Identification and expression of immune genes in the flat oyster Ostrea edulis in response to bonamiosis

Understanding Crassostrea virginica tolerance of Perkinsus marinus through global gene expression analysis

Disease tolerance, a host's ability to limit damage from a given parasite burden, is quantified by the relationship between pathogen load and host survival or reproduction. Dermo disease, caused by the protozoan parasite P. marinus, negatively impacts survival in both wild and cultured eastern oyster (C. virginica) populations. Resistance to P. marinus has been the focus of previous studies, but tolerance also has important consequences for disease management in cultured and wild populations. In this study we measured dermo tolerance and evaluated global expression patterns of two sensitive and two tolerant eastern oyster families experimentally challenged with distinct doses of P. marinus (0, 106, 107, and 108 parasite spores per gram wet weight, n = 3-5 individuals per family per dose). Weighted Gene Correlation Network Analysis (WGCNA) identified several modules correlated with increasing parasite dose/infection intensity, as well as phenotype. Modules positively correlated with dose included transcripts and enriched GO terms related to hemocyte activation and cell cycle activity. Additionally, these modules included G-protein coupled receptor, toll-like receptor, and tumor necrosis factor pathways, which are important for immune effector molecule and apoptosis activation. Increased metabolic activity was also positively correlated with treatment. The module negatively correlated with infection intensity was enriched with GO terms associated with normal cellular activity and growth, indicating a trade-off with increased immune response. The module positively correlated with the tolerant phenotype was enriched for transcripts associated with "programmed cell death" and contained a large number of tripartite motif-containing proteins. Differential expression analysis was also performed on the 108 dosed group using the most sensitive family as the comparison reference. Results were consistent with the network analysis, but signals for "programmed cell death" and serine protease inhibitors were stronger in one tolerant family than the other, suggesting that there are multiple avenues for disease tolerance. These results provide new insight for defining dermo response traits and have important implications for applying selective breeding for disease management.

Shotgun analysis to identify differences in protein expression between granulocytes and hyalinocytes of the European flat oyster Ostrea edulis

Recovery of wild populations of the European flat oyster Ostrea edulis is important for ecosystem health and conservation of this species, because native oyster populations have dramatically declined or disappeared in most European waters. Diseases have contributed to oyster decline and are important constrains for oyster recovery. Understanding oyster immune system should contribute to design effective strategies to fight oyster diseases. Haemocytes play a pivotal role in mollusc immune responses protecting from infection. Two main types of haemocytes, granulocytes and hyalinocytes, are distinguished in O. edulis. A study aiming to explore differential functions between both haemocyte types and, thus, to enrich the knowledge of Ostrea edulis immune system, was performed by comparing the proteome of the two haemolymph cell types, using a shotgun approach through liquid chromatography (LC) coupled to mass spectrometry (MS). Cells from oyster haemolymph were differentially separated by Percoll density gradient centrifugation. Shotgun LC-MS/MS performance allowed the identification of 145 proteins in hyalinocytes and 138 in the proteome of granulocytes. After a comparative analysis, 55 proteins with main roles in defence were identified, from which 28 were representative of granulocytes and 27 of hyalinocytes, plus 11 proteins shared by both cell types. Different proteins involved in signal transduction, apoptosis, oxidative response, processes related with the cytoskeleton and structure, recognition and wound healing were identified as representatives of each haemocyte type. Important signalling pathways in the immune response such as MAPK, Ras and NF-κβ seemed to be more relevant for granulocytes, while the Wnt signalling pathway, particularly relevant for wound healing, more relevant in hyalinocytes. The differences in proteins involved in recognition and in cytoskeleton and structure suggest differential specialisation in processes of phagocytosis and internalisation of pathogens between haemocyte types. Apoptosis seemed more active in granulocytes. The differences in proteins involved in oxidative response also suggest different redox processes in each cell type.

What do the terms resistance, tolerance, and resilience mean in the case of Ostrea edulis infected by the haplosporidian parasite Bonamia ostreae

The decline of the European flat oyster Ostrea edulis represents a loss to European coastal economies both in terms of food security and by affecting the Good Environmental Status of the marine environment as set out by the European Council's Marine Strategy Framework Directive (2008/56/EC). Restoration of O. edulis habitat is being widely discussed across Europe, addressing key challenges such as the devastating impact of the haplosporidian parasite Bonamia ostreae. The use of resistant, tolerant, or resilient oysters as restoration broodstock has been proposed by restoration practitioners, but the definitions and implications of these superficially familiar terms have yet to be defined and agreed by all stakeholders. This opinion piece considers the challenges of differentiating Bonamia resistance, tolerance, and resilience; challenges which impede the adoption of robust definitions. We argue that, disease-resistance is reduced susceptibility to infection by the parasite, or active suppression of the parasites ability to multiply and proliferate. Disease-tolerance is the retention of fitness and an ability to neutralise the virulence of the parasite. Disease-resilience is the ability to recover from illness and, at population level, tolerance could be interpreted as resilience. We concede that further work is required to resolve practical uncertainty in applying these definitions, and argue for a collaboration of experts to achieve consensus. Failure to act now might result in the future dispersal of this disease into new locations and populations, because robust definitions are important components of regulatory mechanisms that underpin marine management.

Haplosporidian host:parasite interactions

The host:parasite interactions of the 3 serious haplosporidian pathogens of oysters, on which most information exists, are reviewed. They are Bonamia ostreae in Ostrea spp. and Crassostrea gigas; Bonamia exitiosa in Ostrea spp.; and Haplosporidium nelsoni in Crassostrea spp. Understanding the haemocytic response to pathogens is constrained by lack of information on haematopoiesis, haemocyte identity and development. Basal haplospridians in spot prawns are probably facultative parasites. H. nelsoni and a species infecting Haliotis iris in New Zealand (NZAP), which have large extracellular plasmodia that eject haplosporosomes or their contents, lyse surrounding cells and are essentially extracellular parasites. Bonamia spp. have small plasmodia that are phagocytosed, haplosporosomes are not ejected and they are intracellular obligate parasites. Phagocytosis by haemocytes is followed by formation of a parasitophorous vacuole, blocking of haemocyte lysosomal enzymes and the endolysosomal pathway. Reactive oxygen species (ROS) are blocked by antioxidants, and host cell apoptosis may occur. Unlike susceptible O. edulis, the destruction of B. ostreae by C. gigas may be due to higher haemolymph proteins, higher rates of granulocyte binding and phagocytosis, production of ROS, the presence of plasma β-glucosidase, antimicrobial peptides and higher levels of haemolymph and haemocyte enzymes. In B.exitiosa infection of Ostrea chilensis, cytoplasmic lipid bodies (LBs) containing lysosomal enzymes accumulate in host granulocytes and in B. exitiosa following phagocytosis. Their genesis and role in innate immunity and inflammation appears to be the same as in vertebrate granulocytes and macrophages, and other invertebrates. If so, they are probably the site of eicosanoid synthesis from arachidonic acid, and elevated numbers of LBs are probably indicative of haemocyte activation. It is probable that the molecular interaction, and role of LBs in the synthesis and storage of eicosanoids from arachidonic acid, is conserved in innate immunity in vertebrates and invertebrates. However, it seems likely that haplosporidians are more diverse than realized, and that there are many variations in host parasite interactions and life cycles.

Rapid transmission of Bonamia exitiosa by cohabitation causes mortality in Ostrea angasi

The haplosporidian Bonamia was first detected in Australian shellfish in 1991. Australian isolates in Ostrea angasi Sowerby, 1871 were identified as Bonamia exitiosa Hine, Cochennac and Berthe, 2001, which threatens development of an O. angasi aquaculture industry. European field data suggest that Bonamia ostreae Pichot, Comps, Tigé, Grizel and Rabouin, 1980 infections in Ostrea edulis Linnaeus, 1758 build slowly, but infection dynamics of B. exitiosa in O. angasi are unknown. We investigated B. exitiosa infection in O. angasi by cohabiting uninfected juvenile O. angasi with adults infected with B. exitiosa. Oysters were sampled at 10, 21 and 40 days after cohabitation, and B. exitiosa prevalence and intensity were assessed. Bonamia exitiosa rapidly infected and caused disease in O. angasi. Mortalities began at 12 days, with ˜50% mortality by day 21 and >85% mortality by day 40. Mortalities displayed pathology consistent with clinical B. exitiosa infection. Time to first infection is likely influenced by a combination of parasite infectivity, host exposure and host immune capacity. Host death is not required for transmission, but probably facilitates release of parasites from decaying tissue. Understanding B. exitiosa transmission informs design and interpretation of field studies and aids development of management strategies for oyster aquaculture.

Molecular evolution of the tnfr gene family and expression profiles in response to pathogens in lamprey(Lethenteron reissneri)

Tumor necrosis factor receptor superfamilies (TNFRSF) are one of essential cytokines and can trigger inflammation, apoptosis, participating lymphocyte homeostasis and tissue development in vertebrates. To gain insights into the evolution and characterization of tnfr genes in lamprey, a jawless vertebrate, we performed a genome-wide and transcriptome survey and identified 7 tnfr genes in the lamprey (Lethenteron reissneri) database. Based on the molecular phylogenetic analysis, 7 L-tnfr genes are divided into three different clusters, and multiple members of tnfr genes family have appeared in lamprey. Meanwhile, protein domains and motifs analysis reveals that TNFRSF are conserved and have typical cysteine-rich domains (CRDs). Synteny results indicates that the L-tnfr neighborhood genes have taken place great changes compared to jawed vertebrates. Real-time quantitative results demonstrate that tnfr gene family plays an important role in the immune defense. This study has a new understanding for origin and evolution of the tnfr gene family in different vertebrates.

Molecular cloning and characterization of a putative mitogen-activated protein kinase (Erk1/2) gene: Involvement in mantle immunity of Pinctada fucata

Extracellular signal-regulated kinases (ERKs) are conserved and related with protein-serine/threonine kinases that participate in the regulation of multiple biological processes, such as cell survival, cell differentiation, proliferation, metabolism, and inflammation. However, little is known about the roles of this kinase in the pearl oyster. In this study, we cloned and identified an ERK homolog from Pinctada fucata (PfErk). Furthermore, we have unraveled its expressional kinetics after lipopolysaccharide (LPS) and polyinosinic-epolycytidylic acid (poly I:C) immune challenge. Pferk harbored a 5' untranslated region (UTR) of 12 bp, a coding sequence of 1074 bp, and a 3' UTR of 882 bp. The putative peptide comprised a predicted molecular mass of 41.19 kDa, with a theoretical pI of 6.15. Sequence analysis showed that it possesses one STK catalytic domain and a conserved His-Arg-Asp (HRD) domain. In addition, a canonical Thr-Glu-Tyr (TEY) dual phosphorylation motif and an ATRW substrate binding site were also identified in the coding protein. Homology assessment of PfErk showed high similarity to Homo sapiens ERK. Phylogenetic analysis supported a close evolutionary relationship with molluscan orthologs. The expression patterns of Pferk were observed in seven different tissues of pearl oyster, with highest expression in the mantle and lowest expression in the digestive gland. Pferk mRNA expression levels were detected at developmental stages, with the highest expression in D-shaped larvae, followed by the 32-cell stage. The mRNA expression of Pferk was upregulated significantly in P. fucata mantle primary cells and mantle tissue after LPS and poly (I:C) treatment, and PfErk phosphorylation levels were activated by LPS and poly (I:C) challenges. Overall, our results suggested that PfErk may play important roles in pearl oyster innate immunity, and provided a new understanding of mantle immunity in the pearl oyster.

Identification of potential key genes associated with severe pneumonia using mRNA-seq

This study aimed to identify the potential key genes associated with severe pneumonia using mRNA-seq. Nine peripheral blood samples from patients with severe pneumonia alone (SP group, n=3) and severe pneumonia accompanied with chronic obstructive pulmonary disease (COPD; CSP group, n=3), as well as volunteers without pneumonia (control group, n=3) underwent mRNA-seq. Based on the sequencing data, differentially expressed genes (DEGs) were identified by Limma package. Following the pathway enrichment analysis of DEGs, the genes that were differentially expressed in the SP and CSP groups were selected for pathway enrichment analysis and coexpression analysis. In addition, potential genes related to pneumonia were identified based on the information in the Comparative Toxicogenomics Database. In total, 645 and 528 DEGs were identified in the SP and CSP groups, respectively, compared with the normal controls. Among these DEGs, 88 upregulated genes and 80 downregulated genes were common between the two groups. The functions of the common DEGs were similar to those of the DEGs in the SP group. In the coexpression network, the commonly downregulated genes (including ND1, ND3, ND4L, and ND6) and the commonly upregulated genes (including TSPY6P and CDY10P) exhibited a higher degree. In addition, 131 DEGs (including ND1, ND3, ND6, MIR449A and TAS2R43) were predicted to be potential pneumonia-related genes. In conclusion, the present study demonstrated that the common DEGs may be associated with the progression of severe pneumonia.

Proteomic profile of Ostrea edulis haemolymph in response to bonamiosis and identification of candidate proteins as resistance markers

European flat oyster Ostrea edulis populations have suffered extensive mortalities caused by bonamiosis. The protozoan parasite Bonamia ostreae is largely responsible for this disease in Europe, while its congener B. exitiosa has been detected more recently in various European countries. Both of these intracellular parasites are able to survive and proliferate within haemocytes, the main cellular effectors of the immune system in molluscs. Two-dimensional electrophoresis was used to compare the haemolymph protein profile between Bonamia spp.-infected and non-infected oysters within 3 different stocks, a Galician stock of oysters selected for resistance against bonamiosis, a non-selected Galician stock and a selected Irish stock. Thirty-four proteins with a presumably relevant role in the oyster-Bonamia spp. interaction were identified; they were involved in major metabolic pathways, such as energy production, respiratory chain, oxidative stress, signal transduction, transcription, translation, protein degradation and cell defence. Furthermore, the haemolymph proteomic profiles of the non-infected oysters of the 2 Galician stocks were compared. As a result, 7 proteins representative of the non-infected Galician oysters selected for resistance against bonamiosis were identified; these 7 proteins could be considered as candidate markers of resistance to bonamiosis, which should be further assessed.

Long-term affected flat oyster (Ostrea edulis) haemocytes show differential gene expression profiles from naïve oysters in response to Bonamia ostreae

European flat oyster (Ostrea edulis) production has suffered a severe decline due to bonamiosis. The responsible parasite enters in oyster haemocytes, causing an acute inflammatory response frequently leading to death. We used an immune-enriched oligo-microarray to understand the haemocyte response to Bonamia ostreae by comparing expression profiles between naïve (NS) and long-term affected (AS) populations along a time series (1 d, 30 d, 90 d). AS showed a much higher response just after challenge, which might be indicative of selection for resistance. No regulated genes were detected at 30 d in both populations while a notable reactivation was observed at 90 d, suggesting parasite latency during infection. Genes related to extracellular matrix and protease inhibitors, up-regulated in AS, and those related to histones, down-regulated in NS, might play an important role along the infection. Twenty-four candidate genes related to resistance should be further validated for selection programs aimed to control bonamiosis.

Gene expression analysis of Ruditapes philippinarum haemocytes after experimental Perkinsus olseni zoospore challenge and infection in the wild

The production of Manila clam (Ruditapes philippinarum) is seriously threatened by the protistan parasite Perkinsus olseni. We characterized and compared gene expression of Manila clam haemocytes in response to P. olseni in a time-course (10 h, 24 h, 8 d) controlled laboratory challenge (LC), representing the first step of infection, and in a more complex infection in the wild (WI), using a validated oligo-microarray containing 11,232 transcripts, mostly annotated. Several immune-genes involved in NIK/NF-kappaB signalling, Toll-like receptor signalling and apoptosis were activated at LC-10 h. However, down-regulation of genes encoding lysozyme, histones, cathepsins and heat shock proteins indicated signals of immunodepression, which persisted at LC-24 h, when only down-regulated genes were detected. A rebound of haemocyte activity occurred at LC-8 d as shown by up-regulation of genes involved in cytoskeleton organization and cell survival. The WI study showed a more complex picture, and several immune-relevant processes including cytoskeleton organization, cell survival, apoptosis, encapsulation, cell redox- and lipid-homeostasis were activated, illustrating the main mechanism of host response. Our results provide useful information, including potential biomarkers, to develop strategies for controlling Manila clam perkinsosis.

Recent findings on phenoloxidases in bivalves

The production of melanin is a complex process involving biochemical cascades, such as the pro-phenoloxidase (proPO) system, and enzymes, such as phenoloxidases (POs). Different studies have shown a strong correlation between the decrease in PO activities and the occurrence of diseases in bivalve invertebrates, leading to mortalities in the host. Results of these studies suggest that POs could play a fundamental role in defense mechanisms in bivalves. This article reviews the fundamental knowledge on the proPO system in bivalves and the methods used to assess PO activities. Finally, this is the first report on the major findings of laboratory and field studies that indicate that a type of PO in bivalves, the laccase enzyme, is inducible and involved in the 1) immune 2) antioxidant and 3) detoxification roles in bivalves, and might be an ecological potential biomarker of environmental stress.

Differential expression of novel metabolic and immunological biomarkers in oysters challenged with a virulent strain of OsHV-1

Early lifestages of the Pacific oyster (Crassostrea gigas) are highly susceptible to infection by OsHV-1 μVar, but little information exists regarding metabolic or pathophysiological responses of larval hosts. Using a metabolomics approach, we identified a range of metabolic and immunological responses in oyster larvae exposed to OsHV-1 μVar; some of which have not previously been reported in molluscs. Multivariate analyses of entire metabolite profiles were able to separate infected from non-infected larvae. Correlation analysis revealed the presence of major perturbations in the underlying biochemical networks and secondary pathway analysis of functionally-related metabolites identified a number of prospective pathways differentially regulated in virus-exposed larvae. These results provide new insights into the pathogenic mechanisms of OsHV-1 infection in oyster larvae, which may be applied to develop disease mitigation strategies and/or as new phenotypic information for selective breeding programmes aiming to enhance viral resistance.

Construction of an Ostrea edulis database from genomic and expressed sequence tags (ESTs) obtained from Bonamia ostreae infected haemocytes: Development of an immune-enriched oligo-microarray

The flat oyster, Ostrea edulis, is one of the main farmed oysters, not only in Europe but also in the United States and Canada. Bonamiosis due to the parasite Bonamia ostreae has been associated with high mortality episodes in this species. This parasite is an intracellular protozoan that infects haemocytes, the main cells involved in oyster defence. Due to the economical and ecological importance of flat oyster, genomic data are badly needed for genetic improvement of the species, but they are still very scarce. The objective of this study is to develop a sequence database, OedulisDB, with new genomic and transcriptomic resources, providing new data and convenient tools to improve our knowledge of the oyster's immune mechanisms. Transcriptomic and genomic sequences were obtained using 454 pyrosequencing and compiled into an O. edulis database, OedulisDB, consisting of two sets of 10,318 and 7159 unique sequences that represent the oyster's genome (WG) and de novo haemocyte transcriptome (HT), respectively. The flat oyster transcriptome was obtained from two strains (naïve and tolerant) challenged with B. ostreae, and from their corresponding non-challenged controls. Approximately 78.5% of 5619 HT unique sequences were successfully annotated by Blast search using public databases. A total of 984 sequences were identified as being related to immune response and several key immune genes were identified for the first time in flat oyster. Additionally, transcriptome information was used to design and validate the first oligo-microarray in flat oyster enriched with immune sequences from haemocytes. Our transcriptomic and genomic sequencing and subsequent annotation have largely increased the scarce resources available for this economically important species and have enabled us to develop an OedulisDB database and accompanying tools for gene expression analysis. This study represents the first attempt to characterize in depth the O. edulis haemocyte transcriptome in response to B. ostreae through massively sequencing and has aided to improve our knowledge of the immune mechanisms of flat oyster. The validated oligo-microarray and the establishment of a reference transcriptome will be useful for large-scale gene expression studies in this species.

Precambrian origins of the TNFR superfamily

The evolution of the tumor necrosis factor/tumor necrosis factor receptor superfamily (TNF/TNFR) is complicated and not well understood. To date, most TNFR studies have focused on vertebrate models leaving the role of TNFRs in invertebrates largely unexplored. The evolution of important cellular processes including stress response, apoptosis, development, and inflammation will be better understood by examining the TNF/TNFR superfamily in ancient invertebrate phyla. How widespread is this gene family within the evolutionary tree of life and is there evidence for similar function in invertebrates? A first step is to identify the presence or absence of these genes within basal metazoan taxa using the signature cysteine-rich domain (CRD) of the TNFR superfamily. In this perspective, we will start by examining what is currently known about the function of TNFRs in invertebrates. Then, we will assess the role of TNFRs in apoptosis and explore the origins of the domains found in TNFRs including the death domain (DD) and CRD. Finally, we will examine the phylogenetic relationship between TNFRs containing DDs identified to date. From these data, we propose a model for a Precambrian origin of TNFRs and their functional role in apoptosis.

High throughput sequencing of small RNAs transcriptomes in two Crassostrea oysters identifies microRNAs involved in osmotic stress response

Increasing evidence suggests that microRNAs post-transcriptionally regulate gene expression and are involved in responses to biotic and abiotic stress. However, the role of miRNAs involved in osmotic plasticity remains largely unknown in marine bivalves. In the present study, we performed low salinity challenge with two Crassostrea species (C. gigas and C. hongkongensis), and conducted high-throughput sequencing of four small RNA libraries constructed from the gill tissues. A total of 202 and 87 miRNAs were identified from C. gigas and C. hongkongensis, respectively. Six miRNAs in C. gigas and two in C. hongkongensis were differentially expressed in response to osmotic stress. The expression profiles of these eight miRNAs were validated by qRT-PCR. Based on GO enrichment and KEGG pathway analysis, genes associated with microtubule-based process and cellular component movement were enriched in both species. In addition, five miRNA-mRNA interaction pairs that showed opposite expression patterns were identified in the C. hongkongensis, Differential expression analysis identified the miRNAs that play important regulatory roles in response to low salinity stress, providing insights into molecular mechanisms that are essential for salinity tolerance in marine bivalves.

Transcriptome analysis of the pearl oyster (Pinctada fucata) hemocytes in response to Vibrio alginolyticus infection

The pearl oyster Pinctada fucata is cultured widely for production of marine pearls in China, while mass mortalities, likely related to pathogenic infections, have occurred frequently in juvenile, mother and operated oysters. To address this issue, understanding host defense mechanisms of P. fucata against pathogenic challenge is extremely important. In the present study, a comparative analysis of hemocyte transcriptomes of P. fucata before and after Vibrio alginolyticus infection was conducted using the Illumina/Hiseq-2000 RNA-Seq technology. A total of 56,345,139 clean reads were generated and then assembled into 74,007 unigenes with an average length of 680 bp and an N50 of 1197 bp. Unigenes were annotated by comparing against non-redundant protein sequence (nr), non-redundant nucleotide (nt), Swiss-Prot, Pfam, Gene Ontology database (GO), Clusters of Orthologous Groups (COG) and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases, and 29,615 unigenes (40.01%) were annotated in at least one database. There were 636 genes (518 up-regulated and 118 down-regulated) that were significantly differentially expressed after bacterial challenge, and among which 369 were associated with 122 pathways, including classical immune-related pathways, such as 'MAPK signaling pathway', 'Chemokine signaling pathway', 'Apoptosis' and 'Wnt signaling pathway'. These findings provide information on the pearl oyster innate immunity and may contribute to developing strategies for management of diseases and long-term sustainability of P. fucata culture.

Immune and stress responses in oysters with insights on adaptation

Oysters are representative bivalve molluscs that are widely distributed in world oceans. As successful colonizers of estuaries and intertidal zones, oysters are remarkably resilient against harsh environmental conditions including wide fluctuations in temperature and salinity as well as prolonged air exposure. Oysters have no adaptive immunity but can thrive in microbe-rich estuaries as filter-feeders. These unique adaptations make oysters interesting models to study the evolution of host-defense systems. Recent advances in genomic studies including sequencing of the oyster genome have provided insights into oyster's immune and stress responses underlying their amazing resilience. Studies show that the oyster genomes are highly polymorphic and complex, which may be key to their resilience. The oyster genome has a large gene repertoire that is enriched for immune and stress response genes. Thousands of genes are involved in oyster's immune and stress responses, through complex interactions, with many gene families expanded showing high sequence, structural and functional diversity. The high diversity of immune receptors and effectors may provide oysters with enhanced specificity in immune recognition and response to cope with diverse pathogens in the absence of adaptive immunity. Some members of expanded immune gene families have diverged to function at different temperatures and salinities or assumed new roles in abiotic stress response. Most canonical innate immunity pathways are conserved in oysters and supported by a large number of diverse and often novel genes. The great diversity in immune and stress response genes exhibited by expanded gene families as well as high sequence and structural polymorphisms may be central to oyster's adaptation to highly stressful and widely changing environments.

Oral immunostimulation of the oyster Ostrea edulis: Impacts on the parasite Bonamia ostreae

Bioactive compounds were orally administered to the native European oyster Ostrea edulis to evaluate the immune response and the progression of infection of the protozoan parasite Bonamia ostreae. The immunostimulants lipopolysaccharide and zymosan directly administrated to the water column induced an increase in lysozyme activity and the percentage of granulocytes in naïve oysters over a period of 7 days. In another set of experiments, zymosan and curdlan were microencapsulated in alginate and also administered to the water column to naïve and B. ostreae infected O. edulis. Oyster mortality, prevalence and intensity of infection and several immune parameters were evaluated up to 28 days post-administration. Lysozyme activity, nitric oxide production and the expression of galectin, lysozyme and superoxide dismutase increased after 24 h in both infected and uninfected oysters. Zymosan immunostimulated oysters displayed a decrease in the prevalence of B. ostreae infection not attributed to mortalities but which could be associated to the enhanced ability of immunostimulants to evoke an enhanced immune response in the oysters and reduce infection.

The use of -omic tools in the study of disease processes in marine bivalve mollusks

Our understanding of disease processes and host-pathogen interactions in model species has benefited greatly from the application of medium and high-throughput genomic, metagenomic, epigenomic, transcriptomic, and proteomic analyses. The rate at which new, low-cost, high-throughput -omic technologies are being developed has also led to an expansion in the number of studies aimed at gaining a better understanding of disease processes in bivalves. This review provides a catalogue of the genetic and -omic tools available for bivalve species and examples of how -omics has contributed to the advancement of marine bivalve disease research, with a special focus in the areas of immunity, bivalve-pathogen interactions, mechanisms of disease resistance and pathogen virulence, and disease diagnosis. The analysis of bivalve genomes and transcriptomes has revealed that many immune and stress-related gene families are expanded in the bivalve taxa examined thus far. In addition, the analysis of proteomes confirms that responses to infection are influenced by epigenetic, post-transcriptional, and post-translational modifications. The few studies performed in bivalves show that epigenetic modifications are non-random, suggesting a role for epigenetics in regulating the interactions between bivalves and their environments. Despite the progress -omic tools have enabled in the field of marine bivalve disease processes, there is much more work to be done. To date, only three bivalve genomes have been sequenced completely, with assembly status at different levels of completion. Transcriptome datasets are relatively easy and inexpensive to generate, but their interpretation will benefit greatly from high quality genome assemblies and improved data analysis pipelines. Finally, metagenomic, epigenomic, proteomic, and metabolomic studies focused on bivalve disease processes are currently limited but their expansion should be facilitated as more transcriptome datasets and complete genome sequences become available for marine bivalve species.

Meta-analysis of studies using suppression subtractive hybridization and microarrays to investigate the effects of environmental stress on gene transcription in oysters

Many microarray and suppression subtractive hybridization (SSH) studies have analyzed the effects of environmental stress on gene transcription in marine species. However, there have been no unifying analyses of these data to identify common stress response pathways. To address this shortfall, we conducted a meta-analysis of 14 studies that investigated the effects of different environmental stressors on gene expression in oysters. The stressors tested included chemical contamination, hypoxia and infection, as well as extremes of temperature, pH and turbidity. We found that the expression of over 400 genes in a range of oyster species changed significantly after exposure to environmental stress. A repeating pattern was evident in these transcriptional responses, regardless of the type of stress applied. Many of the genes that responded to environmental stress encoded proteins involved in translation and protein processing (including molecular chaperones), the mitochondrial electron transport chain, anti-oxidant activity and the cytoskeleton. In light of these findings, we put forward a consensus model of sub-cellular stress responses in oysters.

Identification and expression of antioxidant and immune defense genes in the surf clam Mesodesma donacium challenged with Vibrio anguillarum

The immune system in marine invertebrates is mediated through cellular and humoral components, which act together to address the action of potential pathogenic microorganisms. In bivalve mollusks biomolecules implicated in oxidative stress and recognition of pathogens have been involved in the innate immune response. To better understand the molecular basis of the immune response of surf clam Mesodesma donacium, qPCR approaches were used to identify genes related to its immune response against Vibrio anguillarum infection. Genes related to oxidative stress response and recognition of pathogens like superoxide dismutase (MdSOD), catalase (MdCAT), ferritin (MdFER) and filamin (MdFLMN) were identified from 454-pyrosequencing cDNA library of M. donacium and were evaluated in mantle, adductor muscle and gills. The results for transcripts expression indicated that MdSOD, MdFLMN and MdFER were primarily expressed in the muscle, while MdCAT was more expressed in gills. Challenge experiments with the pathogen V. anguillarum had showed that levels of transcript expression for MdSOD, MdCAT, MdFER, and MdFLMN were positively regulated by pathogen, following a time-dependent expression pattern with significant statistical differences between control and challenge group responses (p<0.05). These results suggest that superoxide dismutase, catalase, ferritin and filamin, could be contributing to the innate immune response of M. donacium against the pathogen V. anguillarum.

Microcell parasites of molluscs: introduction to DAO Special 7

First discovered decades ago, microcell protistan parasites of the genera Bonamia and Mikrocytos remain relevant today for their economic impacts on growing molluscan aquaculture industries and fisheries. Bonamia parasites have received more attention over the years in part because they are more widespread and thus of wider concern, but there has been renewed interest in Mikrocytos recently with the generation of important new findings. Among these has been the surprising observation that Mikrocytos has phylogenetic affinities to the Rhizaria, which includes the haplosporidian protists and the genus Bonamia. This Diseases of Aquatic Organisms Special, emerging from the 5th Meeting of the Microcell Working Group held at the Central Veterinary Institute, Lelystad, the Netherlands, in February 2012, presents new insights into Mikrocytos and Bonamia diversity, distributions, diagnostics, ultrastructure, and infection dynamics, and captures major developments in the field since the last review of these genera in 2004.

Bonamia parasites: a rapidly changing perspective on a genus of important mollusc pathogens

Organisms of the genus Bonamia are intracellular protistan parasites of oysters. To date, 4 species have been described (B. ostreae, B. exitiosa, B. perspora and B. roughleyi), although the status of B. roughleyi is controversial. Introduction especially of B. ostreae and B. exitiosa to naïve host populations has been shown to cause mass mortalities in the past and has had a dramatic impact on oyster production. Both B. ostreae and B. exitiosa are pathogens notifiable to the World Organisation for Animal Health (OIE) and the European Union. Effective management of the disease caused by these pathogens is complicated by the extensive nature of the oyster production process and limited options for disease control of the cultured stocks in open water. This review focuses on the recent advances in research on genetic relationships between Bonamia isolates, geographical distribution, susceptible host species, diagnostics, epizootiology, host-parasite interactions, and disease resistance and control of this globally important genus of oyster pathogens.

Role of microRNAs in the immunity process of the flat oyster Ostrea edulis against bonamiosis

MicroRNAs (miRNAs) are small (∼22nt) non-coding regulatory single strand RNA molecules that reduce stability and/or translation of sequence-complementary target. miRNAs are a key component of gene regulatory networks and have been involved in a wide variety of biological processes, such as signal transduction, cell proliferation and apoptosis. Many miRNAs are broadly conserved among the animal lineages and even between invertebrates and vertebrates. The European flat oyster Ostrea edulis is highly susceptible to infection with Bonamia ostreae, an intracellular parasite able to survive and proliferate within oyster haemocytes. Mollusc haemocytes play a key role in the immune response of molluscs as main cellular effectors. The roles of miRNAs in the immune response of O. edulis to bonamiosis were analysed using a commercial microarray platform (miRCURY LNA™ v2, Exiqon) for miRNAs. Expression of miRNAs in haemocytes from oysters with different bonamiosis intensity was compared. Differential expression was detected in 63 and 76 miRNAs when comparing heavily-affected with non-affected oysters and with lightly-affected ones, respectively. Among them, 19 miRNAs are known to be linked to immune response, being responsible of proliferation and activation of macrophages, inflammation, apoptosis and/or oxidative damage, which is consistent with the modulation of their expression in oyster haemocytes due to bonamiosis.

Deep transcriptome sequencing of Pecten maximus hemocytes: a genomic resource for bivalve immunology

Pecten maximus, the king scallop, is a bivalve species with important commercial value for both fisheries and aquaculture, traditionally consumed in several European countries. Major problems in larval rearing, however, still limit hatchery-based seed production. High mortalities during early larval stages, likely related to bacterial pathogens, represent the most relevant bottleneck. To address this issue, understanding host defense mechanisms against microbes is extremely important. In this study next-generation RNA-sequencing was carried on scallop hemocytes. To enrich for immune-related transcripts, cDNA libraries from hemocytes challenged in vivo with inactivated-Vibrio anguillarum and in vitro with pathogen-associated molecular patterns, as well as unchallenged controls, were sequenced yielding 216,444,674 sequence reads. De novo assembly of the scallop hemocyte transcriptome consisted of 73,732 contigs (31% annotated). A total of 934 contigs encoded proteins with a known immune function, grouped into several functional categories. Particular attention was reserved to Toll-like receptors (TLRs), a family of pattern recognition receptors (PRRs) involved in non-self recognition. Through mining the scallop hemocyte transcriptome, at least four TLRs could be identified. The organization of canonical TLR domains demonstrated that single cysteine cluster and multiple cysteine cluster TLRs co-exist in this species. In addition, preliminary data concerning their mRNA level following bacterial challenge suggested that different members of this family could exhibit opposite responses to pathogenic stimuli. Finally, a global analysis of differential expression comparing gene-expression levels in in vitro and in vivo stimulated hemocytes against controls provided evidence on a large set of transcripts involved in the great scallop immune response.

Brown muscle disease: impact on Manila clam Venerupis (=Ruditapes) philippinarum biology

This study assessed the effect of Brown Muscle Disease (BMD) on Manila clam Venerupis philippinarum fitness. BMD was discovered in 2005. It affects the posterior adductor muscle and leads to clam gaping and eventually death. Three statuses of clams were compared: buried individuals with no signs of BMD (BUR); clams at the surface of the sediment with no signs of BMD (SURF) and clams at the surface of the sediment exhibiting signs of brown muscle disease (BMD). Physiological (condition index), immune (hemocyte parameters) and molecular (gene expressions) parameters collected seasonally were analyzed and compared. Results demonstrated a seasonal pattern in condition index (CI) with peaks in spring/summer and decreases in autumn/winter. At each season, the highest CI was observed in BUR and the lowest CI was observed in BMD. In terms of immune response, phagocytosis rate and capacity were higher in clams with BMD whereas the health status of the clams did not influence the total hemocyte count. Genes involved in the immune system (comp, tnf, inter) were upregulated in clams with BMD. The molecular analysis of gill and posterior muscle showed higher mitochondrial metabolism (cox-1, 16S) in cells of infected clams, suggesting a stronger energetic demand by these cells. Finally, genes involved in oxidative stress response (cat, sod), detoxification (mt) and DNA repair (gadd45) were also overexpressed due to reactive oxygen species production. Most of the studied parameters underlined a cause-effect correlation between Manila clam health status (BUR, SUR, BMD) and physiological parameters. An important stress response was observed in BMD-infected clams at different scales, i.e. condition index, immune parameters and stress-related gene expression.

Molecular characterisation of TNF, AIF, dermatopontin and VAMP genes of the flat oyster Ostrea edulis and analysis of their modulation by diseases

Bonamiosis and disseminated neoplasia (DN) are the most important diseases affecting cultured flat oysters (Ostrea edulis) in Galicia (NW Spain). Previous research of the response of O. edulis against bonamiosis by suppression subtractive hybridisation yielded a partial expressed sequence tag of tumour necrosis factor (TNF) and allograft inflammatory factor (AIF), as well as the whole open reading frame for dermatopontin and vesicle-associated membrane (VAMP). Herein, the complete open reading frames of TNF and AIF genes were determined by the rapid amplification of cDNA, and the deduced amino acid sequences of the four genes were characterised. Phylogenetic relationships for each gene were studied using maximum likelihood parameters. Quantitative-PCR assays were also performed in order to analyse the modulation of the expression of these genes by bonamiosis and disseminated neoplasia. Gene expression profiles were studied in haemolymph cells and in various organs (gill, gonad, mantle and digestive gland) of oysters affected by bonamiosis, DN, and both diseases with regard to non-affected oysters (control). TNF expression in haemolymph cells was up-regulated at heavy stage of bonamiosis but its expression was not affected by DN. AIF expression was up-regulated at heavy stage of bonamiosis in haemolymph cells and mantle, which is associated with heavy inflammatory response, and in haemolymph cells of oysters affected by DN. AIF expression was, however, down-regulated in other organs as gills and gonads. Dermatopontin expression was down-regulated in haemolymph cells and digestive gland of oysters affected by bonamiosis, but DN had no significant effect on its expression. Gills and gonads showed up-regulation of dermatopontin expression associated with bonamiosis. There were significant differences in the expression of TNF and VAMP depending on the bonamiosis intensity stage whereas no significant differences were detected between light and heavy severity degrees of DN for the studied genes. VAMP expression showed also differences among haemolymph cells and the organs studied. The occurrence of both diseases in oysters involved haemolymph cell gene expression patterns different from those associated to each disease separately: no significant effect was observed in TNF expression, dermatopontin was up-regulated and marked up-regulation of AIF and VAMP was recorded, which suggests a multiplier effect of the combination of both diseases for the latter two genes.

Identification of molecular and physiological responses to chronic environmental challenge in an invasive species: the Pacific oyster, Crassostrea gigas

Understanding the environmental responses of an invasive species is critical in predicting how ecosystem composition may be transformed in the future, especially under climate change. In this study, Crassostrea gigas, a species well adapted to the highly variable intertidal environment, was exposed to the chronic environmental challenges of temperature (19 and 24°C) and pH (ambient seawater and a reduction of 0.4 pH units) in an extended 3-month laboratory-based study. Physiological parameters were measured (condition index, shell growth, respiration, excretion rates, O:N ratios, and ability to repair shell damage) alongside molecular analyses. Temperature was by far the most important stressor, as demonstrated by reduced condition indexes and shell growth at 24°C, with relatively little effect detected for pH. Transcriptional profiling using candidate genes and SOLiD sequencing of mantle tissue revealed that classical “stress” genes, previously reported to be upregulated under acute temperature challenges, were not significantly expressed in any of the treatments, emphasizing the different response between acute and longer term chronic stress. The transcriptional profiling also elaborated on the cellular responses underpinning the physiological results, including the identification of the PI3K/AKT/mTOR pathway as a potentially novel marker for chronic environmental challenge. This study represents a first attempt to understand the energetic consequences of cumulative thermal stress on the intertidal C. gigas which could significantly impact on coastal ecosystem biodiversity and function in the future.

cgMolluscidin, a novel dibasic residue repeat rich antimicrobial peptide, purified from the gill of the Pacific oyster, Crassostrea gigas

A 5.5 kDa antimicrobial peptide consisting of 55 amino acids, cgMolluscidin, was purified from the acidified gill extract of the Pacific oyster, Crassostrea gigas, by ion-exchange and C18 reversed-phase high performance liquid chromatography. By comparing the N-terminal amino acid sequences and the molecular weight of this peptide with those of other known antimicrobial peptides, it has been revealed that this peptide had no homology with any known peptides. cgMolluscidin showed potent antimicrobial activity against both Gram-positive bacteria, including Bacillus subtilis, Micrococcus luteus, and Staphylococcus aureus (minimal effective concentrations [MECs]; 1.3-31.3 μg/mL), and Gram-negative bacteria, including Escherichia coli, Salmonella enterica, and Vibrio parahaemolyticus ([MECs]; 0.4-2.3 μg/mL), without hemolytic activity. However, cgMolluscidin did not show any significant activity against Candida albicans. The deduced amino acid sequence of the cgMolluscidin showed no hit in public protein databases, while the nucleotide sequence had a 99% homology (E value = 0) with only the unknown ESTs sequenced by C. gigas EST project. Tissue distribution of the cgMolluscidin mRNA suggests that it is constitutively expressed as a mature form in a non-tissue-specific manner. The cgMolluscidin mRNA expression level was significantly up-regulated at 12 h (2.8-fold) post injection with Vibrio sp. This peptide is highly basic and contains several dibasic residue repeats including Lysine-Lysine or Lysine-Arginine in the sequence, but may not form an ordered structure. These results suggest that cgMolluscidin might be an oyster-specific novel antimicrobial peptide.

Mucosal immunity in the gut: the non-vertebrate perspective

Much is now known about the vertebrate mechanisms involved in mucosal immunity, and the requirement of commensal microbiota at mucosal surfaces for the proper functioning of the immune system. In comparison, very little is known about the mechanisms of immunity at the barrier epithelia of non-vertebrate organisms. The purpose of this review is to summarize key experimental evidence illustrating how non-vertebrate immune mechanisms at barrier epithelia compare to those of higher vertebrates, using the gut as a model organ. Not only effector mechanisms of gut immunity are similar between vertebrates and non-vertebrates, but it also seems that the proper functioning of non-vertebrate gut defense mechanisms requires the presence of a resident microbiota. As more information becomes available, it will be possible to obtain a more accurate picture of how mucosal immunity has evolved, and how it adapts to the organisms' life styles.