Different cytotypes in fishes of the genus Hypostomus Lcépède, 1803, (Siluriformes: Loricariidae) from Xingu river (Amazon region, Brazil)

We analyzed the karyotypes of three specimens of fiof the genus Hy- postomus Lacepede, 1803 (Loricaridae) from Xingu River (Amazon region). We used conventional staining techniques, including C-banding, Ag-NOR staining, CMA 3 - and DAPI-banding. Each specimen had a different cytotype: Hypostomus sp. Xingu-1 had 2n=64 (32M/SM, 32ST/A); Hypostomus sp. Xingu-2 has 2n=66 (32M/SM, 34ST/A), and Hypostomus sp. Xingu-3 had 2n=65 (38M/SM, 26ST/A + 1 B). The three cyto- types showed similar C-, CMA 3 - and DAPI-banding patterns. The nucleolus organiz- ing regions were located in the short arm of chromosome pair 25 of Hypostomus sp. Xingu-1 and pair 29 of Hypostomus sp. Xingu-2, and in the long arm of pair 30 of Hypostomus sp. Xingu-3, probably because of a pericentric inversion. A fusion/fi ssion rearrangement explains the difference in the diploid number and number of M/SM and ST/A chromosomes between the 2n= 64 and 2n=66 cytotypes. The B chromo- some most probably explains the difference between the 2n= 64 and 2n=65 cytotypes. The cytotype with 2n=65 had a signifi cantly larger number of M/SM chromosomes, probably because of pericentric inversions. These three cytotypes may represent dif- ferent species.