Fig. 2. (A-D) Meiotic pairing in pachytene nuclei of the hybrid SLY2006. (A) Extensive stretches of SC delineated by anti-Zip1 immunostaining. (B) Electron microscopy of silver-stained nuclei shows that the synapsed chromosome portions are connected by axial elements (arrows) and that most chromosomes are engaged in multiple pairing partner switches. (C,D) GISH on spread pachytene nuclei. (C) Pairing between S. cerevisiae and S. paradoxus chromosome regions prevails as most red and green threads are running side-by-side; (left) corresponding DAPI image. The rDNA tract, whose labelling is blocked in GISH is clearly visible (arrow). (D) In this nucleus uniformly red or green staining structures (arrows) indicate synapsis of chromosomes from one and the same genome. (E) Pachytene of the diploid chromosome III substitution strain SLY2007. The chromosome III bivalent is painted green with S. paradoxus genomic DNA; (left) corresponding DAPI image. Arrow denotes the bivalent. Bar, 2 µm.