Abstract
In the case of the pests inhabiting metal polluted or fields where the use of pesticides is common, a natural selection of resistant individuals can occur. This may pose serious problems for humans, agriculture, as well as the economies of many countries. In this study, the hypothesis that multigenerational (120 generations) exposure to cadmium of a beet armyworm population could be a selecting factor toward a more efficient DNA protection was verified. The hemocytes of individuals from two culture strains (control and Cd-exposed) were treated with H2O2 (a DNA-damaging agent) or PBS (reference). The level of DNA damage was assessed using the Comet assay immediately and 5, 15 and 30 min. after the treatment. The immediate result of the contact with H2O2 was that the level of DNA damage in the hemocytes of the insects from both strains increased significantly. However, in the cells of the Cd-exposed individuals, the level of DNA damage decreased over time, while in the cells from the control insects it remained at the same level with no evidence of repair. These results suggest that efficient defense mechanisms may exist in the cells of insects that have prolonged contact with cadmium. Some evolutionary and trade-off aspects of the phenomenon are discussed. In a wider context, comparing the results obtained in the laboratory with field studies may be beneficial for understanding basic mechanisms of the resistance of an organism. To summarize, the high potential for the repair of DNA damage that was observed in the insects from the cadmium strain may confirm the hypothesis that multigenerational exposure to that metal may possibly contribute to the selection of insects that have a wider tolerance to oxidative stress. However, our investigations of polymorphism using AFLP did not reveal differences between the two main insect strains.
Highlights
The connections between laboratory investigations and field research on pest insects create serious problems for science today
The experiments that were performed during this study enabled us to test the following, alternative hypotheses: 1. The amount of DNA damage (DNA strand breaks) in the hemocytes of insects from the cadmium strain will not significantly differ compared to those from the reference group
The exposure of control insects to cadmium-containing diet for one generation appeared to cause a tendency to decrease the percentage of living cells in the hemolymph compared to the remaining insect groups that had been exposed to cadmium (Cd-exposed for one week and Cdexposed for 120 generations)
Summary
The connections between laboratory investigations and field research on pest insects create serious problems for science today. Investigation of the defense mechanisms in laboratory-bred organisms selected for a sufficient time toward tolerance to stressors could be a good solution Such a procedure can serve as a good, simplified model, which can simulate the processes that exist in natural conditions. It is important, to stress that long-term exposure of animals to a particular stressor might lead to the selection of individuals that cope better with this stressor in the same manner as in their natural environment. This phenomenon is of particular (and practical) importance in the case of the insect species that are usually recognized as a dangerous, highly expansive crop pests, e.g. Spodoptera exigua [1,2,3]
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