A NEW METHOD FOR DETECTING NUCLEOLUS ORGANIZER REGIONS IN FISH CHROMOSOMES USING DENATURATION AND PROPIDIUM IODIDE STAINING

A rapid method for detecting nucleolus organizer regions (NORs) in fis h chromosomes based on thermal denaturation and staining with propidiu m iodide is described, Under epifluorescence, the NORs of 15 fish spec ies from six families could be detected. This protocol differentiates constitutive heterochromatin in mammalian and avian chromosomes, and i n some cases, heterochromatic blocks in fish chromosomes. The staining of NORs of fish chromosomes with propidium iodide following denaturat ion with formalin is likely the result of differential denaturation of the rDNA due to the thermal characteristics of AT- and GC-rich domain s of the rDNA cistron. This technique provides a new useful marker for descriptive fish cytogenetic studies.