A Drosophila melanogaster cell line tested for the presence of active NORs by silver staining.

Silver staining was used to detect active NORs in a Drosophila melanogaster cell line (C1 82) characterized by dimorphic X chromosomes (XXL), one of the two Xs showing a marked increase in heterochromatin where the nucleolar organizer (NO) is located. The Q-banding technique was used to determine the karyotype characteristics of the line. Ag-positive NORs appeared only on structurally changed X chromosomes (XL), both in diploid and tetraploid cells, indicating that rRNA genes of XL are more active or numerous than those on normal homologues. A possible relationship between NOR stainability, the presence of an increased heterochromatic portion and the selective advantage of XXL cells, recurrent in numerous Drosophila female lines, is discussed.