Abstract
An X-linked regulatory mutation in Drosophila melanogaster which gives rise to constitutive expression of the G 6 PD gene appears to have resulted from insertion of a novel class of transposable genetic elements into the vicinity of the G 6 PD locus. Flies carrying this element exhibit a very high level of G 6 PD activity, and linkage of this element to the X chromosome is stable as long as the mutants are inbred. However, loss of the high-G 6 PD activity trait, probably due to excision of the inserted element, often occurs when the mutant males are mated with females of other strains showing a normal level of G 6 PD activity (hence lacking in the element), but not in the reciprocal crosses. This suggests that the cytoplasm of the mutant might possess some factors which stabilize the regulatory element, and that the element itself might be responsible for the production of this stabilizing factor.
