Spontaneous second-chromosome recessive lethals in a Drosophila melanogaster repair-deficient (mei-9a) mutant

The frequency of spontaneous second-chromosome recessive lethal mutations was determined in a Drosophila melanogaster strain kept homozygous for the X-linked mei-9 mutant for 50 generations. The data show that the rate of lethals of independent origin is 9 times higher and that of semilethals 4 times higher than the control values. The mei-9 mutant, originally isolated on the ground of its abnormal behaviour during meiosis, was later found to be defective in excision repair in somatic cells and to yield, after exposure to different physical and chemical agents, higher frequencies of genetic damage than mei-9+ flies. Our results are taken as evidence that the normal functioning of the mei-9 locus plays an important role in the repair of spontaneous lesions in the second chromosome and give support to positive findings in the X chromosome. Monitoring the accumulation of mutations in an experimental population could be a useful methodological approach to detect slight, though relevant, genetic damage induced by long-term exposure to environmental mutagens, whose detection is beyond the resolving power of conventional tests in Drosophila.