We have identified molecular lesions associated with six mutations, w(IR2) and w(IR4-8), of the white gene of Drosophila melanogaster. These mutations arose in flies subject to I-R hybrid dysgenesis. Four of the mutations give rise to coloured eyes and are associated with insertions of 5.4-kb elements indistinguishable from the I factor controlling I-R dysgenesis. The insertion associated with w(IR4) is at a site which, within the resolution of these experiments, is identical to that of two previously studied I factors. This appears to be a hot-spot for I factor insertion. We have compared the sites of these insertions with sequences complementary to white gene mRNA identified by Pirrotta and Bröckl. The hot-spot is in the fourth intron. The insertion carried by w(IR5) is either within, or just beyond, the last exon. The insertion carried by w(IR6) is near the junction of the first exon and first intron. The w(IR2) mutation is a derivative of w(1). It contains an insertion of I factor DNA within, or immediately adjacent to, the F-like element associated with w(1), and results in restoration of some eye colour. This insertion is just upstream of the start of the white mRNA. Mutations w(IR7) and w(IR8) are deletions removing mRNA coding sequences. Both determine a bleached white phenotype
