Mitotic recombination induced by six alkylating agents has been studied in the wing-spot test of Drosophila melanogaster. The model mutagens chosen have different modes of action at the DNA level. These are: the direct-acting small alkylating agent methylmethanesulfonate (MMS), the small promutagens N-dimethylnitrosamine (DMN), and N-diethylnitrosamine (DEN), the bifunctional cross-linking alkylating agents mitomycin C (MMC), chlorambucil (CLA) and monocrotaline (MCT). Flies of the standard cross (flr(3)/TM3, Bd(S) females and mwh males) were used to produce the larvae to be treated. Three-day-old Drosophila larvae were exposed by chronic feeding for 48 h to three different concentrations of all six alkylating agents. Acute feeding for only 2 h was used in addition with DEN and MMC. Wings of the marker-heterozygous (mvh+/+flr(3)) as well as of the balancer-heterozygous (mwh+/TM3, Bd(S)) progeny were analysed. The ranking of the compounds with respect to their genotoxic potency, based on mwh clone formation frequency in marker-heterozygous wings, was: MMS > MMC > DMN > CLA similar to MCT > DEN. The ranking with respect to the induction of twin spots, which are produced by mitotic recombination exclusively, was: MMS > DMN > MMC > MCT > CLA > DEN. The quantitative determination of recombinagenic activity, based on mwh clone formation frequencies obtained in both types of wings, gave the following values: MMS, 93%
MCT, 87%
CLA, 80%
MMC, 73%
DMN, 67%
DEN, 22%. A clear relationship exists between the extent of N-alkylation of DNA and the efficiency of the monofunctional agents MMS and DMN as well of the bifunctional agents MCT, CLA and MMC to induce mitotic recombination. This contrasts with the ethylation of base oxygen atoms and the resulting lower efficiency of DEN to produce mitotic recombination.
