Polymorphism in a serine protease inhibitor gene and its association with disease resistance in the eastern oyster (Crassostrea virginica Gmelin)

MassArray Genotyping as a Selection Tool for Extending the Shelf-Life of Fresh Gilthead Sea Bream and European Seabass

In modern aquaculture, genomics-driven breeding programs have emerged as powerful tools for optimizing fish quality. This study focused on two emblematic Mediterranean fish species, the European seabass (Dicentrarchus labrax) and the gilthead sea bream (Sparus aurata), with a primary aim of exploring the genetic basis of white muscle/fillet degradation in fresh fish following harvest. We identified 57 and 44 missense SNPs in gilthead sea bream and European seabass, respectively, located within genes encoding for endogenous proteases responsible for fillet quality. These SNPs were cherry-picked based on their strategic location within the catalytic/regulatory domains of endogenous proteases that are expressed in the white muscle. Using MassArray technology, we successfully associated differentiated enzymatic activity of those endogenous proteases post-harvest as a phenotypic trait with genetic polymorphism of six SNPs in gilthead sea bream and nine in European seabass. These findings can be valuable attributes in selective breeding programs toward the extension of freshness and shelf life of these species. The integration of MassArray technology into breeding programs offers a cost-effective strategy for harnessing the potential of these genetic variants to enhance the overall quality of the final product. Recognizing that fresh fish perishability is a challenge, extending shelf-life is pivotal in reducing losses and production costs.

Two common nanoparticles exert immunostimulatory and protective effects in Tegillarca granosa against Vibrio parahaemolyticus

There are many studies revealed that metal-based nanoparticles (NPs) possess excellent bactericidal effect on multitudinous bacteria and fungi. However, the control effect of NPs as antimicrobial agents to against Vibrio parahaemolyticus infection remain in poorly understood for blood clam, Tegillarca granosa. In order to evaluate the effect, the changes in six physiological parameters and the immune-related genes expression of clams exposed to V. parahaemolyticus alone or along with NPs (nZnO or nCuO) were investigated in present study. Results showed that both tested NPs exerted prominent redemptive or mitigative effect in an inverse dose-dependent way on physiological indexes of clam, especially in the total counts, phagocytosis and the cell viability of haemocytes, as well as the concentration and activity of lysozymes, when co-exposed with Vibrio. Gene expression analysis showed NPs at a concentration of 0.1 mg/L generally mitigated the downregulation of immune-related genes after clam exposure to V. parahaemolyticus. The combination of 0.1 mg/mL nZnO and nCuO additives has been shown to significantly enhance the humoral immunity of blood clam, suggesting its potential as a protective measure against V. parahaemolyticus infection in T. granosa.

Development and Evaluation of High-Density SNP Arrays for the Eastern Oyster Crassostrea virginica

The eastern oyster Crassostrea virginica is a major aquaculture species for the USA. The sustainable development of eastern oyster aquaculture depends upon the continued improvement of cultured stocks through advanced breeding technologies. The Eastern Oyster Breeding Consortium (EOBC) was formed to advance the genetics and breeding of the eastern oyster. To facilitate efficient genotyping needed for genomic studies and selection, the consortium developed two single-nucleotide polymorphism (SNP) arrays for the eastern oyster: one screening array with 566K SNPs and one breeders' array with 66K SNPs. The 566K screening array was developed based on whole-genome resequencing data from 292 oysters from Atlantic and Gulf of Mexico populations; it contains 566,262 SNPs including 47K from protein-coding genes with a marker conversion rate of 48.34%. The 66K array was developed using best-performing SNPs from the screening array, which contained 65,893 oyster SNPs including 22,984 genic markers with a calling rate of 99.34%, a concordance rate of 99.81%, and a much-improved marker conversion rate of 92.04%. Null alleles attributable to large indels were found in 13.1% of the SNPs, suggesting that copy number variation is pervasive. Both arrays provided easy identification and separation of selected stocks from wild progenitor populations. The arrays contain 31 mitochondrial SNPs that allowed unambiguous identification of Gulf mitochondrial genotypes in some Atlantic populations. The arrays also contain 756 probes from 13 oyster and human pathogens for possible detection. Our results show that marker conversion rate is low in high polymorphism species and that the two-step process of array development can greatly improve array performance. The two arrays will advance genomic research and accelerate genetic improvement of the eastern oyster by delineating genetic architecture of production traits and enabling genomic selection. The arrays also may be used to monitor pedigree and inbreeding, identify selected stocks and their introgression into wild populations, and assess the success of oyster restoration.

Immune diversity in lophotrochozoans, with a focus on recognition and effector systems

Lophotrochozoa is one of the most species-rich but immunologically poorly explored phyla. Although lack of acquired response in a narrow sense, lophotrochozoans possess various genetic mechanisms that enhance the diversity and specificity of innate immune system. Here, we review the recent advances of comparative immunology studies in lophotrochozoans with focus on immune recognition and effector systems. Haemocytes and coelomocytes are general important yet understudied player. Comparative genomics studies suggest expansion and functional divergence of lophotrochozoan immune reorganization systems is not as "homogeneous and simple" as we thought including the large-scale expansion and molecular divergence of pattern recognition receptors (PRRs) (TLRs, RLRs, lectins, etc.) and signaling adapters (MyD88s etc.), significant domain recombination of immune receptors (RLR, NLRs, lectins, etc.), extensive somatic recombination of fibrinogenrelated proteins (FREPs) in snails. Furthermore, there are repeatedly identified molecular mechanisms that generate immune effector diversity, including high polymorphism of antimicrobial peptides and proteins (AMPs), reactive oxygen and nitrogen species (RONS) and cytokines. Finally, we argue that the next generation omics tools and the recently emerged genome editing technicism will revolutionize our understanding of innate immune system in a comparative immunology perspective.

Chromosomal assembly of the flat oyster (Ostrea edulis L.) genome as a new genetic resource for aquaculture

The European flat oyster (Ostrea edulis L.) is a native bivalve of the European coasts. Harvest of this species has declined during the last decades because of the appearance of two parasites that have led to the collapse of the stocks and the loss of the natural oyster beds. O. edulis has been the subject of numerous studies in population genetics and on the detection of the parasites Bonamia ostreae and Marteilia refringens. These studies investigated immune responses to these parasites at the molecular and cellular levels. Several genetic improvement programs have been initiated especially for parasite resistance. Within the framework of a European project (PERLE 2) that aims to produce genetic lines of O. edulis with hardiness traits (growth, survival, resistance) for the purpose of repopulating natural oyster beds in Brittany and reviving the culture of this species in the foreshore, obtaining a reference genome becomes essential as done recently in many bivalve species of aquaculture interest. Here, we present a chromosome-level genome assembly and annotation for the European flat oyster, generated by combining PacBio, Illumina, 10X linked, and Hi-C sequencing. The finished assembly is 887.2 Mb with a scaffold-N50 of 97.1 Mb scaffolded on the expected 10 pseudochromosomes. Annotation of the genome revealed the presence of 35,962 protein-coding genes. We analyzed in detail the transposable element (TE) diversity in the flat oyster genome, highlighted some specificities in tRNA and miRNA composition, and provided the first insight into the molecular response of O. edulis to M. refringens. This genome provides a reference for genomic studies on O. edulis to better understand its basic physiology and as a useful resource for genetic breeding in support of aquaculture and natural reef restoration.

Genome Wide Identification and Expression Profiling Indicate Expansion of Family I84 Protease Inhibitor via Gene Tandem Duplication and Divergence in Razor Clam Sinonovacula constricta

Family I84 protease inhibitors represent a novel family in the MEROPS peptidase database and are likely unique for molluscan host defense. Two Family I84 members, scSI-1 and scSI-2, were reported from the razor clam Sinonovacula constricta in a previous research. In the present study, 12 additional genes, named scSI-3 to scSI-14, were identified via genome wide sequence analyses. Among them, 10 genes were predicted to have a signal sequence, but one (scSI-7) was not. Besides, one sequence (scSI-14) was likely to encode a prematurely terminated peptide. The predicted mature peptides shared characteristics including 12 conserved cysteine residues, isoelectric points of 4.98 to 6.11, and molecular weights of 7.1 to 9.3 kDa with previously reported family members. Four motifs were characterized in 13 predicted mature peptides (with exception of scSI-14), which shared two to four conserved cysteine residues, are possibly to form two functional domain comprised 6 cysteine residues, respectively. At genomic level, all the 14 razor clam Family I84 genes were organized into 3 exons and 2 introns; 13 of them clustered in 3 regions of 100 kb on 3 separate chromosomes, suggesting tandem duplications of related genes. The promoter region of all the 14 genes was predicted to share some transcription factor binding sites, in particular those responsive to pathological and physiological stimuli, but no shared motifs were identified. Analyses also revealed differences in expression patterns among the genes. One gene in a tandem duplicated gene pairs usually showed a higher expression level than the other whereas non-tandem duplicated genes exhibited a higher degree of correlation in expression level. In addition, 8 of the 14 genes demonstrated higher level of expression in Vibrio tolerant clams than in non-tolerant clams following challenges with Vibrio parahaemolyticus. These results generated important information about the evolution of Family I84 protease inhibitors in S. constricta.

High spatial resolution mapping of the mucosal proteome of the gills of Crassostrea virginica: implication in particle processing

In the oyster Crassostrea virginica, the organization of the gill allows bidirectional particle transport where a dorsal gill tract directs particles meant to be ingested while a ventral tract collects particles intended to be rejected as pseudofeces. Previous studies showed that the transport of particles in both tracts is mediated by mucus. Consequently, we hypothesized that the nature and/or the quantity of mucosal proteins present in each tract is likely to be different. Using endoscopy-aided micro-sampling of mucus from each tract followed by multidimensional protein identification technologies, and in situ hybridization, a high spatial resolution mapping of the oyster gill proteome was generated. Results showed the presence in gill mucus of a wide range of molecules involved in non-self recognition and interactions with microbes. Mucus composition was different between the two tracts, with mucus from the ventral tract shown to be rich in mucin-like proteins, providing an explanation of its high viscosity, while mucus from the dorsal tract was found to be enriched in mannose-binding proteins, known to be involved in food particle binding and selection. Overall, this study generated high-resolution proteomes for C. virginica gill mucus and demonstrated that the contrasting functions of the two pathways present on oyster gills are associated with significant differences in their protein makeup.

Contrasting Immunomodulatory Effects of Probiotic and Pathogenic Bacteria on Eastern Oyster, Crassostrea Virginica, Larvae

Several Vibrio spp. cause acute and severe mortality events in hatcheries where larvae of bivalve mollusks are reared, potentially leading to subsequent shortage of bivalve seed for the grow-out industry. In particular, strains of Vibrio coralliilyticus have been identified as a major cause of disease in Pacific, Crassostrea gigas, and eastern, C. virginica, oyster hatcheries in the United States of America. Probiotic bacteria are an inexpensive, practical, and natural method of disease control. Previous research shows that pretreatment of larval oysters with probiotic bacteria Bacillus pumilus RI06-95 (RI) and Phaeobacter inhibens S4 (S4) significantly decreases mortality caused by experimental challenge with the bacterial pathogen V. coralliilyticus RE22 (RE22). This study aims to characterize the immune response of 6-10-day-old eastern oyster larvae to experimental challenge with pathogen V. coralliilyticus RE22 and probionts RI and S4. Treatments included (a) pathogen and probiont exposure at a concentration of 5 × 10⁴ CFU per mL (~2500 bacterial cells per larva) for a duration of 6 h, (b) probiont exposure at the same concentration for a duration of 24 h, and (c) probiont RI daily treatment of larvae in the hatchery for 4, 11, and 15 days. Differential gene expression analysis compared pathogen or probiotic-treated transcriptomes to unexposed controls. Probiotic and pathogen treatment led to upregulation of transcripts coding for several immune pattern recognition receptors (PRRs) involved in environmental sensing and detection of microbes in oyster larvae. Larval oyster responses to pathogen RE22 suggested suppression of expression of genes in immune signaling pathways (myd88, tak1, nkap), failure in upregulation of immune effector genes, high metabolic demand, and oxidative stress that potentially contributed to mortality. On the other hand, the transcriptomic response to probiotic bacteria RI and S4 suggested activation of immune signaling pathways and expression of immune effectors (e.g., Cv-spi2, mucins and perforin-2). These key features of the host immune response to probiotic bacteria were shared despite the length of probiotic exposure, probiotic species, and the type of environment in which exposures were conducted. This study suggests that pre-exposure of eastern oyster larvae to probiotics for 6-24 h prior to pathogenic challenge leads to a robust and effective immune response that may contribute to protecting larvae from subsequent challenge with V. coralliilyticus RE22. This research provides new insights into host-microbe interactions in larval oysters that could be applied in the management of vibriosis in bivalve hatcheries.

Variation in global transcriptomic response to Perkinsus marinus infection among eastern oyster families highlights potential mechanisms of disease resistance

Dermo disease, caused by the protozoan parasite Perkinsus marinus, negatively impacts wild and cultured Eastern oyster populations, yet our knowledge of the mechanistic bases for parasite pathogenicity and the Eastern oyster's response to it is limited. To better understand host responses to the parasite and identify molecular mechanisms underlying disease-resistance phenotypes, we experimentally challenged two families exhibiting divergent Dermo-resistance phenotypes with the parasite, generated global expression profiles using RNAseq and identified differentially expressed transcripts between control and challenged oysters from each family at multiple time points post-parasite injection. The susceptible and resistant families exhibited strikingly different transcriptomic responses to the parasite over a 28-day time period. The resistant family exhibited a strong, focused, early response to P. marinus infection, where many significantly upregulated transcripts were associated with the biological processes "regulation of proteolysis" and "oxidation-reduction process." P. marinus virulence factors are mainly comprised of proteases that facilitate parasite invasion and weaken host humoral defenses, thus host upregulation of transcripts associated with negative regulation of proteolysis is consistent with a Dermo-resistant phenotype. In contrast, the susceptible family mounted a very weak, disorganized, initial response to the parasite. Few transcripts were differentially expressed between control and injected oysters, and no functional enrichment was detected among them. At the final 28 d time point 2450 differentially expressed transcripts were identified and were associated with either "G-protein coupled receptor activity" (upregulated) or "microtubule-based process" (downregulated). A handful of protease inhibitors were differentially expressed between control and injected susceptible oysters, but this function was not enriched in the susceptible data set. The differential expression patterns observed in this study provide valuable insight into the functional basis of Dermo resistance and suggest that the timing of expression is just as important as the transcripts being expressed.

Pathogen proteases and host protease inhibitors in molluscan infectious diseases

The development of infectious diseases represents an outcome of dynamic interactions between the disease-producing agent's pathogenicity and the host's self-defense mechanism. Proteases secreted by pathogenic microorganisms and protease inhibitors produced by host species play an important role in the process. This review aimed at summarizing major findings in research on pathogen proteases and host protease inhibitors that had been proposed to be related to the development of mollusk diseases. Metalloproteases and serine proteases respectively belonging to Family M4 and Family S8 of the MEROPS system are among the most studied proteases that may function as virulence factors in mollusk pathogens. On the other hand, a mollusk-specific family (Family I84) of novel serine protease inhibitors and homologues of the tissue inhibitor of metalloprotease have been studied for their potential in the molluscan host defense. In addition, research at the genomic and transcriptomic levels showed that more proteases of pathogens and protease inhibitor of hosts are likely involved in mollusk disease processes. Therefore, the pathological significance of interactions between pathogen proteases and host protease inhibitors in the development of molluscan infectious diseases deserves more research efforts.

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.

Two I84 family protease inhibitors from Chinese razor clams Sinonovacula constricta expressed in response to environmental challenges

Protease inhibitors play critical roles in numerous biological processes including host defense in all multicellular organisms. Eighty three evolutionary families of protease inhibitors are currently accommodated in the MEROPS database and the I84 family currently consists of 3 novel serine protease inhibitors from the eastern oyster Crassostrea virginica. In this study, we identified 2 new I84 family members from the Chinese razor clam Sinonovacula constricta, scSI-1 and scSI-2, using cDNA cloning and sequencing. The scSI-1 cDNA consisted of 494 bp with a 282 bp ORF encoding a 93-amino acid polypeptide that was predicted to have a 19-amino acid signal peptide and a 74-residue mature protein with a calculated molecular mass of 8248.5 Da. The scSI-2 cDNA was 490 bp long with a 273 bp ORF encoding a 90-amino acid polypeptide that was predicted to have an 18-amino acid signal peptide and a 72-residue nature protein with a calculated molecular mass of 7528.4 Da. ScSI-1 and scSI-2 shared high sequence similarity with the 3 known members of I84 family and both expressed primarily in the clam digestive glands. Protease inhibitory activity in the clam plasma also exhibited the signature kinetic characteristics of the I84 members from the oyster. In addition, levels of scSI-1 and scSI-2 gene expression in digestive glands and the protease inhibitory activity in plasma elevated significantly in clams challenged by bacterial injections and Vibrio harveyi was more effective than Staphylococcus epidermidis in inducing the gene expression and plasma protease inhibitory activity. Moreover, drastic changes of salinity and temperature also caused significant changes in the gene expression and plasma activity. These results indicated that scSI-1 and scSI-2 represented 2 new members of the I84 family and they likely play a role in clam host defense against infections and in reactions against physiochemical stressors.

Identification of cvSI-3 and evidence for the wide distribution and active evolution of the I84 family of protease inhibitors in mollusks

Protease inhibitors are an extremely diverse group of proteins that control the proteolytic activities of proteases and play a crucial role in biological processes including host defenses. The I84 family of protease inhibitors in the MEROPS database currently consists of cvSI-1 and cvSI-2, two novel serine protease inhibitors purified and characterized from the eastern oyster Crassostrea virginica plasma and believed to play a role in host defense and disease resistance. In the present study, a third member of I84 family, named cvSI-3, was identified from C. virginica by cDNA cloning and sequencing. The full cvSI-3 cDNA was composed of 342 bp including a 255 bp open reading frame (ORF) that encodes an 84-amino acid peptide. The mature cvSI-3 molecule was predicted to have 68 amino acid residues after removal of a 16-amino acid signal peptide, with a calculated molecular mass of 7724.5 Da and a theoretical isoelectric point (pI) of 6.28. CvSI-3 amino acid sequence shared 41% identity with cvSI-2 and 37% identity with cvSI-1, which included 12 conserved cysteines. Quantitative real-time PCR determined that cvSI-3 gene expressed primarily in oyster digestive glands. Real-time PCR also detected that cvSI-1, cvSI-2 and cvSI-3 expression levels in digestive glands varied significantly, with cvSI-2 showing the highest expression level and cvSI-3 the lowest. Additionally, a significant correlation was detected between cvSI-2 and cvSI-3 mRNAs levels. Searches into sequence databases using cvSI-1, cvSI-2 and cvSI-3 as queries retrieved ESTs suggesting the possible existence of at least 9 more I84 family members in eastern oysters and of I84 family protease inhibitors in various bivalve and gastropod species. Moreover, orthologs of all C. virginica I84 family members or potential member genes were found to be present in the C. gigas genome, and their distributions among species provided important information about the evolution of the I84 family of protease inhibitors. It appears that the I84 family of protease inhibitors is widely distributed and actively evolving in the Phylum Mollusca.

Infectious diseases of marine molluscs and host responses as revealed by genomic tools

More and more infectious diseases affect marine molluscs. Some diseases have impacted commercial species including MSX and Dermo of the eastern oyster, QPX of hard clams, withering syndrome of abalone and ostreid herpesvirus 1 (OsHV-1) infections of many molluscs. Although the exact transmission mechanisms are not well understood, human activities and associated environmental changes often correlate with increased disease prevalence. For instance, hatcheries and large-scale aquaculture create high host densities, which, along with increasing ocean temperature, might have contributed to OsHV-1 epizootics in scallops and oysters. A key to understanding linkages between the environment and disease is to understand how the environment affects the host immune system. Although we might be tempted to downplay the role of immunity in invertebrates, recent advances in genomics have provided insights into host and parasite genomes and revealed surprisingly sophisticated innate immune systems in molluscs. All major innate immune pathways are found in molluscs with many immune receptors, regulators and effectors expanded. The expanded gene families provide great diversity and complexity in innate immune response, which may be key to mollusc's defence against diverse pathogens in the absence of adaptive immunity. Further advances in host and parasite genomics should improve our understanding of genetic variation in parasite virulence and host disease resistance.

Polymorphism in a serine protease inhibitor gene and its association with the resistance of bay scallop (Argopecten irradians) to Listonella anguillarum challenge

Serine protease inhibitors (SPIs) play a crucial role in regulation of both host and bacterial serine protease. They are classified into several protein families, where Kazal-type inhibitors are one of families with multi-domain. In the present study, the polymorphism of AiSPI from Bay scallop Argopecten irradians was found to be associated with disease resistance of bay scallop against Listonella anguillarum. Nine single nucleotide polymorphisms (SNPs) were identified in the exon region of AiSPI, where five SNPs were non-synonymous mutation. Three of these mutations were located in "kazal-like 3"domain, two SNP loci positioned at +536, +1312 were selected for further association studies. For the locus +536, the genotype frequency of A/G in the resistant stock (12.8%) was significantly lower (p < 0.05) than that in the susceptible stock (35.1%), while, the genotype A/A in the resistant stock (87.2%) was significantly higher in comparison with susceptible stock (64.9%) (p < 0.05). The G allele frequencies were 6.4% and 17.6% in resistant stock and susceptible stock, respectively, and χ²-test revealed a significant difference in the frequency distribution between the two stocks (p < 0.05). But there was no significant association between the mutation C-T at locus +1312 with either resistant or susceptible group (p > 0.05). The genotype frequencies of T/T, T/C, C/C at locus +1312 were 94.6%, 2.7% and 2.7% respectively in the susceptible stock, while 100%, 0% and 0% respectively in the resistant stock. The amino acid change for the mutation at locus +536 A-G was from asparagine to serine, and the predicted homology model of this amino acid variation could affect its function as well as the structural integrity of the domain. In vitro elastase inhibition assay of the protein variants at locus +536 was conducted to explicate the effect of SNP. The increasing concentration of protein (0 mmol/L- 2.93 mmol/L) was incubated with 80 nmol/L elastase where the residual enzyme activity values for rAiSPI (N) with A variant and rAiSPI (S) with G variant were started to reduce from 0.40 to 0.215 and 0.435 to 0.356, respectively. The elastase inhibition ability of rAiSPI (N) variant was significantly higher than that of rAiSPI (S) (p < 0.01). The results suggested that the mutation at locus +536A/A significantly associated with disease resistance of bay scallop would shed light for selective breeding program.

Protein expression profiling in haemocytes and plasma of the Manila clam Ruditapes philippinarum in response to infection with Perkinsus olseni

The protein expression profiling in clam haemocytes and plasma in response to Perkinsus olseni was addressed. Adult Manila clams from a P. olseni-free bed were experimentally challenged with parasite zoospores to analyse immune response. In another experiment, the effects of longer term infection were assessed in adult clams collected from a P. olseni-affected bed, by comparing moderate to very heavily infected clams with non-infected ones. Haemocyte and plasma proteins were separated by two-dimensional electrophoresis; spot patterns were qualitatively compared between treatments within each experiment and the spots indicating differential protein expression associated with P. olseni challenge or with field infection were processed for protein identification. Fifteen clam proteins (four in haemocytes and eleven in plasma) of which expression was markedly affected by P. olseni were identified. Some of the identified proteins have a well-known role in clam immune response against the parasite, such as lysozyme and lectins. Rho GTPase-activating protein 6 could be a marker of resistance against P. olseni, which should be further studied.

Characterization of two novel gadd45a genes in hybrid tilapia and their responses to the infection of Streptococcus agalactiae

Diseases are one of the major challenges in tilapia aquaculture. Identification of DNA markers associated with disease resistance may facilitate the acceleration of the selection for disease resistance. Gadd45a (growth arrest and DNA damage 45 A), a stress-inducible gene in humans and mice, has not been studied in fish. We characterized the two prologues of Gadd45a genes in hybrid tilapia. Gadd45a1 and Gadd45a2 shared an identical gene structure and showed an amino acid sequence identity of 73.8%. Their expressions were detected in all 10 tissues examined, with the kidney and gill having high transcriptional expressions. The expression levels of Gadd45a1 were significantly lower than those of Gadd45a2 in all examined tissues. After a challenge with a bacterial pathogen Streptococcus agalactiae, the expressions of the two genes were up-regulated significantly in the spleen, kidney, liver and intestine. These findings suggest that the two Gadd45a genes play an important role in the resistance to S. agalactiae in tilapia. We identified 10 SNPs in the two genes. The SNP markers in the two Gadd45a genes could be used to examine whether they are associated with resistance to S. agalactiae.

Identification of the MmeHairy gene and expression analysis affected by two SNPs in the 3'-untranslated region in the clam Meretrix meretrix

As a bHLH transcriptional repressor, Hairy-related proteins can bind to DNA sites in target gene promoters and negatively regulate gene transcription. In the present study, the full-length cDNA of Hairy was obtained from the clam Meretrix meretrix (MmeHairy), and two SNPs in the 3'-untranslated region (UTR) of this gene, SNP1066 and 1067, were identified and characterized. Multiple sequence alignment and phylogenetic analysis revealed that MmeHairy belongs to the Hairy protein subfamily. Analysis of tissue expression patterns showed that the mRNA of MmeHairy had the highest expression level in the hepatopancreas. The expression levels of MmeHairy were up-regulated in the hepatopancreas after Vibrio challenge. Genotyping and quantitative analysis showed that the mRNA levels of MmeHairy were significantly different among individual clams with different genotypes at SNP1066 and 1067 (P < 0.05), which indicated that these two SNP loci may affect the expression of MmeHairy and could be used as candidate markers for future selection in M. meretrix breeding programs.

Clam focal and systemic immune responses to QPX infection revealed by RNA-seq technology

Background

The hard clam Mercenaria mercenaria is an important seafood species widely exploited along the eastern coasts of the United States and play a crucial role in coastal ecology and economy. Severe hard clam mortalities have been associated with the protistan parasite QPX (Quahog Parasite Unknown). QPX infection establishes in pallial organs with the lesions typically characterized as nodules, which represent inflammatory masses formed by hemocyte infiltration and encapsulation of parasites. QPX infection is known to induce host changes on both the whole-organism level and at specific lesion areas, which imply systemic and focal defense responses, respectively. However, little is known about the molecular mechanisms underlying these alterations.

Results

RNA-seq was performed using Illumina Hiseq 2000 (641 Million 100 bp reads) to characterize M. mercenaria focal and systemic immune responses to QPX. Transcripts were assembled and the expression levels were compared between nodule and healthy tissues from infected clams, and between these and tissues from healthy clams. De novo assembly reconstructed a consensus transcriptome of 62,980 sequences that was functionally-annotated. A total of 3,131 transcripts were identified as differentially expressed in different tissues. Results allowed the identification of host immune factors implicated in the systemic and focal responses against QPX and unraveled the pathways involved in parasite neutralization. Among transcripts significantly modulated upon host-pathogen interactions, those involved in non-self recognition, signal transduction and defense response were over-represented. Alterations in pathways regulating hemocyte focal adhesion, migration and apoptosis were also demonstrated.

Conclusions

Our study is the first attempt to thoroughly characterize M. mercenaria transcriptome and identify molecular features associated with QPX infection. It is also one of the first studies contrasting focal and systemic responses to infections in invertebrates using high-throughput sequencing. Results identified the molecular signatures of clam systemic and focal defense responses, to collectively mediate immune processes such as hemocyte recruitment and local inflammation. These investigations improve our understanding of bivalve immunity and provide molecular targets for probing the biological bases of clam resistance towards QPX.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2493-9) contains supplementary material, which is available to authorized users.

A Kazal-type serine proteinase inhibitor from Cyclina sinensis is involved in immune response and signal pathway initiation

Serine protease inhibitors (SPIs) are an important group of protease inhibitors involved in a variety of biological processes. In the present study, a Kazal-type serine protease inhibitor homolog gene (designated as CsKPI) was identified from a Cyclina sinensis cDNA library. The open reading frame consists of 456 bp and encodes a protein of 151 amino acid residues with a theoretical molecular mass of 16.85 kDa and an isoelectric point of 5.74. Furthermore, using quantitative real-time PCR, we focused on the expression patterns of CsKPI found in tissues and on the stimulation of this gene's expression by bacteria. The results show that a higher-level mRNA expression of CsKPI was detected in hemocytes (P < 0.05) and was significantly upregulated at 3 h (P < 0.01) upon receiving bacterial challenges with Vibrio anguillarum. In addition, after the CsKPI gene was silenced by RNA interference, the expression of the CsTLR2 and CsMyD88 genes was extremely significantly decreased (P < 0.01) in C. sinensis. Finally, the recombinant CsKPI (rCsKPI) protein was purified and shown to exhibit less inhibitory activity than C-lyz against V. anguillarum in vitro. Hence, we propose that CsKPI plays an important role in the innate immunity and mediates TLR2 and MyD88-dependent pathway initiation in C. sinensis.

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.

Characterization of the duodenase-1 gene and its associations with resistance to Streptococuus agalactiae in hybrid tilapia (Oreochromis spp.)

Tilapia is a group of cultured teleost fishes whose production is threatened by some diseases. Identification of DNA markers associated with disease resistance in candidate genes may facilitate to accelerate the selection of disease resistance. The gene encoding a duodenase, which can trigger immune response, has not been studied in fish. We characterized the cDNA of duodenase-1 gene of hybrid tilapia. Its ORF is 759 bp, encoding a serine protease of 252 amino acids. This gene consisted of five exons and four introns. Its expression was detected in all 10 tissues examined, and it was highly expressed in the intestine and kidney. After a challenge with the bacterial pathogen, Streptococcus agalactiae, its expression was up-regulated significantly in the intestine, liver and spleen. We identified seven SNPs in the gene and found that four of them were significantly associated with the resistance to S. agalactiae (P < 0.05). The CGTCC haplotype, CAGTC/CGGTC and CGTCC/CGTCC diplotype were significantly associated with the resistance to S. agalactiae (P = 0.00, 0.04 and < 0.0001, respectively). In addition, one SNP was associated significantly with growth traits (P < 0.05). These findings suggest that the duodenase-1 gene plays an important role in the resistance to S. agalactiae in tilapia. The SNP markers in the duodenase-1 gene associated with resistance to the bacterial pathogen, may facilitate the selection of tilapia resistant to the bacterial disease.

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.

Immune responses to infectious diseases in bivalves

Many species of bivalve mollusks (phylum Mollusca, class Bivalvia) are important in fisheries and aquaculture, whilst others are critical to ecosystem structure and function. These crucial roles mean that considerable attention has been paid to the immune responses of bivalves such as oysters, clams and mussels against infectious diseases that can threaten the viability of entire populations. As with many invertebrates, bivalves have a comprehensive repertoire of immune cells, genes and proteins. Hemocytes represent the backbone of the bivalve immune system. However, it is clear that mucosal tissues at the interface with the environment also play a critical role in host defense. Bivalve immune cells express a range of pattern recognition receptors and are highly responsive to the recognition of microbe-associated molecular patterns. Their responses to infection include chemotaxis, phagolysosomal activity, encapsulation, complex intracellular signaling and transcriptional activity, apoptosis, and the induction of anti-viral states. Bivalves also express a range of inducible extracellular recognition and effector proteins, such as lectins, peptidoglycan-recognition proteins, thioester bearing proteins, lipopolysaccharide and β1,3-glucan-binding proteins, fibrinogen-related proteins (FREPs) and antimicrobial proteins. The identification of FREPs and other highly diversified gene families in bivalves leaves open the possibility that some of their responses to infection may involve a high degree of pathogen specificity and immune priming. The current review article provides a comprehensive, but not exhaustive, description of these factors and how they are regulated by infectious agents. It concludes that one of the remaining challenges is to use new "omics" technologies to understand how this diverse array of factors is integrated and controlled during infection.

An immune-enriched oligo-microarray analysis of gene expression in Manila clam (Venerupis philippinarum) haemocytes after a Perkinsus olseni challenge

Parasites of the genus Perkinsus cause high mortality and economic losses in bivalves commonly produced in global aquaculture. Although the immune responses of oysters and clams naturally infected with Perkinsus marinus or Perkinsus olseni have been extensively studied, there is not much information on host response at the early stages of infection. In this study, we analysed how P. olseni influences the gene expression profiles of haemocytes from the Manila clam (Venerupis philippinarum) using temporal experimental infections and an immune-enriched microarray. We identified an early phase of infection that was characterised by no mortality and by the increased expression of genes associated with pathogen recognition, production of nitrogen radicals and antimicrobial activity. Cellular processes such as inhibition of serine proteases and proliferation were also involved in this early response. This phase was followed by an intermediate stage, when the pathogen was most likely multiplying and infecting new areas of the body, and animals began to die. In this stage, many genes related to cell movement were over-expressed. Thirty days after infection metabolic pathway genes were the most affected. Apoptosis appears to be important during pathogenesis. Our results provide novel observations of the broader innate immune response triggered by P. olseni at different infection stages.

Identification of potential general markers of disease resistance in American oysters, Crassostrea virginica through gene expression studies

Several diseases have a significant impact on American oyster populations in the Atlantic coasts of North America. Knowledge about the responses of oysters to pathogenic challenge could help in identifying potential markers of disease resistance and biomarkers of the health status of an oyster population. A previous analysis of the transcriptome of resistant and susceptible American oysters in response to challenge with the bacterial pathogen Roseovarius crassostreae, as well as sequencing of suppression subtractive hybridization libraries from oysters challenged with the protozoan parasite Perkinsus marinus, provided a list of genes potentially involved in disease resistance or susceptibility. We investigated the patterns of inducible gene expression of several of these genes in response to experimental challenge with the oyster pathogens R. crassostreae, Vibrio tubiashii, and P. marinus. Oysters showing differential susceptibility to R. crassostreae demonstrated differential patterns of expression of genes coding for immune (serine protease inhibitor-1, SPI1) and stress-related (heat shock protein 70, HSP70; arginine kinase) proteins 30 days after challenge with this bacterial pathogen. Differential patterns of expression of immune (spi1, galectin and a matrix metalloproteinase) and stress-related (hsp70, histone H4, and arginine kinase) genes was observed in hemocytes from adult oysters challenged with P. marinus, but not with V. tubiashii. While levels of spi1 expression in hemocytes collected 8 and 21 days after P. marinus challenge were negatively correlated with parasite load in oysters tissues at the end of the challenge (62 days), levels of expression of hsp70 in hemocytes collected 1-day after challenge were positively correlated with oyster parasite load at 62 days. Our results confirm previous research on the role of serine protease inhibitor-1 in immunity and disease resistance in oysters. They also suggest that HSP70 and histone H4 could be used as a markers of health status or disease susceptibility in oysters.

Identification of MMV Malaria Box inhibitors of Perkinsus marinus using an ATP-based bioluminescence assay

"Dermo" disease caused by the protozoan parasite Perkinsus marinus (Perkinsozoa) is one of the main obstacles to the restoration of oyster populations in the USA. Perkinsus spp. are also a concern worldwide because there are limited approaches to intervention against the disease. Based on the phylogenetic affinity between the Perkinsozoa and Apicomplexa, we exposed Perkinsus trophozoites to the Medicines for Malaria Venture Malaria Box, an open access compound library comprised of 200 drug-like and 200 probe-like compounds that are highly active against the erythrocyte stage of Plasmodium falciparum. Using a final concentration of 20 µM, we found that 4 days after exposure 46% of the compounds were active against P. marinus trophozoites. Six compounds with IC50 in the µM range were used to compare the degree of susceptibility in vitro of eight P. marinus strains from the USA and five Perkinsus species from around the world. The three compounds, MMV666021, MMV665807 and MMV666102, displayed a uniform effect across Perkinsus strains and species. Both Perkinsus marinus isolates and Perkinsus spp. presented different patterns of response to the panel of compounds tested, supporting the concept of strain/species variability. Here, we expanded the range of compounds available for inhibiting Perkinsus proliferation in vitro and characterized Perkinsus phenotypes based on their resistance to six compounds. We also discuss the implications of these findings in the context of oyster management. The Perkinsus system offers the potential for investigating the mechanism of action of the compounds of interest.

Transcriptome of American oysters, Crassostrea virginica, in response to bacterial challenge: insights into potential mechanisms of disease resistance

The American oyster Crassostrea virginica, an ecologically and economically important estuarine organism, can suffer high mortalities in areas in the Northeast United States due to Roseovarius Oyster Disease (ROD), caused by the gram-negative bacterial pathogen Roseovarius crassostreae. The goals of this research were to provide insights into: 1) the responses of American oysters to R. crassostreae, and 2) potential mechanisms of resistance or susceptibility to ROD. The responses of oysters to bacterial challenge were characterized by exposing oysters from ROD-resistant and susceptible families to R. crassostreae, followed by high-throughput sequencing of cDNA samples from various timepoints after disease challenge. Sequence data was assembled into a reference transcriptome and analyzed through differential gene expression and functional enrichment to uncover genes and processes potentially involved in responses to ROD in the American oyster. While susceptible oysters experienced constant levels of mortality when challenged with R. crassostreae, resistant oysters showed levels of mortality similar to non-challenged oysters. Oysters exposed to R. crassostreae showed differential expression of transcripts involved in immune recognition, signaling, protease inhibition, detoxification, and apoptosis. Transcripts involved in metabolism were enriched in susceptible oysters, suggesting that bacterial infection places a large metabolic demand on these oysters. Transcripts differentially expressed in resistant oysters in response to infection included the immune modulators IL-17 and arginase, as well as several genes involved in extracellular matrix remodeling. The identification of potential genes and processes responsible for defense against R. crassostreae in the American oyster provides insights into potential mechanisms of disease resistance.

Transcriptomic analysis of candidate osmoregulatory genes in the eastern oyster Crassostrea virginica

BACKGROUND

The eastern oyster, Crassostrea virginica, is a euryhaline species that can thrive across a wide range of salinities (5-35). As with all estuarine species, individual oysters must be able to regulate their osmotic balance in response to constant temporal variation in salinity. At the population level, recurrent viability selection may be an additional mechanism shaping adaptive osmoregulatory phenotypes at the margins of oyster salinity tolerance. To identify candidate genes for osmoregulation, we sequenced, assembled, and annotated the transcriptome of wild juvenile eastern oysters from 'high' and 'low' salinity regimes. Annotations and candidates were mostly based on the Pacific oyster (Crassostrea gigas) genome sequence so osmoregulatory relevance in C. virginica was explored by testing functional enrichment of genes showing spatially discrete patterns of expression and by quantifying coding sequence divergence.

RESULTS

The assembly of sequence reads and permissive clustering of potentially oversplit alleles resulted in 98,729 reftigs (contigs and singletons). Of these, 50,736 were annotated with 9,307 belonging to a set of candidate osmoregulatory genes identified from the C. gigas genome. A total of 218,777 SNPs (0.0185 SNPs/bp) were identified in annotated reftigs of C. virginica. Amino acid divergence between translations of C. virginica annotated reftigs and C. gigas coding sequence averaged 23.2 % with an average dN/dS ratio of 0.074, suggesting purifying selection on protein sequences. The high and low salinity source oysters each expressed a subset of genes unique to that group, and the functions for these annotated genes were consistent with known molecular mechanisms for osmotic regulation in molluscs.

CONCLUSIONS

Most of the osmoregulatory gene candidates experimentally identified in C. gigas are present in this C. virginica transcriptome. In general these congeners show coding sequence divergence too high to make the C. gigas genome a useful reference for C. virginica bioinformatics. However, strong purifying selection is characteristic of the osmoregulatory candidates so functional annotations are likely to correspond. An initial examination of C. virginica presence/absence expression patterns across the salinity gradient in a single estuary suggests that many of these candidates have expression patterns that co-vary with salinity, consistent with osmoregulatory function in C. virginica.

Characterization of 40 single nucleotide polymorphism (SNP) via Tm-shift assay in the mud crab (Scylla paramamosain)

In this study, single nucleotide polymorphism (SNP) were identified, confirmed and genotyped in the mud crab (Scylla paramamosain) using Tm-shift assay. High quality sequences (13, 311 bp long) were obtained by re-sequencing that contained 91 SNPs, with a density of one SNP every 146 bp. Of all 91 SNPs, 40 were successfully genotyped and characterized using 30 wild specimens by Tm-shift assay. The minor allele frequency per locus ranged from 0.017 to 0.500. The observed and expected heterozygosity, and polymorphism information content (PIC) ranged from 0.000 to 0.600, from 0.033 to 0.509, and from 0.033 to 0.375, respectively, with an average of 0.142, 0.239 and 0.198 per locus. Seventeen SNPs were significantly deviated from Hardy-Weinberg equilibrium. No significant linkage disequilibrium between pairs of loci was detected after sequential Bonferroni correction (P > 0.00125). Seventeen SNPs were related with known function genes. This study provided new molecular markers for investigation of population genetic diversity, construction of genetic linkage maps and molecular marker-assisted selection in this important crustacean species.

Transcriptome analysis reveals a rich gene set related to innate immunity in the Eastern oyster (Crassostrea virginica)

As a benthic filter-feeder of estuaries, the eastern oyster, Crassostrea virginica, faces tremendous exposure to microbial pathogens. How eastern oysters without adaptive immunity survive in pathogen-rich environments is of fundamental interest, but studies on its immune system are hindered by the lack of genomic resources. We sequenced the transcriptome of an adult oyster with short Illumina reads and assembled 66,229 contigs with a N50 length of 1,503 bp. The assembly covered 89.4 % of published ESTs and 97.9 % of mitochondrial genes demonstrating its quality. A set of 39,978 contigs and unigenes (>300 bp) were identified and annotated by searching public databases. Analysis of the gene set yielded a diverse set of 657 genes related to innate immunity, including many pertaining to pattern recognition, effectors, signal transduction, cytokines, and apoptosis. Gene families encoding C1q domain containing proteins, CTLD, IAPs, Ig_I-set, and TRAFs expanded in C. virginica and Crassostrea gigas. Many key genes of the apoptosis system including IAP, BAX, BAC-2, caspase, FADD, and TNFR were identified, suggesting C. virginica possess advanced apoptosis and apoptosis-regulating systems. Our results show that short Illumina reads can produce transcriptomes of highly polymorphic genomes with coverage and integrity comparable to that from longer 454 reads. The expansion and high diversity in gene families related to innate immunity, point to a complex defense system in the lophotrochozoan C. virginica, probably in adaptation to a pathogen-rich environment.

Identification of immune-related SNPs in the transcriptome of Mytilus chilensis through high-throughput sequencing

Single nucleotide polymorphisms (SNPs) identified in coding regions represent a useful tool for understanding the immune response against pathogens and stressful environmental conditions. In this study, a SNPs database was generated from transcripts involved in the innate immune response of the mussel Mytilus chilensis. The SNPs were identified through hemocytes transcriptome sequencing from 18 individuals, and SNPs mining was performed in 225,336 contigs, yielding 20,306 polymorphisms associated to immune-related genes. Classification of identified SNPs was based on different pathways of the immune response for Mytilus sp. A total of 28 SNPs were identified in the Toll-like receptor pathway and included 5 non-synonymous polymorphisms; 19 SNPs were identified in the apoptosis pathway and included 3 non-synonymous polymorphisms; 35 SNPs were identified in the Ubiquitin-mediated proteolysis pathway and included 4 non-synonymous variants; and 54 SNPs involved in other molecular functions related to the immune response, such as molecular chaperones, antimicrobial peptides, and genes that interacts with marine toxins were also identified. The molecular markers identified in this work could be useful for novel studies, such as those related to associations between high-resolution molecular markers and functional response to pathogen agents.

Concholepas concholepas Ferritin H-like subunit (CcFer): Molecular characterization and single nucleotide polymorphism associated to innate immune response

Ferritin has been identified as the principal protein of iron storage and iron detoxification, playing a pivotal role for the cellular homeostasis in living organisms. However, recent studies in marine invertebrates have suggested its association with innate immune system. In the present study, one Ferritin subunit was identified from the gastropod Concholepas concholepas (CcFer), which was fully characterized by Rapid Amplification of cDNA Ends technique. Simultaneously, a challenge test was performed to evaluate the immune response against Vibrio anguillarum. The full length of cDNA Ccfer was 1030 bp, containing 513 bp of open reading frame that encodes to 170 amino acid peptide, which was similar to the Ferritin H subunit described in vertebrates. Untranslated Regions (UTRs) were identified with a 5'UTR of 244 bp that contains iron responsive element (IRE), and a 3'UTR of 273 bp. The predicted molecular mass of deduced amino acid of CcFer was 19.66 kDa and isoelectric point of 4.92. Gene transcription analysis revealed that CcFer increases against infections with V. anguillarum, showing a peak expression at 6 h post-infection. Moreover, a single nucleotide polymorphism was detected at -64 downstream 5'UTR sequence (SNP-64). Quantitative real time analysis showed that homozygous mutant allele (TT) was significantly associated with higher expression levels of the challenged group compared to wild (CC) and heterozygous (CT) variants. Our findings suggest that CcFer is associated to innate immune response in C. concholepas and that the presence of SNPs may involve differential transcriptional expression of CcFer.

Immunological responses of the mangrove oysters Crassostrea gasar naturally infected by Perkinsus sp. in the Mamanguape Estuary, Paraíba state (Northeastern, Brazil)

Perkinsus genus includes protozoan parasites of marine mollusks, especially bivalves. In the last four years, this parasite has been detected in mangrove oysters Crassostrea rhizophorae and Crassostrea gasar from the Northeastern region of Brazil. Hemocytes are the key cells of the oyster immune system, being responsible for a variety of cellular and humoral reactions, such as phagocytosis, encapsulation and the release of several effector molecules that control the invasion and proliferation of microorganisms. In Brazil, there is little information on perkinsosis and none on the immune responses of native oysters' species against Perkinsus spp. The objective of this study was to determine the effects of natural infection by Perkinsus sp. on the immunological parameters of mangrove oysters C. gasar cultured in the Mamanguape River Estuary (Paraíba, Brazil). Adults oysters (N = 40/month) were sampled in December 2011, March, May, August and October 2012. Gills were removed and used to determine the presence and intensity of the Perkinsus sp. infection, according to a scale of four levels (1-4), using the Ray's fluid thioglycollate medium assay. Immunological parameters were measured in hemolymph samples by flow cytometry, including: total hemocyte count (THC), differential hemocyte count (DHC), cell mortality, phagocytic capacity, and production of Reactive Oxygen Species (ROS). The plasma was used to determine the hemagglutination activity. The results showed the occurrence of Perkinsus sp. with the highest mean prevalence (93.3%) seen so far in oyster populations in Brazil. Despite that, no oyster mortality was associated. In contrast, we observed an increase in hemocyte mortality and a suppression of two of the main defense mechanisms, phagocytosis and ROS production in infected oysters. The increase in the percentage of blast-like cells on the hemolymph, and the increase in THC in oysters heavily infected (at the maximum intensity, 4) suggest an induction of hemocytes proliferation. The immunological parameters varied over the studied months, which may be attributed to the dynamics of infection by Perkinsus sp. The results of the present study demonstrate that Perkinsus sp. has a deleterious effect on C. gasar immune system, mainly in high intensities, which likely renders oysters more susceptible to other pathogens and diseases.

Differential expression of genes involved in immunity and biomineralization during Brown Ring Disease development and shell repair in the Manila clam, Ruditapes philippinarum

Severe drop in Manila clams production in French aquacultured fields since the end of the 1980's is associated to Brown Ring Disease (BRD). This disease, caused by the bacteria Vibrio tapetis, is characterized by specific symptoms on the inner face of the shell. Diseased animals develop conchiolin deposit to enrobe bacteria and form new calcified layers on the shell. Suppression subtractive hybridization was performed to identify genes differentially expressed during the early interaction of V. tapetis and Ruditapes philippinarum. Results revealed 301 unique genes differentially expressed during V. tapetis challenge. Several candidates involved in immune and biomineralization processes were selected from libraries. Transcriptional expression of selected candidates was determined in hemolymph and mantle tissues and revealed spatial and temporal variations. At 56 days after infection, when clams were in phase of shell repair, transcripts of galectin and ferritin in hemocytes showed higher expression. Ca-like and serpin transcripts were specifically expressed in mantle and could contribute to defense against BRD.

Polymorphisms of anti-lipopolysaccharide factors in the swimming crab Portunus trituberculatus and their association with resistance/susceptibility to Vibrio alginolyticus

Anti-lipopolysaccharide factor (ALF) is an important antimicrobial peptide (AMP) that can bind and neutralize major component of Gram-negative bacteria cell wall, lipopolysaccharide (LPS). Seven isoforms of anti-lipopolysaccharide factors (PtALF1-7) were previously identified from the swimming crab Portunus trituberculatus in our laboratory. Here, polymorphisms of PtALF1-7 were detected and their association with resistance/susceptibility to Vibrio alginolyticus (a main Gram-negative bacteria causing high mortality in P. trituberculatus) were investigated. We identified 127, 96, 103, 53 and 158 single nucleotide polymorphisms (SNPs) in genomic fragments of PtALF1-3, PtALF4, PtALF5, PtALF6 and PtALF7, respectively. Among them, totally sixteen SNPs were significantly associated with resistance/susceptibility to V. alginolyticus (P < 0.05). Of these sixteen SNPs, most were located in introns and noncoding exons, while two synonymous SNPs and one nonsynonymous SNP were in coding exons. Additionally, simple sequence repeats (SSRs) were only identified in introns and noncoding exons of PtALF4, PtALF5 and PtALF7. Although no significant difference of allele frequencies was found, these SSRs had different polymorphic alleles according to the repeat number between susceptible and resistant stocks. After further confirmation, polymorphisms investigated here might be applied as potential molecular markers for future selection of resistant strains to diseases caused by Gram-negative bacteria.

A new haplotype variability in complement C6 is marginally associated with resistance to Aeromonas hydrophila in grass carp

Aeromonas hydrophila, a widespread bacterium in the aquatic environment, causes haemorrhagic septicemia in fish. In the last decade, the disease has caused mass mortality and tremendous economic loss in cultured grass carp in the mainland China. The complement component C6 is a constituent of a biochemical cascade that serves as a major effector of the human innate and adaptor immunity, and eliminates infected cells. The objective of this study was to identify single nucleotide polymorphisms (SNPs) in the C6 gene and to assess their association with A. hydrophila resistance in grass carp. A resource population consisting of 186 susceptible and 191 resistant grass carp was constructed. The gcC6 genomic sequence is composed of 9292 bp, containing 18 exons and 17 introns. The promoter sequence of gcC6 gene contained several consensus sequences for hepatic-specific transcription factors. We sequenced a total of 9744 bp of the C6 gene from a diverse population of grass carp and identified 8 SNPs that were genotyped in the resource population. Statistical analysis revealed a lack of association between any individual SNPs and resistance to A. hydrophila in grass carp. The SNPs 1214G>A, 1380G>C, 2095A>C and 2167T>C were linked together (r(2) > 0.8). The haplotype GCCC generated with these four SNPs was associated marginally with resistance to A. hydrophila in grass carp. These findings suggest a lack of strong association of the C6 polymorphisms with the A. hydrophila resistance in grass carp.

Polymorphism of the multiple hemoglobins in blood clam Tegillarca granosa and its association with disease resistance to Vibrio parahaemolyticus

Hemoglobin (Hb) is the major protein component of erythrocytes in animals with red blood, but it can serve additional functions beyond the transport of oxygen. In this study, we identified polymorphism in the blood clam Tegillarca granosa Hb (Tg-Hb) genes and investigated the association of this polymorphism with resistance/susceptibility to Vibrio parahaemolyticus. Analysis of the 540 sequences revealed 28 SNPs in the coding region of three Tg-Hbs, corresponding to about one SNP per 48 bp. Three SNPS: HbIIA-E2-146, HbIIB-E2-23, HbIIB-E2-121 showed a significant association with resistance/susceptibility to V. parahaemolyticus (P < 0.05). To further demonstrate that three significant SNPs of Tg-Hbs is associated with resistance of clams to V. parahaemolyticus, SNPs were genotyped in V. parahaemolyticus resistant strain clams and the wild base population from which this strain was derived. The results indicated that the nonsynonymous mutation T allele at HbIIA-E2-146 and A allele at HbIIB-E2-23 are associated with V. parahaemolyticus resistance in the blood clam, and its association with disease resistance may be due to its cause changes in amino acid sequences to a functional polymorphism. Together with previous bacterial challenge study, these results provides direct evidence that variation at HbIIA-E2-146 and HbIIB-E2-23 are associated with disease resistance in the blood clam, and these two polymorphic loci could be potential gene markers for the future molecular selection of strains that are resistant to diseases caused by V. parahaemolyticus.

Ubiquitin-conjugating enzyme E2-like gene associated to pathogen response in Concholepas concholepas: SNP identification and transcription expression

Ubiquitin-conjugated E2 enzyme (UBE2) is one of the main components of the proteasome degradation cascade. Previous studies have shown an increase of expression levels in individuals challenged to some pathogen organism such as virus and bacteria. The study was to characterize the immune response of UBE2 gene in the gastropod Concholepas concholepas through expression analysis and single nucleotide polymorphisms (SNP) discovery. Hence, UBE2 was identified from a cDNA library by 454 pyrosequencing, while SNP identification and validation were performed using De novo assembly and high resolution melting analysis. Challenge trials with Vibrio anguillarum was carried out to evaluate the relative transcript abundance of UBE2 gene from two to thirty-three hours post-treatment. The results showed a partial UBE2 sequence of 889 base pair (bp) with a partial coding region of 291 bp. SNP variation (A/C) was observed at the 546th position. Individuals challenged by V. anguillarum showed an overexpression of the UBE2 gene, the expression being significantly higher in homozygous individuals (AA) than (CC) or heterozygous individuals (A/C). This study contributes useful information relating to the UBE2 gene and its association with innate immune response in marine invertebrates.

Association between the polymorphism of CfPGRP-S1 gene and disease susceptibility/resistance of zhikong scallop (Chlamys farreri) to Listonella anguillarum challenge

Peptidoglycan recognition protein (PGRP) is a pattern recognition receptor, playing important roles in the innate immune response against invasive pathogens. The single nucleotide polymorphism (SNP) loci in scallop PGRP gene (CfPGRP) were screened from Chlamys farreri to investigate their association with disease resistance of scallop against Listonella anguillarum. Thirteen SNP sites were identified in PGRP domain of CfPGRP, and two of them at positions 4407 and 4408 which are located in the same codon resulted in a nonsynonymous substitution. The genotype frequency of CG/CG in the resistant stock was significantly lower than that in susceptible stock (0% vs 32.4%), while that of CG/TA in the resistant stock was significantly higher than that in susceptible stock (P < 0.01). The pathogen-associated molecular patterns (PAMP) binding activity of two recombinant proteins, rCfPGRP-S1 (R) with CG variant in 4407-4408 site, rCfPGRP-S1 (Y) with TA variant in 4407-4408 site, were elucidated by examining their P/N value at 405 nm with ELISA assay. The in vitro binding activities of the two rCfPGRP-S1 variants to both lipopolysaccharide (LPS) and peptidoglycan (PGN) varied (P < 0.05) in a dose-dependent manner, and rCfPRPP-S1(Y) exhibited significantly higher affinity to PGN and LPS than that of rCfPGRP-S1(R) (P < 0.05). The growth inhibition assay was conducted to find the antibacterial activities of the two variants. Both rCfPGRP-S1(R) and rCfPGRP-S1 (Y) displayed obvious activity to suppress the growth of Escherichia coli, but there was no significant difference in suppression activity of two variants (P > 0.05). The results suggested that the polymorphism at locus 4407-4408 of CfPGRP-S1 considerably affected its PAMP binding activity, and the SNP locus 4407-4408 CG/TA was associated with disease resistance of scallop against L. anguillarum infection, which could be used as a candidate marker for future selection in zhikong scallop breeding program.

Mutation in promoter region of a serine protease inhibitor confers Perkinsus marinus resistance in the eastern oyster (Crassostrea virginica)

Protease inhibitors from the host may inhibit proteases from invading pathogens and confer resistance. We have previously shown that a single-nucleotide polymorphism (SNP198C) in a serine protease inhibitor gene (cvSI-1) is associated with Perkinsus marinus resistance in the eastern oyster. As SNP198 is synonymous, we studied whether its linkage to polymorphism at the promoter region could explain the resistance. A 631 bp fragment of the promoter region was cloned by genome-walking and resequenced, revealing 22 SNPs and 3 insertion/deletions (indels). A 25 bp indel at position -404 was genotyped along with SNP198 for association analysis using before- and after-mortality samples. After mortalities that were primarily caused by P. marinus, the frequency of deletion allele at -404indel increased by 15.6% (p = 0.0437), while that of SNP198C increased by only 3.4% (p = 0.5756). The resistance alleles at the two loci were coupled in 79.6% of the oysters. Oysters with the deletion allele at -404indel showed significant (p = 0.0189) up-regulation of cvSI-1 expression under P. marinus challenge. Our results suggest that mutation at the promoter region causes increased transcription of cvSI-1, which in turn confers P. marinus resistance in the eastern oyster likely through inhibiting pathogenic proteases from the parasite.

Characterisation and genetic polymorphism of metallothionein gene CgMT4 in experimental families of Pacific oyster Crassostrea gigas displaying summer mortality

Summer mortality events have been observed in Pacific oyster Crassostrea gigas for several decades. This paper examines the selective pressure exerted by summer mortality on the polymorphism of a newly identified oyster metallothionein gene. CgMT4 cDNA and genomic sequences were obtained. CgMT4 was studied in two generations of oysters reared in three sites on the French Atlantic coast, using single strand conformation polymorphism analysis. Four alleles were detected. Individuals carrying genotype MT4-CD seem to have higher susceptibility to summer risk conditions. The MT4 gene could be a potential new genetic marker for susceptibility; further validation studies are recommended.