Polymorphism in natural populations for genes controlling radioresistancein Drosophila

A Drosophila model to investigate the neurotoxic side effects of radiation exposure

Children undergoing cranial radiation therapy (CRT) for pediatric central nervous system malignancies are at increased risk for neurological deficits later in life. We have developed a model of neurotoxic damage in adult Drosophila following irradiation during the juvenile stages with the goal of elucidating underlying neuropathological mechanisms and of ultimately identifying potential therapeutic targets. Wild-type third-instar larvae were irradiated with single doses of γ-radiation, and the percentage that survived to adulthood was determined. Motor function of surviving adults was examined with a climbing assay, and longevity was assessed by measuring lifespan. Neuronal cell death was assayed by using immunohistochemistry in adult brains. We also tested the sensitivity at different developmental stages by irradiating larvae at various time points. Irradiating late third-instar larvae at a dose of 20 Gy or higher impaired the motor activity of surviving adults. A dose of 40 Gy or higher resulted in a precipitous reduction in the percentage of larvae that survive to adulthood. A dose-dependent decrease in adult longevity was paralleled by a dose-dependent increase in activated Death caspase-1 (Dcp1) in adult brains. Survival to adulthood and adult lifespan were more severely impaired with decreasing larval age at the time of irradiation. Our initial survey of the Drosophila Genetic Reference Panel demonstrated that differences in genotype can confer phenotypic differences in radio-sensitivity for developmental survival and motor function. This work demonstrates the usefulness of Drosophila to model the toxic effects of radiation during development, and has the potential to unravel underlying mechanisms and to facilitate the discovery of novel therapeutic interventions.

Highlighted Article: To model delayed neurological deficits resulting from pediatric cranial radiation therapy, neurotoxic damage in adult Drosophila is assessed following larval irradiation with the goal of elucidating underlying pathological mechanisms.

Survival and longevity improvements at extreme ages: an interpretation assuming an ecological stress theory of aging

The primary determinant of survival during aging is the energetic efficiency and metabolic stability required to counter the accumulated internal and external stresses of a lifetime. Hence, genetically stress-resistant individuals should accumulate with age; frailer, less robust, less energetically efficient and less metabolically stable individuals should succumb in parallel. This selection process implies the accumulation of energetically efficient stress-resistant individuals with age to the exclusion of all others. High additive genetic variability for survival is expected under extreme circumstances, however there is limited evidence close to the absolute extremes of life that diversity may fall. At this stage, only a few highly adaptive, oxidative-stress-resistant and presumably somewhat homozygous genotypes should remain. Therefore a fall in variability may occur in these outliers, when frailer individuals are unable to cope and are eliminated at extreme ages. This process could provide an explanation of mortality-rate declines in domesticated (laboratory) and free-living populations of the extremely old. That is, mortality-rate declines may be an expectation from a process of genetic sorting resulting from the accumulated responses to environmental stress over time. Application of an ecological stress theory of aging, which combines the external stresses to which organisms are exposed with internal stresses, appears to be the prerequisite for this conclusion.

The association among gene expression responses to nine abiotic stress treatments in Arabidopsis thaliana

The identification and analysis of genes exhibiting large expression responses to several different types of stress may provide insights into the functional basis of multiple stress tolerance in plant species. This study considered whole-genome transcriptional profiles from Arabidopsis thaliana root and shoot organs under nine abiotic stress conditions (cold, osmotic stress, salt, drought, genotoxic stress, ultraviolet light, oxidative stress, wounding, and high temperature) and at six different time points of stress exposure (0.5, 1, 3, 6, 12, and 24 hr). In roots, genomewide correlations between transcriptional responses to different stress treatments peaked following 1 hr of stress exposure, while in shoots, correlations tended to increase following 6 hr of stress exposure. The generality of stress responses at the transcriptional level was therefore time and organ dependent. A total of 67 genes were identified as exhibiting a statistically significant pattern of gene expression characterized by large transcriptional responses to all nine stress treatments. Most genes were identified from early to middle (1-6 hr) time points of stress exposure. Analysis of this gene set indicated that cell rescue/defense/virulence, energy, and metabolism functional classes were overrepresented, providing novel insight into the functional basis of multiple stress tolerance in Arabidopsis.

A selection response plateau for radiation resistance in the cotton boll weevil

Twenty generations of family selection in the cotton boll weevil for 14-day postirradiation survival to 10,000 rads of gamma irradiation has increased survival to nearly 90% as compared with about 35% in the unselected control population. Mean survival time has increased to 21.2 days in the selected population, as compared with 12.8 days in the unselected control. Nearly all of the response to selection occurred in the first 12 generations of selection, with no significant improvement beyond that point. A relaxed selection line was established in generation 12 and has been maintained as a population cage with discrete generations since that time. A comparison in generation 17 between the relaxed selection population and the selected population where both populations were managed in the same way indicated that none of the increase in resistance had been lost due to relaxation of selection (89.2% survival in the relaxed population as compared with 86.0% in the selected population). The rapid increase in response to selection followed by a quick plateau and no decline in the mean following relaxation of selection support the hypothesis that the increased resistance to irradiation resulted from changes in allelic frequencies for a relatively small number of genes. Alleles for increased resistance were either fixed by the selection process or, if still segregating, were not negatively correlated with fitness. Estimates of heritability for other fitness traits indicate selection should be effective for several other traits of importance in the efficiency of a mass rearing program.

[Repair of damage induced by x-rays to embryos of Drosophila (D. melanogaster Meig)]

The sensitivity of Drosophila eggs to the lethal action of X-rays does not remain constant during embryogenesis. The X-ray doses used in the present investigation may retard the hatching of the larvae but do not block development immediately after irradiation. A fraction of the damage induced in young embryos is repaired during gastrulation. The amount of repair is independent of the X-ray dose but is influenced by the temperature. The damage can be repaired even after the cells of the embryo have undergone many mitotic cycles.

Radioresistance in natural populations of Drosophila nebulosa from a Brazilian area of high background radiation

D. nebulosa, collected in two woods of a high background radiation area (both in Iron Hills, State of Minas Gerais, Brazil), were compared to and found to be more resistant than flies collected as controls in two other woods of an adjacent area. This was the second time that average differences in radioresistance between natural populations were established. Previous experiments were carried out with D. willistoni, in the same area and in comparable fashion. In spite of their higher radiation resistance the flies from the radiation area carried a higher expressed load than the controls. The following tests were performed to estimate the differences: (a) survival, after whole body exposure to 90 000 R of 60Co-gamma-rays on 120 strains set up from single inseminated females and (b) reproductive performance, in 240 duplicate croses, measured in terms of the difference between irradiated series (males received 3000 R of 60Co-gamma-rays) and their unirradiated counterparts. The data are based on an offspring of 293784 individuals. Futhermore, two diallel crosses between sensitive and resistant strains have shown that the differences probably are due mainly to additive genes.

Repair replication and photorepair of DNA in larvae of Drosophila melanogaster

Repair replication of DNA has been studied in first instar larvae of Drosophila melanogaster with isopycnic centrifugation techniques. Larvae were fed BUdR, FUdR, streptomycin, penicillin, and Fungazone for two to four hours prior to exposure to UV, X-rays, MMS, or EMS. Feeding was continued for four hours in the presence of (3)HBUdR and DNA was isolated from whole larvae. Repair replication is stimulated by each of these agents. MMS is about 10 times as potent as EMS in stimulating repair synthesis. A dose of 200 ergs/mm(2) largely saturates the level of repair replication observed after UV irradiation. Repair replication rises between 0 and 80,000 R of X-rays before falling off. Semiconservative synthesis is seriously inhibited above a dose of 40,000 R of X-rays. Photorepair has been detected as a reduction in repair synthesis resulting from post-irradiation exposure to photoreactivating light. The same treatment has no detectable effect on X-ray-stimulated repair replication. Repair replication is insensitive to the presence of caffeine or hydroxyurea during the final incubation, although semiconservative synthesis is strongly inhibited by these agents. A mixture of BUdR and (3)HTdR can be used to replace (3)HBUdR in detecting repair replication.

Variations in genetic architecture at different doses of gamma-radiation as measured by longevity in Drosophila melanogaster

Longevity (as a measure of resistance to irradiation) has been examined after different doses of60Co γ-rays, in four inbred lines and their hybrids in D. melanogaster. The control data showed some additive and non-additive effects. At 40, 60, 80, 100 and 120 krads, non-additive effects were highly significant, but additive effects were significant only at 100 and 120 krads, in particular the latter. Thus the genetic architecture varies with dose. Detailed genetic studies probably are simplest to carry out at high doses, because of the importance of additive effects.

The genetics of resistance to long-term exposure to CO2 in Drosophila melanogaster; an environmental stress leading to anoxia

Genetic heterogeneity in populations of D. melanogaster has been described for resistance to long-term exposure to CO2 (4 to 5 hours). Crosses between inbred strains, and between strains set up from single inseminated females collected in the wild show the importance of additive genes. Genetic activity for resistance and sensitivity was found on the X, 2 and 3 chromosomes.The mechanism of resistance was shown to be an anoxia effect since the effect of an N2 atmosphere was the same as that of CO2. A study of 18 strains collected in the wild revealed a positive correlation between metabolic rate as measured by O2 uptake and mortality under CO2, and negative correlations were found between body weight, and both mortality under CO2 and metabolic rate. These results are consistent with an anoxia effect. A further variable correlated with body weight is resistance to desiccation. Thus the anoxia effect is correlated with factors determining the distribution of the species in the wild.

Ether resistance in Drosophila melanogaster

Strains set up from single inseminated females of D. melanogaster from the wild differ in their resistance to the anaesthetics, ether and chloroform. The main differences between four selected extreme strains could be explained by additive genes, which in the case of ether resistance were located to regions of chromosomes 2 and 3. The lack of correspondence between ether and chloroform resistance between strains indicates that although the type of genetic architecture controlling the traits is similar, the actual genes differ, which is reasonable in view of their differing chemical structures. Quite high heritabilities were found for resistance to ether based on five inbred strains. No significant associations between resistance to ether and body weight, developmental rate or longevity were found.It is clear that resistance to both anaesthetics would be amenable to more detailed genetic analyses. It is pointed out that the general conclusions reached from such studies will have implications with respect to the effect of chemicals such as insecticides, not naturally present in nature.

Genetic heterogeneity among the founders of laboratory populations of Drosophila melanogaster : V. Sternopleural and abdominal chaetae in the same strains

1. Single inseminated females of Drosophila melanogaster derived from the same population led to discrete strains for both Sternopleural and abdominal chaeta number in agreement with other work, which indicates that the wild populations have genes polymorphic for these traits. Variability varied significantly between strains for Sternopleural but not abdominal chaeta number. 2. Overall, the data show a weak correlation between the two traits. Considering this by strain, the correlations go from negative (but not significantly < 0) to positive (but significantly > 0). Therefore just as the rate of response to directional selection has been shown to be extremely rapid when based on strains extreme in the direction of the required selection response, it seems that a similar process may be useful for directional selection based on two traits simultaneously, by using only those strains showing a positive correlation.

Genetic heterogeneity in natural populations of Drosophila melanogaster for ability to withstand dessication

Strains set up from single inseminated females of D. melanogaster derived from two wild populations have been shown to differ in their ability to withstand dessication, as measured by mortalities after 16 hours in a dry environment, thus there are genes segregating in wild populations for ability to withstand dessication. A more detailed study on strains from one of the wild populations, showed that strains with high wet and dry weights lose water by dessication relatively less rapidly and have lower mortalities, than strains with lower wet and dry weights.Variability within and between five inbred strains was studied with results as above. Heritabilities for wet weight, dry weight, and mortality were 0.40, 0.41 and 0.60 respectively, showing the likelihood that the traits would be amenable to further genetic analysis.The relevance of the results are discussed in relation to stress to high temperatures, and the ecology of the species in general.