Photoperiod-dependent effects of zinc oxide nanoparticles on the growth and reproduction of Daphnia pulex.

The discharge of metal nanoparticles into the water inevitably poses a threat to aquatic organisms and the balance of the aquatic ecosystem. Photoperiod is one of the most important ecological factors for the development of cladocerans. In addition, different light conditions can also affect the toxicity of metal nanoparticles. In this study, we studied the effects of four photoperiods (8L/16D, 10L/14D, 14L/10D, and 16L/8D) combined with three concentrations of ZnO NPs (0 mg L-1, 0.05 mg L-1, and 0.10 mg L-1) on the growth and reproduction of Daphnia pulex. With the increase of photoperiod, the maternal body size and growth rate increased first and then decreased; the first time to reproduction was advanced, and broods and the total offspring also increased. Under the influence of ZnO NPs, growth rate and reproductive capacity were inhibited. The photoperiod 8L/16D and 16L/8D interacted with ZnO NPs on the growth of D. pulex, which significantly decreased the growth rate. Besides, the interaction between photoperiod 8L/16D and ZnO NPs decreased the reproduction ability of D. pulex. These results suggest that the effects of zinc oxide nanoparticles on the growth and reproduction of D. pulex is photoperiod dependent, which is useful for assessing the risk of pollutants to cladoceras under different light conditions.

Effects of environmental concentrations of 6PPD and its quinone metabolite on the growth and reproduction of freshwater cladoceran.

The widespread occurrence and accumulation of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its quinone metabolite, 6PPD quinone (6PPD-Q), have been globally recognized as a critical environmental issue. However, knowledge on the adverse effects of 6PPD and 6PPD-Q on freshwater invertebrates is limited. This study investigated the effects of 6PPD and its oxidative byproduct, 6PPD-Q, on the growth and reproduction of Daphnia pulex. Through 21-day exposure experiments, we measured the uptake of 0.1, 1, and 10 µg/L 6PPD and 6PPD-Q by D. pulex and assessed the effects on growth and fecundity of D. pulex. While 6PPD and 6PPD-Q did not affect the mortality rate of D. pulex, 6PPD-Q exposure inhibited the growth of D. pulex, indicating potential ecological risks. In particular, the reproductive capacity of D. pulex remained unaffected across the tested concentrations of 6PPD and 6PPD-Q, suggesting specific toxicological pathways that warrant further investigation. This study underscored the importance of evaluating the sublethal effects of emerging contaminants such as 6PPD and 6PPD-Q on aquatic invertebrates, and highlighted the need for comprehensive risk assessments to better understand their environmental impacts.

Copy number and sequence variation in rDNA of Daphnia pulex from natural populations: insights from whole-genome sequencing.

Ribosomal DNA (rDNA) has a vital role in ribosome biogenesis as it contains the genes that encode ribosomal RNA (rRNA) separated by intergenic spacers (IGSs). The rRNA genes occur in hundreds to tens of thousands of copies per haploid genome in eukaryotes and are generally highly conserved with low variation within species. Due to the repetitive nature and large size of rDNA arrays, detecting intraindividual variation can be difficult. In this study, we use whole-genome sequences of 169 Daphnia pulex individuals from 10 natural populations to measure the copy number and sequence variation in rDNA. This revealed that variation in rDNA copy number between individuals spans an order of magnitude. We further observed a substantial level of sequence variation within individual genomes. As expected, single-nucleotide polymorphisms occurred in regions of lower functional constraint such as the IGS and expansion segments of the rRNA genes. The presence of strong linkage disequilibrium among variants facilitated identification of haplotypes within each population. Although there was evidence of recombination among haplotypes from different populations, it is insufficient to eliminate linkage disequilibrium within populations. Estimating copy number and haplotype diversity within individuals revealed that the level of intraindividual sequence variation is not strongly correlated with copy number. The observed patterns of variation highlight a complex evolutionary history of rDNA in D. pulex. Future research should explore the functional implications of rDNA copy number and sequence variation on organismal phenotypes.

Differential interference effects of thermal pollution on the induced defense of different body-sized cladocerans.

Climate warming influences the biological activities of aquatic organisms, including feeding, growth, and reproduction, thereby affecting predator-prey interactions. This study explored the variation in thermal sensitivity of anti-predator responses in two cladoceran species with varying body sizes, Daphnia pulex and Ceriodaphnia cornuta. These species were cultured with or without the fish (Rhodeus ocellatus) kairomone at temperatures of 15, 20, 25, and 30 °C for 15 days. Results revealed that cladocerans of different body sizes exhibited varying responses to fish kairomones in aspects such as individual size, first-brood neonate size, total offspring number, average brood size, growth rate, and reproductive effort. Notably, low temperature differently affected defense responses in cladocerans of different body sizes. Both high and low temperatures moderated the intensity of the kairomone-induced response on body size at maturity. Additionally, low temperature reversed the reducing effect of fish kairomone on the total offspring number, average brood size, and reproductive effort in D. pulex. Conversely, it enhanced the increasing effect of fish kairomone on these parameters in C. cornuta. These results suggest that inducible anti-predator responses in cladocerans are modifiable by temperature. The differential effects of fish kairomones on various cladocerans under temperature influence offer crucial insights for predicting changes in predator-prey interactions within freshwater ecosystems under future climate conditions.

Combined effects of spinetoram and Microcystis aeruginosa on Daphnia pulex offspring: Maternal effects and multigenerational implications.

The increasing occurrence of harmful algal blooms (HABs) in freshwater ecosystems detrimentally affect global water environments. Zooplankton's role in controlling HABs is hindered by contaminant exposure, necessitating research into combined stressors' ecological impacts. The response of Daphnia, a freshwater keystone species, to environmental stressors can be influenced by its maternal effects. Here, we investigated the combined effects of the world-widely used insecticide spinetoram and non-toxic HABs species Microcystis aeruginosa on the life-history traits of D. pulex offspring produced from different maternal food conditions. Four maternal groups were established, with each group receiving a specific blend of C. vulgaris (Ch) and M. aeruginosa (Ma) in varying proportions: A (100% Ch), B (90% Ch + 10% Ma), C (80% Ch + 20% Ma), and D (70% Ch + 30% Ma). The offspring from the third brood were gathered, and a 21-day experiment was carried out, involving various feeding groups (AA, AD, BA, BB, CA, CC, DA, and DD). Results demonstrated that grazing on M. aeruginosa by D. pulex induced maternal effects on their offspring, with the continuous exposure group showing an enhanced tolerance to M. aeruginosa. This study also unveiled that spinetoram could interfere with the molting of D. pulex, leading to developmental retardation. The Recovery Group exhibited an intriguing phenomenon: under the influence of both concentrations of the pesticide spinetoram (0.18, 0.35 µg L-1), D. pulex produced more offspring. This might be due to a combined strategy of allocating more energy towards reproduction in response to low-quality food and a potential hormetic effect from low concentrations of spinetoram. Assessing the interplay of combined stressors across multiple generations, encompassing harmful algal blooms (HABs) and environmental pollutants, is essential for predicting population responses to evolving environmental conditions. This understanding is vital for the protection and management of aquatic environments and ecosystems.

Evolutionary Insights from a Large-Scale Survey of Population-Genomic Variation.

The field of genomics has ushered in new methods for studying molecular-genetic variation in natural populations. However, most population-genomic studies still rely on small sample sizes (typically, <100 individuals) from single time points, leaving considerable uncertainties with respect to the behavior of relatively young (and rare) alleles and, owing to the large sampling variance of measures of variation, to the specific gene targets of unusually strong selection. Genomic sequences of ∼1,700 haplotypes distributed over a 10-year period from a natural population of the microcrustacean Daphnia pulex reveal evolutionary-genomic features at a refined scale, including previously hidden information on the behavior of rare alleles predicted by recent theory. Background selection, resulting from the recurrent introduction of deleterious alleles, appears to strongly influence the dynamics of neutral alleles, inducing indirect negative selection on rare variants and positive selection on common variants. Temporally fluctuating selection increases the persistence of nonsynonymous alleles with intermediate frequencies, while reducing standing levels of variation at linked silent sites. Combined with the results from an equally large metapopulation survey of the study species, classes of genes that are under strong positive selection can now be confidently identified in this key model organism. Most notable among rapidly evolving Daphnia genes are those associated with ribosomes, mitochondrial functions, sensory systems, and lifespan determination.

The Genome-wide Signature of Short-term Temporal Selection.

Despite evolutionary biology's obsession with natural selection, few studies have evaluated multi-generational series of patterns of selection on a genome-wide scale in natural populations. Here, we report on a nine-year population-genomic survey of the microcrustacean Daphnia pulex. The genome-sequences of > 800 isolates provide insights into patterns of selection that cannot be obtained from long-term molecular-evolution studies, including the pervasiveness of near quasi-neutrality across the genome (mean net selection coefficients near zero, but with significant temporal variance about the mean, and little evidence of positive covariance of selection across time intervals), the preponderance of weak negative selection operating on minor alleles, and a genome-wide distribution of numerous small linkage islands of observable selection influencing levels of nucleotide diversity. These results suggest that fluctuating selection is a major determinant of standing levels of variation in natural populations, challenge the conventional paradigm for interpreting patterns of nucleotide diversity and divergence, and motivate the need for the development of new theoretical expressions for the interpretation of population-genomic data.

Evolutionary Insights from a Large-scale Survey of Population-genomic Variation.

Results from data on > 1000 haplotypes distributed over a nine-year period from a natural population of the microcrustacean Daphnia pulex reveal evolutionary-genomic features at a refined scale, including key population-genetic properties that are obscured in studies with smaller sample sizes. Background selection, resulting from the recurrent introduction of deleterious alleles, appears to strongly influence the dynamics of neutral alleles, inducing indirect negative selection on rare variants and positive selection on common variants. Fluctuating selection increases the persistence of nonsynonymous alleles with intermediate frequencies, while reducing standing levels of variation at linked silent sites. Combined with the results from an equally large metapopulation survey of the study species, regions of gene structure that are under strong purifying selection and classes of genes that are under strong positive selection in this key species can be confidently identified. Most notable among rapidly evolving Daphnia genes are those associated with ribosomes, mitochondrial functions, sensory systems, and lifespan determination.

Divergence of the Host-Associated Microbiota with the Genetic Distance of Host Individuals Within a Parthenogenetic Daphnia Species.

The taxonomic composition of the microbiota in the gut and epidermis of animals is known to vary among genetically and physiologically different host individuals within the same species. However, it is not clear whether the taxonomic composition diverges with increasing genetic distance of the host individuals. To unveil this uncertainty, we compared the host-associated microbiota among the genotypes within and between genetically distant lineages of parthenogenetic Daphnia cf. pulex across different physiological states, namely, well-fed, starved, and dead. Metagenomic analysis with 16S rRNA showed that, regardless of the host genotypes, diversity of the host-associated microbiota was high when the host individuals were fed food and gradually decreased when they were starved until they died. However, the difference in the host-associated microbiota, that is, ß-diversity, was significant among the genotypes within and between the host lineages when they were fed. Although some bacteria in the microbiota, such as Limnohabitans, Rhodococcus, and Aeromicrobium, were found abundantly and commonly in all host genotypes; others, such as those of Holosoporacea, were found only in the genotypes of a specific lineage. Accordingly, the ß-diversity tended to increase with increasing genetic distance of the host individuals. These results support an idea that the host-associated microbiota diverged with genetic divergence in the host species and that at least some bacteria are highly dependent on the genetically specific metabolites produced by the host individuals.

Anthropogenic noise does not strengthen multiple-predator effects in a freshwater invasive fish.

Anthropogenic noise has the potential to alter community dynamics by modifying the strength of nested ecological interactions such as predation. Direct effects of noise on per capita predation rates have received much attention but the context in which predation occurs is often oversimplified. For instance, many animals interact with conspecifics while foraging and these nontrophic interactions can positively or negatively influence per capita predation rates. These effects are often referred to as multiple-predator effects (MPEs). The extent to which noise can modulate MPEs and thereby indirectly alter per capita predation remains unknown. To address this question, we derived the relationship between per capita predation rate and prey density, namely the functional response (FR), of single and pairs of the invasive topmouth gudgeon Pseudorasbora parva when feeding on water fleas under two noise conditions: control ambient noise estimated at 95 dB re 1 µPa and ambient noise supplemented with motorboat sounds whose relative importance over ambient noise ranged from 4.81 to 27 dB. In addition, we used video recordings to track fish movements. To detect MPEs, we compared the observed group-level FRs to predicted group-level FRs inferred from the individual FRs and based on additive effects only. Regardless of the number of fish and the noise condition, the FR was always of type II, showing predation rate in a decelerating rise to an upper asymptote. Compared to the noiseless condition, the predation rate of single fish exposed to noise did not differ at high prey densities but was significantly lower at low prey densities, resulting in an FR with the same asymptote but a less steep initial slope. Noise also reduced fish mobility, which might explain the decrease in predation rate at low prey densities. Conspecific presence suppressed the individual response to noise, the FRs of two fish (observed group-level FRs) being perfectly similar between the two noise conditions. Although observed and predicted group-level FRs did not differ significantly, observed group-level FRs tended to fall in the low range of predicted group-level FRs, suggesting antagonism and a negative effect of nontrophic interactions on individual foraging performance. Interestingly, the difference between predicted and observed group-level FRs was not greater with noise, which means that noise did not strengthen MPEs. Our results show that when considering the social context of foraging, here through the presence of a conspecific, anthropogenic noise does not compromise foraging in the invasive P. parva.

Genome-wide identification glutathione-S-transferase gene superfamily in Daphnia pulex and its transcriptional response to nanoplastics.

Glutathione S-transferases (GSTs) are key multifunctional phase II detoxification enzymes involved in the regulation of growth, development, and stress responses. However, the knowledge of GSTs in the model invertebrate organism Daphnia pulex at the genomic level remains limited. In the present study, 35 GST genes were identified in D. pulex (Dp-GST), belonging to eight subfamilies, with the sigma, mu, and delta/epsilon subfamilies constituting approximately 29 %, 20 %, and 20 % of the GST superfamily, respectively. Chromosome tandem duplication of genes within the same subfamily was observed, which may be the main force driving GST expansion in D. pulex. DpGST genes showed different expression patterns in response to nanoplastic exposure for 96 h and 21 days. Some homologous GST genes in D. pulex showed similar expression patterns in response to nanoplastic exposure, likely owing to their unique motifs. For example, motif 9 is found in all delta/epsilon GST genes, whereas motifs 1, 2, 3, 5, and 7 are highly conserved in sigma GST genes. The characterization of D. pulex GSTs extending the knowledge of GST-mediated environmental contaminants, especially nanoplastics.

Performance of Daphnia simultaneously exposed to nitrite and predation risk: Reduced nitrite tolerance and aggravated predation-induced miniaturization.

Nitrogenous pollutants derived from human activities not only pose direct risk on aquatic organisms but may also indirectly endanger the stability of interspecific relations. To date, the effects of the nitrogen-containing pollutants on the induced defense remain unclear. Here, we aim to investigate the induced defense of an aquatic keystone species, Daphnia pulex, which responds to predation risk under nitrite pollution at environmentally relevant concentrations and simultaneously evaluate the effects of their induced defenses on nitrite tolerance. Results showed that increasing nitrite significantly reduced the survival time of D. pulex and posed severe reproductive toxicity, consequently reducing the offspring and broods. In the morphological defensive responses, early nitrite exposure interfered with the spine elongation, but the relative spine length induced by the predation risk was unaffected by the nitrite concentrations with exposure time prolonged, although high-dose nitrite inhibited the spine elongation and the increase of the body size. The integration of biomarker response index analyses further indicated that the reproductive capacity was more seriously impaired than the morphology and the survival. Moreover, the sensitivity analyses of growth and reproduction indicated that predation risk significantly reduced Daphnia's tolerance to nitrite. Conclusively, these findings highlight that long-term nitrite exposure exacerbates the predator-induced miniaturization of zooplanktons, and predation risk also reduces their tolerance to nitrite, which provides new insights into the performance changes of zooplanktons exposed to pollutants under predation risk and the vulnerability of predator-prey interspecific relationships in polluted environments.

An Efficient Solid-Phase Microextraction-Gas Chromatography-Mass Spectrometry Method for the Analysis of Methyl Farnesoate Released in Growth Medium by Daphnia pulex.

Methyl farnesoate (MF), a juvenile hormone, can influence phenotypic traits and stimulates male production in daphnids. MF is produced endogenously in response to stressful conditions, but it is not known whether this hormone can also be released into the environment to mediate stress signaling. In the present study, for the first time, a reliable solid-phase microextraction-gas chromatography-mass spectrometry (SPME-GC-MS) method was developed and validated for the ultra-trace analysis of MF released in growth medium by Daphnia pulex maintained in presence of crowding w/o MK801, a putative upstream inhibitor of MF endogenous production. Two different clonal lineages, I and S clones, which differ in the sensitivity to the stimuli leading to male production, were also compared. A detection limit of 1.3 ng/L was achieved, along with good precision and trueness, thus enabling the quantitation of MF at ultra-trace level. The achieved results demonstrated the release of MF by both clones at the 20 ng/L level in control conditions, whereas a significant decrease in the presence of crowding was assessed. As expected, a further reduction was obtained in the presence of MK801. These findings strengthen the link between environmental stimuli and the MF signaling pathway. Daphnia pulex, by releasing the juvenile hormone MF in the medium, could regulate population dynamics by means of an autoregulatory feedback loop that controls the intra- and extra-individual-level release of MF produced by endogenous biosynthesis.

A Non-Stressful Temperature Rise and Greater Food Availability Could Increase Tolerance to Calcium Limitation of Daphnia cf. pulex (Sensu Hebert, 1995) Populations in Cold Soft-Water Lakes.

Calcium (Ca) is an important driver of community structure in freshwaters. We examined the combined effects of increased temperatures and variations in food quantity on the tolerance to low Ca of Daphnia pulex. The aim was to predict the impact of climate warming on this keystone zooplanktonic species in cold-climate lakes. We conducted a factorial life-history experiment in a clone of North American Daphnia cf. pulex to analyse the interaction effects of a temperature increase (17.5 °C−21 °C) within their physiological preferred range and expected by climate warming over the next few decades and a narrow Ca gradient (0.25−1.74 mg Ca L−1) under stressful vs. abundant food conditions. We found a striking positive synergistic effect of Ca and temperature on D. pulex reproduction at high food conditions. Although the increase in temperature to 21 °C greatly reduced survival, high energy allocation to reproduction at high food levels allowed the population to succeed in poor Ca (<0.25 mg Ca L−1). Results suggest that climate warming and higher food availability will make the populations of many cold and Ca-limited lakes more tolerant to low Ca levels with higher growth population rates, thereby altering zooplanktonic community structures and inducing potential cascading effects on the food web.

Daphnia as a versatile model system in ecology and evolution.

Water fleas of the genus Daphnia have been a model system for hundreds of years and is among the best studied ecological model organisms to date. Daphnia are planktonic crustaceans with a cyclic parthenogenetic life-cycle. They have a nearly worldwide distribution, inhabiting standing fresh- and brackish water bodies, from small temporary pools to large lakes. Their predominantly asexual reproduction allows for the study of phenotypes excluding genetic variation, enabling us to separate genetic from non-genetic effects. Daphnia are often used in studies related to ecotoxicology, predator-induced defence, host-parasite interactions, phenotypic plasticity and, increasingly, in evolutionary genomics. The most commonly studied species are Daphnia magna and D. pulex, for which a rapidly increasing number of genetic and genomic tools are available. Here, I review current research topics, where the Daphnia model system plays a critical role.

The stress effect of atrazine on the inducible defense traits of Daphnia pulex in response to fish predation risk: Evidences from morphology, life history traits, and expression of the defense-related genes.

Herbicide pollution is persistent, which not only has a negative impact on individual organisms, but also may endanger the interspecific relationship between predators and prey. Cladocerans, i.e. zooplankton that plays an important role in the energy flow and material circulation in freshwater ecosystem, usually develop induced defense in response to predation risk. We used atrazine, one of the most used herbicides in the world, and Daphnia pulex, a representative cladocerans, to test the possible interference effect of herbicides on the induced defensive traits of cladocerans in response to predator fish (Rhodeus ocellatus) kairomone, including morphological defense, life history strategies, and the expression of defense-related genes. Atrazine reduced the body size, spine size, growth rate, total offspring, and the relative reproductive output of D. pulex, which further affected the response strength of the morphological and life history defenses, i.e., atrazine significantly reduced the spine size, relative spine size, and fecundity of D. pulex in response to R. ocellatus kairomone. Exposure to atrazine affected the expression of defense-related genes, and we speculated that atrazine affected the signaling process in the induced anti-predation defense of cladocerans. Specially, fish kairomone attenuated the negative effects of high concentrations of atrazine on the life history traits of D. pulex. Our results will help to accurately assess the potential risk of artificial compounds in freshwater ecosystems from the perspective of interspecific relationships, and help to understand the impact of environmental changes on the inducible anti-predator defense of prey in aquatic ecosystems.

Evolutionary Genomics of a Subdivided Species.

The ways in which genetic variation is distributed within and among populations is a key determinant of the evolutionary features of a species. However, most comprehensive studies of these features have been restricted to studies of subdivision in settings known to have been driven by local adaptation, leaving our understanding of the natural dispersion of allelic variation less than ideal. Here, we present a geographic population-genomic analysis of 10 populations of the freshwater microcrustacean Daphnia pulex, an emerging model system in evolutionary genomics. These populations exhibit a pattern of moderate isolation-by-distance, with an average migration rate of 0.6 individuals per generation, and average effective population sizes of ∼650,000 individuals. Most populations contain numerous private alleles, and genomic scans highlight the presence of islands of excessively high population subdivision for more common alleles. A large fraction of such islands of population divergence likely reflect historical neutral changes, including rare stochastic migration and hybridization events. The data do point to local adaptive divergence, although the precise nature of the relevant variation is diffuse and cannot be associated with particular loci, despite the very large sample sizes involved in this study. In contrast, an analysis of between-species divergence highlights positive selection operating on a large set of genes with functions nearly nonoverlapping with those involved in local adaptation, in particular ribosome structure, mitochondrial bioenergetics, light reception and response, detoxification, and gene regulation. These results set the stage for using D. pulex as a model for understanding the relationship between molecular and cellular evolution in the context of natural environments.

A Study on the Bio-responses of a Freshwater Snail (Biomphalaria alexandrina) to Fungal-derived Compounds.

BACKGROUND: Biomphalaria alexandrina snails, as transitional hosts of schistosomiasis, plays an essential part in the spread of the illness. Control of these snails by the substance molluscicides antagonistically influences the oceanic climate, causing poisonous and cancer-causing consequences for non-target life forms. OBJECTIVE: Looking for new naturally safe substances that can treat schistosomiasis disease with minimal side effects on the environment and plants, fish wealth and do not affect vital human functions. METHODS: Fifty fungal species were used to evaluate their activity against Biomphalaria alexandrina. Study the effect of the fungal extract on vital functions of Biomphalaria alexandrina and fish wealth. Purification of active substances and identification of their chemical structures. RESULTS: Cladosporium nigrellum and Penicillium aurantiogresium metabolites were effective against B. alexandrina snails, and the effects of promising fungal extracts sublethal concentrations (IC10 & IC25) on the levels of steroid sex hormones, liver enzymes, total protein, lipids, albumin and glucose were determined. Chemical analyses of this filtrate separated a compound effective against snails; it was identified. Protein electrophoresis showed that fungal filtrate affects the protein pattern of snails' haemolymph. Little or no mortality of Daphnia pulex individuals was observed after their exposure to sublethal concentrations of each treatment. CONCLUSION: Certain compounds from fungal cultures could be safely used for biological control of Biomphalaria alexandrina snails.

Pollution induces epigenetic effects that are stably transmitted across multiple generations.

It has been hypothesized that the effects of pollutants on phenotypes can be passed to subsequent generations through epigenetic inheritance, affecting populations long after the removal of a pollutant. But there is still little evidence that pollutants can induce persistent epigenetic effects in animals. Here, we show that low doses of commonly used pollutants induce genome-wide differences in cytosine methylation in the freshwater crustacean Daphnia pulex. Uniclonal populations were either continually exposed to pollutants or switched to clean water, and methylation was compared to control populations that did not experience pollutant exposure. Although some direct changes to methylation were only present in the continually exposed populations, others were present in both the continually exposed and switched to clean water treatments, suggesting that these modifications had persisted for 7 months (>15 generations). We also identified modifications that were only present in the populations that had switched to clean water, indicating a long-term legacy of pollutant exposure distinct from the persistent effects. Pollutant-induced differential methylation tended to occur at sites that were highly methylated in controls. Modifications that were observed in both continually and switched treatments were highly methylated in controls and showed reduced methylation in the treatments. On the other hand, modifications found just in the switched treatment tended to have lower levels of methylation in the controls and showed increase methylation in the switched treatment. In a second experiment, we confirmed that sublethal doses of the same pollutants generate effects on life histories for at least three generations following the removal of the pollutant. Our results demonstrate that even low doses of pollutants can induce transgenerational epigenetic effects that are stably transmitted over many generations. Persistent effects are likely to influence phenotypic development, which could contribute to the rapid adaptation, or extinction, of populations confronted by anthropogenic stressors.

Population changes of Daphnia caused by declined calcium concentration: Evidences from population dynamics and sexual reproduction.

The decline in freshwater calcium has become a new environmental stressor to Daphnia with high calcium demand, however, the population dynamics and sexual reproduction of Daphnia under low calcium stress are still lack of deep understanding. To evaluate the impact of declined calcium on Daphnia from population level, we respectively exposed two clones of Daphnia pulex (CH and SH) to different calcium concentrations (0.5, 1.0, 1.5, 5.0, 10.0, 25.0 mg L-1) for 30 days and recorded the population indicators. Results showed that total biomass, average dry weight per individual, total number of ephippia, total number of resting eggs of Daphnia pulex CH clone at 1.0 mg L-1 calcium decreased by 75.5%, 34.0%, 83.6%, and 77.6% compared with those at 25 mg L-1 calcium, while SH clone at 1.0 mg L-1 calcium decreased by 64.6%, 26.1%, 94.5%, and 82.2%, respectively. Importantly, Ca content in dry Daphnia pulex population of CH clone at 1.0 and 1.5 mg L-1 calcium decreased by 32.7% and 6.7% compared to those at 25 mg L-1 calcium, and SH clone at 1.0 mg L-1 and 1.5 mg L-1 calcium also decreased by 30.9% and 10.5%, respectively. Furthermore, low calcium significantly decreased the perimeter and surface area of ephippia. Interestingly, observation by scanning electron microscope found that low calcium changed the surface of ephippia. The negative impact of calcium decline on Daphnia population and sexual reproduction will inevitably endanger the persistence of species and genes at meta population level.