| The concentrations of heavy metals in the topsoil can increase with the addition of chemical fertilizers, pesticides, and sewage from urban and metal mill. Heavy metal contamination from these sources has become an important global environmental problem, which threatens the ecological equilibrium. Effect of heavy metal stress on insects under the short-time exposure is involved in the previous researches. However, under the stress of heavy metal for long time, the research on defense mechanism of insects is not enough at present. As the destructive pest throughout the world, Sitobion avenae can be the good material of researching adaptation and evolution of insects as their biological characteristics of parthenogernesis and high fecundity. Therefore, the exploration about defense mechanism of S. avenae exposed to heavy metal for long time has the theoretical and practical significance for the integrated pest management.In the simulative natural environment, this study evaluated the population dynamic of5. avenae. The activity and gene expression patterns of antioxidant enzymes, carboxylesterase (CarE) and acetylcholin esterase (AchE) were carried out under the long-term exposure of Cd and Zn. We aimed to explore the defense mechanism of S. avenae under the stress of heavy metal. The following are the conclusions:1) The result of estimating concentration in wheat showed that the concentration of Cd and Zn increased with the increasing of that in soil. When it reached a certain concentration, the absorbed dose in wheat would be saturated. According to Holling Ⅱ functional response, the largest absorbed doses of Cd and Zn in wheat were97.087mg/kg and1000mg/kg respectively. It was concluded that host plant would accumulate heavy metal from contaminated soil within a certain range.2) The population dynamic of S. avenae was evaluated under the short-term exposure of Cd. The result showed that the survival rate and fecundity were reduced in the later stage of aphids. The life table parameters, such as the net reproductive rate (Ro), innate capacity of increase (r), and finite rate of increase (λ), were also inhibited by Cd. The most significant influence was occurred at the concentration of20mg/kg. This concentration was more sensitive for S. avenae. 3) The effect on activity and the transcript level of superoxide dismutase (SOD) gene of S. avenae were estimated under the exposure of Cd for20generation consequently. It was resulted that, they were inhibited at5th generation, of which this inhibition was more obvious at high concentration of Cd. Although the SOD activity and gene expression of S. avenae were increased at10th generation, they were still inhibited after treated for10generation. SOD gene of S. avenae was more sensitive at the concentration higher than40mg/kg. Therefore,40mg/kg and10th generation were the critical dose and stage for S. avenae to transform gene expressive pattern from up-regulation to down-regulation4) The effect on CarE activity and gene expression of S. avenae were estimated under the exposure of Cd for20generations consequently. The CarE activity and transcript level were induced by Cd in10th generation at all the concentration. However, the higher concentration than40mg/kg of Cd had no significant effect on the CarE activity and gene expression in20generation.40mg/kg and10th generation were the critical dose and stage. It was concluded that, under the stress of low-level of Cd, CarE played an important role of resisting damage, and this role of detoxification was limited under the high-level Cd.5) The effect on the activity and transcript level of SOD gene of S. avenae were estimated under the exposure of Zn for30generations consequently. SOD activities of all the treatments within the generation were higher than the control, but decreased with increasing of generation treated significantly. The expression of SOD gene was up-regulated, of which5th generation was more significant in1600mg/kg. However, the sensibility was reduced after15generations for1600mg/kg of Zn. No difference was observed for SOD activity and gene expression in the30th generation of aphids. Therefore, the antioxidation of SOD was lost under the long-term exposure of Zn with high concentration.6) The effect on CarE activity and gene expression of&avenae were estimated under the exposure of Zn for30generations consequently. The CarE activity and transcript level were induced by Cd in the lower generations, but decreased with increasing of generation treated significantly.1600mg/kg of Zn inhibited CarE activity after20generations and no difference was found for transcript level. It was conclude that CarE could be the sensitive biomarker for S. avenae response to the presence of Zn. In order to adapt to the stress of heavy metal Zn, the S. avenae had the particular patterns of gene expression under the long-time stress.7) The effect on activity of peroxidase (POD) and catalase (CAT) of S. avenae was estimated under the exposure of Cd and Zn for long term. It was found that the activities of POD were reduced by Cd and no significant difference of POD activity was found in all generations. Under the stress of Cd, the peak value of CAT activity was occurred in15th generation, after that, it was decreased and lower than the control. The activity of POD and CAT of S. avenae was still affected by Zn significantly. They have the tendency of decrease with the increasing of generations. When aphids were treated for30generations, no obvious influence had been found on POD activity, CAT activity was inhibited by Zn evenly. The results demonstrated that S. avenae would maintain the metabolic balance through inducing antioxidant enzyme, but could not at the high-level toxicity.8) The effect on AchE gene expression of S. avenae under the exposure of Cd and Zn for long term:gene expression of AchE was induced and then reduced with the increasing of generation and concentration of Cd and Zn. At5th generation, gene expression of AchE was down-regulated by Cd and Zn. Under the stress of Cd for10generations, the expressive pattern was transformed to up-regulated, but still inhibited by long-term exposure of Cd (20generations).40mg/kg and10th generation were the critical dose and stage, which was inconsistent with SOD and CarE. However, the critical stage was15th generation for S. avenae under the stress of Zn. The expressive pattern was down-regulated after15th generation. It was concluded that, the resisting role of AchE to heavy metals was limited under the high-level toxicity.In conclusion, the development and population parameters of S. avenae were inhibited under the exposure of Cd and Zn though the accumulation in wheat. But S. avenae could adapt and compensate the toxicity of low-level heavy metals through inducing the expression of antioxidant enzymes, CarE and AchE. However, the physiological confusion would occur under the high-level stress. Under the long-term exposure of heavy metals, the defense of antioxidant enzymes, CarE and AchE would be restricted due to the high-level toxicity. Maybe the new advanced pattern of adaptation emerged... |
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