Interindividual size heteromorphism of NOR and chromosomal location of 5S rRNA genes in Iheringichthys labrosus

Twenty-five specimens of Iheringichthys labrosus from the Capivara Reservoir were analysed cytogenetically. AgNORs were detected in a pair of ST chromosomes, in the telomeric region of the long arm. Some individuals showed size heteromorphism of this region between homologous chromosomes. Treatment with CMA3 displayed GC-rich regions corresponding to the AgNOR pair, plus other fluorescent staining. In situ hybridization by fluorescence (FISH) with the 18S rDNA probe showed only one pair of stained chromosomes, confirming the heteromorphism observed with AgNO3 and CMA3 in some individuals. The 5S rDNA probe revealed telomeric staining on the long arm of a pair of chromosomes of the ST-A group, probably different from the NOR pair