Super-unstable mutations associated with P-M hybrid dysgenesis in Drosophila melanogaster.

Super-unstable mutations occasionally appear either in natural populations of Drosophila melanogaster or in P-M hybrid dysgenesis. We found that they may be reproducibly obtained with a high frequency from crosses between males from the pi 2 strain and females from the waG* strain or its derivatives. Super-unstable mutations in the ocelliless, singed, white, yellow and other loci have been obtained. Each super-unstable mutation gives rise to a large family of new super-unstable mutations with a wide range of phenotypic expression. Mutations with the same phenotype often differ in the specificity of their potential for further mutation. As a rule, a super-unstable mutation is associated with a specific reversible mutation and paired alleles are formed in this way. Other mutations are usually irreversible, but new mutations of these may also form paired alleles. Active transposase encoded by transposable P elements is necessary to maintain super-instability. Finally, some preliminary molecular data are discussed which suggest that this type of super-instability is a result of interaction between P elements and a novel mobile element, designated as X.