Rapid phenotypic changes in Caenorhabditis elegans under uranium exposure

Fatty acid- and retinol-binding protein 6 does not control worm fatty acid content in Caenorhabditis elegans but might play a role in Haemonchus contortus parasitism

BACKGROUND

Nematodes have lost the ability to synthesise necessary lipids de novo and have complementally evolved the capacity to acquire fatty acids and their derivatives from a diet or host animal. Nematode-specific fatty acid- and retinol-binding protein (FAR) family is one approach that facilitates lipid acquisition, representing an Achilles heel and potential target against roundworms of socioeconomic significance. However, little is known about their detailed functional roles in either free-living or parasitic nematodes.

METHODS

A genome-wide identification and curation were performed to screen the FAR family members of Haemonchus contortus. Their transcription patterns in worms were also analysed to identify the targets. Ligand binding assay and molecular docking were conducted to verify the fatty acid binding activities of FAR proteins of interest. RNA interference (RNAi) and heterologous expression (rescuing) experiments were designed to explore the potential roles of the selected FAR protein in nematodes. Localisation of the protein was shown in sections of paraffin-embedded worms after an immunohistochemistry (IHC) assay.

RESULTS

Here, an orthologue of far-6 in the model organism Caenorhabditis elegans (Ce-far-6) was functionally characterised in a parasitic nematode, H. contortus (Hc-far-6). It is demonstrated that knockdown of Ce-far-6 gene did not affect worm fat content, reproduction, or lifespan, but decreased worm body length at an early life stage of C. elegans. In particular, the Ce-far-6 mutant associated phenotype was completely rescued by Hc-far-6, suggesting a conserved functional role. Surprisingly, there were distinct tissue expression patterns of FAR-6 in the free-living C. elegans and parasitic H. contortus. High transcriptional level of Hc-far-6 and dominant expression of FAR-6 in the intestine of the parasitic stage of H. contortus link this gene/protein to nematode parasitism.

CONCLUSIONS

These findings substantially enhance our understanding of far genes and the associated lipid biology of this important parasitic nematode at a molecular level, and the approaches established are readily applicable to the studies of far genes in a broad range of parasites.

Continuous and discontinuous multi-generational disturbances of tetrabromobisphenol A on longevity in Caenorhabditis elegans

Tetrabromobisphenol A (TBBPA) is one of the most prevalently used brominated flame retardants. Due to its persistence, it is predominantly found in environmental matrices and has the potential to generate multi-generational toxicity. However, knowledge of its adaptive response or long-term residual effect in multi-generations, and molecular mechanisms remain understudied. In the current study, the model animal nematode Caenorhabditis elegans (C. elegans) was exposed to TBBPA at environmentally realistic concentrations (0.1-1000 μg L^-1) for four consecutive generations (G₀ to G₃). Degenerative age-related multiple endpoints including lifespan, locomotion behaviors, growth, reproduction, oxidative stress-related biochemical responses, cell apoptosis, and stress related gene expressions were assessed in the continuous exposure generations (G₀ and G₃) and the discontinuously exposed generations (T₃ and T'₃). The results showed that changes in degenerative age-related response monitored four generations varied in direction and magnitude depending on the TBBPA concentrations, and the response intensify ranked as G₀ > T'₃/G₃ > T₃. TBBPA at 1 μg L^-1 dosage was detected as the lowest observed effect concentration in multi-biomarkers. The underlying mechanism of aging phenotypes was that reactive oxygen species accumulation led to cell apoptosis regulated by gene ape-1, and confirmed catalase enzyme and superoxide dismutase activity played a crucial role in the detoxification process of TBBPA at the molecular level. This study provided insights into the underlying mechanism of TBBPA-interfered longevity and its environmental multi-generational potential risks.

Ionizing radiation affects the demography and the evolution of Caenorhabditis elegans populations

Ionizing radiation can reduce survival, reproduction and affect development, and lead to the extinction of populations if their evolutionary response is insufficient. However, demographic and evolutionary studies on the effects of ionizing radiation are still scarce. Using an experimental evolution approach, we analyzed population growth rate and associated change in life history traits across generations in Caenorhabditis elegans populations exposed to 0, 1.4, and 50.0 mGy.h^-1 of ionizing radiation (gamma external irradiation). We found a higher population growth rate in the 1.4 mGy.h^-1 treatment and a lower in the 50.0 mGy.h^-1 treatment compared to the control. Realized fecundity was lower in both 1.4 and 50.0 mGy.h^-1 than control treatment. High irradiation levels decreased brood size from self-fertilized hermaphrodites, specifically early brood size. Finally, high irradiation levels decreased hatching success compared to the control condition. In reciprocal-transplant experiments, we found that life in low irradiation conditions led to the evolution of higher hatching success and late brood size. These changes could provide better tolerance against ionizing radiation, investing more in self-maintenance than in reproduction. These evolutionary changes were with some costs of adaptation. This study shows that ionizing radiation has both demographic and evolutionary consequences on populations.

Ketamine induction of physiological functions alterations in Caenorhabditis elegans by chronic and multigenerational exposure and corresponding aquatic environmental risk assessment

Although ketamine (KET) has been widely detected in aquatic environments, the ecotoxicity data in aquatic invertebrates and associated risk remained unclear. This study aimed to investigate the adverse effects on benthos (Caenorhabditis elegans (C.elegans)) posed by KET from chronic (10 days) and multigenerational (four generations) exposure. Such exposure induced dose-dependent alterations on apoptosis, reactive oxygen species (ROS) induction, locomotion activity, feeding rate, chemotaxis, and brood size of nematodes, showing a cumulative damage through generations. KET posed vulva deformations and worm bags of C. elegans with a dosed-dependent increase. As a consequence, the fecundity and viability of worms would be impaired, which could eventually impact aquatic ecosystem equilibrium. Meanwhile, the bioactivation/detoxification process of xenobiotics and longevity regulating pathway induced by KET might be responsible for the physiological function disorders. Accordingly, the risk quotients (RQ) of KET in surface water in China were calculated using the 90% indicator protection concentration (C_0.1) derived from multiple toxicity indicators cumulative analyses. The results would be more objective considering numerous biomarkers changes of one species in comparison with traditional method using no observed effect concentrations (NOEC) of teratogenesis. The risk in surface water in southern China was up to high level (RQ > 1), suggesting long-term monitoring was imperative.

Assessment on chronic and transgenerational toxicity of methamphetamine to Caenorhabditis elegans and associated aquatic risk through toxicity indicator sensitivity distribution (TISD) analysis

Evidence about the adverse effects of methamphetamine (METH) on invertebrates is scarce. Hence, C. elegans, a representative invertebrate model, was exposed to METH at environmental levels to estimate chronic and transgenerational toxicity. The results of chronic exposure were integrated into an underlying toxicity framework of METH in invertebrates (e.g., benthos) at environmentally relevant concentrations. The induction of cellular oxidative damage-induced apoptosis and fluctuation of ecologically important traits (i.e., feeding and locomotion) might be attributed by the activation of the longevity regulating pathway regulated by DAF-16/FOXO, and detoxification by CYP family enzymes. The adverse effects to the organism level included impaired viability and decreased fecundity. The results from transgenerational exposure elucidated the cumulative METH-induced damage in invertebrates. Finally, a new risk assessment method named toxicity indicator sensitivity distribution (TISD) analysis was proposed by combining multiple toxicity indicator test data (EC_x) to derive the hazardous concentration for 10% indicators (C₁₀) of one species. The risk quotient (RQ) values calculated by measured environmental concentrations and C₁₀ in southern China, southeastern Australia, and the western US crossed the alarm line (RQ = 5), suggesting a need for long-term monitoring.

Exploring the Toxicology of Depleted Uranium with Caenorhabditis elegans

Depleted uranium (DU) is an emerging heavy metal pollutant with considerable environmental and occupational concerns. Its radiotoxicity is known to be low. However, its chemical toxicity should not be ignored. In order to explore the chemical toxicity of DU, the effects of uranyl nitrate, prepared from DU, on the model organism Caenorhabditis elegans were investigated. Chronic exposure to DU did not affect the lifespan or reproduction of the worm. DU had little effect on the physiological processes of C. elegans. Additionally, DU treatment did not make C. elegans more susceptible to UV, heat, or oxidative stress. Interestingly, chronic exposure of DU decreased the in vivo reactive oxygen species-scavenging ability through inhibiting the expression of antioxidant genes ctl-1, ctl-2, ctl-3, gst-7, and gst-10. Chronic but not acute exposure of DU induced a statistically significant degeneration of the dopaminergic (DAergic) neurons of treated worms and promoted the increase of α-synuclein aggregation and DAergic neurotoxicity. These findings may raise the public concerns regarding DU as an etiologic agent of Parkinson's disease and underline its potential neurotoxicity.

Transgenerational reproductive and developmental toxicity of tebuconazole in Caenorhabditis elegans

The transgenerational reproductive and developmental toxicity of tebuconazole (TEB) in Caenorhabditis elegans was investigated over five generations (P0 - F4). Only parental C.elegans (P0) were exposed to TEB (0, 0.01, 0.1, 1, and 10 μg/L) for 24 h and the subsequent offspring (F1-F4) were grown under TEB-free conditions. TEB exposure caused dose-dependent reproductive defects and developmental impairments in C.elegans. In the P0 generation reproductive defects were observed such as: reduced brood size and embryo hatchability, prolonged generation time, retarded gonadal development, and slower germline proliferation, even at 0.01 μg/L, together with developmental toxicity with significant reduced body length and narrowed body width at 10 μg/L. Additionally, the brood size significantly reduced in F2, which began to recover from F3, but was still lower than the control in F4. The proportion of abnormalities increased significantly in F2 and reduced from F3, but was still higher than the control, suggesting that TEB could have cumulative potential and be passed to offspring through parental exposure. Furthermore, exposure to TEB (10 μg/L) in P0 significantly reduced the body length in F1, which began to recover from F2, and was the same level as the control in F4. There was a concentration-dependent increase in body width in F1-F4, with a significant increase only observed in F1 at 10 μg/L. Thus, parental exposure to TEB induced transgenerational defects in both reproduction and development, emphasizing the significance of considering bio-toxicity over multiple generations to conduct accurate assessment of environmental risks of toxicants.

Advances on the toxicity of uranium to different organisms

The extensive application of radioactive element uranium (U) and its compounds in the nuclear industry has significantly increased the risk of exposure to the environment. Therefore, research on the safety risks and toxicity mechanisms of U exposure has received increasing attention. This paper reviews the toxic effects of U on different species under different conditions, and summarizes the potential toxicity mechanisms. Under the exposure of U, reactive oxygen species (ROS) produced in cells will damage membrane structure in cells, and inhibit respiratory chain reaction by reducing the production of NADH and ATP. It also induce the expression of apoptosis factors such as Bcl-2, Bid, Bax, and caspase family to cause apoptosis cascade reaction, leading to DNA degradation and cell death. We innovatively list some methods to reduce the toxicity of U because some microorganisms can precipitate uranyl ions through biomineralization or reduction processes. Our work provides a solid foundation for further risk assessment of U.

Mixed evidence for adaptation to environmental pollution

Adaptation to pollution has been studied since the first observations of heavy metal tolerance in plants decades ago. To document micro-evolutionary responses to pollution, researchers have used phenotypic, molecular genetics, and demographic approaches. We reviewed 258 articles and evaluated the evidence for adaptive responses following exposure to a wide range of pollutants, across multiple taxonomic groups. We also conducted a meta-analysis to calculate the magnitude of phenotypic change in invertebrates in response to metal pollution. The majority of studies that reported differences in responses to pollution were focused on phenotypic responses at the individual level. Most of the studies that used demographic assays in their investigations found that negative effects induced by pollution often worsened over multiple generations. Our meta-analysis did not reveal a significant relationship between metal pollution intensity and changes in the traits studied, and this was probably due to differences in coping responses among different species, the broad array of abiotic and biotic factors, and the weak statistical power of the analysis. We found it difficult to make broad statements about how likely or how common adaptation is in the presence of environmental contamination. Ecological and evolutionary responses to contamination are complex, and difficult to interpret in the context of taxonomic, and methodological biases, and the inconsistent set of approaches that have been used to study adaptation to pollution in the laboratory and in the field. This review emphasizes the need for: (a) long-term monitoring programs on exposed populations that link demography to phenotypic, genetic, and selection assays; (b) the use of standardized protocols across studies especially for similar taxa. Approaches that combine field and laboratory studies offer the greatest opportunity to reveal the complex eco-evolutionary feedback that can occur under selection imposed by pollution.

Adaptation costs to constant and alternating polluted environments

Some populations quickly adapt to strong and novel selection pressures caused by anthropogenic stressors. However, this short-term evolutionary response to novel and harsh environmental conditions may lead to adaptation costs, and evaluating these costs is important if we want to understand the evolution of resistance to anthropogenic stressors. In this experimental evolution study, we exposed Caenorhabditis elegans populations to uranium (U populations), salt (NaCl populations) and alternating uranium/salt treatments (U/NaCl populations) and to a control environment (C populations), over 22 generations. In parallel, we ran common-garden and reciprocal-transplant experiments to assess the adaptive costs for populations that have evolved in the different environmental conditions. Our results showed rapid evolutionary changes in life history characteristics of populations exposed to the different pollution regimes. Furthermore, adaptive costs depended on the type of pollutant: pollution-adapted populations had lower fitness than C populations, when the populations were returned to their original environment. Fitness in uranium environments was lower for NaCl populations than for U populations. In contrast, fitness in salt environments was similar between U and NaCl populations. Moreover, fitness of U/NaCl populations showed similar or higher fitness in both the uranium and the salt environments compared to populations adapted to constant uranium or salt environments. Our results show that adaptive evolution to a particular stressor can lead to either adaptive costs or benefits once in contact with another stressor. Furthermore, we did not find any evidence that adaptation to alternating stressors was associated with additional adaption costs. This study highlights the need to incorporate adaptive cost assessments when undertaking ecological risk assessments of pollutants.

Caenorhabditis elegans as a powerful alternative model organism to promote research in genetic toxicology and biomedicine

In view of increased life expectancy the risk for disturbed integrity of genetic information increases. This inevitably holds the implication for higher incidence of age-related diseases leading to considerable cost increase in health care systems. To develop preventive strategies it is crucial to evaluate external and internal noxae as possible threats to our DNA. Especially the interplay of DNA damage response (DDR) and DNA repair (DR) mechanisms needs further deciphering. Moreover, there is a distinct need for alternative in vivo test systems for basic research and also risk assessment in toxicology. Especially the evaluation of combinational toxicity of environmentally present genotoxins and adverse effects of clinically used DNA damaging anticancer drugs is a major challenge for modern toxicology. This review focuses on the applicability of Caenorhabditis elegans as a model organism to unravel and tackle scientific questions related to the biological consequences of genotoxin exposure and highlights methods for studying DDR and DR. In this regard large-scale in vivo screens of mixtures of chemicals and extensive parallel sequencing are highlighted as unique advantages of C. elegans. In addition, concise information regarding evolutionary conserved molecular mechanisms of the DDR and DR as well as currently available data obtained from the use of prototypical genotoxins and preferential read-outs of genotoxin testing are discussed. The use of established protocols, which are already available in the community, is encouraged to facilitate and further improve the implementation of C. elegans as a powerful genetic model system in genetic toxicology and biomedicine.

Neurodegeneration Induced by Metals in Caenorhabditis elegans

Metals are a component of a variety of ecosystems and organisms. They can generally be divided into essential and nonessential metals. The essential metals are involved in physiological processes once the deficiency of these metals has been associated with diseases. Although iron, manganese, copper, and zinc are important for life, it has been evidenced that they are also involved in neuronal damage in many neurodegenerative disorders. Nonessential metals, which are metals without physiological functions, are present in trace or higher levels in living organisms. Occupational, environmental, or deliberate exposures to lead, mercury, aluminum, and cadmium are clearly correlated with the increase of toxicity and varied kinds of pathological situations. Actually, the field of neurotoxicology needs to satisfy two opposing demands: the testing of a growing list of chemicals and resource limitations and ethical concerns associated with testing using traditional mammalian species. Toxicological assays using alternative animal models may relieve some of this pressure by allowing testing of more compounds while reducing expenses and using fewer mammals. The nervous system is by far the more complex system in C. elegans. Almost a third of their cells are neurons (302 neurons versus 959 cells in adult hermaphrodite). It initially underwent extensive development as a model organism in order to study the nervous system, and its neuronal lineage and the complete wiring diagram of its nervous system are stereotyped and fully described. The neurotransmission systems are phylogenetically conserved from nematodes to vertebrates, which allows for findings from C. elegans to be extrapolated and further confirmed in vertebrate systems. Different strains of C. elegans offer a new perspective on neurodegenerative processes. Some genes have been found to be related to neurodegeneration induced by metals. Studying these interactions may be an effective tool to slow neuronal loss and deterioration.

Transgenerational Reproductive Effects of Arsenite Are Associated with H3K4 Dimethylation and SPR-5 Downregulation in Caenorhabditis elegans

Arsenic is a prevalent environmental toxin. Arsenic is associated with a wide variety of adverse effects; however, studies on whether As-induced toxicities can be transferred from parents to offspring have received little attention. Caenorhabditis elegans has become an important animal model in biomedical and environmental toxicology research. In this study, transgenerational reproductive toxicity by arsenite exposure and the underlying mechanisms in C. elegans were investigated over six generations (F0-F5). Following arsenite maternal exposure of the F0 generation, subsequent generations (F1-F5) were cultured under arsenite-free conditions. We found that the brood size of C. elegans was significantly reduced by arsenite exposure in F0 and that this reduction in brood size was also observed in the offspring generations (F1-F5), after the toxicant had been removed from the diet. In addition, adult worms from F0 and F1 generations accumulated arsenite and arsenate when F0 L4 larvae were exposed to arsenite for 24 h. We found that the mRNA level of H3K4me2 demethylase LSD/KDM1, spr-5, was significantly reduced in the F0 exposed generation and subsequent unexposed generations (F1-F3). Likewise, the mRNA levels of spr-5 were also significantly decreased in the F1-F3 generations. Moreover, dimethylation of global H3K4 was increased in the F0-F3 generations. Our study demonstrates that maternal arsenite exposure causes transgenerational reproductive effects in C. elegans, which might be associated with H3K4 dimethylation and SPR-5 downregulation.

Ecotoxicity of bisphenol A to Caenorhabditis elegans by multigenerational exposure and variations of stress response in vivo across generations

In order to understand how bisphenol A (BPA) exposure acts on the evolutionary dynamics of populations and changes of stress response across generations, the model animal Caenorhabditis elegans was used to conduct the multigenerational testing. Multiple endpoints at the physiological (growth, reproduction, and locomotion behaviors) and molecular (stress-related gene expressions) levels were examined by multigenerational exposure to low-concentration BPA (0.001-10 μM) across four generations. The results showed that changes of physiological-level effects across four generations varied in magnitude and direction, depending on the exposure concentrations. C. elegans individuals in the first generation grew smaller, moved slower, and produced less offsprings than the controls by BPA exposure. As for each trait tested, the first generation response could be commonly mirrored in the subsequent generations at the highest concentration of 10 μM. However, at lower concentrations, response of parental generation was a relatively poor predictor of the effects on progeny, as acclimation or cumulative damage could occur in the subsequent generations. The integrated gene expression profiles visually illustrated that the tested gene expressions at low concentrations (0.001-0.01 μM) were more obviously changed in both G1 and G4 generations, and the G1 generation showed a much greater degree of increase in stress-related gene expressions than the G4 generation. The multigenerational toxicity data emphasize the need of considering biological effects over multiple generations to conduct accurate assessment of environmental risks of toxicants on population dynamics.

Nested interactions in the combined toxicity of uranium and cadmium to the nematode Caenorhabditis elegans

Uranium is a natural, ubiquitous radioactive element for which elevated concentrations can be found in the vicinity of some nuclear fuel cycle facilities or intensive farming areas, and most often in mixtures with other contaminants such as cadmium, due to co-occurrence in geological ores (e.g. U- or P-ore). The study of their combined effects on ecosystems is of interest to better characterize such multi-metallic polluted sites. In the present study, the toxicity of binary mixture of U and Cd on physiological parameters of the soil nematode Caenorhabditis elegans was assessed over time. Descriptive modeling using concentration and response addition reference models was applied to compare observed and expected combined effects and identify possible synergistic or antagonistic interactions. A strong antagonism between U and Cd was identified for length increase and brood size endpoints. The study revealed that the combined effects might be explained by two nested antagonistic interactions. We demonstrate that the first interaction occurred in the exposure medium. We also identified a significant second antagonistic interaction which occurred either during the toxicokinetic or toxicodynamic steps. These findings underline the complexity of interactions that may take place between chemicals and thus, highlight the importance of studying mixtures at various levels to fully understand underlying mechanisms.

Pollution breaks down the genetic architecture of life history traits in Caenorhabditis elegans

When pollution occurs in an environment, populations present suffer numerous negative and immediate effects on their life history traits. Their evolutionary potential to live in a highly stressful environment will depend on the selection pressure strengths and on the genetic structure, the trait heritability, and the genetic correlations between them. If expression of this structure changes in a stressful environment, it becomes necessary to quantify these changes to estimate the evolutionary potential of the population in this new environment. We studied the genetic structure for survival, fecundity, and early and late growth in isogenic lines of a Caenorhabditis elegans population subject to three different environments: a control environment, an environment polluted with uranium, and a high salt concentration environment. We found a heritability decrease in the polluted environments for fecundity and early growth, two traits that were the most heritable in the control environment. The genetic structure of the traits was particularly affected in the uranium polluted environment, probably due to generally low heritability in this environment. This could prevent selection from acting on traits despite the strong selection pressures exerted on them. Moreover, phenotypic traits were more strongly affected in the salt than in the uranium environment and the heritabilities were also lower in the latter environment. Consequently the decrease in heritability was not proportional to the population fitness reduction in the polluted environments. Our results suggest that pollution can alter the genetic structure of a C. elegans population, and thus modify its evolutionary potential.

Energy-based modelling to assess effects of chemicals on Caenorhabditis elegans: a case study on uranium

The ubiquitous free-living nematode Caenorhabditis elegans is a powerful animal model for measuring the evolutionary effects of pollutants which is increasingly used in (eco) toxicological studies. Indeed, toxicity tests with this nematode can provide in a few days data on the whole life cycle. These data can be analysed with mathematical tools such as toxicokinetic-toxicodynamic modelling approaches. In this study, we assessed how a chronic exposure to a radioactive heavy metal (uranium) affects the life-cycle of C. elegans using a mechanistic model. In order to achieve this, we exposed individuals to a range of seven concentrations of uranium. Growth and reproduction were followed daily. These data were analysed with a model for nematodes based on the Dynamic Energy Budget theory, able to handle a wide range of plausible biological parameters values. Parameter estimations were performed using a Bayesian framework. Our results showed that uranium affects the assimilation of energy from food with a no-effect concentration (NEC) of 0.42 mM U which would be the threshold for effects on both growth and reproduction. The sensitivity analysis showed that the main contributors to the model output were parameters linked to the feeding processes and the actual exposure concentration. This confirms that the real exposure concentration should be measured accurately and that the feeding parameters should not be fixed, but need to be reestimated during the parameter estimation process.

Rapid evolutionary responses of life history traits to different experimentally-induced pollutions in Caenorhabditis elegans

BACKGROUND

Anthropogenic disturbances can lead to intense selection pressures on traits and very rapid evolutionary changes. Evolutionary responses to environmental changes, in turn, reflect changes in the genetic structure of the traits, accompanied by a reduction of evolutionary potential of the populations under selection. Assessing the effects of pollutants on the evolutionary responses and on the genetic structure of populations is thus important to understanding the mechanisms that entail specialization to novel environmental conditions or resistance to novel stressors.

RESULTS

Using an experimental evolution approach we exposed Caenorhabditis elegans populations to uranium, salt and alternating uranium-salt environments over 22 generations. We analyzed the changes in the average values of life history traits and the consequences at the demographic level in these populations. We also estimated the phenotypic and genetic (co)variance structure of these traits at different generations. Compared to populations in salt, populations in uranium showed a reduction of the stability of their trait structure and a higher capacity to respond by acclimation. However, the evolutionary responses of traits were generally lower for uranium compared to salt treatment; and the evolutionary responses to the alternating uranium-salt environment were between those of constant environments. Consequently, at the end of the experiment, the population rate of increase was higher in uranium than in salt and intermediate in the alternating environment.

CONCLUSIONS

Our multigenerational experiment confirmed that rapid adaptation to different polluted environments may involve different evolutionary responses resulting in demographic consequences. These changes are partly explained by the effects of the pollutants on the genetic (co)variance structure of traits and the capacity of acclimation to novel conditions. Finally, our results in the alternating environment may confirm the selection of a generalist type in this environment.

Genes required for alleviation of uranium toxicity in sulfate reducing bacterium Desulfovibrio alaskensis G20 [corrected]

The sulfate reducing bacterium Desulfovibrio alaskensis strain G20 can grow in lactate sulfate medium with up to 4 mM uranyl acetate. In order to identify the genes that are required for the growth of strain G20 at toxic levels of uranium(VI) (U(VI)), 5,760 transposon insertion mutants were screened for U(VI) resistance defects, and 24 of them showed loss of U(VI) resistance in lactate sulfate medium with 2 mM uranyl acetate. In the 24 mutants, 23 genes were disrupted by transposon insertions, and one transposon is located in a non-coding region. In the ten mutants that were completely inhibited by 2 mM uranyl acetate, the disrupted genes are involved in DNA repair, rRNA methylation, regulation of expression and RNA polymerase renaturation. The remaining 14 mutants showed partial inhibition of growth by 2 mM U(VI), in which the disrupted genes participate in DNA repair, regulation of transcription, membrane transport, etc. In addition, none except one of these 24 mutants showed loss in its ability to reduce U(VI) to U(IV) in the washed cell test. These results altogether suggest that U(VI) toxicity mainly involves damage to nucleic acids and proteins.

Consequences of a multi-generation exposure to uranium on Caenorhabditis elegans life parameters and sensitivity

The assessment of toxic effects at biologically and ecologically relevant scales is an important challenge in ecosystem protection. Indeed, stressors may impact populations at much longer term than the usual timescale of toxicity tests. It is therefore important to study the evolutionary response of a population under chronic stress. We performed a 16-generation study to assess the evolution of two populations of the ubiquitous nematode Caenorhabditis elegans in control conditions or exposed to 1.1 mM of uranium. Several generations were selected to assess growth, reproduction, survival, and dose-responses relationships, through exposure to a range of concentrations (from 0 to 1.2 mM U) with all endpoints measured daily. Our experiment showed an adaptation of individuals to experimental conditions (increase of maximal length and decrease of fecundity) for both populations. We also observed an increase of adverse effects (reduction of growth and fertility) as a function of uranium concentration. We pointed out the emergence of population differentiation for reproduction traits. In contrast, no differentiation was observed on growth traits. Our results confirm the importance of assessing environmental risk related to pollutant through multi-generational studies.