Metaphase plates and selected chromosomes of C. albula and C. fontanae showing FISH experiments. FISH with ITS2 (red) as probe hybridized to C. albula (a) and to C. fontanae (b), counterstained with DAPI. Double-FISH analysis with the Rex1 retrotransposon (red) and the 800 bp 28S rDNA (green) to C. albula (c), detail of the chromosome with Rex1 and 28S rDNA co-localization in inset. FISH with the 18S rDNA (red) to C. fontanae chromosomes (d) counterstained with DAPI. Chromosomes bearing the ITS2 (red) signal in C. albula (e). Chromosomes bearing the 18S rDNA signals (red) in C. fontanae (f). Chromosomes on (f) represent the hypothetically ancestral condition of rDNA distribution prior to its multiplication in C. fontanae. Bar = 5 μm.