Linear Dose-response Relationship and No Inverse Dose-rate Effect Observed for Low X-ray Dose-induced Mitotic Recombination in Drosophila Melanogaster

Mitotic recombination has emerged lately as a surprisingly common cause of recessive functional gene loss in mammalian cells and has been implicated in tumour suppressor gene loss in human neoplasms. In an assay, primarily monitoring mitotic recombination in Drosophila melanogaster, the ability of low dose acute- and chronic X-ray irradiation to induce clonal expression of recessive mutations of formally heterozygous loci was investigated. Mosaic spots of recessive wing-hair misshape mutations (mwh and fir) and of hair-into-bristles transforming mutation (zw3tic) were enhanced by a factor of two over control level following irradiation of heterozygous larvae to doses as low as 0·01, 0·03 or 0·1 Gy X-rays. The frequencies of mosaic spots induced with eight doses in the interval 0·01-2·0 Gy was linearly related to the dose. The regression lines show no significant intercept at zero dose. During the entire larval developmental period exposure of the exponentially growing target cell population to conditions of chronic irradiation at dose-rate of 15·7 × 10−5 Gy/min provided no evidence of an inverse dose-rate effect as reported in yeast. In Drosophila, the probability of mitotic recombination per induced DNA double-strand break appears to be at least one order of magnitude higher than in man.