GENE EXPRESSION DURING IMIDACLOPRID-INDUCED HORMESIS IN GREEN PEACH APHID

Imidacloprid-induced hormesis in the form of stimulated reproduction has previously been reported in green peach aphid, Myzus persicae. Changes in gene expression accompanying this hormetic response have not been previously investigated. In this study, expression of stress response (Hsp60), dispersal (OSD, TOL and ANT), and developmental (FPPS I) genes were examined for two generations during imidacloprid-induced reproductive stimulation in M. persicae. Global DNA methylation was also measured to test the hypothesis that changes in gene expression are heritable. At hormetic concentrations, down-regulation of Hsp60 was followed by up-regulation of this gene in the subsequent generation. Likewise, expression of dispersal-related genes and FPPS I varied with concentration, life stage, and generation. These results indicate that reproductive hormesis in M. persicae is accompanied by a complex transgenerational pattern of up- and down-regulation of genes that likely reflects trade-offs in gene expression and related physiological processes during the phenotypic dose-response. Moreover, DNA methylation in second generation M. persicae occurred at higher doses than in first-generation aphids, suggesting that heritable adaptability to low doses of the stressor might have occurred

Transgenerational shifts in reproduction hormesis in green peach aphid exposed to low concentrations of imidacloprid.

Hormesis is a biphasic phenomenon that in toxicology is characterized by low-dose stimulation and high-dose inhibition. It has been observed in a wide range of organisms in response to many chemical stressors, including insects exposed to pesticides, with potential repercussions for agriculture and pest management. To address questions related to the nature of the dose-response and potential consequences on biological fitness, we examined transgenerational hormesis in the green peach aphid, Myzus persicae, when exposed to sublethal concentrations of the insecticide imidacloprid. A hormetic response in the form of increased reproduction was consistently observed and a model previously developed to test for hormesis adequately fit some of our data. However, the nature of the dose-response differed within and across generations depending upon the duration and mode of exposure. Decreased reproduction in intermediate generations confirmed that fitness tradeoffs were a consequence of the hormetic response. However, recovery to levels of reproduction equal to that of controls in subsequent generations and significantly greater total reproduction after four generations suggested that biological fitness was increased by exposure to low concentrations of the insecticide, even when insects were continuously exposed to the stressor. This was especially evident in a greenhouse experiment where the instantaneous rate of population increase almost doubled and total aphid production more than quadrupled when aphids were exposed to potato plants systemically treated with low amounts of imidacloprid. Our results show that although fitness tradeoffs do occur with hormetic responses, this does not necessarily compromise overall biological fitness

Multigenerational effects of low doses of imidacloprid on aphids.

<p>Multigenerational effects of continuous exposure to sublethal concentration of imidacloprid on the (A) longevity and (B) length of adult <i>M. persicae.</i></p

Least-squares means of multigenerational fecundity^a following two-day exposure of <i>M. persicae</i> to sublethal concentrations of imidacloprid.

a<p>24 h old nymphs were placed on treated potato leaf discs and fecundity of each resulting adult was recorded every 2 days until it died. In the succeeding generation, 5 randomly selected 24 h old nymphs were tracked and fecundity of the resulting adults was recorded every 2 days until they died. G₀ is initial generation, G₁ is progeny of G₀, G₂ is progeny of G₁, and G₃ is progeny of G₂. G₀ nymphs were exposed to treated discs for two days and all aphids were thereafter exposed to untreated leaf discs.</p>b<p>Progeny per adult data were log transformed before analysis. Backtransformed means are presented. Values followed by different letters are significantly different (LSD, α = 0.05). SEM values are not backtransformed.</p>c<p>Mean total number of nymphs produced over four generations.</p>d<p>G₀ nymphs did not survive to adulthood when treated with 25 µg L⁻¹. This concentration was not included in the analysis.</p

Regression parameters of model-fitting hormetic responses (G₀, G₁ fecundity and <i>r_i</i>) in <i>M. persicae</i> exposed to sublethal concentrations of imidacloprid.

a<p><i>b,</i> steepness of the curve after the maximal hormetic effect; <i>d</i>, untreated control; <i>e,</i> lower bound on the ED50 level; <i>f,</i> rate of stimulation; RSE, residual standard error; <i>df,</i> degrees of freedom. In the model, <i>c</i> was set to zero and α set at 0.25 <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0074532#pone.0074532-Cedergreen1" target="_blank">[32]</a>.</p>b<p>denotes fecundity of <i>M. persicae</i> adults continuously exposed to sublethal concentrations of imidacloprid. G₀ is initial generation, G₁ is progeny of G₀.</p>c<p>data were square-root transformed before analysis.</p>d<p><i>r_i</i> is the instantaneous rate of increase of a <i>M. persicae</i> population exposed to low-dose imidacloprid treated potato plants.</p

Preharvest Spray Hexanal Formulation Enhances Postharvest Quality in ‘Honeycrisp’ Apples by Regulating Phospholipase D and Calcium Sensor Proteins Genes

‘Honeycrisp’ (Malus domestica Borkh.), a premium applecultivar, is highly susceptible to bitter pit and decline in quality during long-term storage. In order to enhance the quality, an aqueous composition containing hexanal was applied as a preharvest spray. The effects of hexanal were assessed on the treated fruit and compared with HarvistaTM (a sprayable 1-Methylcyclopropene based commercial formulation) applied and control fruit under both cold (2.5 °C; four months) and cold after room temperature storage (20 °C; 14 days) conditions. Color, firmness, and total soluble solids (TSS) did not show a significant change in response to any treatment at harvest, while abscisic acid (ABA) significantly reduced and tryptophan increased in response to hexanal, compared to HarvistaTM and control. The treatment effects on quality traits were observed during storage. Both hexanal and HarvistaTM sprayed apples had higher TSS under both cold and room temperature storage. In addition, both sprays enhanced firmness at room temperature storage. However, the effects of sprays on other quality traits showed a different pattern. Apples sprayed with hexanal had lower phospholipase D enzyme (PLD) activity, lower incidence of bitter pit, and decreased expression of MdPLDα1 compared to HarvistaTM and control. On the other hand, HarvistaTM treated fruit produced lower ethylene. Both sprays decreased the expression of MdPLDα4, MdCaM2, MdCaM4 and MdCML18 genes. Generally, PLD alpha has a direct role in promoting fruit senescence, whereas the calcium senor proteins (CaM/CMLs) may involve in fruit ripening process via calcium and ethylene interactions. Therefore, improved postharvest qualities, including the lower incidence of bitter pit in hexanal treated ‘Honeycrisp’, may be associated with lower membrane damage due to lower PLD enzyme activity and decreased expression of MdPLDα1 and MdPLDα4 genes throughout the storage period

Least-squares means of the instantaneous rate of increase (<i>r_i</i>) and total number of aphids per plant after 21 days following infestation of <i>M. persicae</i> on to potato plants treated with sublethal concentrations of imidacloprid in a greenhouse.

a<p>Data were square root transformed before analysis. Backtransformed means are presented. Values followed by different letters are significantly different (LSD, α = 0.05). SEM values are not backtransformed.</p

Least-squares means of two-generational fecundity^a following topical exposure of <i>M. persicae</i> to sublethal concentrations of imidacloprid.

a<p>24 h old nymphs were topically treated and thereafter reared on untreated potato leaf discs. Fecundity of each resulting adult was recorded every 2 days until it died. In the succeeding generation, 5 randomly selected 24 h old nymphs were tracked and fecundity of the resulting adults was recorded every 2 days until they died. G₀ is initial generation, G₁ is progeny of G₀.</p>b<p>Progeny per adult data were log transformed before analysis. Backtransformed means are presented. Values followed by different letters are significantly different (LSD, α = 0.05). SEM values are not backtransformed.</p>c<p>Mean total number of nymphs produced over four generations.</p

Hormesis model-fitting of low doses of imidacloprid on fecundity and <i>r_i</i> of aphids.

<p>Four-parameter biphasic model <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0074532#pone.0074532-Cedergreen1" target="_blank">[32]</a> for reproductive hormetic responses of <i>M. persicae</i> in an initial (A) and second (B) generation when continuously exposed to sublethal concentrations of imidacloprid on potato leaf discs, and (C) the instantaneous rate of increase (<i>r_i</i>) of <i>M. persicae</i> populations developing on whole potato plants treated with sublethal concentrations of imidacloprid. * indicates data were square-root transformed before analysis.</p

Least-squares means of multigenerational fecundity^a following continuous exposure of <i>M. persicae</i> to sublethal concentrations of imidacloprid.

a<p>24 h old nymphs were placed on treated potato leaf discs and fecundity of each resulting adult was recorded every 2 days until it died. In the succeeding generation, 5 randomly selected 24 h old nymphs were tracked and fecundity of the resulting adults was recorded every 2 days until they died. G₀ is initial generation, G₁ is progeny of G₀, G₂ is progeny of G₁, and G₃ is progeny of G₂. Leaf discs were replaced every two days over all generations.</p>b<p>Progeny per adult data were log transformed before analysis. Backtransformed means are presented. Values followed by different letters are significantly different (LSD, α = 0.05). SEM values are not backtransformed.</p>c<p>Mean total number of nymphs produced over four generations.</p