Mutations induced at the white and vermilion loci in Drosophila melanogaster.

The white and vermilion loci in D. melanogaster were selected as target genes for the study of the mutational specificity of ionizing radiation and N-ethyl-N-nitrosourea (ENU) in a whole organism. Analysis of X-ray- and neutron-induced white mutants by a combination of genetic and molecular techniques showed that ionizing radiation induces primarily break-type mutations against a repair-proficient background, the majority of these alterations being deletions. Both very large multi-locus deficiencies and deletions of only a few base pairs were observed. These small deletions are flanked by repeats of 2-3 nucleotides, one copy of which is retained at the new junction. Presumably these small repeats are involved in the generation of the X-ray-induced deletions. In excision-repair-deficient mus201D1 flies, the frequency of whole-body white mutants recovered after X-ray irradiation is the same as in the wild-type strain. The percentage of mosaic mutations, however, is enhanced by a factor 3-4. Analysis by blot hybridization of ENU-induced white mutants strongly indicates that most mutations are due to base-pair changes. This was confirmed by sequence analysis of 25 ENU-induced vermilion mutants. In all mutants the alterations are due to base-pair changes, the majority being GC to AT transitions (61%).