ABSTRACT
Fish cytogenetics has incorporated during the last years new methodologies that employ molecular techniques, such as in situ hybridization with DNA probes for localization of chromosomal segments, chromosome banding using in situ digestion with restriction enzymes, in vivo incorporation of DNA base analogues, base-specific fluorochrome staining and, more recently, immunolabelling of chromosomes with antinucleoside antibodies. These techniques, associated with results obtained by using classical cytogenetic markers such as C-bands and nucleolus organizer regions, have allowed a more comprehensive understanding of the molecular structure of chromosomes and enlarged the area of cytogenetic evolutive studies. In neotropical fish the use of these modern techniques resulted in the localization of rDNA segments and the analysis of their activity; the localization of telomeric DNA segments; the identification of C-band modified patterns in species analyzed by in situ digestion of chromosomes with restriction endonucleases; the identification of R-handing patterns after incorporation of 5-bromodeoxyuridine; and, using the antibody antimethylcytosine, the identification of the methylated state of the heterochromatin in diploids and triploids.