Abstract
We report here the genetical and X chromosome rDNA molecular study of two Drosophila melanogaster Oregon R lines. These lines differ extensively in the degree of resistance of the females both to the lethal effect of an increased temperature and to that of Bacillus thuringiensis beta-exotoxin, which is an inhibitor of the nucleolar RNA polymerase. The 3B line, whose females are resistant, came from an Oregon R population subjected over several generations to increased temperature, 28 degrees C or over, while the other line is derived from the initial stock. Twofold variation was observed in the total number of ribosomal genes between the two lines. This variation applied to most ribosomal units, including the active ones. Variations among X chromosome rDNA content in a wild type population have thus been revealed using tests of resistance to the Bacillus thuringiensis beta-exotoxin. Additive variations in specific unit types between the two lines indicate that modifications to the rDNA content are not rare events.
