Andreata, Artur AntonioDe Almeida-Toledo, Lurdes ForestiDe Almeida, SilvioFilho, ToledoOliveira, Claudio2022-04-292022-04-291992-01-01Cytologia, v. 57, n. 3, p. 369-372, 1992.0011-4545http://hdl.handle.net/11449/230704The cytogenetic analysis of 13 specimens (9 males and 4 females) of Pseudotocinclus tietensis showed that this species has 2n=54 chromosomes (26M+20SM+6ST) plus one heteromorphic pair (SM/M) in the males and one homomorphic pair (SM/SM) in the females. Ag-NORs are located interstitially on the long arm of the largest metacentric pair and a small amount of constitutive heterochromatin is present in the karyotypes. The occurrence of a heteromorphic pair in the males is demonstrable by the presence of one extra C-banded block in the Y chromosome. The presence of one heterochromatin block distinguishing the sexual chromosomes suggests that this species presents a very primitive mechanism of sex chromosome differentiation. © 1992, Japan Mendel Society, International Society of Cytology. All rights reserved.369-372engC-bandKaryotypeNORPseudotocinclusXX/XY sex chromosomesArtigo10.1508/cytologia.57.3692-s2.0-0000326443