O-ethylthymidine adducts are the most relevant damages for mutation induced by N-ethyl-N-nitrosourea in female germ cells of Drosophila melanogaster

Responses to genotoxic agents vary not only among organisms, test systems, and cellular stages, but also between sexes; little, however, is known about the mutagenic consequences of chemical exposures to female germ cells. In this study, the mutagenicity of N-ethyl-N-nitrosourea (ENU) was analyzed in female germ cells of Drosophila melanogaster using the recessive-lethal test and the vermilion system, which simultaneously generates information on induced mutation frequency and mutation spectrum. ENU was mutagenic in all stages of oogenesis, although there were differences among the stages. In mature and immature oocytes, ENU-induced mutations in the vermilion locus were 43.5% A:T-->G:C transitions, 39.1% A:T-->T:A transversions, 8.7% G:C-->A:T transitions, and 8.7% A:T-->C:G transversions, indicating that the most important premutagenic lesions induced by this chemical are O(4)-ethylthymine and O(2)-ethylthymine. The low frequency of mutation involving O(6)-ethylguanine (i.e., G:C-->A:T transitions) could be a consequence of the repair of these lesions by O(6)-methylguanine DNA methyltransferase. Comparison of these results with those previously obtained in male germ cells stresses the importance of the repair activity of the analyzed cells, because the mutation spectrum in female germ cells was similar to the spectrum obtained with repair-proficient spermatogonial cells and different from repair-deficient postmeiotic cells. The results also indicate that studies with female germ cells could be an alternative to the use of premeiotic male germ cells, especially when the analysis of these cells is difficult or almost impossible and when studies of in vivo DNA repair in premeiotic germ cells are performed.