Temperature Effects on Survival and DNA Repair in Four Freshwater CladoceranDaphniaSpecies Exposed to UV Radiation

Mechanisms underlying the impact and interaction of temperature and UV-B on the hatching of spider mite and phytoseiid mite eggs

BACKGROUND

A spider mite control method using night-time ultraviolet (UV)-B irradiation was recently developed for strawberry greenhouses (UV method). The control effect of this UV method is negatively affected by increasing temperature. Tetranychus urticae eggs are more resistant to a single dose of UV-B irradiation than Neoseiulus californicus eggs. By contrast, N. californicus can better survive nightly UV-B irradiation with the UV method compared with T. urticae. To elucidate the mechanism underlying these phenomena, we explored the hypotheses that higher temperature promotes photoenzymatic repair (PER) and that mortality is determined by UV-B susceptibility in the embryonic stage exposed to UV-B.

RESULTS

PER efficacy was not promoted by increasing temperature. The lowest hatchability (around zero) of T. urticae eggs after a single dose of UV-B irradiation (0.288 and 0.432 kJ m^-2 ) without photoreactivation was seen in the morphogenesis stages between "cleavage ended" and "eye points became colored". Based on these results, we developed a linear function of daily UV-B irradiance and deviation of cumulative irradiance during vulnerable embryonic developmental phases from 50% lethal dose (LD₅₀ ) after a single dose of UV-B irradiation. The difference between T. urticae and N. californicus and changes in UV-B vulnerability due to temperature could be explained by this simple relationship.

CONCLUSION

Slower development in T. urticae than N. californicus in nature and developmental delay under low temperatures increase the ovicidal effects of the UV method. This shows the advantage of the simultaneous use of the UV method and biological control, contributing to the development of integrated pest management. © 2022 Society of Chemical Industry.

Fitness cost from fluctuating ultraviolet radiation in Daphnia magna

Solar ultraviolet radiation (UVR) is an important environmental threat for organisms in aquatic systems, but its temporally variable nature makes the understanding of its effects ambiguous. The aim of our study was to assess potential fitness costs associated with fluctuating UVR in the aquatic zooplankter Daphnia magna. We investigated individual survival, reproduction and behaviour when exposed to different UVR treatments. Individuals exposed to fluctuating UVR, resembling natural variations in cloud cover, had the lowest fitness (measured as the number of offspring produced during their lifespan). By contrast, individuals exposed to the same, but constant UVR dose had similar fitness to control individuals (not exposed to UVR), but they showed a significant reduction in daily movement. The re-occurring threat response to the fluctuating UVR treatment thus had strong fitness costs for D. magna, and we found no evidence for plastic behavioural responses when continually being exposed to UVR, despite the regular, predictable exposure schedule. In a broader context, our results imply that depending on how variable a stressor is in nature, populations may respond with alternative strategies, a framework that could promote rapid population differentiation and local adaptation.

Effect of chronic UVR exposure on zooplankton molting and growth

Molting is a crucial physiological process in arthropods development, growth, and adult reproduction, where the chitinolytic enzyme chitobiase (CB) and the apoptosis process (caspase-3 activity) play crucial roles. Both molecular endpoints have been observed to be affected by different toxics that may be present in aquatic environments. However, the role of ultraviolet radiation (UVR) in the molting process remains with poor evidence and the possible effect of the previous exposure on F1 generation is unknown. Here, we conducted laboratory experiments with chronic UVR exposure to test the effect on the molting process of Daphnia commutata. Our results showed a clear negative effect of the UVR that affected the molting process with a reduction in individual growth. This trend was also observed in CB and caspase-3 activities. Our results also suggest that the UV dose received by the mother and eggs has an additive effect with the dose received by the offspring. These results imply that the cumulative impact of small UVR doses (2 h per day, daily dose: 2520 J m^-2 of 340 nm) on mothers and eggs (which cannot be discriminated in our experiments) can have an additive or synergistic effect along with the generations through a potential increase in lethal effect. Finally, the observed desynchronization in the molting process by UVR will affect the fitness of individuals and population dynamics.

De Novo Development of a Quantitative Adverse Outcome Pathway (qAOP) Network for Ultraviolet B (UVB) Radiation Using Targeted Laboratory Tests and Automated Data Mining

Ultraviolet B (UVB) radiation is a natural nonchemical stressor posing potential hazards to organisms such as planktonic crustaceans. The present study was conducted to revisit the lethal effects of UVB on crustaceans, generate new experimental evidence to fill in knowledge gaps, and develop novel quantitative adverse outcome pathways (qAOPs) for UVB. A combination of laboratory and computational approaches was deployed to achieve the goals. For targeted laboratory tests, Daphnia magna was used as a prototype and exposed to a gradient of artificial UVB. Targeted bioassays were used to quantify the effects of UVB at multiple levels of biological organization. A toxicity pathway network was assembled based on the new experimental evidence and previously published data extracted using a novel computational tool, the NIVA Risk Assessment Database (NIVA RAdb). A network of AOPs was developed, and weight of evidence was assessed based on a combination of the current and existing data. In addition, quantitative key event relationships in the AOPs were developed by fitting the D. magna data to predefined models. A complete workflow for assembly and evaluation of qAOPs has been presented, which may serve as a good example for future de novo qAOP development for chemical and nonchemical stressors.

Dose-Response and Temperature Dependence of the Mortality of Spider Mite and Predatory Mite Eggs Caused by Daily Nighttime Ultraviolet-B Irradiation

The two-spotted spider mite, Tetranychus urticae, is an economically important agricultural pest. A novel physical control method involving daily nighttime UV-B irradiation was recently developed for use in strawberry greenhouses. However, the overlapping of leaves after March prevents direct irradiation to T. urticae on the lower leaf surface, decreasing control effect. Excessive UV-B irradiation causes leaf sunscald in winter. Therefore, optimization of UV-B irradiance and a compensatory control agent are desired. Temperature may affect the survival of organisms exposed to UV-B, although the temperature dependence of UV-B damage is controversial. A phytoseiid mite, Neoseiulus californicus, is a prominent predator but vulnerable to a single UV-B irradiation. We compared dose-response and temperature dependence of UV-B damage between T. urticae and N. californicus eggs under daily nighttime UV-B irradiation. Unexpectedly, N. californicus showed greater resistance to UV-B than T. urticae, and the mortality was increased and decreased at low and high temperatures, respectively. This makes possible the application of UV-B doses that are lethal for spider mites but safe for phytoseiid mites. Overall, we concluded that combined use of phytoseiid mites with UV-B lamps is advantageous to spider mite management in strawberry greenhouses.

Melanism protects alpine zooplankton from DNA damage caused by ultraviolet radiation

Melanism is widely observed among animals, and is adaptive in various contexts for its thermoregulatory, camouflaging, mate-attraction or photoprotective properties. Many organisms exposed to ultraviolet radiation show increased fitness resulting from melanin pigmentation; this has been assumed to result in part from reduced UV-induced damage to DNA. However, to effectively test the hypothesis that melanin pigmentation reduces UV-induced DNA damage requires quantification of UV-specific DNA damage lesions following UV exposure under controlled conditions using individuals that vary in pigmentation intensity. We accomplished this using alpine genotypes of the freshwater microcrustacean Daphnia melanica, for which we quantified cyclobutane pyrimide dimers in DNA, a damage structure that can only be generated by UV exposure. For genotypes with carapace melanin pigmentation, we found that individuals with greater melanin content sustained lower levels of UV-induced DNA damage. Individuals with more melanin were also more likely to survive exposure to ecologically relevant levels of UV-B radiation. Parallel experiments with conspecific genotypes that lack carapace melanin pigmentation provide additional support for our conclusion that melanism protects individuals from UV-induced DNA damage. Finally, within-genotype comparisons with asexually produced clonal siblings demonstrate that melanin content influences DNA damage even among genetically identical individuals raised in the same environment.

Will the Antarctic tardigrade Acutuncus antarcticus be able to withstand environmental stresses related to global climate change?

Because conditions in continental Antarctica are highly selective and extremely hostile to life, its biota is depauperate, but well adapted to live in this region. Global climate change has the potential to impact continental Antarctic organisms because of increasing temperatures and ultraviolet radiation. This research evaluates how ongoing climate changes will affect Antarctic species, and whether Antarctic organisms will be able to adapt to the new environmental conditions. Tardigrades represent one of the main terrestrial components of Antarctic meiofauna; therefore, the pan-Antarctic tardigrade Acutuncus antarcticus was used as model to predict the fate of Antarctic meiofauna threatened by climate change. Acutuncus antarcticus individuals tolerate events of desiccation, increased temperature and UV radiation. Both hydrated and desiccated animals tolerate increases in UV radiation, even though the desiccated animals are more resistant. Nevertheless, the survivorship of hydrated and desiccated animals is negatively affected by the combination of temperature and UV radiation, with the hydrated animals being more tolerant than desiccated animals. Finally, UV radiation has a negative impact on the life history traits of successive generations of A. antarcticus, causing an increase in egg reabsorption and teratological events. In the long run, A. antarcticus could be at risk of population reductions or even extinction. Nevertheless, because the changes in global climate will proceed gradually and an overlapping of temperature and UV increase could be limited in time, A. antarcticus, as well as many other Antarctic organisms, could have the potential to overcome global warming stresses, and/or the time and capability to adapt to the new environmental conditions.

Effects of Low Temperature on Spider Mite Control by Intermittent Ultraviolet-B Irradiation for Practical Use in Greenhouse Strawberries

The application of ultraviolet-B (UVB) radiation to control spider mites is challenging as a key technology for integrated pest management (IPM) in greenhouse strawberries in Japan. To address this, concurrent use of phytoseiid mites and reduced UVB irradiance is desirable to ensure control effects in areas shaded from UVB radiation and to minimize the sunscald in winter, respectively. We designed experiments reproducing the UVB dose on the lower leaf surfaces in strawberry and evaluated the effects of intermittent UVB irradiation at midnight for practical application in the greenhouse and low temperature on the survival of the spider mite Tetranychus urticae Koch (Acari: Tetranychidae) and damage to the phytoseiid mite Neoseiulus californicus (McGregor) (Acari: Phytoseiidae). The midnight intermittent UVB irradiation effectively suppressed egg hatching and development of larvae of T. urticae, and the control effect was reinforced at 20°C (no eggs hatched at 0.13 kJ m-2 d-1) rather than, at 25°C (70.8% eggs hatched). In contrast, the hatchability of N. californicus eggs was unaffected by intermittent UVB irradiation at 0.27 kJ m-2 d-1 at 25°C and 20°C. However, residual effects of UVB irradiation to N. californicus eggs on survival of hatched larvae were seen, so that reducing the UVB dose is also advantageous for this phytoseiid mite. N. californicus showed a photoreactivation capacity, whereas their UVB tolerance was improved by prey species, suggesting the possibility of the improvement of phytoseiid mites by diet. The reduction of UVB dose and concurrent use of phytoseiid mites increase reliability of the UVB method in IPM strategies in strawberry greenhouse.

DNA double-strand break repair in Penaeus monodon is predominantly dependent on homologous recombination

DNA double-strand breaks (DSBs) are mostly repaired by nonhomologous end joining (NHEJ) and homologous recombination (HR) in higher eukaryotes. In contrast, HR-mediated DSB repair is the major double-strand break repair pathway in lower order organisms such as bacteria and yeast. Penaeus monodon, commonly known as black tiger shrimp, is one of the economically important crustaceans facing large-scale mortality due to exposure to infectious diseases. The animals can also get exposed to chemical mutagens under the culture conditions as well as in wild. Although DSB repair mechanisms have been described in mammals and some invertebrates, its mechanism is unknown in the shrimp species. In the present study, we show that HR-mediated DSB repair is the predominant mode of repair in P. monodon. Robust repair was observed at a temperature of 30 °C, when 2 µg of cell-free extract derived from hepatopancreas was used for the study. Although HR occurred through both reciprocal recombination and gene conversion, the latter was predominant when the bacterial colonies containing recombinants were evaluated. Unlike mammals, NHEJ-mediated DSB repair was undetectable in P. monodon. However, we could detect evidence for an alternative mode of NHEJ that uses microhomology, termed as microhomology-mediated end joining (MMEJ). Interestingly, unlike HR, MMEJ was predominant at lower temperatures. Therefore, the results suggest that, while HR is major DSB repair pathway in shrimp, MMEJ also plays a role in ensuring the continuity and stability of the genome.

Effects of UVB radiation on grazing of two cladocerans from high-altitude Andean lakes

Climate change and water extraction may result in increased exposition of the biota to ultraviolet-B radiation (UVB) in high-altitude Andean lakes. Although exposition to lethal doses in these lakes is unlikely, sub-lethal UVB doses may have strong impacts in key compartments such as zooplankton. Here, we aimed at determining the effect of sub-lethal UVB doses on filtration rates of two cladoceran species (Daphnia pulicaria and Ceriodaphnia dubia). We firstly estimated the Incipient Limiting Concentration (ILC) and the Gut Passage Time (GPT) for both species. Thereafter we exposed clones of each species to four increasing UVB doses (treatments): i) DUV-0 (Control), ii) DUV-1 (0.02 MJ m²), iii) DUV-2 (0.03 MJ m²) and iv) DUV-3 (0.15 MJ m²); and estimated their filtration rates using fluorescent micro-spheres. Our results suggest that increasing sub-lethal doses of UVB radiation may strongly disturb the structure and functioning of high-altitude Andean lakes. Filtration rates of D. pulicaria were not affected by the lowest dose applied (DUV-1), but decreased by 50% in treatments DUV-2 and DUV-3. Filtration rates for C. dubia were reduced by more than 80% in treatments DUV-1 and DUV-2 and 100% of mortality occurred at the highest UVB dose applied (DUV-3).

Photo-enzymatic repair of UVB-induced DNA damage in the two-spotted spider mite Tetranychus urticae

Ambient ultraviolet-B (UVB) radiation induces lethal effects in the two-spotted spider mite Tetranychus urticae, whereas photoreactivation by irradiation with ultraviolet-A and visible light (VIS) plays an important role to increase survival of mites irradiated by UVB. The physiological mechanisms and ecological significance of photoreactivation in terrestrial arthropods have not been shown clearly. We verified the biological impact and accumulation of DNA lesions by UVB irradiation and the repair of them by photoreactivation in T. urticae larvae. Survival of UVB-irradiated larvae decreased with increasing UVB dose, but recovered remarkably with VIS exposure after UVB irradiation (photoreactivation). The DNA lesions, cyclobutane pyrimidine dimers (CPDs) and 6-4 pyrimidine-pyrimidine photoproducts (6-4PPs) linearly increased with the UVB dose. The CPDs were repaired after exposure to VIS, whereas the frequency of 6-4PPs was unaffected by VIS; CPD photolyase genes, but not (6-4) photolyase genes, have been found in the T. urticae genome. Therefore, DNA damage and CPD photo enzymatic repair (PER) is significant for survival in this mite under ambient UVB radiation. Unexpectedly, gene expression of CPD photolyase was unaffected by irradiation with UVB and VIS. Instead, expression of xeroderma pigmentosum A (XPA) was increased by irradiation. XPA is a core factor in nucleotide excision repair (NER), which is a repair system unrelated to photo energy. The relationship between gene expression and enzymatic repair remains unclear. To elucidate the PER process in T. urticae, further study will be necessary on the gene expression patterns and molecular functions of CPD photolyase in PER and of XPA in NER.

The impact of UVB radiation on the glycoprotein glue of orb-weaving spider capture thread

Many spider orb-webs are exposed to sunlight and the potentially damaging effects of ultraviolet B (UVB) radiation. We examined the effect of UVB on the viscoelastic glycoprotein core of glue droplets deposited on the prey capture threads of these webs, hypothesizing that webs built by species that occupy sunny habitats are less susceptible to UVB damage than are webs built by species that prefer shaded forest habitats or by nocturnal species. Threads were tested shortly after being collected in the early morning and after being exposed to UVB energy equivalent to a day of summer sun and three times this amount. Droplets kept in a dark chamber allowed us to evaluate post-production changes. Droplet volume was unaffected by treatments, indicating that UVB did not damage the hygroscopic compounds in the aqueous layer that covers droplets. UVB exposure did not affect energies of droplet extension for species from exposed and partially to mostly shaded habitats (Argiope aurantia, Leucauge venusta and Verrucosa arenata). However, UVB exposure reduced the energy of droplet extension in Micrathena gracilis from shaded forests and Neoscona crucifera, which forages at night. Only in L. venusta did the energy of droplet extension increase after the dark treatment, suggesting endogenous molecular alignment. This study adds UVB irradiation to the list of factors (humidity, temperature and strain rate) known to affect the performance of spider glycoprotein glue, factors that must be more fully understood if adhesives that mimic spider glycoprotein glue are to be produced.

Effect of Clonal Selection on Daphnia Tolerance to Dark Experimental Conditions

Recent studies have demonstrated substantial effects of environmental stress that vary among clones. Exposure to ultraviolet radiation (UV) is an important abiotic stressor that is highly variable in aquatic ecosystems due to diel and seasonal variations in incident sunlight as well as to differences in the UV transparency of water among water bodies, the depth distribution of organisms, and the ability of organisms to detect and respond to UV. In contrast to the convention that all UV is damaging, evidence is accumulating for the beneficial effects of exposure to low levels of UV radiation. Whereas UV has been frequently observed as the primary light-related stressor, herein we present evidence that dark conditions may be similarly “stressful” (reduction of overall fitness), and stress responses vary among clones of the freshwater crustacean Daphnia parvula. We have identified a significant relationship between survivorship and reduced fecundity of clones maintained in dark conditions, but no correlation between tolerance of the clones to dark and UV radiation. Low tolerance to dark conditions can have negative effects not only on accumulated stresses in organisms (e.g. the repair of UV-induced damage in organisms with photolyase), but potentially on the overall physiology and fitness of organisms. Our results support recent evidence of the beneficial effects of low-level UV exposure for some organisms.

Co-exposure of ZnO nanoparticles and UV radiation to Daphnia magna and Danio rerio: Combined effects rather than protection

The application of nanoparticles (NPs) in consumer products has been increasing over the past few years. Their release into the environment is likely to happen at any stage of production or during the use of products containing NPs. Zinc oxide NPs (ZnO-NP) are among the most-used NPs on the market due to its intrinsic properties, such as ultraviolet (UV) absorption. The aim of the present study was to assess the combined effects of ZnO-NP and UV radiation on 2 freshwater species: Daphnia magna and Danio rerio. The initial hypothesis was that the presence of ZnO-NP in the aquatic media would decrease the damaging effects of UV radiation for both species. The endpoints assessed for D. magna were immobilization, feeding inhibition, and reproduction output. For D. rerio, egg development was studied during 96 h and mortality, hatching delay, and abnormal development were the endpoints recorded. Combined exposures were designed based on the single toxicity of both stressors and analyzed based on the independent action concept and exploring possible deviations for synergism/antagonism, dose level, and dose ratio. Combined exposures with D. magna induced synergism on reproduction, decreasing the number of neonates produced more than expected based on both stressors' individual toxicity. Single exposures of D. rerio embryos to both stressors induced negative effects. The combined exposures caused a dose-ratio deviation pattern on mortality and hatching, with a synergism observed when ZnO-NP was the dominant stressor, changing to antagonism when UV radiation dominated the combined exposure. Regarding the results attained, studying ZnO toxicity under laboratory conditions may underestimate the risks when considering the potential interaction on effects when combined with UV radiation.

Divergence in DNA photorepair efficiency among genotypes from contrasting UV radiation environments in nature

Populations of organisms routinely face abiotic selection pressures, and a central goal of evolutionary biology is to understand the mechanistic underpinnings of adaptive phenotypes. Ultraviolet radiation (UVR) is one of earth's most pervasive environmental stressors, potentially damaging DNA in any organism exposed to solar radiation. We explored mechanisms underlying differential survival following UVR exposure in genotypes of the water flea Daphnia melanica derived from natural ponds of differing UVR intensity. The UVR tolerance of a D. melanica genotype from a high-UVR habitat depended on the presence of visible and UV-A light wavelengths necessary for photoenzymatic repair of DNA damage, a repair pathway widely shared across the tree of life. We then measured the acquisition and repair of cyclobutane pyrimidine dimers, the primary form of UVR-caused DNA damage, in D. melanica DNA following experimental UVR exposure. We demonstrate that genotypes from high-UVR habitats repair DNA damage faster than genotypes from low-UVR habitats in the presence of visible and UV-A radiation necessary for photoenzymatic repair, but not in dark treatments. Because differences in repair rate only occurred in the presence of visible and UV-A radiation, we conclude that differing rates of DNA repair, and therefore differential UVR tolerance, are a consequence of variation in photoenzymatic repair efficiency. We then rule out a simple gene expression hypothesis for the molecular basis of differing repair efficiency, as expression of the CPD photolyase gene photorepair did not differ among D. melanica lineages, in both the presence and absence of UVR.

UV-Stressed Daphnia pulex Increase Fitness through Uptake of Vitamin D3

Ultraviolet radiation is known to be highly variable in aquatic ecosystems. It has been suggested that UV-exposed organisms may demonstrate enough phenotypic plasticity to maintain the relative fitness of natural populations. Our long-term objective is to determine the potential photoprotective effect of vitamin D3 on Daphnia pulex exposed to acute or chronic UV radiation. Herein we report our initial findings in this endeavor. D. pulex survival and reproduction (fitness) was monitored for 5 d as a proof of concept study. Significantly higher fitness was observed in the D. pulex with D3 than those without (most extreme effects observed were 0% survival in the absence of D3 and 100% with 10 ppm D3). Vitamin D3 was isolated from the culture media, the algal food (Pseudokirchneriella), and the D. pulex and quantified using high performance liquid chromatography (HPLC). Vitamin D3 was fluorescently labeled using a phenothiazinium dye and added to cultures of D. pulex. Images demonstrating the uptake of D3 into the tissues and carapace of the D. pulex were acquired. Our initial findings suggest a positive role for D3 in ecosystems as both UV-stressed algae and Daphnia sequester D3, and D. pulex demonstrate increased fitness in the presence of D3.

UV-B radiation-induced oxidative stress and p38 signaling pathway involvement in the benthic copepod Tigriopus japonicus

Ultraviolet B (UV-B) radiation presents an environmental hazard to aquatic organisms. To understand the molecular responses of the intertidal copepod Tigriopus japonicus to UV-B radiation, we measured the acute toxicity response to 96 h of UV-B radiation, and we also assessed the intracellular reactive oxygen species (ROS) levels, glutathione (GSH) content, and antioxidant enzyme (GST, GR, GPx, and SOD) activities after 24 h of exposure to UV-B with LD50 and half LD50 values. Also, expression patterns of p53 and hsp gene families with phosphorylation of p38 MAPK were investigated in UV-B-exposed copepods. We found that the ROS level, GSH content, and antioxidant enzyme activity levels were increased with the transcriptional upregulation of antioxidant-related genes, indicating that UV-B induces oxidative stress by generating ROS and stimulating antioxidant enzymatic activity as a defense mechanism. Additionally, we found that p53 expression was significantly increased after UV-B irradiation due to increases in the phosphorylation of the stress-responsive p38 MAPK, indicating that UV-B may be responsible for inducing DNA damage in T. japonicus. Of the hsp family genes, transcriptional levels of hsp20, hsp20.7, hsp70, and hsp90 were elevated in response to a low dose of UV-B radiation (9 kJ m(-2)), suggesting that these hsp genes may be involved in cellular protection against UV-B radiation. In this paper, we performed a pathway-oriented mechanistic analysis in response to UV-B radiation, and this analysis provides a better understanding of the effects of UV-B in the intertidal benthic copepod T. japonicus.

Interspecific variation in the repair of UV damaged DNA in the genus Xiphophorus as a factor in the decline of the Rio Grande Platyfish

The fish genus Xiphophorus consists of 26 species distributed along the eastern slopes of mountain ranges extending from northern Mexico to Belize and Nicaragua. We analyzed light-dependent repair of UV-induced DNA damage in at least two species from each of the four monophyletic Xiphophorus groups. We found that the northern platyfish had significantly reduced photoenzymatic repair compared to the other three groups, including the northern swordtails, southern platyfish and southern swordtails. All of the species of the northern platyfish, including the Marbled (meyeri), Northern (gordoni) and Monterrey Platyfish (couchianus) are the northernmost species in the genus and are the only three species in the genus that are currently found on the IUCN Red List of Threatened Species. Satellite data from the past 30 years (1979-2008) correlate greater increases in shorter wavelength UVB with higher latitudes within the Xiphophorus range. We suggest that, combined with other consequences of human population growth, anthropogenic deozonation resulting in a disproportionate increase in UVB in temperate latitudes may be a contributing factor in the decline and extirpation of the northern platyfish.

Functional genomics of acclimation and adaptation in response to thermal stress in Daphnia

Background

Gene expression regulation is one of the fundamental mechanisms of phenotypic plasticity and is expected to respond to selection in conditions favoring phenotypic response. The observation that many organisms increase their stress tolerance after acclimation to moderate levels of stress is an example of plasticity which has been long hypothesized to be based on adaptive changes in gene expression. We report genome-wide patterns of gene expression in two heat-tolerant and two heat-sensitive parthenogenetic clones of the zooplankton crustacean Daphnia pulex exposed for three generations to either optimal (18°C) or substressful (28°C) temperature.

Results

A large number of genes responded to temperature and many demonstrated a significant genotype-by-environment (GxE) interaction. Among genes with a significant GxE there were approximately equally frequent instances of canalization, i.e. stronger plasticity in heat-sensitive than in heat-tolerant clones, and of enhancement of plasticity along the evolutionary vector toward heat tolerance. The strongest response observed is the across-the-board down-regulation of a variety of genes occurring in heat-tolerant, but not in heat-sensitive clones. This response is particularly obvious among genes involved in core metabolic pathways and those responsible for transcription, translation and DNA repair.

Conclusions

The observed down-regulation of metabolism, consistent with previous findings in yeast and Drosophila, may reflect a general compensatory stress response. The associated down-regulation of DNA repair pathways potentially creates a trade-off between short-term benefits of survival at high temperature and long-term costs of accelerated mutation accumulation.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-859) contains supplementary material, which is available to authorized users.

Relationship between heat shock protein 70 expression and life span in Daphnia

The longevity of an organism is directly related to its ability to effectively cope with cellular stress. Heat shock response (HSR) protects the cells against accumulation of damaged proteins after exposure to elevated temperatures and also in aging cells. To understand the role of Hsp70 in regulating life span of Daphnia, we examined the expression of Hsp70 in two ecotypes that exhibit strikingly different life spans. Daphnia pulicaria, the long lived ecotype, showed a robust Hsp70 induction as compared to the shorter lived Daphnia pulex. Interestingly, the short-lived D. pulex isolates showed no induction of Hsp70 at the mid point in their life span. In contrast to this, the long-lived D. pulicaria continued to induce Hsp70 expression at an equivalent age. We further show that the Hsp70 expression was induced at transcriptional level in response to heat shock. The transcription factor responsible for Hsp70 induction, heat shock factor-1 (HSF-1), although present in aged organisms did not exhibit DNA-binding capability. Thus, the decline of Hsp70 induction in old organisms could be attributed to a decline in HSF-1's DNA-binding activity. These results for the first time, present a molecular analysis of the relationship between HSR and life span in Daphnia.

UV-induced DNA damage in Cyclops abyssorum tatricus populations from clear and turbid alpine lakes

Zooplankton from clear alpine lakes thrive under high levels of solar UV radiation (UVR), but in glacially turbid ones they are more protected from this damaging radiation. Here, we present results from experiments done with Cyclops abyssorum tatricus to assess UV-induced DNA damage and repair processes using the comet assay. Copepods were collected from three alpine lakes of differing UV transparency ranging from clear to glacially turbid, and exposed to artificial UVR. In addition, photoprotection levels [mycosporine-like amino acids (MAAs) and lipophilic antioxidant capacity] were estimated in the test populations. Similar UV-induced DNA damage levels were observed among the copepods from all lakes, but background DNA damage (time zero and dark controls) was lowest in the copepods from the glacially turbid lake, resulting in a higher relative DNA damage accumulation. Most DNA strand breaks were repaired after recovery in the dark. Low MAA concentrations were found in the copepods from the glacially turbid lake, while the highest levels were observed in the population from the most UV transparent lake. However, the highest lipophilic antioxidant capacities were measured in the copepods from the lake with intermediate UV transparency. Photoprotection and the ability to repair DNA damage, and consequently reducing UV-induced damage, are part of the response mechanisms in zooplankton to changes in water transparency caused by glacier retreat.

Using the comet assay to assess the combined and separate genotoxic effects of Cd and Zn in Eisenia andrei (Oligochaeta) at different temperatures

Using the comet assay, the genotoxicity of Cd, Zn and Cd/Zn mixtures in Eisenia andrei was assessed after 4 weeks of exposure at 15, 20 and 25 °C. Relative to the controls, significant increases in TDNA% were observed in exposures to Cd alone at 500 and 1,000 mg/kg soil at both 20 and 25 °C, while a general decrease occurred at 15 °C. For Zn alone, a decreasing trend in TDNA% occurred at all three temperatures with increasing Zn concentration. For the Cd/Zn mixtures at 15 °C, genotoxicity was reduced at all mixture concentrations relative to the control. At 20 °C, the genotoxic response was similar to the control at all exposures. At 25 °C, the response was elevated at the 50 + 50 and 250 + 250 mg/kg mixture concentrations. In the remaining treatments at 25 °C, TDNA% was similar to the values in the respective control. The lack of consistently significant mixture genotoxicity may indicate antagonistic interactions between Cd and Zn in the mixtures. However, this was not conclusively determined because temperature alone had an inconsistent effect upon TDNA% readings in the control exposures.

The Bunsen-Roscoe reciprocity law in ultraviolet-B-induced mortality of the two-spotted spider mite Tetranychus urticae

To determine whether the Bunsen-Roscoe reciprocity law (i.e., the extent of photochemical effects is determined by cumulative irradiance) is applicable to ultraviolet-B (UVB) damage in the twospotted spider mite Tetranychus urticae, egg hatchability and survival of individuals were assessed after irradiation with a UVB lamp using various combinations of intensity and time length. A positive linear correlation between probit mortality and cumulative UVB irradiance was detected in eggs, larvae, teleiochrysalis females, and adult females, regardless of UVB intensity (0.19-0.58 Wm(-2)). LD50 values were clearly higher in adult females, followed by teleiochrysalis females, larvae, and eggs. In eggs, reciprocity was obeyed not only at the UVB intensities listed above, but also at very low UVB intensity (0.014-0.023 Wm(-2)). Such reciprocity in the negative effects of UVB radiation was also observed for the developmental rate of juveniles and egg production of adult females. However, the LD50 value of eggs obtained using the UVB lamp (0.58 kJm(-2)) was lower than that elicited by solar UVB radiation in a previous outdoor experiment (about 50 kJm(-2)). These results suggest that a photoreactivation mechanism plays an important role in the survival of this mite under solar radiation.

An integrated multi-disciplinary approach for studying multiple stressors in freshwater ecosystems: Daphnia as a model organism

The increased overexploitation of freshwater ecosystems and their extended watersheds often generates a cascade of anthropogenic stressors (e.g., acidification, eutrophication, metal contamination, Ca decline, changes in the physical environment, introduction of invasive species, over-harvesting of resources). The combined effect of these stressors is particularly difficult to study, requiring a coordinated multi-disciplinary effort and insights from various sub-disciplines of biology, including ecology, evolution, toxicology, and genetics. It also would benefit from a well-developed and broadly accepted model systems. The freshwater crustacean Daphnia is an excellent model organism for studying multiple stressors because it has been a chosen focus of study in all four of these fields. Daphnia is a widespread keystone species in most freshwater ecosystems, where it is routinely exposed to a multitude of anthropogenic and natural stressors. It has a fully sequenced genome, a well-understood life history and ecology, and a huge library of responses to toxicity. To make the case for its value as a model species, we consider the joint and separate effects of natural and three anthropogenic stressors-climatic change, calcium decline, and metal contaminants on daphniids. We propose that integrative approaches marrying various subfields of biology can advance our understanding of the combined effects of stressors. Such approaches can involve the measuring of multiple responses at several levels of biological organization from molecules to natural populations. For example, novel interdisciplinary approaches such as transcriptome profiling and mutation accumulation experiments can offer insights into how multiple stressors influence gene transcription and mutation rates across genomes, and, thus, help determine the causal mechanism between environmental stressors and population/community effects as well as long-term evolutionary patterns.

Transcriptional responses in neonate and adult Daphnia magna in relation to relative susceptibility to genotoxicants

Little information is available on the responses of lower animals to genotoxic chemicals or on their sensitivity for detecting genotoxic chemicals, especially at different life-stages, despite the established use of the water flea Daphnia magna in ecotoxicity testing. Comet assay methodology was developed and applied to daphnid cells but only limited, non-statistically significant responses to the genotoxicants sodium dichromate (0.2-1 μM), chrysoidine (0.1-2 μM), and mixtures of benzo-a-pyrene (BaP) and sodium dichromate were found (from 0.01 μM BaP & 0.1 μM sodium dichromate to 0.25 μM BaP & 0.75 μM sodium dichromate). Transcriptomic analyses using Agilent D. magna oligonucleotide microarrays were undertaken to assess the effect of a mixture of sodium dichromate and BaP (designed to produce both adducted and oxidised DNA) on gene transcription. Neonates (<24h) and adults (day 7) were exposed for 6h and 24h at two combination concentration levels (0.02 μM BaP & 0.15 μM sodium dichromate and 0.1 μM BaP & 0.75 μM sodium dichromate). The greatest differences in transcriptional profile occurred between adults and neonates. Subsets of the transcriptional profiles distinguished genotoxicant-exposed animals from controls, both for neonates and adults. Higher transcript levels of DNA repair genes were found in adults and adults also displayed significant induction of DNA repair gene transcripts in response to exposure whereas neonates did not. Transcriptional changes in response to genotoxicant exposure proved more sensitive than measurement of DNA strand breaks by the Comet assay and the extensive differences in transcription between adults and neonates emphasized the importance of life stage in toxicant testing with Daphnia.

Genomic analyses of the Daphnia pulex peptidome

Genome mining has provided a valuable tool for peptide discovery in many species, yet no crustacean has undergone this analysis. Currently, the only crustacean with a sequenced genome is the cladoceran Daphnia pulex, a model organism in many fields of biology. Here, we have mined the D. pulex genome for peptide-encoding genes. For each gene identified, the encoded precursor protein was deduced, and its mature peptides predicted. Twenty-four peptide-encoding genes were identified, including ones predicted to produce members of the A-type allatostatin, B-type allatostatin, C-type allatostatin, allatotropin (ATR), bursicon α, bursicon β, calcitonin-like diuretic hormone, corazonin, crustacean cardioactive peptide, crustacean hyperglycemic hormone, ecdysis-triggering hormone, eclosion hormone (EH), insulin-like peptide (ILP), molt-inhibiting hormone, neuropeptide F, orcokinin (two genes), pigment-dispersing hormone, proctolin, red pigment concentrating hormone/adipokinetic hormone (RPCH/AKH), short neuropeptide F, SIFamide, sulfakinin, and tachykinin-related peptide (TRP) families/subfamilies. In total, 96 peptides were predicted from these genes. Our identification of isoforms of corazonin, EH, ILP, proctolin, RPCH/AKH, sulfakinin and TRP are the first for D. pulex, while our prediction of ATR from this species is the first from any crustacean. The number of peptides predicted in our study shows the power of genome mining for peptide discovery, and provides a model for future genomic analyses of the peptidomes of other crustaceans. In addition, the data presented in our study provide foundations for future molecular, biochemical, anatomical, and physiological investigation of peptidergic signaling in D. pulex and other cladoceran species.

UV radiation and freshwater zooplankton: damage, protection and recovery

While many laboratory and field studies show that zooplankton are negatively affected when exposed to high intensities of ultraviolet radiation (UVR), most studies also indicate that zooplankton are well adapted to cope with large variations in their UVR exposure in the pelagic zone of lakes. The response mechanisms of zooplankton are diverse and efficient and may explain the success and richness of freshwater zooplankton in optically variable waters. While no single behavioural or physiological protection mechanism seems to be superior, and while several unexplained and contradictory patterns exist in zooplankton UVR ecology, recent increases in our understanding are consistent with UVR playing an important role for zooplankton. This review examines the variability in freshwater zooplankton responses to UVR, with a focus on crustacean zooplankton (Cladocera and Copepoda). We present an overview of UVR-induced damages, and the protection and recovery mechanisms freshwater zooplankton use when exposed to UVR. We review the current knowledge of UVR impact on freshwater zooplankton at species and community levels, and discuss briefly how global change over the last three decades has influenced the UVR milieu in lakes.

Direct effects of UV-B radiation on the freshwater heterotrophic nanoflagellate Paraphysomonas sp

The formation of DNA photoproducts in organisms exposed to ambient levels of UV-B radiation can lead to death and/or reduced population growth in aquatic systems. Dependence on photoenzymatic repair to reverse DNA damage caused by UV-B radiation is demonstrated for Paraphysomonas sp., a member of a widely distributed genus of heterotrophic nanoflagellates. At 20 degrees C, Paraphysomonas sp. was exposed to a range of UV-B intensities encountered in natural systems. Populations of the flagellate survived and grew in a dose-dependent manner, but only when simultaneously exposed to photorepair radiation (PRR). In contrast, flagellates exposed to UV-B at 15 degrees C suffered 100% mortality except at the lowest UV-B level (with PRR) tested, which suggested a photorepair temperature optimum above 15 degrees C. After acute UV-B exposures, DNA damage (measured as the formation of pyrimidine dimers) was reduced only in organisms that underwent subsequent exposure to PRR. Populations kept in the dark after UV-B exposure maintained the initial levels of pyrimidine dimers. These results are the first to demonstrate the reliance of a heterotrophic flagellate on photoenzymatic DNA repair for survival from UV-B exposure.