Structure of the Drosophila mutable allele, white-crimson, and its white-ivory and wild-type derivatives.

The white locus in Drosophila is required for a normal brick-red eye color; deletions of this locus result in a bleached-white eye color. The white-crimson (wc) allele of white was isolated as a partial revertant of another mutant white allele, white-ivory (wi), a mutation due to duplication of sequences within the white locus. The wc allele reverts at high frequencies to wild-type and wi phenotypes and generates white-eyed derivatives, including deletions with one endpoint at the white locus. We analyzed the structure of the wc allele by molecular cloning and by Southern blot analysis of genomic DNA and found that the wc phenotype results from the insertion of a 10 kilobase DNA sequence into the wi duplication. Five independent phenotypic revertants of wc to wi were examined, and in each case reversion was accompanied by apparently precise excision of the insertion. Reversion of wc to a wild-type phenotype in each of the six cases examined was mediated by excision of both the insertion and one copy of the wi duplication, restoring gene structure to wild-type.