Analysis of UV-stimulated recombination in the Drosophila SMART assay

The UV component of solar radiation is classified into UVA (320-400 nm), UVB (290-320 nm), and UVC (200-290 nm). Although all three types of UV light are capable of damaging biological systems, the earth's atmosphere filters out UVC, and a portion of UVB. In this study, we evaluated the induction of mutation and recombination by different wavelengths of UV light in the wing spot test of Drosophila melanogaster (Somatic Mutation and Recombination Test, SMART). Third-instar larvae that were trans-heterozygous for the third chromosome recessive markers, multiple wing hairs (mwh) and flare-3 (flr(3)), were exposed to different doses of UVA (at 365 nm), UVB (at 312 nm) or UVC (at 254 nm), and transferred to standard Drosophila culture medium. Feeding ended with pupation of the surviving larvae, and the genetic changes induced in the somatic cells of the wing's imaginal discs lead to the formation of mutant clones on the wing blade. Point mutation, chromosome breakage, and mitotic recombination produce single spots, while twin spots are produced only by mitotic recombination. Exposure to 500-4,000 J/cm(2) UVA did not increase the frequency of mutant spots. UVB doses of 200, 250, 300, 350, and 400 J/cm(2) increased the frequency of all categories of spots, indicating that UVB was potentially both mutagenic and recombinogenic. Assays run in balancer-heterozygous flies (which are insensitive to recombination) indicated that the fraction of mutants in trans-heterozygous flies due to recombination increased from 48.57% at 200 J/cm(2) UVB to 98.30% at 400 J/cm(2) UVB. While 140-480 J/cm(2) of UVC was not genotoxic, UVC produced a strong toxic response at doses higher than 140 J/cm(2). The results of this study indicate that UVB was much more active than UVC or UVA in the SMART assay, and that UVB was highly recombinogenic.