Determination of mRNA expression of DMRT93B, vitellogenin, and cuticle 12 in Daphnia magna and their biomarker potential for endocrine disruption

Epigenetic and Gene Expression Responses of Daphnia magna to Polyethylene and Polystyrene Microplastics

Microplastics (MPs), ubiquitous environmental pollutants, pose substantial threats to aquatic ecosystems and organisms, including the model species Daphnia magna. This study examined the effects of polyethylene (PE) and polystyrene (PS) MPs on D. magna, focusing on their ingestion, epigenetic alterations, and transcriptional responses. Exposure experiments revealed a concentration-dependent accumulation of MPs, with PS particles showing higher ingestion rates due to their higher density and propensity for aggregation. Epigenetic analyses demonstrated that exposure to PE MPs significantly reduced the global DNA methylation (5-mC) of Daphnia magna, suggesting hypomethylation as a potential stress response. Conversely, the DNA hydroxymethylation (5-hmC) of Daphnia magna displayed variability under PS exposure. Transcriptional analysis identified a marked downregulation of Vitellogenin 1 (v1) and upregulation of Ecdysone Receptor B (ecr-b), highlighting the occurrence of stress-related and adaptive molecular responses. These findings enhance our understanding of the molecular and epigenetic effects of MPs on aquatic organisms, offering critical insights for the development of effective environmental management and conservation strategies in the face of escalating MP pollution.

Toxicity comparison of polylactic acid and polyethylene microplastics co-exposed with methylmercury on Daphnia magna

The prevalence of microplastics (MPs) in aquatic ecosystems has become a significant environmental concern due to their persistence and potential toxicity. Although bioplastics, such as polylactic acid (PLA), are promoted as eco-friendly alternatives to conventional plastics, their toxicity remains poorly understood. This study compares the toxicity and pollutant vector roles of polar PLA-derived bio-microplastics (bio-MPs) with apolar low-density polyethylene (LDPE) MPs, both individually and in combination with methylmercury (MeHg), in Daphnia magna. PLA bio-MPs, both alone and in combination with MeHg, significantly reduced survival rates and reproduction while inducing oxidative stress. Additionally, PLA bio-MPs increased Hg accumulation and negatively impacted acetylcholinesterase activity and vitellogenin gene expression compared to LDPE MPs. The findings of this study suggest that PLA bio-MPs, despite being in vivo biodegradable, may pose similar or even greater environmental risks than fossil fuel-based MPs, particularly due to their potential to enhance the bioaccumulation and toxicity of coexisting pollutants.

Genes of filter-feeding species as a potential toolkit for monitoring microplastic impacts

Microplastics (MPs) are ubiquitous in the marine environment and impact organisms at multiple levels. Understanding their actual effects on wild populations is urgently needed. This study develops a toolkit to monitor changes in gene expression induced by MPs in natural environments, focusing on filter-feeding and bioindicator species from diverse ecological and taxonomic groups. Six candidate genes -Caspase, HSP70, HSP90, PK, SOD, and VTG- and nine filter-feeding species -two branchiopods, one copepod, five bivalves and one fish- were selected based on differential expression in response to MPs exposure (mainly the widely used polystyrene and polyethylene polymers) reported in over 30 publications. Some genes are particularly determinant, such as HSP70 and HSP90 (key to managing a wide range of stressors) and SOD (critical for addressing oxidative stress), as they are more directly related to stress. PK is related to carbohydrate metabolism (alterations in energy metabolism); VTG is associated with reproductive problems; Caspase mediates in apoptosis. Each gene in the toolkit plays a role depending on the type of stress assessed, and their combination provides a comprehensive understanding of the impacts of MPs. Differences in gene expressions between species and the exposure thresholds were found. These genes were examined in various scenarios with different types, concentrations, and sizes of MPs, alone or with other stressors. The toolkit offers significant advantages, allowing a comprehensive study of the impact of MPs and focusing on filtering bioindicator species, thus enabling pollution assessment and long-term monitoring. It will outperform traditional methods like tissue counts of MPs where only physical damage is visible, providing a deeper understanding. To our knowledge, this is the first toolkit of its kind.

Maternal Daphnia magna exposure to the antidepressant sertraline causes molting disorder, multi-generational reproductive and serotonergic dysfunction

Sertraline, one of the most commonly used antidepressants, has exhibited a progressively escalating trend in usage over the course of the last decades years, which have been exacerbated by the COVID-19 pandemic. Here, this study assessed the transgenerational effects of sertraline on the aquatic microcrustacean Daphnia magna, a parthenogenetic model species. The parental D. magna (G0) were exposed to environmentally relevant concentrations of sertraline (0.1 and 10 μg/L) for 21 days at individual and population level, and observed exposure triggered specific increased fecundity and desynchronized molting. These alterations were partially inherited through three subsequent non-exposed generations (G1, G2, and G3), as evidenced by increased fecundity and disordered molting in G1, reduced fecundity in G2, and reduced body size of G3-offspring. The molt-related genes neverland 1 and hormone receptor 3 were significantly different to the control group simultaneously only in the exposed generation, which may well be responsible for the molting asynchrony. Vitellogenin plays an important role in reproduction, and our results indicate that its abnormal expression persists up to G3, which was highly correlated with the expression of serotonin transporter, the drug target of sertraline. This finding suggested that sertraline possesses a sustained reproductive toxicity and disrupting potential and may be associated with serotonin dysregulation caused by compensatory feedback of serotonin transporter. In combination with male birth and upregulation of doublesex and vitellogenin, sertraline was deemed to trigger a self-defense response of D. magna, known as "abandon-ship" by increasing reproductive inputs. However, no males was found in individual reproduction test in each generation, which may suggest some interaction between sertraline and population density. Our findings emphasize that the toxic effects of sertraline can be transferred to unexposed generations, even with different adverse consequences, implying that future studies need to focus on transgenerational delayed effects and the underlying mechanisms.

Screening of morphology-related genes based on predator-induced transcriptome sequencing and the functional analysis of Dagcut gene in Daphnia galeata

High fish predation pressure can trigger "induced defense" in Daphnia species, resulting in phenotypic plasticity in morphology, behavior, or life-history traits. The molecular mechanisms of defense morphogenesis (e.g., the tail spine and helmet) in Daphnia remain unclear. In the present study, the tail spine, helmet, and body of Daphnia galeata under fish and non-fish kairomones conditions were collected for transcriptome analysis. A total of 24 candidate genes related to the morphological defense of D. galeata were identified, including 2 trypsin, one cuticle protein, 1 C1qDC protein, and 2 ferritin genes. The function of the Dagcut gene (D. galeata cuticle protein gene) in relation to tail spine morphology was assessed using RNA interference (RNAi). Compared with the EGFP (Enhanced green fluorescent protein) treatment, after RNAi, the expression levels of the Dagcut gene (D. galeata cuticle protein gene) showed a significant decrease. Correspondingly, the tail spines of the offspring produced by D. galeata after RNAi of the Dagcut gene appeared curved during the experiment. In whole-mount in situ hybridization, a clear signal site was detected on the tail spine of D. galeata before RNAi which disappeared after RNAi. Our results suggest that the Dagcut gene may play an important role in tail spine formation of D. galeata, and will provide a theoretical basis for studying the molecular mechanisms of the morphological plasticity in cladocera in the future.

Thermal stress-stimulated ZnO toxicity inhibits reproduction of freshwater crustacean Daphnia magna

Elevated temperatures due to climate change pose a variety of environmental risks to the freshwater ecosystem. At the same time, zinc oxide (ZnO) has become widely used and has entered the freshwater environment. As thermal stress may potentially impact the physicochemical properties of ZnO, its toxicity to freshwater organisms in the face of global warming is poorly understood. The potential effects on reproductive performances, including oogenesis, are of particular concern. In this study, we investigate the reproductive performances and related mRNA abundance of the zooplankton Daphnia magna under conditions of ZnO exposure and heat stress. The results revealed that ZnO and elevated temperature delayed maturity and juvenile production of D. magna. Histological observations indicated that oogenesis was inhibited, and the number and size of oocytes were reduced in the condition of ZnO exposure under heat stress. Eventual offspring in the same treatment exhibited decreased numbers, size, and quality. Congenital juvenile anomalies were increased, such as deformed eye, and impaired antenna and tail spine. Moreover, both ZnO and elevated temperature treatments inhibited expression levels of reproduction-related genes (vtg, EcR and VMO1) and induced the dmrt93b gene involved in the production of male offspring. Furthermore, we found that D. magna tried to cope with ZnO and thermal stress by upregulating hsp90, HIF-1α and HIF-1β. ZnO and heat stress inhibited the reproductive capacity of D. magna, produced deleterious effects on reproduction-associated physiological pathways, and damaged reproductive outcomes.

Mono(2-ethylhexyl) phthalate modulates lipid accumulation and reproductive signaling in Daphnia magna

Mono(2-ethylhexyl) phthalate (MEHP) is a primary metabolite of di(2-ethylhexyl) phthalate (DEHP), which is widely used in industry as a plasticizer. Both DEHP and MEHP have been identified as endocrine disruptors affecting reproduction systems in natural aquatic environments. However, the effects of MEHP exposure on aquatic invertebrates such as Daphnia magna are still poorly understood. In the present study, lipid alterations caused by MEHP in D. magna were identified by analyzing lipid accumulation and nontarget metabolomics. In addition, reproductive endpoints were investigated. MEHP exposure under any conditions upto 2 mg/L was not associated with mortality of D. magna; yet, the number of lipid droplets and the adult female daphnids reproduction rates increased after 96 h of exposure and 21 days of exposure, respectively. MEHP also enhanced lipid metabolism, as evident from 283 potential lipid metabolites, including glycerolipids, glycerophospholipids, and sphingolipids, identified following 48 h of exposure. The MEHP-treated group exhibited significantly higher ecdysone receptor (EcR) and vitellogenin 2 (Vtg2) expression levels at 6 and 24 h. At 48 h, EcR and Vtg2 expression levels were downregulated in the 1 and 2 mg/L MEHP exposure groups. Our data reveal that the EcR pathway changes over MEHP exposure could be associated with lipid accumulation, owing to increased lipid levels and the subsequent increase in the reproduction of MEHP-exposed D. magna.

Juvenile hormone synthesis and signaling disruption triggering male offspring induction and population decline in cladocerans (water flea): Review and adverse outcome pathway development

Juvenile hormone (JH) are a family of multifunctional hormones regulating larval development, molting, metamorphosis, reproduction, and phenotypic plasticity in arthropods. Based on its importance in arthropod life histories, many insect growth regulators (IGRs) mimicking JH have been designed to control harmful insects in agriculture and aquaculture. These JH analogs (JHAs) may also pose hazards to nontarget species by causing unexpected endocrine-disrupting (ED) effects such as molting and metamorphosis defects, larval lethality, and disruption of the sexual identity. This critical review summarizes the current knowledge of the JH-mediated effects in the freshwater cladoceran crustaceans such as Daphnia species on JHA-triggered endocrine disruptive outputs to establish a systematic understanding of JHA effects. Based on the current knowledge, adverse outcome pathways (AOPs) addressing the JHA-mediated ED effects in cladoceran leading to male offspring production and subsequent population decline were developed. The weight of evidence (WoE) of AOPs was assessed according to established guidelines. The review and AOP development aim to present the current scientific understanding of the JH pathway and provide a robust reference for the development of tiered testing strategies and new risk assessment approaches for JHAs in future ecotoxicological research and regulatory processes.

Long-term exposure of Daphnia magna to polystyrene microplastic (PS-MP) leads to alterations of the proteome, morphology and life-history

In the past years, the research focus on the effects of MP on aquatic organisms extended from marine systems towards freshwater systems. An important freshwater model organism in the MP field is the cladoceran Daphnia, which plays a central role in lacustrine ecosystems and has been established as a test organism in ecotoxicology. To investigate the effects of MP on Daphnia magna, we performed a chronic exposure experiment with polystyrene MP under strictly standardized conditions. Chronic exposure of D. magna to PS microparticles led to a significant reduction in body length and number of offspring. To shed light on underlying molecular mechanisms induced by microplastic ingestion in D. magna, we assessed the effects of PS-MP at the proteomic level, as proteins, e.g., enzymes, are especially relevant for an organism's physiology. Using a state-of-the-art mass spectrometry based approach, we were able to identify 28,696 different peptides, which could be assigned to 3784 different proteins. Using a customized bioinformatic workflow, we identified 41 proteins significantly altered in abundance (q-value <0.05) in the PS exposed D. magna. Among the proteins increased in the PS treated group were several sulfotransferases, involved in basic biochemical pathways, as well as GABA transaminase catalyzing the degradation of the neurotransmitter GABA. In the abundance decreased group, we found essential proteins such as the DNA-directed RNA polymerase subunit and other proteins connected to biotic and inorganic stress and reproduction. Strikingly, we further identified several digestive enzymes that are significantly downregulated in the PS treated animals, which could have interfered with the affected animal's nutrient supply. This may explain the altered morphological and life history traits of the PS exposed daphnids. Our results indicate that long-term exposure to PS microplastics, which are frequently detected in environmental samples, may affect the fitness of daphnids.

The thermal regime modifies the response of aquatic keystone species Daphnia to microplastics: Evidence from population fitness, accumulation, histopathological analysis and candidate gene expression

The water bodies are greatly influenced by chemical contamination and global increasing temperature. As an emerging pollutant, microplastics are widely distributed in the freshwater environment, raising concerns regarding their potential toxicity to organisms. Especially for zooplankton filter feeders, many of microplastics are in similar size as their food. Individually, both microplastics and temperature have profound effects on zooplankton populations and their function in ecosystems. However, the strength and direction of their interactive effects are still not clear. Here, we performed a comprehensive biotoxicity assessment providing empirical evidence that the temperature played a key role in shaping the sensitivity of the zooplankter, Daphnia magna, against microplastic toxicity. We found that exposure to microplastics generally caused negative effects on Daphnia individual fitness, such as increased lethality, declined fecundity and reduced population growth rate. This microplastic toxicity was more prominent at 30 °C than at 20 °C, and was rather minor at 15 °C. Moreover, the warming accelerated the ingestion of microplastics, and triggered abnormal ultrastructure of intestinal epithelial cells. In addition, the expression profiling of candidate genes revealed oxidative damage, fecundity impairment and energy retardation by microplastics were amplified with increasing temperature, which may contribute to the enhancement of microplastic toxicity under warming. Given that high temperature fluctuations are becoming more common and difficult to predict, the interactive effects of microplastics and climate warming on Daphnia population dynamics and biomass production may become increasingly aggravated in nature. Collectively, extrapolation for environmental risk assessment studies conducted under different temperature contexts may broaden our knowledge microplastic toxicity on aquatic organism fitness.

Effects of butyl benzyl phthalate exposure on Daphnia magna growth, reproduction, embryonic development and transcriptomic responses

Butyl benzyl phthalate (BBP) is widely used as a plasticizer to increase the plasticity and flexibility of plastic products. Although the potential health hazards of BBP have recently received extensive attention, its toxicological properties and mechanisms remain largely undefined. In the present work, growth, reproductive and developmental toxicity of BBP to Daphnia magna were evaluated, and the transcriptomic alteration of early embryos upon BBP exposure was analyzed. In a 21-day chronic toxicity test, reduced survival ratio, decreased body length, increased abnormal ratio, advanced time to first brood, and reduced offspring of D. magna were observed. BBP exposure inhibited expression of the vitellogenin gene. In addition, embryotoxicity of BBP was observed, which showed not only in the induction of abnormal neonates, but also in the shortened embryonic development cycle. RNA-Seq of early embryo treated with 0.1 mg/L BBP indicated that the pathways involved in signal transduction, cell communication, and embryonic development were significantly down-regulated, while those of biosynthesis, metabolism, cell homeostasis, redox homeostasis were remarkably up-regulated upon BBP exposure, which was consistent with the above phenotypic results. Taken together, our results highlight the toxic effects of BBP on the embryonic development and larval growth of D. magna.

Reproduction stage specific dysregulation of Daphnia magna metabolites as an early indicator of reproductive endocrine disruption

Rapid biomolecular observation in model indicator organisms has been considered as a potential predictor of water pollution from chronic and trace toxicants. This study evaluated the use of Daphnia magna metabolomic measurements as indicators for exposure to reproductive endocrine disruptors by using the model juvenile hormone analogue fenoxycarb. Because D. magna reproduction controls metabolic regulation, the reproduction stage was also carefully considered in metabolic observations and data analysis to examine differences. Comparisons of metabolite abundance regulation between 1 and 12 days of fenoxycarb exposure were performed to investigate the predictability of the sub-chronic (12 days) adverse impacts on reproduction and metabolic regulation based on acute (1 day) metabolic observations. ANOVA-simultaneous component analysis (ASCA) detected reversed patterns in direction of time-course metabolite abundance regulation with fenoxycarb exposure. For example, decreases in the abundances of leucine, asparagine, methionine, and isoleucine which then changed to increases were observed with time during fenoxycarb exposures. The reversed regulation pattern was observed at the last reproduction stage (stage 3), exclusively. Pearson correlation analysis showed that correlations of pairwise metabolites were disrupted with fenoxycarb exposure. Similar to ASCA, data normalization based on the reproduction stage improved the detectability of significant correlations. The disruption on ambient metabolite regulation patterns and pairwise metabolite correlations was consistently observed with both 1 and 12 days of fenoxycarb exposures for sets of select metabolites. The observed regulatory disruptions to these specific metabolites suggest altered oogenesis as the affected metabolites and the specific reproduction stage are related to successful oogenesis. This study demonstrates that D. magna metabolic dysregulation is a predictor of water contamination by endocrine disrupting compounds. The high predictability of sub-chronic (12 days) endocrine disruption was confirmed based on acute (1 day) metabolic observations. Furthermore, integration of the reproduction cycle information in D. magna metabolomics was validated by observing a reproduction stage specific dysregulation in metabolite abundance regulation, which was not observable from the broader data analysis. Consequently, this study confirms the potential for establishing a quantitative relationship between water quality and indicator species metabolic observations. Additionally, it was found that constraining variables relevant to toxicity mechanisms of interest, such as the reproduction stage, is a key consideration for extraction of ecologically meaningful information in environmental metabolomics.

Calmodulin inhibition as a mode of action of antifungal imidazole pharmaceuticals in non-target organisms

To improve assessment of risks associated with pharmaceutical contamination of the environment, it is crucial to understand effects and mode of action of drugs in non-target species. The evidence is accumulating that species with well-conserved drug targets are prone to be at risk when exposed to pharmaceuticals. An interesting group of pharmaceuticals released into the environment is imidazoles, antifungal agents with inhibition of ergosterol synthesis as a primary mode of action in fungi. However, imidazoles have also been identified as competitive antagonists of calmodulin (CaM), a calcium-binding protein with phylogenetically conserved structure and function. Therefore, imidazoles would act as CaM inhibitors in various organisms, including those with limited capacity to synthesize sterols, such as arthropods. We hypothesized that effects observed in crustaceans exposed to imidazoles are related to the CaM inhibition and CaM-dependent nitric oxide (NO) synthesis. To test this hypothesis, we measured (i) CaM levels and its gene expression, (ii) NO accumulation and (iii) gene expression of NO synthase (NOS1 and NOS2), in the cladoceran Daphnia magna exposed to miconazole, a model imidazole drug. Whereas significantly increased CaM gene expression and its cellular allocation were observed, supporting the hypothesized mode of action, no changes occurred in either NO synthase expression or NO levels in the exposed animals. These findings suggest that CaM inhibition by miconazole leads to protein overexpression that compensates for the loss in the protein activity, with no measurable downstream effects on NO pathways. The inhibition of CaM in D. magna may have implications for effect assessment of exposure to mixtures of imidazoles in aquatic non-target species.

The effects of 1-hexyl-3-methylimidazolium bromide on embryonic development and reproduction in Daphnia magna

Ionic liquids (ILs) are acknowledged as green chemicals and favorable substitutes for volatile organic solvents, which are currently used. However, previous studies have shown that these compounds had toxicological impacts on aquatic organisms. To investigate the effects of 1-hexyl-3- methylimidazolium bromide ionic liquid ([C₆mim]Br) on embryonic development and reproduction in water flea (Daphnia magna), a series of exposure experiments were conducted, including acute toxicity, maternal exposure, and chronic exposure tests. In acute toxicity experiment, D. magna neonates exhibited developmental abnormalities in the shell spine and the second antennae in a concentration-dependent manner after exposure to [C₆mim]Br. The results in maternal exposure test also revealed a certain embryo-toxicity in response to [C₆mim]Br in D. magna. However, the toxicity was lower than that conveyed by direct acute exposure, this indicated that the IL could act directly on organism. During the 21 days chronic exposure, the 1.6 mg/L exposure caused marked drop in the survival, molts and the number of the first brood of D. magna. Meanwhile, the total number of offspring was significantly declined in 1.6 mg/L concentration treatment groups, whereas increased in 0.2 mg/L groups. Generally, abnormalities in the offspring were significantly increased across all of the treatment groups in contrast to the control. No effect on sex differentiation was found during the experiments. These findings suggested that [C₆mim]Br could affect embryonic development and reproduction in D. magna, and provided references for further study on the mechanisms underlying toxicological effects of ILs and the assessment of their potential environmental risks.

Detoxification and reproductive system-related gene expression following exposure to Cu(OH)2 nanopesticide in water flea (Daphnia magna Straus 1820)

The extensive use of copper-based nanopesticides in agriculture has led to their release into the aquatic environment and causes a potential risk to aquatic biota. However, there is a lack of knowledge regarding the possible toxic effect of these nanopesticides on non-target aquatic organisms including invertebrates. Therefore, in this study, effects of commonly used copper-based nanopesticide "Kocide 3000" on gene expression related to detoxification (cyp360a8, gst, P-gp, and hr96) and reproductive system (cut, cyp314, dmrt93, and vtg) in Daphnia magna was investigated through an acute toxicity test. In general, exposure to the nanopesticide caused significant down-regulation of detoxification genes after 24 h and then significant up-regulation after 48 h. Exposure to the nanopesticide, however, significantly induced cut expression after 24 h. Moreover, dmrt93 and vtg genes were up-regulated after 48 h exposure to the nanopesticide. On the other hand, the expression of dmrt93 and vtg down-regulated at high concentration of Cu(OH)₂ nanopesticide (1.5 ppm) after 96 h. The results of this study provide first evidence into the crucial role of genes related to detoxification and reproductive system in response to Cu(OH)₂ nanopesticide. The use of physiological, biochemical bioassays, as well as gene expression, can help explain the toxic effect of copper-based nanopesticides and provide more insight into the exact mechanism of toxicity in non-target aquatic organisms.

Effects of two juvenile hormone analogue insecticides, fenoxycarb and methoprene, on Neocaridina davidi

Juvenile hormone analogue (JHA) insecticides are endocrine disrupters that interfere with hormonal action in insects by mimicking their juvenile hormones (JH). As the structure and functions of methyl farnesoate in crustaceans are similar to those of JH in insects, exogenous JHA insecticides could have adverse effects on the development and reproduction of crustaceans. This study examined the toxic effects of two JHA insecticides, fenoxycarb and methoprene, on a freshwater shrimp model of cherry shrimp, Neocaridina davidi. Both insecticides had detrimental effects on cherry shrimp, but fenoxycarb was more toxic than methoprene. Chronic exposure to these insecticides reduced the shrimp's body length and molting frequency. Based on transcriptome annotations for N. davidi, we identified important gene homologues that were active in both insect JH biosynthetic and degradative pathways as well as JH and ecdysteroid signaling pathways. Chronic treatments with JHAs had significant effects on these genes in N. davidi. Our transcriptomic analysis showed that genes involved in the pathways related to cuticle development, serine protease activity, and carbohydrate, peptide and lipid metabolic processes were differentially expressed in shrimp exposed to JHAs. These results demonstrate the toxicity of fenoxycarb and methoprene to freshwater crustaceans and indicate the need to monitor the use of JHA insecticides.

Phylogenetic analysis and embryonic expression of panarthropod Dmrt genes

Background

One set of the developmentally important Doublesex and Male-abnormal-3 Related Transcription factors (Dmrt) is subject of intense research, because of their role in sex-determination and sexual differentiation. This likely non-monophyletic group of Dmrt genes is represented by the Drosophila melanogaster gene Doublesex (Dsx), the Caenorhabditis elegans Male-abnormal-3 (Mab-3) gene, and vertebrate Dmrt1 genes. However, other members of the Dmrt family are much less well studied, and in arthropods, including the model organism Drosophila melanogaster, data on these genes are virtually absent with respect to their embryonic expression and function.

Results

Here we investigate the complete set of Dmrt genes in members of all main groups of Arthropoda and a member of Onychophora, extending our data to Panarthropoda as a whole. We confirm the presence of at least four families of Dmrt genes (including Dsx-like genes) in Panarthropoda and study their expression profiles during embryogenesis. Our work shows that the expression patterns of Dmrt11E, Dmrt93B, and Dmrt99B orthologs are highly conserved among panarthropods. Embryonic expression of Dsx-like genes, however, is more derived, likely as a result of neo-functionalization after duplication.

Conclusions

Our data suggest deep homology of most of the panarthropod Dmrt genes with respect to their function that likely dates back to their last common ancestor. The function of Dsx and Dsx-like genes which are critical for sexual differentiation in animals, however, appears to be much less conserved.

Electronic supplementary material

The online version of this article (10.1186/s12983-019-0322-0) contains supplementary material, which is available to authorized users.

Age-dependent survival, stress defense, and AMPK in Daphnia pulex after short-term exposure to a polystyrene nanoplastic

The widespread occurrence and accumulation of micro- and nanoplastics in aquatic environments has become a growing global concern. Generally, natural aquatic populations are characterized by a variety of multi-structured age groups, for which physiological and biochemical responses typically differ. The freshwater cladoceran, Daphnia pulex, is a model species used extensively in environmental monitoring studies and ecotoxicology testing. Here, the effects of a polystyrene nanoplastic on the physiological changes (i.e., survival) and expression levels of stress defense genes (i.e., those encoding antioxidant-mediated and heat shock proteins) in this freshwater flea were measured. Results from acute bioassays were used to determine the respective nanoplastic LC50 values for five age groups (1-, 4-, 7-, 14- and 21-day-old individuals): the obtained values for the 1- and 21-day-old D. pulex groups were similar (i.e., not significantly different). The expression levels of genes encoding key stress defense enzymes and proteins-SOD, CAT, GST, GPx, HSP70, and HSP90-were influenced by the nanoplastic in all the age groups, but not in the same way for each. Significant differences were observed among all age groups in their expression of the gene encoding the energy-sensing enzyme AMPK (adenosine monophosphate-activated protein kinase) α, β, and γ following exposure to the nanoplastic. Moreover, the expression of AMPK α was significantly increased in the 1-, 7-, and 21-day-old individuals exposed to nanoplastic relative to the control group. Together, these results indicate that age in D. pulex affects the sensitivity of its individuals to pollution from this nanoplastic, primarily via alterations to vital physiological and biochemical processes, such as cellular energy homeostasis and oxidation, which were demonstrated in vivo. We speculate that such age-related effects may extend to other nanoplastics and forms of pollution in D. pulex and perhaps similar marine organisms.

An in situ toxicity identification and evaluation water analysis system: Laboratory validation

It is difficult to assess the toxicity of a single stressor and establish a strong stressor-causality link when multiple stressors coexist. Toxicity identification evaluation (TIE) methodology uses a series of chemical and physical manipulations to fractionate compounds within a matrix and systematically identify potential toxicants. The current US Environmental Protection Agency application of TIE can provide valuable information but often lacks ecological realism and is subject to laboratory-related artifacts. An in situ TIE device (iTIED) was designed to assess the sources of toxicity in aquatic ecosystems. For this laboratory validation, each unit was equipped with a sorbent resin chamber, an organism exposure chamber, a water collection container, and a peristaltic pump. Chemical analyses of water processed by each iTIED unit were compared with both lethal and sublethal molecular responses of the organisms. The compound removal effectiveness of different sorbent resins was also compared. In addition to successfully fractionating diverse chemical mixtures, the iTIED demonstrated a potential for early detection of molecular biomarkers, which could identify chronic toxicity that may go unnoticed in traditional TIE assays. Utilizing this novel in situ system will reduce the uncertainty associated with laboratory-based simulations and aid management efforts in targeting compounds that pose the greatest threat. Environ Toxicol Chem 2017;36:1636-1643. © 2016 SETAC.

Transcriptomic, cellular and life-history responses of Daphnia magna chronically exposed to benzotriazoles: Endocrine-disrupting potential and molting effects

Benzotriazoles (BZTs) are ubiquitous aquatic contaminants used in a wide range of industrial and domestic applications from aircraft deicers to dishwasher tablets. Acute toxicity has been reported in aquatic organisms for some of the BZTs but their mode of action remains unknown. The objectives of this study were to evaluate the transcriptomic response of D. magna exposed to sublethal doses of 1H-benzotriazole (BTR), 5-methyl-1H-benzotriazole (5MeBTR) and 5-chloro-1H-benzotriazole (5ClBTR) using RNA-sequencing and quantitative real-time PCR. Cellular and life-history endpoints (survival, number of neonates, growth) were also investigated. Significant effects on the molting frequency were observed after 21-d exposure to 5MeBTR and 5ClBTR. No effects on molting frequency were observed for BTR but RNA-seq results indicated that this BZT induced the up-regulation of genes coding for cuticular proteins, which could have compensated the molting disruption. Molting in cladocerans is actively controlled by ecdysteroid hormones. Complementary short-term temporal analysis (4- and 8-d exposure) of the transcription of genes related to molting and hormone-mediated processes indicated that the three compounds had specific modes of action. BTR induced the transcription of genes involved in 20-hydroxyecdysone synthesis, which suggests pro-ecdysteroid properties. 5ClBTR exposure induced protein activity and transcriptional levels of chitinase enzymes, associated with an impact on ecdysteroid signaling pathways, which could explain the decrease in molt frequency. Finally, 5MeBTR seemed to increase molt frequency through epigenetic processes. Overall, results suggested that molting effects observed at the physiological level could be linked to endocrine regulation impacts of BZTs at the molecular level.

Endocrine-disruption potential of perfluoroethylcyclohexane sulfonate (PFECHS) in chronically exposed Daphnia magna

Perfluoroethylcyclohexane sulfonate (PFECHS), mainly used in hydraulic fluids in aircrafts, is a member of the perfluoroalkyl sulfonate family which includes the regulated perfluorooctane sulfonate (PFOS). PFECHS has been reported in environmental samples but its toxicity to aquatic organisms is unknown. The objectives of this study were to identify biological pathways altered by sublethal exposure (12 d) of D. magna to PFECHS (0.06, 0.6, and 6 mg/L) using microarray and quantitative real-time PCR and to identify potential biomarkers to link transcriptomic to phenotypic responses. PFECHS was also quantified in surface water samples (1.04-1.38 ng/L) collected from the St. Lawrence River, Canada. Transcriptomic analyses indicated the under-regulation of vitellogenin-related genes (VTG1) in PFECHS-exposed groups. PFECHS exposure also led to the up-regulation of genes related to cuticle. VTG was selected as a potential cellular marker and identified in D. magna using an immuno-specific assay and quantified using Western blot and LC/MS/MS. Results indicated a decrease of VTG content in exposed D. magna which was in concordance with the transcription of VTG-related genes. No effects were observed on survival, molting, or reproduction at the individual/population levels. Overall, results suggest endocrine disruption potential for PFECHS in D. magna at concentrations higher than levels reported in the aquatic environment.

Transcriptomic alterations in Daphnia magna embryos from mothers exposed to hypoxia

Hypoxia occurs when dissolved oxygen (DO) falls below 2.8mgL(-1) in aquatic environments. It can cause trans-generational effects not only in fish, but also in the water fleas Daphnia. In this study, transcriptome sequencing analysis was employed to identify transcriptomic alterations induced by hypoxia in embryos of Daphnia magna, with an aim to investigate the mechanism underlying the trans-generational effects caused by hypoxia in Daphnia. The embryos (F1) were collected from adults (F0) that were previously exposed to hypoxia (or normoxia) for their whole life. De novo transcriptome assembly identified 18270 transcripts that were matched to the UniProtKB/Swiss-Prot database and resulted in 7419 genes. Comparative transcriptome analysis showed 124 differentially expressed genes, including 70 up- and 54 down-regulated genes under hypoxia. Gene ontology analysis further highlighted three clusters of genes which revealed acclimatory changes of haemoglobin, suppression in vitellogenin gene family and histone modifications. Specifically, the expressions of histone H2B, H3, H4 and histone deacetylase 4 (HDAC4) were deregulated. This study suggested that trans-generational effects of hypoxia on Daphnia may be mediated through epigenetic regulations of histone modifications.

Transcriptional profiling of predator-induced phenotypic plasticity in Daphnia pulex

Background

Predator-induced defences are a prominent example of phenotypic plasticity found from single-celled organisms to vertebrates. The water flea Daphnia pulex is a very convenient ecological genomic model for studying predator-induced defences as it exhibits substantial morphological changes under predation risk. Most importantly, however, genetically identical clones can be transcriptionally profiled under both control and predation risk conditions and be compared due to the availability of the sequenced reference genome. Earlier gene expression analyses of candidate genes as well as a tiled genomic microarray expression experiment have provided insights into some genes involved in predator-induced phenotypic plasticity. Here we performed the first RNA-Seq analysis to identify genes that were differentially expressed in defended vs. undefended D. pulex specimens in order to explore the genetic mechanisms underlying predator-induced defences at a qualitatively novel level.

Results

We report 230 differentially expressed genes (158 up- and 72 down-regulated) identified in at least two of three different assembly approaches. Several of the differentially regulated genes belong to families of paralogous genes. The most prominent classes amongst the up-regulated genes include cuticle genes, zinc-metalloproteinases and vitellogenin genes. Furthermore, several genes from this group code for proteins recruited in chromatin-reorganization or regulation of the cell cycle (cyclins). Down-regulated gene classes include C-type lectins, proteins involved in lipogenesis, and other families, some of which encode proteins with no known molecular function.

Conclusions

The RNA-Seq transcriptome data presented in this study provide important insights into gene regulatory patterns underlying predator-induced defences. In particular, we characterized different effector genes and gene families found to be regulated in Daphnia in response to the presence of an invertebrate predator. These effector genes are mostly in agreement with expectations based on observed phenotypic changes including morphological alterations, i.e., expression of proteins involved in formation of protective structures and in cuticle strengthening, as well as proteins required for resource re-allocation. Our findings identify key genetic pathways associated with anti-predator defences.

Electronic supplementary material

The online version of this article (doi:10.1186/s12983-015-0109-x) contains supplementary material, which is available to authorized users.

Synergistic interactions of biotic and abiotic environmental stressors on gene expression

Understanding the response of organisms to multiple stressors is critical for predicting if populations can adapt to rapid environmental change. Natural and anthropogenic stressors often interact, complicating general predictions. In this study, we examined the interactive and cumulative effects of two common environmental stressors, lowered calcium concentration, an anthropogenic stressor, and predator presence, a natural stressor, on the water flea Daphnia pulex. We analyzed expression changes of five genes involved in calcium homeostasis — cuticle proteins (Cutie, Icp2), calbindin (Calb), and calcium pump and channel (Serca and Ip3R) — using real-time quantitative PCR (RT-qPCR) in a full factorial experiment. We observed strong synergistic interactions between low calcium concentration and predator presence. While the Ip3R gene was not affected by the stressors, the other four genes were affected in their transcriptional levels by the combination of the stressors. Transcriptional patterns of genes that code for cuticle proteins (Cutie and Icp2) and a sarcoplasmic calcium pump (Serca) only responded to the combination of stressors, changing their relative expression levels in a synergistic response, while a calcium-binding protein (Calb) responded to low calcium stress and the combination of both stressors. The expression pattern of these genes (Cutie, Icp2, and Serca) were nonlinear, yet they were dose dependent across the calcium gradient. Multiple stressors can have complex, often unexpected effects on ecosystems. This study demonstrates that the dominant interaction for the set of tested genes appears to be synergism. We argue that gene expression patterns can be used to understand and predict the type of interaction expected when organisms are exposed simultaneously to natural and anthropogenic stressors.

Are pharmaceuticals with evolutionary conserved molecular drug targets more potent to cause toxic effects in non-target organisms?

The ubiquitous use of pharmaceuticals has resulted in a continuous discharge into wastewater and pharmaceuticals and their metabolites are found in the environment. Due to their design towards specific drug targets, pharmaceuticals may be therapeutically active already at low environmental concentrations. Several human drug targets are evolutionary conserved in aquatic organisms, raising concerns about effects of these pharmaceuticals in non-target organisms. In this study, we hypothesized that the toxicity of a pharmaceutical towards a non-target invertebrate depends on the presence of the human drug target orthologs in this species. This was tested by assessing toxicity of pharmaceuticals with (miconazole and promethazine) and without (levonorgestrel) identified drug target orthologs in the cladoceran Daphnia magna. The toxicity was evaluated using general toxicity endpoints at individual (immobility, reproduction and development), biochemical (RNA and DNA content) and molecular (gene expression) levels. The results provide evidence for higher toxicity of miconazole and promethazine, i.e. the drugs with identified drug target orthologs. At the individual level, miconazole had the lowest effect concentrations for immobility and reproduction (0.3 and 0.022 mg L-1, respectively) followed by promethazine (1.6 and 0.18 mg L-1, respectively). At the biochemical level, individual RNA content was affected by miconazole and promethazine already at 0.0023 and 0.059 mg L-1, respectively. At the molecular level, gene expression for cuticle protein was significantly suppressed by exposure to both miconazole and promethazine; moreover, daphnids exposed to miconazole had significantly lower vitellogenin expression. Levonorgestrel did not have any effects on any endpoints in the concentrations tested. These results highlight the importance of considering drug target conservation in environmental risk assessments of pharmaceuticals.