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The males of the Drosophila melanogaster M115 mutant strain and of its spontaneous revertant strain RM115 are phenotypically similar to those carrying the w(+UZ) and w(+UR) alleles. The molecular description of these mutant strains could be extended to the unstable-zeste system, which has been used as a genotoxicity test, and could be of use for a better understanding of the assay. An FB element in the 3' vicinity of the white gene, actually into the second intron of the newly predicted CG32795 gene, was found and precisely located in our M115 and RM115 strains, and also in w(+UZ) as expected. We demonstrate the presence of NOF sequences in the M115 and w(+UZ) insertions. However, we found that the z1 phenotype in these males might not be due to the FB-NOF interference on the zeste-white interaction but to a duplication of the white gene unnoticed in previous studies. The reversion of RM115 correlates with the loss of the duplication, probably by a complex recombination event. Furthermore, a FISH experiment suggests that the two copies of white are nearby or tandemly duplicated.

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