Different behaviour of Xbb+ and Xbb chromosomes in D. melanogaster with only one nucleolus organizer

We have examined the rDNA content of male and female adult flies having only one nucleolus organizer (NO), using X chromosomes carrying wild or partially deleted bobbed loci (Xbb+/O, Xbb+/XNO- and Xbb/O, Xbb/XNO-). The results show that in Xbb+/O and Xbb+/XNO- flies, where only somatic gene compensation is supposed to occur, the rDNA increase, althought less pronounced than previously reported, is directly proportional to the number of rRNA genes initially present in the nucleolus organizer. In Xbb/O and in Xbb/Xbb/XNO- flies the rDNA increase is relatively much higher than that observed in flies carrying bb+ instead of bb. It is suggested that this may be due to rDNA premagnification and somatic gene compensation occurring simultaneously in the former flies.