Rates of forward and reverse mutation inDrosophila after exposure to mustard gas and X-rays

Abstract

After treatment with mustard gas, reversions of the mutantforked ³ⁿ were observed with a frequency of 1 in 7,500. In the absence of indications for either suppressors or chromosome-rearrangements, these data provide evidence that a chemical mutagen can produce back mutations inDrosophila. Half the number of reversions was characterized by mosaic manifestation. This shows that delayed appearance after chemical treatment also holds for true gene mutations. One partial reversion to nearly normal type was not due to back mutation, but to a rearrangement, presumably involving a duplication of theforked containing region.

The study on reversion off ³ⁿ was combined with tests for recessive visibles at 15 selected loci of the X-chromosome. Mutations at theruby locus were most frequently induced by mustard gas (1 in 1,700). About one quarter of the forward mutations were fractionals. After exposure to 5,000 r X-irradiation both reversions off ³ⁿ and forward mutations at the loci under study were observed with frequencies comparable to those induced by mustard gas. Thus, no indication for mutagenspecific differences in mutational response have been obtained. After treatment with mustard gas a higher ratio of visibles to lethals was observed than after exposure to X-irradiation. It is pointed out that comparisons of the mutagenic effect of a chemical mutagen with that of X-radiation, even if restricted to visible mutations, inevitably involve an underestimate for the chemical, due to delayed effects of the latter.