Superunstable alleles at the cut locus in Drosophila melanogaster

Summary This is a detailed study of the reversions of the ct ^MR2 allele putatively carrying á mobile element (MR-transposon) in the cut locus. Stable, unstable and superunstable revertants have been identified. Besides, a series of multiple unstable visible and lethal ct mutations derived from the ct ^MR2 allele have been obtained. They are shown to include supermutable alleles. The results suggest that the MR-transposon is connected with at least three functions: excision; change of orientation; and change of position within the cut locus, these functions being disturbed in different ways in different unstable ct ⁺ and ct alleles. In some cases the mutant transitions are somehow strongly stimulated leading to superinstability, reaching the rate of 0.5.