Variation of spontaneous and induced mitotic recombination in different Drosophila populations: a pilot study on the effects of polyaromatic hydrocarbons in six newly constructed tester strains.

A set of six Drosophila strains was developed, by inducing by chemical treatment with N-ethyl-N-nitrosourea (ENU) new white and, in some strains, yellow mutations in 3 wild-type (WT) and 3 insecticide-resistant (IR) populations. These strains were previously shown to vary with regard to contents and inducibility of microsomal oxidative enzymes (Zijlstra et al., 1984). In this pilot study results from a first evaluation of these strains in somatic mutation experiments are reported, using as genotoxins an aromatic amine (2-naphthylamine, 2-NA), one substituted (9,10-dimethylanthracene, DA) and one non-substituted (benzo[a]pyrene, BP) polycyclic aromatic hydrocarbon. Developing larvae heterozygous for white were chronically exposed to three different exposure doses of each carcinogen. Adult females were inspected for the occurrence of mosaic light clones in their eyes, using the somatic mutation and recombination test (SMART). Evidence is presented indicating strong genotype-dependent variation in both spontaneous and chemically induced mutational and recombinational events in somatic cells of Drosophila. The spontaneous frequencies varied from 3.5% (Hikone-R), 4.3% (Berlin-K), 6.3% (Oregon-K), 9.1% (91-C), 20.5% (Haag-79) to 49.1% (91-R), corresponding to a 14-fold difference in spot frequencies between the two extremes. BP, DA and 2-NA were readily detectable in both Hikone-R (IR) and Oregon-K (WT), less so in 91-C (WT) and Haag-79 (IR), whereas the performance of strain Berlin-K (WT) was rather poor. The special problem with strain 91-R was the high frequency with which mosaic light spots occur not only in female genotypes heterozygous for white, but also in homozygous condition in the original stock. The up to 20-fold variation in induced spot frequencies between different genotypes poses questions for further investigations with respect to the genetic constitution of the various strains and the role of enzyme induction on somatic cell mutagenicity, which in this system is predominantly the result of mitotic recombination.