Diverse expression regulation of Hsp70 genes in scallops after exposure to toxic Alexandrium dinoflagellates

Ecotoxicological risk of asphalt pavements to aquatic animals associated with pollutant leaching

Contaminants such as heavy metals and polycyclic aromatic hydrocarbons (PAHs) can be released from asphalt pavement and transported through stormwater runoff to nearby water bodies, leading to water pollution and potential harm to living aquatic animals. This study characterizes the heavy metal and PAH leaching from various asphalt paving materials and their potential ecotoxicological effects on zebrafish Danio rerio. Artificial runoffs were prepared in the laboratory concerning the effects of water, temperature, and traffic. The concentrations of heavy metals and PAHs in the leachates were quantified, while the toxicity assessment encompassed mortality, metal stress, PAH toxicity, inflammation, carcinogenicity, and oxidative damage. Gene expressions of related proteins or transcription factors were assessed, including metallothionines, aryl hydrocarbon receptors, interleukin-1β, interleukin-10, nuclear factor-κB, tumor necrosis factor-α, tumor suppressor p53, heat shock protein 70, and reactive oxygen species (ROS). The findings demonstrate that leachates from asphalt pavements containing waste bottom ash, crumb rubber, or specific chemicals could induce notable stress and inflammation responses in zebrafish. In addition, potential carcinogenic effects and the elevation of ROS were identified within certain treatment groups. This study represents the first attempt to assess the ecotoxicity of pavement leachates employing a live fish model, thereby improving the current understanding of the environmental impact of asphalt pavements.

Genome-wide identification of the HSP70 genes in Pacific oyster Magallana gigas and their response to heat stress

Heat shock protein 70 (HSP70), the most prominent and well-characterized stress protein in animals, plays an important role in assisting animals in responding to various adverse conditions. In the present study, a total of 113 HSP70 gene family members were identified in the updated genome of Magallana gigas (designated MgHSP70) (previously known as Crassostrea gigas). There were 75, 12, 11, and 8 HSP70s located in the cytoplasm, nucleus, mitochondria, and endoplasmic reticulum, respectively, and 7 HSP70s were located in both the nucleus and cytoplasm. Among 113 MgHSP70 genes, 107 were unevenly distributed in 8 chromosomes of M. gigas with the greatest number in chromosome 07 (61 genes, 57.01%). The MgHSP70 gene family members were mainly assigned into five clusters, among which the HSPa12 subfamily underwent lineage-specific expansion, consisting of 89 members. A total of 68 MgHSP70 genes (60.18%) were tandemly duplicated and formed 30 gene pairs, among which 14 gene pairs were under strong positive selection. In general, the expression of MgHSP70s was tissue-specific, with the highest expression in labial palp and gill and the lowest expression in adductor muscle and hemocytes. There were 35, 31, and 47 significantly upregulated genes at 6, 12, and 24 h after heat shock treatment (28 °C), respectively. The expression patterns of different tandemly duplicated genes exhibited distinct characteristics after shock treatment, indicating that these genes may have different functions. Nevertheless, genes within the same tandemly duplicated group exhibit similar expression patterns. Most of the tandemly duplicated HSP70 gene pairs showed the highest expression levels at 24 h. This study provides a comprehensive description of the MgHSP70 gene family in M. gigas and offers valuable insights into the functions of HSP70 in the mollusc adaptation of oysters to environmental stress.

Genome-Wide Identification and Transcriptome Analysis of the Hsp70 Gene Family in Monodonta labio Reveals Its Role in Response to Nanoplastics Stress

For marine invertebrates, the disruption of organismal physiology and behavior by nanoplastics (NPs) has been extensively reported. Heat shock proteins (Hsps) are important for redundant protein breakdown, environmental changes, and intracellular protein transport. An exhaustive identification of Hsp70 genes and an experiment where different concentrations of NPs were stressed were performed to study how Hsp70 genes respond to NPs stress in Monodonta labio. Our results identified 15 members of Hsp70 within the genome of M. labio and provided insights into their responses to different concentrations of acute NP stress. Phylogenetic analyses revealed extensive amplification of the Hsp70 genes from the Hsc70 subfamily, with gene duplication events. As a result of NP stress, five of fifteen genes showed significant upregulation or downregulation. Three Hsp70 genes were highly expressed at an NP concentration of 0.1 mg/L, and no genes were downregulated. At 10 mg/L, they showed significant upregulation of two genes and significant downregulation of two genes. At 1 mg/L treatment, three genes were significantly downregulated, and no genes were significantly upregulated. Moreover, a purifying selection was revealed using a selection test conducted on duplicate gene pairs, indicating functional redundancy. This work is the first thorough examination of the Hsp70s in Archaeogastropoda. The findings improve knowledge of Hsp70s in molluscan adaptation to NP stress and intertidal living and offer essential data for the biological study of M. labio.

Expansion of the HSP70 gene family in Tegillarca granosa and expression profiles in response to zinc toxicity

Zinc (Zn) is an essential micronutrient in organisms and an abundant element in the Earth's crust. Trace amounts of Zn released from natural sources can enter aquatic ecosystems through weathering and erosion. Zn accumulates in organisms, and when its intracellular concentration exceeds a certain level, it can induce oxidative stress and trigger oxidative stress-mediated heat shock protein (HSP) modulation. HSP70 is the most evolutionarily conserved among the HSP families. Despite extensive research on HSP70 genes in bivalves, the HSP70 gene family of Tegillarca granosa is still poorly characterized. We identified 65 HSP70 genes belonging to 6 families in the T. granosa genome, with 50 HSPa12 and 11 HSPa B2 genes highly expanded. On chromosome 11, 39 HSP70 (60%) genes were identified, and the HSPa12A genes were highly duplicated. A total of 527 and 538 differentially expressed genes were identified in the gills and mantle based on Zn exposure, respectively. The Gene Ontology of cellular anatomical entities was significantly enriched with upregulated differentially expressed genes in the gills and mantle. Eight of the 11 HSPa B2 genes were upregulated in both tissues. Most of the genes identified in both tissues were involved in "protein homeostasis" and "inhibition of apoptosis," which are associated with the HSP70 family's resistance to extrinsic and intrinsic stress. Hence, this study identified that the HSP70 gene family plays a vital role in the adaptation of aquatic organisms to heavy metal (e.g., Zn) stress in contaminated environments by compiling the different physiological responses to preserve homeostasis.

Effects of acute and chronic chromium stress on the expression of heat shock protein genes and activities of antioxidant enzymes in larvae of Orthetrum albistylum

The dragonfly species Orthetrum albistylum, can accumulate heavy metals from its aquatic environment and thus serves as a biological indicator for monitoring and evaluating water quality. Heat shock proteins (HSPs) play important biological roles in resistance to various types of environmental stress. The full-length cDNA sequences of the heat shock cognate (hsc) 70 and heat shock protein (hsp) 70 genes were cloned from O. albistylum larvae. Relative levels of expression of hsc70 and hsp70 in the head, epidermis, midgut, and adipose tissue were measured by qRT-PCR after chronic and acute contamination of 5-8 instar larvae with chromium (Cr) solution, and under control conditions. Activities of superoxide dismutase (SOD) and catalase (CAT) in chronically contaminated larvae were also measured. Phylogenetic analysis revealed that the cloned hsc70 and hsp70 genes were highly homologous to known HSP70 family members reported in other insects. The mRNA levels of hsc70 and hsp70 did not differ significantly in various larval tissues. Under chronic chromium stress, hsc70 and hsp70 expression were upregulated to a maximum and then downregulated; hsp70 mRNA levels were higher than those of hsc70 at all concentrations of chromium. Under acute chromium stress, hsc70 expression was inhibited at low chromium concentrations and upregulated at chromium concentrations higher than 125 mg/L. However, hsp70 expression was higher than that in the control group and markedly higher than that of hsc70. Changes in SOD and CAT activities displayed consistent trends for different chronic chromium concentrations, first increasing and then decreasing over time. Collectively, these findings demonstrated the response of the HSP family of genes and antioxidant enzymes following exposure to heavy metal stress, as well as their potential applicability as biomarkers for monitoring environmental pollutants.

Tissue-specific antioxidative response and metabolism of paralytic shellfish toxins in scallop (Chlamys farreri) mantle with Alexandrium dinoflagellate exposure

Bivalves show remarkable capacity to acclimate paralytic shellfish toxins (PSTs) produced by dinoflagellates, severely affecting fishery industry and public health. Here, transcriptomic response to PSTs-producing dinoflagellate (Alexandrium minutum) was investigated in Zhikong scallop (Chlamys farreri) mantle. The PSTs accumulated in C. farreri mantle continually increased during the 15 days exposure, with "oxidation-reduction" genes induced compared to the control group at the 1st and 15th day. Through gene co-expression network analysis, 16 PSTs-responsive modules were enriched with up- or down-regulated genes. The concentration of GTXs, major PSTs in A. minutum and accumulated in scallops, was correlated with the up-regulated magenta module, enriching peroxisome genes as the potential mantle-specific PSTs biomarker. Moreover, Hsp70B2s were inhibited throughout the exposure, which together with the expanded neurotransmitter transporter SLC6As, may play essential roles on neurotransmitter homeostasis in scallop mantle. These results paved the way for a comprehensive understanding of defensive mechanism and homeostatic response in scallop mantle against PSTs.

Transcriptome analysis of digestive diverticula of Hong Kong oyster (Crassostrea hongkongesis) infected with Vibrio harveyi

The Hong Kong oyster (Crassostrea hongkongesis), as the main marine aquaculture shellfish in the South China Sea, not only has high economic and ecological value, but also is an ideal model for conducting research on pathogen host interaction. However, diseases caused by Vibrio pose a serious impediment to the culture of C. hongkongesis. In this study, we performed transcriptome analysis of digestive diverticula of C. hongkongesis infected with V. harveyi. A total of 977, 689, 912 high quality reads and 955, 208, 562 valid reads were obtained. At 12, 24, 48 and 72 h post-infection, 1402, 2168, 2727 and 1398 differentially expressed genes (DEGs) were captured, respectively. GO enrichment analysis showed that DEGs were significantly enriched in cellular processes, catalytic activity, cell part and other terms. KEGG enrichment analysis revealed that these DEGs were mainly closely related to Necroptosis, RIG-I-like receptor signaling pathway, NF-kappa B signaling pathway, Toll-like receptor signaling pathway and other pathways are related. The results of WGCNA analysis indicated that THBS1, CA10, Trpm2, THAP12, PTPRT, HSPA12A, and ADAM10 were the hub genes in the gene co-expression network. This study will provide new ideas at the transcriptome level for the immune regulatory mechanisms and adaptability of the C. hongkongesis to V. infection, as well as for achieving selective breeding for Vibrio resistance in the C. hongkongesis.

Closest horizons of Hsp70 engagement to manage neurodegeneration

Our review seeks to elucidate the current state-of-the-art in studies of 70-kilodalton-weighed heat shock proteins (Hsp70) in neurodegenerative diseases (NDs). The family has already been shown to play a crucial role in pathological aggregation for a wide spectrum of brain pathologies. However, a slender boundary between a big body of fundamental data and its implementation has only recently been crossed. Currently, we are witnessing an anticipated advancement in the domain with dozens of studies published every month. In this review, we briefly summarize scattered results regarding the role of Hsp70 in the most common NDs including Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS). We also bridge translational studies and clinical trials to portray the output for medical practice. Available options to regulate Hsp70 activity in NDs are outlined, too.

Cardiac performance and heart gene network provide dynamic responses of bay scallop Argopecten irradians irradians exposure to marine heatwaves

The increased frequency of marine heat waves (MHWs) caused by global climate change is predicted to threaten the survival of economic bivalves, therefore having severely adverse effects on local ecological communities and aquaculture production. However, the study of scallops facing MHWs is still scarce, particularly in the scallop Argopecten irradians irradians, which has a significant share of "blue foods" in northern China. In the present study, bay scallop heart was selected to detect its cardiac performance, oxidative impairment and dynamic molecular responses, accompanied by assessing survival variations of individuals in the simulated scenario of MWHs (32 °C) with different time points (0 h, 6 h, 12 h, 24 h, 3 d, 6 d and 10 d). Notably, cardiac indices heart rate (HR), heart amplitude (HA), rate-amplitude product (RAP) and antioxidant enzyme activities superoxide dismutase (SOD) and catalase (CAT) all peaked at 24 h but sharply dropped on 3 d, coinciding with mortality. Transcriptome analysis revealed that the heart actively defended against heat stress at the acute stage (<24 h) via energy supply, misfolded proteins correction and enhanced signal transduction, whereas regulation of the defense response and apoptotic process combined with twice transcription initiation were the dominant responses at the chronic stage (3-10 d). In particular, HSP70 (heat shock protein 70), HSP90 and CALR (calreticulin) in the endoplasmic reticulum were identified as the hub genes (top 5 %) in the HR-associated module via WGCNA (weighted gene co-expression network analysis) trait-module analysis, followed by characterization of their family members and diverse expression patterns under heat exposure. Furthermore, RNAi-mediated knockdown of CALR expression (after 24 h) significantly weakened the thermotolerance of scallops, as evidenced by a drop of 1.31 °C in ABT (Arrhenius break temperature) between the siRNA-injected group and the control group. Our findings elucidated the dynamic molecular responses at the transcriptome level and verified the cardiac functions of CALR in bay scallops confronted with stimulated MHWs.

Transcriptomic analysis reveals the genes involved in tetrodotoxin (TTX) accumulation, translocation, and detoxification in the pufferfish Takifugu rubripes

Tetrodotoxin (TTX) is a potent marine neurotoxin that exists in a variety of aquatic and terrestrial organisms. Pufferfish in different habitats show great variation in their TTX contents. Exploring the genes involved in TTX metabolism could contribute to our understanding of the molecular mechanisms underlying TTX accumulation, translocation, and detoxification in pufferfish. In this study, transcriptomic analysis was used to identify the functional genes related to TTX metabolism in the blood, liver, and muscle of the toxic and non-toxic tiger puffer (Takifugu rubripes). A total of 6101 differentially expressed genes (DEGs) were obtained after transcriptomic analysis; of these, 2401 were identified in the blood, 2262 in the liver, and 1438 in the muscle. After enrichment analysis, fourteen genes encoding glutathione S-transferases (GSTs), glutathione peroxidase (GPx), thioredoxins (TXNs), superoxide dismutase (SOD), ATP-binding cassettes (ABCs), apolipoproteins (APOs), inhibitors of apoptosis protein (IAP), and solute carrier (SLC), which are mainly antioxidant enzymes, membrane transporters, or anti-apoptotic factors, were revealed in the blood. Thirty-six genes encoding SLCs, ABCs, long-chain-fatty-acid-CoA ligases (ACSLs), interleukin 6 cytokine family signal transducer (IL6ST), endoplasmic reticulum (ER), and heat shock protein family A (Hsp70) were involved in transmembrane transporter activity and innate immune response. Notably, a large number of slc genes were found to play critical and diverse roles in TTX accumulation and translocation in the liver of T. rubripes. Nine genes from the slc, hsp70, complement C5 (c5), acsl, er, and serpin peptidase inhibitor (serpin) gene families were found to participate in the regulation of protein processing and anti-apoptosis. These results reflect the diverse functions of genes closely related to TTX accumulation, translocation, and detoxification in T. rubripes.

Phylogenetic annotation of Caenorhabditis elegans heat shock protein 70 genes

Annotation of the 70 kDa heat shock proteins (Hsp70s) has been chaotic especially in invertebrates. In this study, we validated an emerging nomenclature of Hsp70s, which can be potentially applied to all metazoan Hsp70s, by conducting a genome-wide annotation of Caenorhabditis elegans Hsp70s. Using the phylogenetic annotation, the seven canonical C. elegans Hsp70s were successfully classified into four known lineages, cytosolic A, cytosolic B, endoplasmic reticulum, and mitochondria. Motifs specific to each lineage were all conserved in the C. elegans Hsp70s. From these results, we propose new aliases of C. elegans Hsp70s that should help future annotation of this important molecular chaperone.

Expression Plasticity of Peroxisomal Acyl-Coenzyme A Oxidase Genes Implies Their Involvement in Redox Regulation in Scallops Exposed to PST-Producing Alexandrium

Filter-feeding bivalves can accumulate paralytic shellfish toxins (PST) produced by toxic microalgae, which may induce oxidative stress and lipid peroxidation. Peroxisomal acyl-coenzyme A oxidases (ACOXs) are key enzymes functioning in maintaining redox and lipid homeostasis, but their roles in PST response in bivalves are less understood. Herein, a total of six and six ACOXs were identified in the Chlamys farreri and Patinopecten yessoensis genome, respectively, and the expansion of ACOX1s was observed. Gene expression analysis revealed an organ/tissue-specific expression pattern in both scallops, with all ACOXs being predominantly expressed in the two most toxic organs, digestive glands and kidneys. The regulation patterns of scallop ACOXs after exposure to different PST-producing algaes Alexandrium catenella (ACDH) and A. minutum (AM-1) were revealed. After ACDH exposure, more differentially expressed genes (DEGs) were identified in C. farreri digestive glands (three) and kidneys (five) than that in P. yessoensis (two), but the up-regulated DEGs showed similar expression patterns in both species. In C. farreri, three DEGs were found in both digestive glands and kidneys after AM-1 exposure, with two same CfACOX1s being acutely and chronically induced, respectively. Notably, these two CfACOX1s also showed different expression patterns in kidneys between ACDH (acute response) and AM-1 (chronic response) exposure. Moreover, inductive expression of CfACOXs after AM-1 exposure was observed in gills and mantles, and all DEGs in both tissues were up-regulated and their common DEGs exhibited both acute and chronic induction. These results indicate the involvement of scallop ACOXs in PST response, and their plasticity expression patterns between scallop species, among tissues, and between the exposure of different PST analogs.

Transcriptome and Network Analyses Reveal the Gene Set Involved in PST Accumulation and Responses to Toxic Alexandrium minutum Exposure in the Gills of Chlamys farreri

Bivalve molluscs are filter-feeding organisms that can accumulate paralytic shellfish toxins (PST) through ingesting toxic marine dinoflagellates. While the effects of PST accumulation upon the physiology of bivalves have been documented, the underlying molecular mechanism remains poorly understood. In this study, transcriptomic analysis was performed in the gills of Zhikong scallop (Chlamys farreri) after 1, 3, 5, 10, and 15 day(s) exposure of PST-producing dinoflagellate Alexandrium minutum. Higher numbers of differentially expressed genes (DEGs) were detected at day 1 (1538) and day 15 (989) than that at day 3 (77), day 5 (82), and day 10 (80) after exposure, and most of the DEGs were only regulated at day 1 or day 15, highlighting different response mechanisms of scallop to PST-producing dinoflagellate at different stages of exposure. Functional enrichment results suggested that PST exposure induced the alterations of nervous system development processes and the activation of xenobiotic metabolism and substance transport processes at the acute and chronic stages of exposure, respectively, while the immune functions were inhibited by PST and might ultimately cause the activation of apoptosis. Furthermore, a weighted gene co-expression network was constructed, and ten responsive modules for toxic algae exposure were identified, among which the yellow module was found to be significantly correlated with PST content. Most of the hub genes in the yellow module were annotated as solute carriers (SLCs) with eight being OCTN1s, implying their dominant roles in regulating PST accumulation in scallop gills. Overall, our results reveal the gene set responding to and involved in PST accumulation in scallop gills, which will deepen our understanding of the molecular mechanism of bivalve resistance to PST.

Genome-Wide Analyses of Heat Shock Protein Superfamily Provide New Insights on Adaptation to Sulfide-Rich Environments in Urechis unicinctus (Annelida, Echiura)

The intertidal zone is a transitional area of the land-sea continuum, in which physical and chemical properties vary during the tidal cycle and highly toxic sulfides are rich in sediments due to the dynamic regimes. As a typical species thriving in this habitat, Urechis unicinctus presents strong sulfide tolerance and is expected to be a model species for sulfide stress research. Heat shock proteins (HSPs) consist of a large group of highly conserved molecular chaperones, which play important roles in stress responses. In this study, we systematically analyzed the composition and expression of HSPs in U. unicinctus. A total of eighty-six HSP genes from seven families were identified, in which two families, including sHSP and HSP70, showed moderate expansion, and this variation may be related to the benthic habitat of the intertidal zone. Furthermore, expression analysis revealed that almost all the HSP genes in U. unicinctus were significantly induced under sulfide stress, suggesting that they may be involved in sulfide stress response. Weighted gene co-expression network analysis (WGCNA) showed that 12 HSPs, including 5 sHSP and 4 HSP70 family genes, were highly correlated with the sulfide stress response which was distributed in steelblue and green modules. Our data indicate that HSPs, especially sHSP and HSP70 families, may play significant roles in response to sulfide stress in U. unicinctus. This systematic analysis provides valuable information for further understanding of the function of the HSP gene family for sulfide adaptation in U. unicinctus and contributes a better understanding of the species adaptation strategies of marine benthos in the intertidal zone.

Genomic and Transcriptomic Landscape and Evolutionary Dynamics of Heat Shock Proteins in Spotted Sea Bass (Lateolabrax maculatus) under Salinity Change and Alkalinity Stress

Simple Summary

Heat shock proteins (Hsps) are ubiquitous and conserved in almost all living organisms and are involved in a wide spectrum of cellular responses against diverse environmental stresses. However, our knowledge about the coordinated Hsp co-chaperon interaction is still limited, especially in aquatic animals facing dynamic water environments. In this study, we provided the systematic analysis of 95 Hsp genes (LmHsps) in spotted sea bass (Lateolabrax maculatus), an important aquaculture species in China, under salinity change and alkalinity stress through in silico analysis. The coordinated expression of LmHsps in response to salinity change and alkalinity stress in the gills was determined. Our results confirmed the diverse regulated expression of Hsps in L. maculatus, and that the responses to alkalinity stress may have arisen through the adaptive recruitment of LmHsp40-70-90 co-chaperons. Our results provide vital insights into the function and adaptation of aquatic animal Hsps in response to salinity-alkalinity stress.

Abstract

The heat shock protein (Hsp) superfamily has received accumulated attention because it is ubiquitous and conserved in almost all living organisms and is involved in a wide spectrum of cellular responses against diverse environmental stresses. However, our knowledge about the Hsp co-chaperon network is still limited in non-model organisms. In this study, we provided the systematic analysis of 95 Hsp genes (LmHsps) in the genome of spotted sea bass (Lateolabrax maculatus), an important aquaculture species in China that can widely adapt to diverse salinities from fresh to sea water, and moderately adapt to high alkaline water. Through in silico analysis using transcriptome and genome database, we determined the expression profiles of LmHsps in response to salinity change and alkalinity stress in L. maculatus gills. The results revealed that LmHsps were sensitive in response to alkalinity stress, and the LmHsp40-70-90 members were more actively regulated than other LmHsps and may also be coordinately interacted as co-chaperons. This was in accordance with the fact that members of LmHsp40, LmHsp70, and LmHsp90 evolved more rapidly in L. maculatus than other teleost lineages with positively selected sites detected in their functional domains. Our results revealed the diverse and cooperated regulation of LmHsps under alkaline stress, which may have arisen through the functional divergence and adaptive recruitment of the Hsp40-70-90 co-chaperons and will provide vital insights for the development of L. maculatus cultivation in alkaline water.

Single individual structural variant detection uncovers widespread hemizygosity in molluscs

The advent of complete genomic sequencing has opened a window into genomic phenomena obscured by fragmented assemblies. A good example of these is the existence of hemizygous regions of autosomal chromosomes, which can result in marked differences in gene content between individuals within species. While these hemizygous regions, and presence/absence variation of genes that can result, are well known in plants, firm evidence has only recently emerged for their existence in metazoans. Here, we use recently published, complete genomes from wild-caught molluscs to investigate the prevalence of hemizygosity across a well-known and ecologically important clade. We show that hemizygous regions are widespread in mollusc genomes, not clustered in individual chromosomes, and often contain genes linked to transposition, DNA repair and stress response. With targeted investigations of HSP70-12 and C1qDC, we also show how individual gene families are distributed within pan-genomes. This work suggests that extensive pan-genomes are widespread across the conchiferan Mollusca, and represent useful tools for genomic evolution, allowing the maintenance of additional genetic diversity within the population. As genomic sequencing and re-sequencing becomes more routine, the prevalence of hemizygosity, and its impact on selection and adaptation, are key targets for research across the tree of life. This article is part of the Theo Murphy meeting issue 'Molluscan genomics: broad insights and future directions for a neglected phylum'.

IAPs Gene Expansion in the Scallop Patinopecten yessoensis and Their Expression Profiles After Exposure to the Toxic Dinoflagellate

Inhibitors of apoptosis proteins (IAPs) are conserved regulators involved in cell cycle, cell migration, cell death, immunity and inflammation, should be due to the fact that they can assist with the ability to cope with different kinds of extrinsic or intrinsic stresses. Bivalve molluscs are well adapted to highly complex marine environments. As free-living filter feeders that may take toxic dinoflagellates as food, bivalves can accumulate and put up with significant levels of paralytic shellfish toxins (PSTs). PSTs absorption and accumulation could have a deleterious effect on bivalves, causing negative impact on their feeding and digestion capabilities. In the present study, we analyzed IAP genes (PyIAPs) in Yesso scallop (Patinopecten yessoensis), a major fishery and aquaculture species in China. Forty-seven PyIAPs from five sub-families were identified, and almost half of the PyIAP genes were localized in clusters on two chromosomes. Several sites under positive selection was revealed in the significantly expanded sub-families BIRC4 and BIRC5. After exposure to PST-producing dinoflagellates, Alexandrium catenella, fourteen PyIAPs showed significant responses in hepatopancreas and kidney, and more than eighty-five percent of them were from the expanded sub-families BIRC4 and BIRC5. The regulation pattern of PyIAPs was similar between the two tissues, with more than half exhibited expression suppression within three days after exposure. In contrast to hepatopancreas, more acute changes of PyIAPs expression could be detected in kidney, suggesting the possible involvement of these PyIAPs in tissue-specific PST tolerance. These findings also imply the adaptive expansion of bivalve IAP genes in response to algae derived biotoxins.

Effects of toxic dinoflagellate Alexandrium catenella on sexual maturation and reproductive output in the pacific oyster Crassostrea gigas

Marine bivalves which feed mainly on microalgae could accumulate toxins during harmful algal blooms, posing a health hazard to humans and other animals through food chains. The Pacific oyster (Crassostrea gigas) is the globally dominant farmed bivalves in the world. In order to benefit sustainable development of the oyster aquaculture, we assessed the effects of an artificial bloom of the toxin-producing dinoflagellate, Alexandrium catenella, upon sexual maturation in C. gigas. Oysters were exposed to A. catenella from April to June in 2012, and compared to a control batch of oysters fed with Isochrysis galbana. During the exposure, clearance, histological observations, biochemical composition as well as embryonic development were measured. The results indicated that A. catenella could be a food source, and inhibited the clearance rate of C. gigas for I. galbana. Significant pathological changes in the form of degeneration in adductor muscles, mantle, ovary and tubules and several inflammatory responses were observed in C. gigas when exposed to harmful microalga. The exposure of A. catenella had negative effects on assimilation, biochemical composition and so as the reproduction. The results of this study demonstrated that toxic microalga can affect "quality'' of eggs and the consequences, in terms of fertility, embryo and larval output.

Toxin- and species-dependent regulation of ATP-binding cassette (ABC) transporters in scallops after exposure to paralytic shellfish toxin-producing dinoflagellates

ATP-binding cassette (ABC) transporters are membrane-bound proteins involved in exporting various xenobiotic compounds from living cells. Bivalve mollusks can accumulate large amounts of paralytic shellfish toxins (PSTs) from marine dinoflagellates. For aquatic invertebrates, the importance of ABC proteins in multi-xenobiotic resistance has been demonstrated, however, the systematic identification of ABC transporters is very limited. In this study, 64 and 67 ABC genes containing all eight described subfamilies (A to H) were identified in Yesso scallop (Patinopecten yessoensis) and Zhikong scallop (Chlamys farreri), respectively, with massive gene expansion being observed in the ABCC and ABCG subfamilies. The kidney harbored more specifically expressed ABC genes than other organs/tissues, most of which belonged to ABCB, ABCC, and ABCG subfamilies. After feeding the scallops with PST-producing dinoflagellates, the expression of scallop ABC genes in the kidney was regulated in toxin- and species-dependent manners. In total, 20 and 24 ABC genes in Zhikong scallop (CfABCs) were induced after exposure to Alexandrium minutum and A. catenella, with the up-regulated members from both ABCC and ABCG subfamilies mainly showing acute and chronic induction by A. minutum and A. catenella, respectively, while the up-regulated CfABCBs mainly showing chronic induction by both dinoflagellates. In Yesso scallop, only eight ABC genes (PyABCs) were regulated after A. catenella exposure, and all the five up-regulated PyABCs were acutely induced. Our findings imply the functional diversity of scallop ABC genes in coping with PST accumulation, which may contribute to the lineage-specific adaptation of scallops for dealing with algal toxins challenge.

Tissue-Biased and Species-Specific Regulation of Glutathione Peroxidase (GPx) Genes in Scallops Exposed to Toxic Dinoflagellates

Marine bivalves could accumulate paralytic shellfish toxins (PSTs) produced by toxic microalgae, which might induce oxidative stress. Glutathione peroxidases (GPxs) are key enzymes functioning in the antioxidant defense, whereas our understanding of their roles in PST challenge in bivalves is limited. Herein, through genome-wide screening, we identified nine (CfGPx) and eight (PyGPx) GPx genes in Zhikong scallop (Chlamys farreri) and Yesso scallop (Patinopecten yessoensis), respectively, and revealed the expansion of GPx3 sub-family in both species. RNA-Seq analysis revealed high expression of scallop GPx3s after D stage larva during early development, and in adult hepatopancreas. However, in scallops exposed to PST-producing dinoflagellates, no GPx was significantly induced in the hepatopancreas. In scallop kidneys where PSTs were transformed to higher toxic analogs, most CfGPxs were up-regulated, with CfGPx3s being acutely and chronically induced by Alexandrium minutum and A. catenella exposure, respectively, but only one PyGPx from GPx3 subfamily was up-regulated by A. catenella exposure. Our results suggest the function of scallop GPxs in protecting kidneys against the oxidative stresses by PST accumulation or transformation. The tissue-, species-, and toxin-dependent expression pattern of scallop GPxs also implied their functional diversity in response to toxin exposure.

Transcriptomic responses of Artemia salina exposed to an environmentally relevant dose of Alexandrium minutum cells or Gonyautoxin2/3

Toxicities of the marine algae Alexandrium minutum and its excreted gonyautoxins (GTXs) to the marine crustacean Artemia salina were investigated. Mortality was observed for neither larvae nor adult A. salina exposed to A. minutum at a density of 5000 cells/mL or 0.5 μM GTX2/3. After exposure, the full transcriptome of adult A. salina was assembled and functionally annotated. A total of 599,286 transcripts were obtained, which were clustered into 515,196 unigenes. Results of the transcriptional effect level index revealed that direct exposure to the toxic algae A. minutum caused greater alterations in the transcriptome than did exposure to the extracellular product GTX2/3. Mechanisms of effects were different between exposure of A. salina to A. minutum cells or GTX2/3. Exposure to A. minutum modulated formation of the ribonucleoprotein complex and metabolism of amino acids and lipids in A. salina. Exposure to GTX2/3 exposure inhibited expression of genes related to metabolism of chitin, which might result in disruption of molting process or disturbed sheath morphogenesis. Overall, effects on transcription observed in this study represent the first report based on application of next generation sequencing techniques to investigate the transcriptomic response of A. salina exposed to an environmentally realistic level of A. minutum or GTX2/3.

Genome-Wide Identification and Characterization of SODs in Zhikong Scallop Reveals Gene Expansion and Regulation Divergence after Toxic Dinoflagellate Exposure

As filter-feeding animals mainly ingesting microalgae, bivalves could accumulate paralytic shellfish toxins (PSTs) produced by harmful algae through diet. To protect themselves from the toxic effects of PSTs, especially the concomitant oxidative damage, the production of superoxide dismutase (SOD), which is the only eukaryotic metalloenzyme capable of detoxifying superoxide, may assist with toxin tolerance in bivalves. To better understand this process, in the present study, we performed the first systematic analysis of SOD genes in bivalve Chlamys farreri, an important aquaculture species in China. A total of six Cu/Zn-SODs (SOD1-6) and two Mn-SODs (SOD7, SOD8) were identified in C. farreri, with gene expansion being revealed in Cu/Zn-SODs. In scallops exposed to two different PSTs-producing dinoflagellates, Alexandrium minutum and A. catenella, expression regulation of SOD genes was analyzed in the top ranked toxin-rich organs, the hepatopancreas and the kidney. In hepatopancreas, which mainly accumulates the incoming PSTs, all of the six Cu/Zn-SODs showed significant alterations after A. minutum exposure, with SOD1, 2, 3, 5, and 6 being up-regulated, and SOD4 being down-regulated, while no significant change was detected in Mn-SODs. After A. catenella exposure, up-regulation was observed in SOD2, 4, 6, and 8, and SOD7 was down-regulated. In the kidney, where PSTs transformation occurs, SOD4, 5, 6, and 8 were up-regulated, and SOD7 was down-regulated in response to A. minutum feeding. After A. catenella exposure, all the Cu/Zn-SODs except SOD1 were up-regulated, and SOD7 was down-regulated in kidney. Overall, in scallops after ingesting different toxic algae, SOD up-regulation mainly occurred in the expanded Cu/Zn-SOD group, and SOD6 was the only member being up-regulated in both toxic organs, which also showed the highest fold change among all the SODs, implying the importance of SOD6 in protecting scallops from the stress of PSTs. Our results suggest the diverse function of scallop SODs in response to the PST-producing algae challenge, and the expansion of Cu/Zn-SODs might be implicated in the adaptive evolution of scallops or bivalves with respect to antioxidant defense against the ingested toxic algae.