In the present study, nuclear DNA content was analyzed in 53 species of Characiformes fish from the Neotropical region. Diploid number ranged from 2n = 48 in Astyanax fasciatus, Gymnocorymbus ternetzi and Hyphessobrycon griemi to 2n = 102 in Potamorhina squamoralevis, with a modal number of 54 chromosomes. Nuclear DNA content ranged from 1.70 ± 0.04 pg of DNA per diploid nucleus in Acestrorhynchus pantaneiro to 3.94 ± 0.09 pg in Tetragonopterus chalceus. A general analysis showed a mean value of 2.9 pg of DNA per diploid nucleus. Very similar DNA content values were observed in the species of the family Cynodontidae which showed a variation of 3% between the two genera studied. Small variations were observed between populations of Gymnocorymbus ternetzi, Astyanax fasciatus and Moenkhausia sanctaefilomenae (Characidae, Tetragonopterinae). The subfamilies Tetragonopterinae and Acestrorhynchinae (Characidae) presented the widest range, about 96%. Even in those families in which diploid number and karyotypic formulae were conserved such as the families Anostomidae, Curimatidae, and Prochilodontidae, episodes leading to losses or gains of genetic material became fixed in their evolutionary history.

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The subfamily Stethaprioninae, a monophyletic group among characids, is almost unknown from the cytogenetic viewpoint. This paper shows the first chromosome characterization of Poptella paraguayensis, an endemic species of the Paraguay river basin. Analysis in Giemsa-stained preparations showed 2n = 50: 10 M + 26SM + 8ST +6A. The nucleolar organizing regions (NORs) detected by silver nitrate and GC-specific fluorochrome mithramycin (MM) staining showed two NORbearing chromosomes in the diploid complement. DA/DAPI staining showed no differential fluorescence along the chromosomes. C-banding evidenced small blocks of heterochromatin in the centromeric and telomeric regions. Phylogenetic relationships between the Stethaprioninae and other characids are discussed on the base of chromosome data.