Identification and expression analysis of a doublesex1 gene in Daphnia pulex during different reproductive stages

Evolution of sex determination in crustaceans

Sex determination (SD) involves mechanisms that determine whether an individual will develop into a male, female, or in rare cases, hermaphrodite. Crustaceans harbor extremely diverse SD systems, including hermaphroditism, environmental sex determination (ESD), genetic sex determination (GSD), and cytoplasmic sex determination (e.g., Wolbachia controlled SD systems). Such diversity lays the groundwork for researching the evolution of SD in crustaceans, i.e., transitions among different SD systems. However, most previous research has focused on understanding the mechanism of SD within a single lineage or species, overlooking the transition across different SD systems. To help bridge this gap, we summarize the understanding of SD in various clades of crustaceans, and discuss how different SD systems might evolve from one another. Furthermore, we review the genetic basis for transitions between different SD systems (i.e., Dmrt genes) and propose the microcrustacean Daphnia (clade Branchiopoda) as a model to study the transition from ESD to GSD.

Sublethal effects of DBE-DBCH diastereomers on physiology, behavior, and gene expression of Daphnia magna

1,2-dibromo-4-(1,2-dibromoethyl)-cyclohexane (DBE-DBCH) is a brominated flame retardant used in commercial and industrial applications. The use of DBE-DBCH containing products has resulted in an increased release into the environment. However, limited information is available on the long-term effects of DBE-DBCH and its effects in aquatic invertebrates. Thus, the present study was aimed at determining how DBE-DBCH diastereomers (αβ and γδ) affects aquatic invertebrates using Daphnia magna as a model organism. Survival, reproduction, feeding, swimming behavior and toxicogenomic responses to environmental relevant concentrations of DBE-DBCH were analyzed. Chronic exposure to DBE-DBCH resulted in decreased lifespan, and reduced fecundity. Expression of genes involved in reproductive processes, vtg1 and jhe, were also inhibited. DBE-DBCH also induced hypoxia by inhibiting the transcription of genes involved in heme biosynthesis and oxygen transport. Furthermore, DBE-DBCH also inhibited feeding resulting in emptiness of the alimentary canal. Increased expression of the stress response biomarkers was observed following DBE-DBCH exposure. In addition, DBE-DBCH diastereomers also altered the swimming behavior of Daphnia magna. The present study demonstrates that DBE-DBCH cause multiple deleterious effects on Daphnia magna, including effects on reproduction and hormonal systems. These endocrine disrupting effects are in agreement with effects observed on vertebrates. Furthermore, as is the case in vertebrates, DBE-DBCH γδ exerted stronger effects than DBE-DBCH αβ on Daphnia magna. This indicate that DBE-DBCH γδ has properties making it more toxic to all so far studied animals than DBE-DBCH αβ.

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.

Linking multiple biomarker responses in Daphnia magna under thermal stress

Temperature is an important abiotic variable that greatly influences the performance of aquatic ectotherms, especially under current anthropogenic global warming and thermal discharges. The aim of the present study was to evaluate thermal stress (20 °C vs 28 °C) in Daphnia magna over 21 d, focusing on the linkage among molecular and biochemical biomarker responses. Thermal stress significantly increased the levels of reactive oxygen species (ROS) and lipid peroxidation, especially in the 3-d short-term exposure treatment. This change in the ROS level was also correlated with mitochondrial membrane damage. These findings suggest that oxidative stress is the major pathway for thermally-induced toxicity of D. magna. Additionally, the expression levels of genes related to hypoxia (Hb), development (Vtg1), and sex determination (Dsx1-α, Dsx1-β, and Dsx2) were greatly increased by elevated temperature in a time-dependent manner. The cellular energy allocation was markedly decreased at the elevated temperature in the 3-d exposure treatment, mainly due to carbohydrates consumption for survival (oxidative stress defense). The present study showed that linking multiples biomarker responses are crucial for understanding the underlying mechanism of thermal stress on D. magna.

Mutation of doublesex in Hyphantria cunea results in sex-specific sterility

BACKGROUND

The gene doublesex (dsx) plays pivotal roles in sex determination and controls sexually dimorphic development in certain insects. Importantly, it also displays a potential candidate target for pest management due to its sex-specific splicing. Therefore, we used CRISPR/Cas9-mediated gene disruption to investigate the function of dsx in Hyphantria cunea, an invasive forest pest.

RESULT

In the present study, we identified the dsx gene from H. cunea which showed a sex-biased expression pattern that was different from other lepidopteran insects. Referring to sex-specific functional analyses in Bombyx mori, we performed a site-specific knockout of the Hcdsx gene by using a CRISPR/Cas9 system, which induced severe abnormalities in external genitalia and some incomplete sex reversal phenotypes, which in turn led to reduced sex-specific fecundity. An alternative splicing pattern of Hcdsx was altered by CRISPR/Cas9-induced mutation, and alterations in splicing affected expression of downstream genes encoding pheromone binding protein 1, vg1 and vg2 (encoding vitellogenin), which contributed to the sex-specific sterility phenotypes in the Hcdsx mutants.

CONCLUSION

The Hcdsx gene plays important roles in sexual differentiation in H. cunea. Disruption of Hcdsx induced sex-specific sterility, demonstrating a potential application in control of this pest. © 2019 Society of Chemical Industry.

Transcriptional response provides insights into the effect of chronic polystyrene nanoplastic exposure on Daphnia pulex

Nanoplastic pollution is widespread and persistent across global water systems and can cause a negative effect on aquatic organisms, especially the zooplankter which is the keystone of the food chain. The present study uses RNA sequencing to assess the global change in gene expression caused by 21 days of exposure to 75 nm polystyrene (PS) nanoplastics on Daphnia pulex, a model organism for ecotoxicity. With the threshold value at P value < 0.05 and fold change >2, 244 differentially expressed genes were obtained. Combined with real-time PCR validation of several selected genes, our results indicated that a distinct expression profile of key genes, including downregulated trehalose transporter, trehalose 6-phosphate synthase/phosphatase, chitinase and cathepsin-L as well as upregulated doublesex 1 and doublesex and mab-3 related transcription factor-like protein, contributed to the toxic effects of chronic nanoplastic exposure on Daphnia, such as slowed growth, subdued reproductive ability and reproductive pattern shifting. Our study also showed that chronic exposure to nanoplastic changed the sex ratio of D. pulex neonates. By integrating the gene expression pattern in an important model organism, this study gained insight into the molecular mechanisms of the toxic effect of chronic PS nanoplastic exposure on D. pulex, which may also extend to other nanoplastics or aquatic animals.

Evolution of Gene Expression during a Transition from Environmental to Genetic Sex Determination

Genetic sex determination (GSD) can evolve from environmental sex determination (ESD) via an intermediate state in which both coexist in the same population. Such mixed populations are found in the crustacean Daphnia magna, where non-male producers (NMP, genetically determined females) coexist with male producers (MP), in which male production is environmentally inducible and can also artificially be triggered by exposure to juvenile hormone. This makes Daphnia magna a rare model species for the study of evolutionary transitions from ESD to GSD. Although the chromosomal location of the NMP-determining mutation has been mapped, the actual genes and pathways involved in the evolution of GSD from ESD remain unknown. Here, we present a transcriptomic analysis of MP and NMP females under control (female producing) and under hormone exposure conditions. We found ∼100 differentially expressed genes between MP and NMP under control conditions. Genes in the NMP-determining chromosome region were especially likely to show such constitutive expression differences. Hormone exposure led to expression changes of an additional ∼100 (MP) to ∼600 (NMP) genes, with an almost systematic upregulation of those genes in NMP. These observations suggest that the NMP phenotype is not determined by a simple "loss-of-function" mutation. Rather, homeostasis of female offspring production under hormone exposure appears to be an active state, tightly regulated by complex mechanisms involving many genes. In a broader view, this illustrates that the evolution of GSD, while potentially initiated by a single mutation, likely leads to secondary integration involving many genes and pathways.