The paper represents a comparative cytogenetic analysis of three populations of Hypostomus regani in Brazil.Two populations belong to the Upper Paraná River Basin and the third one, the karyotype of which is described for the first time, was probably introduced into the São Francisco River Basin through transposition from the Piumhi River. Karyotype features of populations of Hypostomus regani from the Piracicaba and Tietê River Basins are also discussed. The occurrence of Hypostomus regani in the São Francisco River Basin is reported for the first time here. The study also revealed distinct differences in the location of the Ag-NORs between the analyzed populations that enable individuals from the Piumhi River, Mogi-Guaçu River and Tietê River to be distinguished from one another. Thus, the data obtained indicate the possibility of geographic variation fixing different karyotypes even in the same basin of origin.

Cytogenetic analysis of Astylus antis using mitotic and meiotic cells was performed to characterize the haploid and diploid numbers, sex determination system, chromosome morphology, constitutive heterochromatin distribution pattern and chromosomes carrying nucleolus organizer regions (NORs). Analysis of spermatogonial metaphase cells revealed the diploid number 2n = 18, with mostly metacentric chromosomes. Metaphase I cells exhibited 2n = 8II+Xyp and a parachute configuration of the sex chromosomes. Spermatogonial metaphase cells submitted to C-banding showed the presence of small dots of constitutive heterochromatin in the centromeric regions of nearly all the autosomes and on the short arm of the X chromosome (Xp), as well as an additional band on one of the arms of pair 1. Mitotic cells submitted to double staining with base-specific fluorochromes (DAPI-CMA3 ) revealed no regions rich in A+T or G+C sequences. Analysis of spermatogonial mitotic cells after sequential Giemsa/AgNO 3 staining did not reveal any specific mark on the chromosomes. Meiotic metaphase I cells stained with silver nitrate revealed a strong impregnation associated to the sex chromosomes, and in situ hybridization with an 18S rDNA probe showed ribosomal cistrons in an autosomal bivalent.