NUCLEOLAR CHROMOSOME VARIANTS IN STERNOPYGUS-MACRURUS (PISCES, STERNOPYGIDAE) FROM 3 BRAZILIAN RIVER BASINS

The karyotypes, location of nucleolus organizer regions (NOR) and cons titutive heterochromatin pattern of Sternopygus macrurus (Pisces, Gymn otoidei) of natural populations from the Amazon River, Sao Francisco R iver and Tiete River (the last belonging to the Upper Parana River sys tem) are reported. All specimens had 2n = 46 chromosomes and presented small differences in karyotypic formulae, but populations of each riv er basin had a different fixed NOR phenotype. The loss of the satellit e and a gradual deletion of the heterochromatin block adjacent to the NOR may be the origin of the variants. The possible mechanism of fixat ion of the NOR phenotypes, and the implications of the occurrence of i ntraspecific differences in fixed NOR phenotype in this species are di scussed.