The effects of monochromatic ultraviolet light on the egg of Drosophila

The Adaptive Significance of Natural Genetic Variation in the DNA Damage Response of Drosophila melanogaster

Despite decades of work, our understanding of the distribution of fitness effects of segregating genetic variants in natural populations remains largely incomplete. One form of selection that can maintain genetic variation is spatially varying selection, such as that leading to latitudinal clines. While the introduction of population genomic approaches to understanding spatially varying selection has generated much excitement, little successful effort has been devoted to moving beyond genome scans for selection to experimental analysis of the relevant biology and the development of experimentally motivated hypotheses regarding the agents of selection; it remains an interesting question as to whether the vast majority of population genomic work will lead to satisfying biological insights. Here, motivated by population genomic results, we investigate how spatially varying selection in the genetic model system, Drosophila melanogaster, has led to genetic differences between populations in several components of the DNA damage response. UVB incidence, which is negatively correlated with latitude, is an important agent of DNA damage. We show that sensitivity of early embryos to UVB exposure is strongly correlated with latitude such that low latitude populations show much lower sensitivity to UVB. We then show that lines with lower embryo UVB sensitivity also exhibit increased capacity for repair of damaged sperm DNA by the oocyte. A comparison of the early embryo transcriptome in high and low latitude embryos provides evidence that one mechanism of adaptive DNA repair differences between populations is the greater abundance of DNA repair transcripts in the eggs of low latitude females. Finally, we use population genomic comparisons of high and low latitude samples to reveal evidence that multiple components of the DNA damage response and both coding and non-coding variation likely contribute to adaptive differences in DNA repair between populations.

A rapid assay for mitochondrial DNA damage and respiratory chain inhibition in the yeast Saccharomyces cerevisiae

There is a need for a rapid assay to identify agents that damage mitochondria because the mitochondrion may be an important target for numerous environmental mitotoxins. Certainly at least one chemotherapeutic regimen (CHOP therapy) that includes doxorubicin can induce cardiomyopathy through mitochondrial genotoxicity in cardiac muscle cells. Yeast cells (1.5 x 10(6)-10(7)) in water are spread on a YEPD plate, and, when the suspension of cells has dried, a small well (12 mm diameter) is cut into the agar; 200-400 microl of a solution of the presumptive mitochondrial genotoxin is placed in the well, and the plates are incubated for 2 days. The genotoxin forms a concentration gradient through the agar and affects the growing cells. An overlay containing tetrazolium chloride is added, and the plates are incubated for 6-24 hr. Respiring cells turn red, and nonrespiring cells, with damaged DNA or inhibited respiratory chains, that are adjacent to the well, are white. A white ring, or a more lightly colored red ring, around the well indicates the presence of cells with lowered respiratory activity which may be fully reversible when the mitochondrial genotoxin is removed. In preliminary experiments, doxorubicin (= adriamycin) shows strong activity with this assay; cyclophosphamide is negative, and 4-hydroxycyclophosphamide, a metabolite of cyclophosphamide, is weakly positive. Ethidium bromide, methotrexate, 5-fluorouracil, and 5-fluorocytosine also are mitochondrial genotoxins. Antifungal agents similar to 5-fluorocytosine and anthelmintic compounds such as pyrvinium iodide can be powerful mitochondrial genotoxins.

Ultraviolet mutagenesis of radiation-sensitive (rad) mutants of the nematode Caenorhabditis elegans

A mutational tester strain (JP10) of the nematode C. elegans was used to capture recessive lethal mutations in a balanced 300 essential gene autosomal region. The probability of converting a radiation interaction into a lethal mutation was measured in young gravid adults after exposure to fluences of 254-nm ultraviolet radiation (UV) ranging from 0 to 300 Jm-2. Mutation frequencies as high as 5% were observed. In addition, three different radiation-hypersensitive mutations, rad-1, rad-3 and rad-7 were incorporated into the JP10 background genotype, which allowed us to measure mutation frequencies in radiation-sensitive animals. The strain homozygous for rad-3 was hypermutable to UV while strains homozygous for rad-1 and rad-7 were hypomutable. Data showing the effects of UV on larval development and fertility for the rad mutants is also shown and compared for wild-type and JP10 backgrounds.

Sensitivity of wild type and recombination-deficient strains of Drosophila melanogaster to ultraviolet radiation

The sensitivity of Drosophila melanogaster to ultraviolet light has been studied in wild type and recombination-deficient strains. Survival was measured as the proportion of irradiated embryos or larvae which developed to adult flies. In view of the fact that males of this species do not participate in meiotic recombination, emphasis was placed on the relative sensitivity of males and females. The results show that young wild type male larvae are more sensitive to UV radiation than are young female larvae. This difference in sensitivity, however, is not apparent in some recombination-deficient strains. In addition, young embryos of the recombination-deficient strain Df(3)sbd105/T(2;E)Xa are exceptionally sensitive to UV radiation.

[Sensitivity of the ova of Drosophila (Drosophila melanogaster, Meig.) to the lethal action of ultraviolet rays. iv. Survival of embryos irradiated at the segmentation stage]

During the early segmentation stage, Drosophila eggs are very resistant to U.V. light. At that time their survival curve presents a plateau. This inflection of the curve cannot be explained by a restoration mechanism, nor by differential sensitivity of mitotic phases. It must be ascribed to the presence in the irradiated population of eggs that have been retained and deposited after the beginning of embryogenesis. Many reasons permit to relate the decline in resistance of the eggs to U.V. light during segmentation to the decrease in the DNA content in the cytoplasm.