ABSTRACT
The composition of heterochromatin classes along the chromosomes of specimens from two populations of the fish Astyanax scabripinnis was examined using fluorescence banding with GC- and AT-DNA specific fluorochromes and fluorescence in situ hybridization (FISH) with an AT-rich satellite DNA (As51) probe. For the pericentromeric heterochromatin blocks neither GC/AT-DNA specific fluorochromes nor the FISH technique produce any response with chromosomes from either of the populations. On the other hand, the telomeric distal heterochromatin blocks of both populations fluoresced when the FISH technique was applied but showed distinct responses after GC-specific fluorochrome treatments, leading us to propose different structural arrangements of the FISH-positive heterochromatins. Such differences in chromosome banding patterns together with other karyotypic differences suggest differentiation of these populations at taxonomic level.