X chromosome variations of Bacillus thuringiensis supernatant resistance and ribosomal DNA content in Drosophila melanogaster wild-type Oregon R lines.

We have used the supernatant of Bacillus thuringiensis cultures to follow variations of the ribosomal DNA content in a wild-type Oregon R line of Drosophila melanogaster. These variations are revealed by differences in the degree of resistance to the lethal effect of the supernatant added in the culture medium. Different X chromosomes, all originated from the same X chromosome, confer different degrees of resistance. Increases and reductions in the number of the X ribosomal DNA transcriptional units have been found and are correlated with variations in the degree of supernatant resistance. However, no significant variations in X rDNA content were observed between the initial line and this line tested 20 generations later. These results show that the X rDNA is subjected to frequent modifications that must be essentially complementary and thus originate from reciprocal molecular events. Variations of resistance have been observed in the absence of recombination between homologous chromosomes; this is in agreement with the hypothesis that unequal recombinations between sister chromatids are largely involved in the variations of the X ribosomal DNA in Oregon R wild-type strains.