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The chromosomes of Steindachneridion sp. (2n = 56) and Rhamdia quelen (2n = 58) were analyzed by C-banding (CB) and Chromomycin A3 (CMA3) and 4,6-diamidino-2-phenylindole (DAPI) staining, separately and consecutively, in order to understand the role of base-specific fluorochrome treatment after CB. Both species' chromosomes shared common staining profiles as follows. CB with Giemsa (CBG) revealed weak heterochromatic blocks in the telomeric regions of some chromosomes and conspicuous bands on the short arms of one chromosome pair, where nucleolar organizer regions (NORs) were evidenced by silver-staining. Without CB pretreatment, the NORs were stained conspicuously with CMA3, but not with DAPI. The latter uniformly stained all chromosomes, but leaving the NORs pale. Combination of CMA3 or DAPI staining with CB showed distinctive fluorescent blocks in the NOR-bearing short arms of the single chromosome pair along with several bright fluorescent signals on other chromosomes, which were not evidenced by single CMA3 or DAPI staining. These results suggest a modification of chromatin structure by CB treatment, which may increase the stainability of CMA3 and DAPI.

Eighteen specimens of Pimelodus maculatus collected from Tibagi River (Sertaneja, PR, Brazil) were analyzed cytogenetically. The diploid number of 56 chromosomes was observed and karyotype was 20 M + 20 SM + 10 ST + 6 A with fundamental number (FN) of 106. Results of analyses from the nucleolus organizer regions (NORs), obtained by AgNO3, CMA3 and C-band staining showed marking in a terminal position on the long arm of a pair of subtelocentric chromosomes. The restriction enzyme AluI produced a linear differentiation similar to C-banding.