Molecular cytogenetics of Oligosarcus hepsetus (Teleostei, Characiformes) from two Brazilian locations.

Karyotype and cytogenetic markers of Oligosarcus hepsetus from two Brazilian locations in the Paraiba do Sul River Basin (Brazil) were investigated using differential staining techniques (C-banding, silver (Ag)- and chromomycin A3 (CMA3)-staining) and fluorescent in situ hybridization (FISH) using 18 S rDNA and 5S rDNA probes. The diploid chromosome number was invariably 2n = 50 with 3 pairs of metacentric, 5 pairs of submetacentric, 8 pairs of subtelocentric and 9 pairs of acrocentric chromosomes. No heteromorphic sex chromosomes were observed. The nucleolar organizer regions (NORs) were detected in the short arms of the largest acrocentric pair using Ag-, CMA3- stainings and FISH with 18S rDNA probe, the latter showing also positive labeling in the short arms of a small acrocentric pair, not visualized by the former methods. FISH with 5S rDNA probe showed positive labeling in the two chromosome pairs. While the CMA3-staining exhibited GC-rich heterochromatin segments in two pairs of chromosomes, including those coincided with Ag-NORs, the DAPI staining did not reveal any signal, indicating the absence of AT-rich heterochromatin. FISH with an As-51 satellite DNA probe derived from the closely related Astyanax scabripinnis did not reveal any positive signal, demonstrating the absence of this class of DNA in the genome of the specimens under study.