Mobilization of two retroelements, ZAM and Idefix, in a novel unstable line of Drosophila melanogaster

We describe a novel transposition system in a line of Drosophila melanogast er called RevI in which two retroelements are mobilized. These elements are the retroelement ZAM, recently described in the literature, and a novel el ement designated Idefix. Like ZAM, Idefix displays the structural features of a vertebrate retrovirus. Its three open reading frames encode predicted products resembling the products of the gag, pol, and env genes of retrovir uses. In situ hybridization and Southern analyses performed on the RevI gen ome revealed the presence of some 20 copies of ZAM and Idefix, whereas ZAM is absent and Idefix is present in only four copies on the chromosomal arms of the original parental line. From RevI, a series of mutations affecting eye coloration has been recovered. The genetic and molecular analyses of th ese mutations have shown that most of them affected the white locus through three rounds of mutational events. The first mutational event was previous ly shown to be caused by a ZAM insertion 3 lib upstream of the transcriptio n start site of white. It confers a red-brick phenotype to the orange eye c oloration of the parental line. The second event results from the insertion of an Idefix copy 1.7 kb upstream of the transcription start site of the w hite gene, which modifies the red-brick phenotype to orange. This second mu tational event was recovered as a recurrent specific mutation in 11 indepen dent individuals. The third event results from an additional Idefix located 1.7 kb upstream of white that is responsible for the full reversion of the orange phenotype to redbrick. The fact that such mutations due to recurren t appearances of both ZAM and Idefix at the white locus result in such a va riety of phenotypes brings to light a new molecular system in which the int erference of mobile elements with the correct expression of the host gene c an be tested.