromosomal locus (region B, see Fig. 5) as determined by FISH. If a FISH probe to a chromosomal region produces two separate dots within a MIC, the chromosomes are considered not to be paired in this region. Pairing is indicated by two closely associated (double dot) FISH signals or by the fusion of FISH signals (single dot). In wild type, the frequency of nuclei with pairing increases throughout meiotic prophase. In the hop2A mutant, the maximal level of pairing achieved in stage IV is somewhat lower. In the spo11 mutant, the maximal pairing (at the stage resembling stage III) achieves a level above the ground level (presumably owing to random contacts) at the beginning of meiosis. A total of 100 nuclei per stage and genotype were evaluated. Pairing levels at region A were similar (supplementary material Table S2).