Shuangying Yu

Lethal and sublethal effects of three insecticides on two developmental stages of xenopus laevis and comparison with other amphibians

It has been suggested that Xenopus laevis is less sensitive than other amphibians to some chemicals, and therefore, that the Frog Embryo Teratogenesis Assay-Xenopus (FETAX) may have limited use in risk assessments for other amphibians. However, comparisons are based mostly on results of FETAX, which emphasizes embryos. Larval X. laevis may be more sensitive to chemicals than embryos and may serve as a better life stage in risk assessments. The present study was conducted to determine the lethal and sublethal effects of 3 insecticides (malathion, endosulfan, and α-cypermethrin) on X. laevis embryos and larvae and to compare toxicity of X. laevis with that of other amphibians. All 3 insecticides have different modes of action, and they caused mortality, malformations, and growth inhibition in both developmental stages. Compared with embryos, larvae were more sensitive to endosulfan and α-cypermethrin but not to malathion. Xenopus laevis larvae had low sensitivity to endosulfan, median sensitivity to malathion, and high sensitivity to α-cypermethrin/cypermethrin relative to other larval amphibians. Our results suggest that X. laevis larvae may generate more protective toxicity estimates in risk assessments than embryos. Xenopus laevis may have limited use in evaluating risk of organochlorine insecticides to other amphibians but may provide useful toxicity thresholds for pyrethroid and perhaps organophosphorus insecticides.

Distinct expression profiles of stress defense and DNA repair genes in Daphnia pulex exposed to cadmium, zinc, and quantum dots.

The ever-increasing production and use of nanocrystaline semiconductors (Quantum dots; QDs) will inevitably result in increased appearance of these nanomaterials in the aquatic environment. However, the behavior and potential toxicity of heavy metal constituted nanoparticulates in aquatic invertebrates is largely unknown, especially with regard to molecular responses. The freshwater crustacean Daphnia pulex is a well-suited toxicological and ecological model to study molecular responses to environmental stressors. In this study, D. pulex were exposed for 48 h to sublethal doses of QDs (25% and 50% of LC50) with differing spectral properties (CdTe and CdSe/ZnS QDs) and Cd and Zn salts. Our data suggest that acute exposure to both CdSO4 and Cd-based QDs leads to Cd uptake in vivo, which was biologically supported by the observation of increased expression of metallothionein (MT-1). Furthermore, Cd, Zn, and CdSe/ZnS QDs induced different patterns of gene expression regarding stress defense and DNA repair, which furthers our knowledge regarding which response pathways are affected by nanoparticulate forms of metals versus ionic forms in aquatic crustaceans.

Effects of chlorothalonil on development and growth of amphibian embryos and larvae

Chlorothalonil is a broad spectrum fungicide widely used in agricultural and urban environments, yet little is known regarding its effects on amphibians. We examined effects of chlorothalonil on growth, malformations, and mortality in embryos and larvae of Xenopus laevis and Spea multiplicata, and assessed variation in sensitivity among aquatic organisms using a species sensitivity distribution (SSD). Chlorothalonil induced gut malformations in X. laevis embryos and inhibited growth. Tail degeneration was observed in larvae of both species and reduced tail length to total length ratios occurred at environmentally relevant concentrations (5.9 and 11.0 μg/L). The mechanism of tail degeneration is unclear, but alteration in the expression of genes involved in tail resorption is a hypothesized mechanism. Larval amphibians were more sensitive than invertebrates and fish. Based on our results and the range of reported environmental concentrations, chlorothalonil may pose a risk to larval amphibians in certain habitats and scenarios.

Acute toxicity of herbicide formulations and chronic toxicity of technical-grade trifluralin to larval green frogs (Lithobates clamitans)

Fewer toxicity studies have been performed on herbicides than on insecticides despite heavier use of herbicides and evidence of herbicide formulation toxicity to amphibians. We conducted acute and chronic toxicity tests with the herbicide trifluralin (2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)aniline) on tadpoles. Herbicide formulations had lower median lethal concentrations than an insecticide formulation and technical-grade trifluralin. Chronic trifluralin exposure resulted in significantly smaller tadpoles at low concentrations (20 µg/L) compared with controls and 200-µg/L treatments.

Bioaccumulation of fullerene (C60) and corresponding catalase elevation in Lumbriculus variegatus

Fullerene (C(60)), with its unique physical properties and nanometer size, has been mass-produced for many applications in recent decades. The increased likelihood of direct release into the environment has raised interest in understanding both the environmental fate and corresponding biological effects of fullerenes to living organisms. Because few studies have emphasized fullerene uptake and resulting biochemical responses by living organisms, a toxicity screening test and a 28-d bioaccumulation test for Lumbriculus variegatus were performed. No mortality was observed in the range of 0.05 mg C(60) /kg dry sediment to 11.33 mg C(60) /kg dry sediment. A biota-sediment accumulation factor of micron-sized fullerene agglomerates (µ-C(60)) was 0.032 ± 0.008 at day 28, which is relatively low compared with pyrene (1.62 ± 0.22). Catalase (CAT) activity, an oxidative stress indicator, was elevated significantly on day 14 for L. variegatus exposed to µ-C(60) (p = 0.034). This peak CAT activity corresponded to the highest body residues observed in the present study, 199 ± 80 µg C(60) /kg dry weight sediment. Additionally, smaller C(60) agglomerate size increased bioaccumulation potential in L. variegatus. The relationship between C(60) body residue and the increased CAT activity followed a linear regression. All results suggest that C(60) has a lower bioaccumulation potential than pyrene but a higher potential to induce oxidative stress in L. variegatus.

Interactive effects of ultraviolet-B radiation and pesticide exposure on DNA photo-adduct accumulation and expression of DNA damage and repair genes in Xenopus laevis embryos

Pesticide use and ultraviolet-B (UVB) radiation have both been suggested to adversely affect amphibians; however, little is known about their interactive effects. One potential adverse interaction could involve pesticide-induced dysregulation of DNA repair pathways, resulting in greater numbers of DNA photo-adducts from UVB exposure. In the present study, we investigated the interactive effects of UVB radiation and two common pesticides (endosulfan and α-cypermethrin) on induction of DNA photo-adducts and expression of DNA damage and repair related genes in African clawed frog (Xenopus laevis) embryos. We examined 13 genes that are, collectively, involved in stress defense, cell cycle arrest, nucleotide excision repair (NER), base excision repair, mismatch repair, DNA repair regulation, and apoptosis. We exposed X. laevis embryos to 0, 25, and 50 μg/L endosulfan or 0, 2.5, and 5.0 μg/L α-cypermethrin for 96 h, with environmentally relevant exposures of UVB radiation during the last 7 h of the 96 h exposure. We measured the amount of cyclobutane pyrimidine dimers (CPDs) and mRNA abundance of the 13 genes among treatments including control, pesticide only, UVB only, and UVB and pesticide co-exposures. Each of the co-exposure scenarios resulted in elevated CPD levels compared to UVB exposure alone, suggesting an inhibitory effect of endosulfan and α-cypermethrin on CPD repair. This is attributed to results indicating that α-cypermethrin and endosulfan reduced mRNA abundance of XPA and HR23B, respectively, to levels that may affect the initial recognition of DNA lesions. In contrast, both pesticides increased transcript abundance of CSA and MUTL. In addition, mRNA abundance of HSP70 and GADD45α were increased by endosulfan and mRNA abundance of XPG was increased by α-cypermethrin. XPC, HR23B, XPG, and GADD45α exhibited elevated mRNA concentrations whereas there was a reduction in MUTL transcript concentrations in UVB-alone treatments. It appeared that even though expression of XPC and CSA were induced by exposure to UVB or pesticides, XPA was the limiting factor in the NER pathway. Our results suggest that pesticides may increase the accumulation of UVB-induced DNA photo-adducts and one likely mechanism is the alteration of critical NER gene expression. The present study provides important implications for evaluating the combined risks of pesticide usage and potentially increasing UVB radiation in aquatic ecosystems.

Joint effects of pesticides and ultraviolet-B radiation on amphibian larvae

A combination of multiple stressors may be linked to global amphibian declines. Of these, pesticides and UVB radiation co-exposures were examined on the African clawed frog (Xenopus laevis) to provide information that may be useful for amphibian conservation. The independent action model and inferential statistics were used to examine interactions between pesticides (malathion, endosulfan, α-cypermethrin, or chlorothalonil) and environmentally relevant UVB exposures. UVB radiation alone caused 35-68% mortality and nearly 100% of malformations. Pesticides and UVB had additive effects on larval mortality; however, several non-additive effects (antagonistic and synergistic interactions) were observed for total body length. Insecticides mainly affected axial development, whereas UVB radiation caused high incidence of edema, gut malformations, and abnormal tail tips. These results suggest that sublethal developmental endpoints were more sensitive for detecting joint effects. This work has implications for amphibian risk assessments for ecosystems where pesticides and high UVB radiation may co-occur.

The effects of pesticide exposure on ultraviolet-B radiation avoidance behavior in tadpoles

Effects of contaminants on behavior may have important consequences on wildlife populations because behaviors such as predation, predator avoidance, reproduction, and social interaction can affect population dynamics. As a common environmental stressor, ultraviolet-B (UVB) radiation causes various deleterious effects and some aquatic organisms actively avoid UVB radiation in water. However, the extent to which environmental contaminants can impair UVB avoidance has not been evaluated, which may cause greater UVB exposure and toxicity. In the present study, we used Xenopus laevis tadpoles to determine if acute exposure to sublethal concentrations of agricultural chemicals can alter tadpole response to UVB radiation. We exposed tadpoles to four pesticides (malathion, endosulfan, α-cypermethrin, and chlorothalonil) for 96 h. At the end of the exposure, tadpoles were transferred to tanks divided into UVB and no-UVB areas. We observed tadpoles for 30 min and recorded time spent in the UVB area. We compared the proportion of time tadpoles spent in the UVB area among different concentrations for each pesticide. There was no significant difference between FETAX control and solvent control tadpoles. When combined, control tadpoles spent less than half of the time in the UVB area indicating that X. laevis tadpoles exhibit UVB avoidance behavior. Tadpoles exposed to 5 μg/L endosulfan spent significantly more time under UVB than control tadpoles. Other pesticides had no effect on tadpole UVB avoidance behavior. Our results suggest that some neurotoxic pesticides can affect UVB avoidance in larval amphibians, which may increase their exposure and subsequently the risk of UVB-induced damage. The present study highlights the importance of examining the interaction between two stressors that co-occur across broad spatial scales and to consider behavioral alteration when evaluating the risk of pesticides to amphibians.