Preponderance of GC-rich sites in silver-stained nucleolus organizing regions of Rita rita (Hamilton) and Mystus gulio (Hamilton) (Bagridae, Pisces), as revealed by chromomycin A3-staining technique and scanning electron microscopic studies.

The karyotypes of two species of catfish, Rita rita (Hamilton) (2n = 54; 14m + 34sm + 6st; NF = 102) and Mystus gulio (Hamilton) (2n = 58; 30m + 12sm + 2st + 14t, NF = 100) were studied through Giemsa-, silver- and chromomycin A(3)-staining techniques. The silver-stained karyotypes in both sexes of R. rita and M. gulio revealed that the nucleolus organizing regions were located terminally at the shorter arms (Tp) of one pair of submetacentric chromosomes, placed at positions Nos. 2 and 1, respectively, which was confirmed by scanning electron microscopy. Staining with a GC-specific fluorochrome, chromomycin A(3), produced bright fluorescence in the Ag-positive nucleolus organizer regions, suggesting thereby that nucleolus organizing regions actually included GC-rich sites of active r-RNA genes in metaphase chromosomes of these two bagrids. Further such studies are needed due to the extreme paucity of data on fish.