Molecular Characterization and Expression Analysis of P38 MAPK Gene and Protein in Aquatic Midge, Chironomus riparius (Diptera: Chironomidae), Exposed to Environmental Contaminants

Salinity-induced ionoregulatory changes in the gill proteome of the mayfly, Neocloeon triangulifer

Ecologists have observed declines in the biodiversity of sensitive freshwater organisms in response to increasing concentrations of major ions (salinization). Yet, how changing salinities physiologically challenge aquatic organisms, such as mayflies, remains remarkably understudied. Moreover, it is not well understood the degree to which species respond and acclimate to salinity changes. Our lab is developing the Baetid mayfly, N. triangulifer, as a model organism for physiological research. We have previously described acclimatory changes in both ion flux rates and altered mRNA transcript levels in response to chronic exposures to elevated major ion concentrations at the whole animal level. In the present study, we use shotgun proteomics to identify the specific proteins associated with apical ion transport and how their abundance changes in response to chronic salinity exposures in gills. Gills were isolated from the penultimate nymphal stage of N. triangulifer reared under control culture conditions, elevated NaCl (157 mg L^-1 Na), elevated CaCl₂ (121 mg L^-1 Ca), elevated Ca/MgSO₄ (735 mg L^-1 SO₄). These conditions mirrored those from previously published physiological work. We also acutely exposed nymphs to dilute (50% dilution of culture water with deionized water) to explore proteomic changes in the gills in response to dilute conditions. We report 710 unique peptide sequences among treatment groups, including important apical ion transporters such as Ca-ATPase, Na/K ATPase, and V-ATPase. Treatment with elevated NaCl and Ca/MgSO₄ appeared to cause more significant differential protein expression (452 and 345, respectively) compared to CaCl₂ and dilute groups (134 and 17, respectively). Finally, we demonstrated the breadth of physiological functions in gills by exploring non-transport related pathways found in our dataset, including ATP synthesis, calcium signaling, and oxidative stress response. We discuss our results in the context of freshwater salinization and the challenges of working with non-model species without fully sequenced and annotated genomes.

A chromosome level genome assembly of Propsilocerus akamusi to understand its response to heavy metal exposure

Chironomidae species are universally used for studying the impact of pollutants in aquatic systems. The nonbiting midge Propsilocerus akamusi is often found in urban streams and is suitable for use as a toxicological bioindicator. However, few studies have previously examined metal stress in this species. We sequenced the genome of this urban midge to address this question. Here, we present the first chromosome-level genome of P. akamusi, obtained from Illumina short-read and PacBio long-read sequences with Hi-C technology. The size of the very small assembled genome was 85.83 Mb with a contig N50 of 6.2 Mb and a scaffold N50 of 26.1 Mb. This assembly revealed significant expansion of haemoglobin (Hb) genes, some of which formed large tandem repeats. Transcriptomic studies for copper tolerance identified four genes in the tandem array that were highly expressed, all of which presented intron loss. This characteristic might highlight the potential role of Hb genes in copper tolerance. Additionally, detoxification genes, chemosensory genes and heat shock protein genes of this midge were identified, some of which are associated with metal stress. The high-quality assembled genome of P. akamusi and the transcriptomic analyses provide new insight into the molecular mechanisms of heavy metal stress. Our comparison of the P. akamusi genome with other dipteran genomes provides valuable resources for understanding the evolutionary history, genetics, and ecology of this species as well as those of other midges.

Multi-Level Gene Expression in Response to Environmental Stress in Aquatic Invertebrate Chironomids: Potential Applications in Water Quality Monitoring

In freshwater ecosystems, aquatic invertebrates are influenced continuously by both physical stress and xenobiotics. Chironomids (Diptera; Chironomidae), or non-biting midges, are the most diverse and abundant invertebrates in freshwater habitats. They are a fundamental link in food chains of aquatic ecosystems. Chironomid larvae tolerate stress factors in their environments via various physiological processes. At the molecular level, environmental pollutants induce multi-level gene responses in Chironomus that regulate cellular protection through the activation of defense processes. This paper reviews literature on the transcriptional responses of biomarker genes to environmental stress in chironomids at the molecular level, in studies conducted from 1991 to 2020 (120 selected literatures of 374 results with the keywords "Chironomus and gene expression" by PubMed search tool). According to these studies, transcriptional responses in chironomids vary depending on the type of stress factor and defensive responses associated with antioxidant activity, the endocrine system, detoxification, homeostasis and stress response, energy metabolism, ribosomal machinery, apoptosis, DNA repair, and epigenetics. These data could provide a comprehensive overview of how Chironomus species respond to pollutants in aquatic environments. Furthermore, the transcriptomic data could facilitate the development of genetic tools for water quality and environmental monitoring based on resident chironomid species.

Effects of single and mixture exposure of cadmium and copper in apoptosis and immune related genes at transcriptional level on the midge Chironomus riparius Meigen (Diptera, Chironomidae)

Metals and heavy metals are natural contaminants with an increasing presence in aquatic ecosystems as a result of human activities. Although they are mixed in the water, research is usually focused on analyzing them in isolation, so there is a lack of knowledge about their combined effects. The aim of this work was to assess the damage produced by mixtures of cadmium and copper, two frequent metals used in industry, in the harlequin midge Chironomus riparius (Diptera). The effects of acute doses of cadmium and copper were evaluated in fourth instar larvae by analyzing the mRNA levels of six genes related to apoptosis (DRONC, IAP1), immune system (PO1, Defensin), stress (Gp93), and copper homeostasis (Ctr1). DRONC, Ctr1, and IAP1 transcripts are described here for first time in this species. Individual fourth instar larvae were submitted to 10 μM, 1 μM and 0.1 μM of CdCl₂ or CuCl_2, and mixture. The employed individuals came from different egg masses. Real-time PCR analysis showed a complex pattern of alterations in transcriptional activity for two genes, DRONC and Gp93, while the rest of them did not show any statistically significant differences. The effector caspase DRONC showed upregulation with the highest concentration tested of the mixture. In case of gp93, chaperone involved in regulation of immune response, differences in expression levels were found with 1 and 10 μM Cu and 0.1 and 10 μM of mixtures, compared to control samples. These results suggest that mixtures affect the transcriptional activity differently and produce changes in apoptosis and stress processes, although it is also possible that Gp93 alteration could be related to the immune system since it is homologous to human protein Gp96, which has been related with Toll-like receptors. In conclusion, cadmium and copper mixtures can affect the population by affecting the ability of larvae to respond to the infection and the apoptosis, an important process in the metamorphosis of insects.

Aberrant gene expression profiles in Mediterranean sea urchin reproductive tissues after metal exposures

Marine organisms are simultaneously exposed to numerous pollutants, among which metals probably represent the most abundant in marine environments. In order to evaluate the effects of metal exposure at molecular level in reproductive tissues, we profiled the sea urchin transcriptional response after non-lethal exposures using pathway-focused mRNA expression analyses. Herein, we show that exposures to relatively high concentrations of both essential and toxic metals hugely affected the gonadic expression of several genes involved in stress-response, detoxification, transcriptional and post-transcriptional regulation, without significant changes in gonadosomatic indices. Even though treatments did not result in reproductive tissues visible alterations, metal exposures negatively affected the main mechanisms of stress-response, detoxification and survival of adult P. lividus. Additionally, transcriptional changes observed in P. lividus gonads may cause altered gametogenesis and maintenance of heritable aberrant epigenetic effects. This study leads to the conclusion that exposures to metals, as usually occurs in polluted coastal areas, may affect sea urchin gametogenesis, thus supporting the hypothesis that parental exposure to environmental stressors affects the phenotype of the offspring.