Silver-stained nucleolus organizer regions (Ag-NORs) in metaphase chromosomes of cultured fibroblasts were compared among 16 inbred strains of the Norway rat, Rattus norvegicus. Ag-NORs were located at the secondary constrictions or juxtacentromeric regions of the short arms of chromosomes 3, 11, and 12. The frequency and relative size of Ag-NORs were found to be strain-specific, providing a genetic marker system useful for characterization of inbred strains. While considerable cell-to-cell variation was observed within a given strain, the strain-specific pattern of Ag-NORs was shown to be consistent in cultured and noncultured cells obtained from different tissues of embryos, newborns, and adults, as well as in successively cultured cells examined up to the 10th subculture generation. The patterns of Ag-NORs in F₁ hybrids made between some of the inbred strains were in general agreement with those expected from the parental strains; some unexpected patterns were noted in F₁ hybrids of a particular cross, suggesting the possible existence of nucleolar interactions in such interstrain hybrids, although this has to be confirmed.

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