Genetic Instability in Drosophila melanogaster: Putative Multiple Insertion Mutants at the Singed Bristle Locus

A series of eleven independent mutants at the X chromosome singed bristle (sn) locus of Drosophila melanogaster is described. All mutants descend from flies caught in the wild and bred in the laboratory. On the basis of their inordinately high spontaneous mutation frequency, ten of the mutants are classified as putative insertion mutants. Reversions to wild type occur at frequencies of 10-4-10-3. Some reversions appear to be losses of the inserted element, others appear (by analogy with prokaryotes) to be changes in the orientation of the inserted elements. Consistent with the insertion hypothesis, some sn mutants generate what are interpreted to be deletions at the sn locus. In their mutational properties, the sn mutants are analogous to insertion sequence (IS) elements and bacteriophage Mu of Escherichia coli, but the precise nature of the insertion remains unknown.