Non-chemical stresses do not strongly induce male offspring in Daphnia magna ascertained using the short-term juvenile hormone activity screening assay

Juvenile hormone (JH), together with ecdysone, regulates molting, metamorphosis, growth, and reproduction in arthropods. The effects of its analogs used as insecticides on nontarget species are of concern. Since JH and JH analogs (JHAs) induce male offspring in daphnids, which generally reproduce by parthenogenesis, short-term JH activity screening assay (JHASA) using the male offspring ratio as an endpoint has been developed as a detection method for JHA. However, the production of male offspring is also induced by environmental stresses such as temperature, short-day length, overcrowding, and food limitation. Thus, it is vital to prevent non-chemical stresses from inducing male offspring during the test to detect chemicals with potential JH activity accurately. Therefore, we investigated the effects of temperature (low and high), hardness, high density with low feeding, and day length on male production utilizing JHASA. Male offspring were not strongly induced by any stresses in JHASA, although the male ratios of 4-12% were observed in the preculture under high density (≥70 daphnid/L) and constant darkness. The Clone A strain was relatively more sensitive to high density and day length compared with the strain from National Institute for Environmental Studies (NIES). The selection of strains that rarely produce males under non-chemical stresses and finding the culturing conditions for each strain appropriate for not-inducing male offspring are recommended to control and prevent male offspring induction during JHASA.

Risk assessment of bisphenol analogues towards mortality, heart rate and stress-mediated gene expression in cladocerans Moina micrura

Bisphenol A (BPA) is a well-known endocrine-disrupting compound that causes several toxic effects on human and aquatic organisms. The restriction of BPA in several applications has increased the substituted toxic chemicals such as bisphenol F (BPF) and bisphenol S (BPS). A native tropical freshwater cladoceran, Moina micrura, was used as a bioindicator to assess the adverse effects of bisphenol analogues at molecular, organ, individual and population levels. Bisphenol analogues significantly upregulated the expressions of stress-related genes, which are the haemoglobin and glutathione S-transferase genes, but the sex determination genes such as doublesex and juvenile hormone analogue genes were not significantly different. The results show that bisphenol analogues affect the heart rate and mortality rate of M. micrura. The 48-h lethal concentration (LC₅₀) values based on acute toxicity for BPA, BPF and BPS were 611.6 µg L^-1, 632.0 µg L^-1 and 819.1 µg L^-1, respectively. The order of toxicity based on the LC₅₀ and predictive non-effect concentration values were as follows: BPA > BPF > BPS. Furthermore, the incorporated method combining the responses throughout the organisation levels can comprehensively interpret the toxic effects of bisphenol analogues, thus providing further understanding of the toxicity mechanisms. Moreover, the output of this study produces a comprehensive ecotoxicity assessment, which provides insights for the legislators regarding exposure management and mitigation of bisphenol analogues in riverine ecosystems.

Male frequency in Caenorhabditis elegans increases in response to chronic irradiation

Outcrossing can be advantageous in a changing environment because it promotes the purge of deleterious mutations and increases the genetic diversity within a population, which may improve population persistence and evolutionary potential. Some species may, therefore, switch their reproductive mode from inbreeding to outcrossing when under environmental stress. This switch may have consequences on the demographic dynamics and evolutionary trajectory of populations. For example, it may directly influence the sex ratio of a population. However, much remains to be discovered about the mechanisms and evolutionary implications of sex ratio changes in a population in response to environmental stress. Populations of the androdioecious nematode Caenorhabditis elegans, are composed of selfing hermaphrodites and rare males. Here, we investigate the changes in the sex ratio of C. elegans populations exposed to radioactive pollution for 60 days or around 20 generations. We experimentally exposed populations to three levels of ionizing radiation (i.e., 0, 1.4, and 50 mGy.h^-1). We then performed reciprocal transplant experiments to evaluate genetic divergence between populations submitted to different treatments. Finally, we used a mathematical model to examine the evolutionary mechanisms that could be responsible for the change in sex ratio. Our results showed an increase in male frequency in irradiated populations, and this effect increased with the dose rate. The model showed that an increase in male fertilization success or a decrease in hermaphrodite self-fertilization could explain this increase in the frequency of males. Moreover, males persisted in populations after transplant back into the control conditions. These results suggested selection favoring outcrossing under irradiation conditions. This study shows that ionizing radiation can sustainably alter the reproductive strategy of a population, likely impacting its long-term evolutionary history. This study highlights the need to evaluate the impact of pollutants on the reproductive strategies of populations when assessing the ecological risks.

Effects of pollution on dormant-stage banks of cladocerans and rotifers in a large tropical reservoir

Here, we describe the abundance and composition of the dormant-stage banks of cladocerans and rotifers at two locations in a tropical reservoir (Furnas Reservoir, Minas Gerais, Brazil) that are subject to different anthropogenic impacts. We hypothesized that at the site that received sewage, the density of resting eggs would be higher, but the species richness would be lower. Sediment samples were collected monthly at the two sampling stations. We counted the number of dormant stages and performed hatching experiments in the laboratory. Combining both sampling locations, we found a total of 26 species, 16 cladocerans, and 10 rotifers. Our hypothesis was partially corroborated, since the abundance and richness of dormant stages were significantly higher in the location subjected to wastewater release. Environmental pollution resulting from wastewater release and the cultivation of tilapia in cages is the most likely factor contributing to the higher values of egg density at this location. In contrast, the presence of aquatic macrophytes (a possible result of nutrient enrichment) at the same site may have contributed to the increase in species richness. Finally, our study indicates that the wastewater release in the reservoir can affect the production of dormant stages by cladocerans and rotifers.

Speciation in Daphnia

The microcrustacean Daphnia is arguably one of the most studied zooplankton species, having a well understood ecology, life history, and a relatively well studied evolutionary history. Despite this wealth of knowledge, species boundaries within closely related species in this genus often remain elusive and the major evolutionary forces driving the diversity of daphniids remain controversial. This genus contains more than 80 species with multiple cryptic species complexes, with many closely related species able to hybridize. Here, we review speciation research in Daphnia within the framework of current speciation theory. We evaluate the role of geography, ecology, and biology in restricting gene flow and promoting diversification. Of the 253 speciation studies on Daphnia, the majority of studies examine geographic barriers (55%). While evidence shows that geographic barriers play a role in species divergence, ecological barriers are also probably prominent in Daphnia speciation. We assess the contribution of ecological and nonecological reproductive isolating barriers between closely related species of Daphnia and found that none of the reproductive isolating barriers are restricting gene flow completely. Research on reproductive isolating barriers has disproportionally focused on two species complexes, the Daphnia pulex and Daphnia longispina species complexes. Finally, we identify areas of research that remain relatively unexplored and discuss future research directions that build our understanding of speciation in daphniids.

A Crab Is Not a Fish: Unique Aspects of the Crustacean Endocrine System and Considerations for Endocrine Toxicology

Crustaceans-and arthropods in general-exhibit many unique aspects to their physiology. These include the requirement to moult (ecdysis) in order to grow and reproduce, the ability to change color, and multiple strategies for sexual differentiation. Accordingly, the endocrine regulation of these processes involves hormones, receptors, and enzymes that differ from those utilized by vertebrates and other non-arthropod invertebrates. As a result, environmental chemicals known to disrupt endocrine processes in vertebrates are often not endocrine disruptors in crustaceans; while, chemicals that disrupt endocrine processes in crustaceans are often not endocrine disruptors in vertebrates. In this review, we present an overview of the evolution of the endocrine system of crustaceans, highlight endocrine endpoints known to be a target of disruption by chemicals, and identify other components of endocrine signaling that may prove to be targets of disruption. This review highlights that crustaceans need to be evaluated for endocrine disruption with consideration of their unique endocrine system and not with consideration of the endocrine system of vertebrates.

Transgenerational reproductive effects of two serotonin reuptake inhibitors after acute exposure in Daphnia magna embryos

The release of pharmaceuticals and personal care products (PPCPs) into aquatic environments has been a major concern for the health of ecosystems. Transgenerational plasticity is a potential mechanism for organisms to respond to changing environmental conditions, including climate change and environmental contaminants. The purpose of the present study was to determine the long-term transgenerational effects of an abundant freshwater zooplankton, Daphnia magna, to acute embryonic exposures of serotonin re-uptake inhibitors (SSRI - fluoxetine and sertraline). Both SSRIs have been used extensively to treat depression and anxiety disorders for decades and persist in freshwater ecosystems at physiologically relevant concentrations. Our results revealed that even short (72 h) embryonic exposures of D. magna embryos had long lasting consequences on life history and expression of 5HT related genes in the unexposed generation (F3). Moreover, we identified direct effects of SSRIs on heart rate and swimming behavior in the first generation that carried over from embryonic exposure. We also found that SSRI exposure resulted in a transient increase of ephippia formation in the F1 and F2 . Our results suggest that SSRI exposure has transgenerational consequences to the unexposed generation and potentially beyond, even at low concentration (10-100× lower than what can be found in natural ecosystems) and as a result of embryonic exposure. Because of the short reproductive period of D. magna and their integral role in aquatic food webs, these findings have population-level implications and deserve further investigation.

Evaluation of transgenerational effects caused by metals as environmental pollutants in Daphnia magna

The present study aimed to evaluate the acute and chronic toxicity of environmentally relevant concentrations of metals (Mn, Al, Fe, and Pb) in Daphnia magna and the generational transposition of reproductive and morphological damages. The effective concentration for 10% of the organisms from each metal was obtained by the acute toxicity test (96 hours); then, another five concentrations lower than this one were defined for the chronic experimentation (21 days), in which the number of neonates generated by each individual was checked daily. At the end of the exposition, the lengths and number of morphological damages were recorded in each adult daphnid. During this, the molt generated on the 14th and 21st days were collected and cultivated for posterior evaluation of the same parameters. Alterations in the reproductive performance were observed in the organisms exposed to manganese and aluminum (4.0 and 0.5 mg L^-1, respectively). Organisms exposed to aluminum (0.05 mg L^-1) and iron (0.27 mg L^-1) showed a reduction in body length. It is also noteworthy that the molt of these adults and their respective offspring also presented reproductive alterations, especially the molt from the 14th day of lead exposure (0.02 mg L^-1) and the 21st day of manganese exposure (4.0 mg L^-1). Such effects allow us to conclude that environments polluted by metals can reduce the ability of the species to maintain themselves in the ecosystem. In addition, there is a need to increase the control and monitoring of metals, such as aluminum, which present risks even in low concentrations.

Two Doublesex1 mutants revealed a tunable gene network underlying intersexuality in Daphnia magna

In recent years, the binary definition of sex is being challenged by repetitive reports about individuals with ambiguous sexual identity from various animal groups. This has created an urge to decode the molecular mechanism underlying sexual development. However, sexual ambiguities are extremely uncommon in nature, limiting their experimental value. Here, we report the establishment of a genetically modified clone of Daphnia magna from which intersex daphniids can be readily generated. By mutating the conserved central sex determining factor Doublesex1, body-wide feminization of male daphniid could be achieved. Comparative transcriptomic analysis also revealed a genetic network correlated with Doublesex1 activity which may account for the establishment of sexual identity in D. magna. We found that Dsx1 repressed genes related to growth and promoted genes related to signaling. We infer that different intersex phenotypes are the results of fluctuation in activity of these Dsx1 downstream factors. Our results demonstrated that the D. magna genome is capable of expressing sex in a continuous array, supporting the idea that sex is actually a spectrum.

Involvement of glutamate and serotonin transmitter systems in male sex determination in Daphnia pulex

Environmental sex determination occurs in many organisms, however the means by which environmental stimuli are translated into endocrine messages remains poorly understood. The N-methyl-ᴅ-aspartate receptor (NMDAR) was evaluated as a candidate neural sensor of environmental signals linking environmental cues to endocrine responses using the crustacean Daphnia pulex. NMDAR agonists, modulators, and antagonists were evaluated for their ability to impact D. pulex male sex determination during early stages of reproductive maturity under conditions that simulated seasonal change. The antagonists MK-801 and desipramine significantly increased male sex determination. Both chemicals are also modulators of serotonergic and noradrenergic systems, thus, we evaluated several modulators of monoamine neurotransmission in an effort to discern which signaling pathways might contribute to male sex determination. Compounds that altered serotonergic signaling also stimulated male sex determination. The involvement of the glutamate and monoamine signaling in male sex determination was supported by the increase in mRNA levels of related receptors and transporters under conditions that stimulate male sex determination. Further, mRNA levels of components of the terminal endocrine pathway responsible for male sex determination were also elevated under stimulatory conditions. Overall, we provide evidence that glutamatergic and serotonergic systems function upstream of the endocrine regulation of male sex determination in early life stage daphnids.

A comprehensive epigenomic analysis of phenotypically distinguishable, genetically identical female and male Daphnia pulex

BACKGROUND

Daphnia species reproduce by cyclic parthenogenesis involving both sexual and asexual reproduction. The sex of the offspring is environmentally determined and mediated via endocrine signalling by the mother. Interestingly, male and female Daphnia can be genetically identical, yet display large differences in behaviour, morphology, lifespan and metabolic activity. Our goal was to integrate multiple omics datasets, including gene expression, splicing, histone modification and DNA methylation data generated from genetically identical female and male Daphnia pulex under controlled laboratory settings with the aim of achieving a better understanding of the underlying epigenetic factors that may contribute to the phenotypic differences observed between the two genders.

RESULTS

In this study we demonstrate that gene expression level is positively correlated with increased DNA methylation, and histone H3 trimethylation at lysine 4 (H3K4me3) at predicted promoter regions. Conversely, elevated histone H3 trimethylation at lysine 27 (H3K27me3), distributed across the entire transcript length, is negatively correlated with gene expression level. Interestingly, male Daphnia are dominated with epigenetic modifications that globally promote elevated gene expression, while female Daphnia are dominated with epigenetic modifications that reduce gene expression globally. For examples, CpG methylation (positively correlated with gene expression level) is significantly higher in almost all differentially methylated sites in male compared to female Daphnia. Furthermore, H3K4me3 modifications are higher in male compared to female Daphnia in more than 3/4 of the differentially regulated promoters. On the other hand, H3K27me3 is higher in female compared to male Daphnia in more than 5/6 of differentially modified sites. However, both sexes demonstrate roughly equal number of genes that are up-regulated in one gender compared to the other sex. Since, gene expression analyses typically assume that most genes are expressed at equal level among samples and different conditions, and thus cannot detect global changes affecting most genes.

CONCLUSIONS

The epigenetic differences between male and female in Daphnia pulex are vast and dominated by changes that promote elevated gene expression in male Daphnia. Furthermore, the differences observed in both gene expression changes and epigenetic modifications between the genders relate to pathways that are physiologically relevant to the observed phenotypic differences.

Different as night and day: Behavioural and life history responses to varied photoperiods in Daphnia magna

Nearly all animal species have utilized photoperiod to cue seasonal behaviours and life history traits. We investigated photoperiod responses in keystone species, Daphnia magna, to identify molecular processes underlying ecologically important behaviours and traits using functional transcriptomic analyses. Daphnia magna were photoperiod‐entrained immediately posthatch to a standard control photoperiod of 16 light/ 8 dark hours (16L:8D) relative to shorter (4L:20D, 8L:16D, 12L:12L) and longer (20L:4D) day length photoperiods. Short‐day photoperiods induced significantly increased light‐avoidance behaviours relative to controls. Correspondingly, significant differential transcript expression for genes involved in glutamate signalling was observed, a critical signalling pathway in arthropod light‐avoidance behaviour. Additionally, period circadian protein and proteins coding F‐box/LRR‐repeat domains were differentially expressed which are recognized to establish circadian rhythms in arthropods. Indicators of metabolic rate increased in short‐day photoperiods which corresponded with broadscale changes in transcriptional expression across system‐level energy metabolism pathways. The most striking observations included significantly decreased neonate production at the shortest day length photoperiod (4L:20D) and significantly increased male production across short‐day and equinox photoperiods (4L:20D, 8L:16D and 12L:12D). Transcriptional expression consistent with putative mechanisms of male production was observed including photoperiod‐dependent expression of transformer‐2 sex‐determining protein and small nuclear ribonucleoprotein particles (snRNPs) which control splice variant expression for genes like transformer. Finally, increased transcriptional expression of glutamate has also been shown to induce male production in Daphnia pulex via photoperiod‐sensitive mechanisms. Overall, photoperiod entrainment affected molecular pathways that underpin critical behavioural and life history traits in D. magna providing fundamental insights into biological responses to this primary environmental cue.