[Genetic instability in the Drosophila melanogaster lozenge locus: characteristics of the lz75V locus].

Genetic properties of lz75V, an unstable allele of the lozenge locus, are described. The lz75V allele appeared in progeny of a male from a Far East natural population of Drosophila melanogaster. Mutation of this allele produces a broad spectrum of mutant derivatives with phenotypes varying from normal to extreme. The arising alleles can be stable or unstable. Some lz75V derivatives continuously preserve their spontaneous mutability in laboratory conditions, whereas other alleles of the same family show progressive stabilization at the intralocus or intrachromosome level. Instability of the lz75V-bearing X chromosome is locus-specific: only the lozenge gene mutates with high frequency, while visible mutations at other loci rarely occur. As shown previously, the lz75V allele appears to be caused by a P-element insertion. The appearance of spontaneous instability is discussed with regard to the general problem of transposition regulation in mobile elements. Different systems of hybrid dysgenesis, and, in particular, P elements are assumed to play an important role in induction of unstable mutations in nature.