Biomineralization-related specialization of hemocytes and mantle tissues of the Pacific oyster Crassostrea gigas

Proteomic changes associated with predator-induced morphological defenses in oysters

Inducible prey defenses occur when organisms undergo plastic changes in phenotype to reduce predation risk. When predation pressure varies persistently over space or time, such as when predator and prey co-occur over only part of their biogeographic ranges, prey populations can become locally adapted in their inducible defenses. In California estuaries, native Olympia oyster (Ostrea lurida) populations have evolved disparate phenotypic responses to an invasive predator, the Atlantic oyster drill (Urosalpinx cinerea). In this study, oysters from an estuary with drills, and oysters from an estuary without drills, were reared for two generations in a laboratory common garden, and subsequently exposed to cues from Atlantic drills. Comparative proteomics was then used to investigate molecular mechanisms underlying conserved and divergent aspects of their inducible defenses. Both populations developed smaller, thicker, and harder shells after drill exposure, and these changes in shell phenotype were associated with up-regulation of calcium transport proteins that could influence biomineralization. Inducible defenses evolve in part because defended phenotypes incur fitness costs when predation risk is low. Immune proteins were down-regulated by both oyster populations after exposure to drills, implying a trade-off between biomineralization and immune function. Following drill exposure, oysters from the population that co-occurs with drills grew smaller shells than oysters inhabiting the estuary not yet invaded by the predator. Variation in the response to drills between populations was associated with isoform-specific protein expression. This trend suggests that a stronger inducible defense response evolved in oysters that co-occur with drills through modification of an existing mechanism.

The Mantle Transcriptome of Chamelea gallina (Mollusca: Bivalvia) and Shell Biomineralization

Simple Summary

Chamelea gallina is a bivalve mollusc that represents one of the most important fishery resources in the Mediterranean basin. In this species, the thickness and sturdiness of the shell valves are two important characteristics as they are decisive for protection against predators and survival of specimens rejected in the sea because caught under commercial size. The aim of this work is to investigate the ability of this species to modulate the expression of genes encoding proteins involved in shell biomineralization process in response to abiotic and biotic factors. Our findings, obtained through a multidisciplinary approach, highlighted a different shell mineralization behaviour in C. gallina clams collected in sampling sites characterized by different salinity and food availability. Moreover, this study provided the first comprehensive transcriptome from mantle, the tissue responsible for shell formation. Therefore, these results contribute to increase knowledge on this process and might help in adopting ad hoc management plans for this fishery resource.

Abstract

The striped venus Chamelea gallina is a bivalve mollusc that represents one of the most important fishery resources of the Adriatic Sea. In this work, we investigated for the first time the ability of this species to modulate the expression of genes encoding proteins involved in biomineralization process in response to biotic and abiotic factors. We provided the first comprehensive transcriptome from the mantle tissue of clams collected in two sampling sites located along the Italian Adriatic coast and characterized by different environmental features. Moreover, the assessment of environmental parameters, scanning electron microscopy (SEM), and X-ray diffraction (XRD) measurements on valves were conducted to better contextualize RNA sequencing (RNA-Seq) data. Functional annotation of differentially expressed genes (DEGs) and SEM observations highlighted a different shell mineralization behaviour in C. gallina clams collected from two selected sites characterized by diverse environmental parameters.

Bivalve Haemocyte Subpopulations: A Review

Bivalve molluscs stand out for their ecological success and their key role in the functioning of aquatic ecosystems, while also constituting a very valuable commercial resource. Both ecological success and production of bivalves depend on their effective immune defence function, in which haemocytes play a central role acting as both the undertaker of the cellular immunity and supplier of the humoral immunity. Bivalves have different types of haemocytes, which perform different functions. Hence, identification of cell subpopulations and their functional characterisation in immune responses is essential to fully understand the immune system in bivalves. Nowadays, there is not a unified nomenclature that applies to all bivalves. Characterisation of bivalve haemocyte subpopulations is often combined with 1) other multiple parameter assays to determine differences between cell types in immune-related physiological activities, such as phagocytosis, oxidative stress and apoptosis; and 2) immune response to different stressors such as pathogens, temperature, acidification and pollution. This review summarises the major and most recent findings in classification and functional characterisation of the main haemocyte types of bivalve molluscs.

Transcriptome analysis of the bivalve Placuna placenta mantle reveals potential biomineralization-related genes

The shells of window pane oyster Placuna placenta are very thin and exhibit excellent optical transparency and mechanical robustness. However, little is known about the biomineralization-related proteins of the shells of P. placenta. In this work, we report the comprehensive transcriptome of the mantle tissue of P. placenta for the first time. The unigenes of the mantle tissue of P. placenta were annotated by using the public databases such as nr, GO, KOG, KEGG, and Pfam. 24,343 unigenes were annotated according to Pfam database, accounting for 21.48% of the total unigenes. We find that half of the annotated unigenes of the mantle tissue of P. placenta are consistent to the annotated unigenes from pacific oyster Crassostrea gigas according to nr database. The unigene sequence analysis from the mantle tissue of P. placenta indicates that 465,392 potential single nucleotide polymorphisms (SNPs) and 62,103 potential indel markers were identified from 60,371 unigenes. 178 unigenes of the mantle tissue of P. placenta are found to be homologous to those reported proteins related to the biomineralization process of molluscan shells, while 18 of them are highly expressed unigenes in the mantle tissue. It is proposed that four unigenes with the highest expression levels in the mantle tissue are very often related to the biomineralization process, while another three unigenes are potentially related to the biomineralization process according to the Quantitative Real-Time Polymerase Chain Reaction (qRT-PCR) analysis. In summary, the transcriptome analysis of the mantle tissue of P. Placenta shows the potential biomineralization-related proteins and this work may shed light for the shell formation mechanism of bivalves.

Transcriptomic, Proteomic, and Functional Assays Underline the Dual Role of Extrapallial Hemocytes in Immunity and Biomineralization in the Hard Clam Mercenaria mercenaria

Circulating hemocytes in the hemolymph represent the backbone of innate immunity in bivalves. Hemocytes are also found in the extrapallial fluid (EPF), the space delimited between the shell and the mantle, which is the site of shell biomineralization. This study investigated the transcriptome, proteome, and function of EPF and hemolymph in the hard clam Mercenaria mercenaria. Total and differential hemocyte counts were similar between EPF and hemolymph. Overexpressed genes in the EPF were found to have domains previously identified as being part of the "biomineralization toolkit" and involved in bivalve shell formation. Biomineralization related genes included chitin-metabolism genes, carbonic anhydrase, perlucin, and insoluble shell matrix protein genes. Overexpressed genes in the EPF encoded proteins present at higher abundances in the EPF proteome, specifically those related to shell formation such as carbonic anhydrase and insoluble shell matrix proteins. Genes coding for bicarbonate and ion transporters were also overexpressed, suggesting that EPF hemocytes are involved in regulating the availability of ions critical for biomineralization. Functional assays also showed that Ca2+ content of hemocytes in the EPF were significantly higher than those in hemolymph, supporting the idea that hemocytes serve as a source of Ca2+ during biomineralization. Overexpressed genes and proteins also contained domains such as C1q that have dual functions in biomineralization and immune response. The percent of phagocytic granulocytes was not significantly different between EPF and hemolymph. Together, these findings suggest that hemocytes in EPF play a central role in both biomineralization and immunity.

Cellular bicarbonate accumulation and vesicular proton transport promote calcification in the sea urchin larva

The sea urchin embryo develops a calcitic endoskeleton through intracellular formation of amorphous calcium carbonate (ACC). Intracellular precipitation of ACC, requires [Formula: see text] concentrating as well as proton export mechanisms to promote calcification. These processes are of fundamental importance in biological mineralization, but remain largely unexplored. Here, we demonstrate that the calcifying primary mesenchyme cells (PMCs) use Na+/H+-exchange (NHE) mechanisms to control cellular pH homeostasis during maintenance of the skeleton. During skeleton re-calcification, pHi of PMCs is increased accompanied by substantial elevation in intracellular [Formula: see text] mediated by the [Formula: see text] cotransporter Sp_Slc4a10. However, PMCs lower their pHi regulatory capacities associated with a reduction in NHE activity. Live-cell imaging using green fluorescent protein reporter constructs in combination with intravesicular pH measurements demonstrated alkaline and acidic populations of vesicles in PMCs and extensive trafficking of large V-type H+-ATPase (VHA)-rich acidic vesicles in blastocoelar filopodial cells. Pharmacological and gene expression analyses underline a central role of the VHA isoforms Sp_ATP6V0a1, Sp_ATP6V01_1 and Sp_ATPa1-4 for the process of skeleton re-calcification. These results highlight novel pH regulatory strategies in calcifying cells of a marine species with important implications for our understanding of the mineralization process in times of rapid changes in oceanic pH.

Comparative genomic analysis of Vibrios yields insights into genes associated with virulence towards C. gigas larvae

BACKGROUND

Vibriosis has been implicated in major losses of larvae at shellfish hatcheries. However, the species of Vibrio responsible for disease in aquaculture settings and their associated virulence genes are often variable or undefined. Knowledge of the specific nature of these factors is essential to developing a better understanding of the environmental and biological conditions that lead to larvae mortality events in hatcheries. We tested the virulence of 51 Vibrio strains towards Pacific Oyster (Crassostreae gigas) larvae and sequenced draft genomes of 42 hatchery-associated vibrios to determine groups of orthologous genes associated with virulence and to determine the phylogenetic relationships among pathogens and non-pathogens of C. gigas larvae.

RESULTS

V. coralliilyticus strains were the most prevalent pathogenic isolates. A phylogenetic logistic regression model identified over 500 protein-coding genes correlated with pathogenicity. Many of these genes had straightforward links to disease mechanisms, including predicted hemolysins, proteases, and multiple Type 3 Secretion System genes, while others appear to have possible indirect roles in pathogenesis and may be more important for general survival in the host environment. Multiple metabolism and nutrient acquisition genes were also identified to correlate with pathogenicity, highlighting specific features that may enable pathogen survival within C. gigas larvae.

CONCLUSIONS

These findings have important implications on the range of pathogenic Vibrio spp. found in oyster-rearing environments and the genetic determinants of virulence in these populations.

Intracellular pH regulation in mantle epithelial cells of the Pacific oyster, Crassostrea gigas

Shell formation and repair occurs under the control of mantle epithelial cells in bivalve molluscs. However, limited information is available on the precise acid–base regulatory machinery present within these cells, which are fundamental to calcification. Here, we isolate mantle epithelial cells from the Pacific oyster, Crassostrea gigas and utilise live cell imaging in combination with the fluorescent dye, BCECF-AM to study intracellular pH (pH_i) regulation. To elucidate the involvement of various ion transport mechanisms, modified seawater solutions (low sodium, low bicarbonate) and specific inhibitors for acid–base proteins were used. Diminished pH recovery in the absence of Na⁺ and under inhibition of sodium/hydrogen exchangers (NHEs) implicate the involvement of a sodium dependent cellular proton extrusion mechanism. In addition, pH recovery was reduced under inhibition of carbonic anhydrases. These data provide the foundation for a better understanding of acid–base regulation underlying the physiology of calcification in bivalves.

DNA from mollusc shell: a valuable and underutilised substrate for genetic analyses

Mollusc shells are an abundant resource that have been long used to predict the structures of ancient ecological communities, examine evolutionary processes, reconstruct paleoenvironmental conditions, track and predict responses to climatic change, and explore the movement of hominids across the globe. Despite the ubiquity of mollusc shell in many environments, it remains relatively unexplored as a substrate for molecular genetic analysis. Here we undertook a series of experiments using the New Zealand endemic greenshell mussel, Perna canaliculus, to explore the utility of fresh, aged, beach-cast and cooked mollusc shell for molecular genetic analyses. We find that reasonable quantities of DNA (0.002-21.48 ng/mg shell) can be derived from aged, beach-cast and cooked mussel shell and that this can routinely provide enough material to undertake PCR analyses of mitochondrial and nuclear gene fragments. Mitochondrial PCR amplification had an average success rate of 96.5% from shell tissue extracted thirteen months after the animal's death. A success rate of 93.75% was obtained for cooked shells. Amplification of nuclear DNA (chitin synthase gene) was less successful (80% success from fresh shells, decreasing to 10% with time, and 75% from cooked shells). Our results demonstrate the promise of mollusc shell as a substrate for genetic analyses targeting both mitochondrial and nuclear genes.

Effects of seawater salinity and pH on cellular metabolism and enzyme activities in biomineralizing tissues of marine bivalves

Molluscan shell formation is a complex energy demanding process sensitive to the shifts in seawater CaCO₃ saturation due to changes in salinity and pH. We studied the effects of salinity and pH on energy demand and enzyme activities of biomineralizing cells of the Pacific oyster (Crassostrea gigas) and the hard-shell clam (Mercenaria mercenaria). Adult animals were exposed for 14 days to high (30), intermediate (18), or low (10) salinity at either high (8.0-8.2) or low (7.8) pH. Basal metabolic cost as well as the energy cost of the biomineralization-related cellular processes were determined in isolated mantle edge cells and hemocytes. The total metabolic rates were similar in the hemocytes of the two studied species, but considerably higher in the mantle cells of C. gigas compared with those of M. mercenaria. Cellular respiration was unaffected by salinity in the clams' cells, while in oysters' cells the highest respiration rate was observed at intermediate salinity (18). In both studied species, low pH suppressed cellular respiration. Low pH led to an upregulation of Na⁺/K⁺ ATPase activity in biomineralizing cells of oysters and clams. Activities of Ca²⁺ ATPase and H⁺ ATPase, as well as the cellular energy costs of Ca²⁺ and H⁺ transport in the biomineralizing cells were insensitive to the variation in salinity and pH in the two studied species. Variability in cellular response to low salinity and pH indicates that the disturbance of shell formation under these conditions has different underlying mechanisms in the two studied species.

Teosin, a novel basic shell matrix protein from Hyriopsis cumingii induces calcium carbonate polycrystal formation

In this study, a novel matrix protein (teosin) was isolated from Hyriopsis cumingii. Gene expression analysis showed that teosin is mainly expressed in the mantle and blood, and a hybridization signal was found in dorsal epithelial cells of the mantle pallial by in situ hybridization. Moreover, teosin expression during pearl formation indicated its participation in initial nacreous layer biomineralization, and suppressing teosin expression resulted in irregular crystal morphology and disordered arrangement in RNAi assay. In vitro crystallization assays indicated teosin could increase the size of calcite. By turning the sample stage about 15°, we got the high-resolution TEM images of the crystals' edges. This is a novel method to observe the crystal which is over 200 nm under TEM. In the control experiment group, the calcite show the character of long range order. The calcite induced by teosin were composed of nano-grains, and the polycrystal character were confirmed by EDS. These results suggested that teosin is involved in regulating crystal morphology regulation and inducing polycrystal formation during nacreous-layer formation.

Molecular adaptation of molluscan biomineralisation to high-CO2 oceans - The known and the unknown

High-CO₂ induced ocean acidification (OA) reduces the calcium carbonate (CaCO₃) saturation level (Ω) and the pH of oceans. Consequently, OA is causing a serious threat to several ecologically and economically important biomineralising molluscs. Biomineralisation is a highly controlled biochemical process by which molluscs deposit their calcareous structures. In this process, shell matrix proteins aid the nucleation, growth and assemblage of the CaCO₃ crystals in the shell. These molluscan shell proteins (MSPs) are, ultimately, responsible for determination of the diverse shell microstructures and mechanical strength. Recent studies have attempted to integrate gene and protein expression data of MSPs with shell structure and mechanical properties. These advances made in understanding the molecular mechanism of biomineralisation suggest that molluscs either succumb or adapt to OA stress. In this review, we discuss the fate of biomineralisation process in future high-CO₂ oceans and its ultimate impact on the mineralised shell's structure and mechanical properties from the perspectives of limited substrate availability theory, proton flux limitation model and the omega myth theory. Furthermore, studying the interplay of energy availability and differential gene expression is an essential first step towards understanding adaptation of molluscan biomineralisation to OA, because if there is a need to change gene expression under stressors, any living system would require more energy than usual. To conclude, we have listed, four important future research directions for molecular adaptation of molluscan biomineralisation in high-CO₂ oceans: 1) Including an energy budgeting factor while understanding differential gene expression of MSPs and ion transporters under OA. 2) Unraveling the genetic or epigenetic changes related to biomineralisation under stressors to help solving a bigger picture about future evolution of molluscs, and 3) Understanding Post Translational Modifications of MSPs with and without stressors. 4) Understanding carbon uptake mechanisms across taxa with and without OA to clarify the OA theories on Ω.

Ocean Acidification and Coastal Marine Invertebrates: Tracking CO2 Effects from Seawater to the Cell

In the last few decades, numerous studies have investigated the impacts of simulated ocean acidification on marine species and communities, particularly those inhabiting dynamic coastal systems. Despite these research efforts, there are many gaps in our understanding, particularly with respect to physiological mechanisms that lead to pathologies. In this review, we trace how carbonate system disturbances propagate from the coastal environment into marine invertebrates and highlight mechanistic links between these disturbances and organism function. We also point toward several processes related to basic invertebrate biology that are severely understudied and prevent an accurate understanding of how carbonate system dynamics influence organismic homeostasis and fitness-related traits. We recommend that significant research effort be directed to studying cellular phenotypes of invertebrates acclimated or adapted to elevated seawater pCO₂ using biochemical and physiological methods.

Recoverable impacts of ocean acidification on the tubeworm, Hydroides elegans: implication for biofouling in future coastal oceans

Ocean uptake of anthropogenic CO₂ causes ocean acidification (OA), which not only decreases the calcification rate, but also impairs the formation of calcareous shells or tubes in marine invertebrates such as the dominant biofouling tubeworm species, Hydroides elegans. This study examined the ability of tubeworms to resume normal tube calcification when returned to ambient pH 8.1 from a projected near-future OA level of pH 7.8. Tubeworms produced structurally impaired and mechanically weaker calcareous tubes at pH 7.8 compared to at pH 8.1, but were able to recover when the pH was restored to ambient levels. This suggests that tubeworms can physiologically recover from the impacts of OA on tube calcification, composition, density, hardness and stiffness when returned to optimal conditions. These results help understanding of the progression of biofouling communities dominated by tubeworms in future oceans with low pH induced by OA.

The global effects of PmRunt co-located and co-expressed with a lincRNA lncRunt in pearl oyster Pinctada fucata martensii

Runt related transcription factors as trans-acting elements play critical roles in the developmental control of cell fate, hematopoiesis, bone formation and cancers. In previous study, the homologue of runt related transcription factor PmRunt has been identified from pearl oyster Pinctada fucata martensii and considered to play an important role in nacre formation. In this study, we used the same samples to perform RNA-seq to detect the global effects after the decrease of PmRunt expression. The transcription levels of several nacre shell matrix protein (NSMP) genes were significantly changed and the potential compensatory effect could happen internal gene families. Downregulation of PmRunt could also influence the biosynthesis of NSMPs through affecting amino acid metabolism, translation, protein processing and export. The inhibition of PmRunt also possibly affected the expression of caspases, IAPs and C1qs that related to apoptosis and immune. In addition, PmRunt highly expressed at 12 h and 12 d after transplantation in hemolymph, which was corresponded to transplantation immunity immune response and the morphology of pearl sac, suggested the cross-talk of biomineralization-immune regulation in hemocytes. Furthermore, a lincRNA (LncRunt) that co-located with PmRunt was identified and showed a significantly relative expression with PmRunt, which suggested the potential regulation. Therefore, these findings provided new idea to find the regulation targets of runt-related transcription factors and offers evidence of lncRNAs in potential biomineralization-immune regulation.

An Evolutionary Perspective of Dopachrome Tautomerase Enzymes in Metazoans

Melanin plays a pivotal role in the cellular processes of several metazoans. The final step of the enzymically-regulated melanin biogenesis is the conversion of dopachrome into dihydroxyindoles, a reaction catalyzed by a class of enzymes called dopachrome tautomerases. We traced dopachrome tautomerase (DCT) and dopachrome converting enzyme (DCE) genes throughout metazoans and we could show that only one class is present in most of the phyla. While DCTs are typically found in deuterostomes, DCEs are present in several protostome phyla, including arthropods and mollusks. The respective DCEs belong to the yellow gene family, previously reported to be taxonomically restricted to insects, bacteria and fungi. Mining genomic and transcriptomic data of metazoans, we updated the distribution of DCE/yellow genes, demonstrating their presence and active expression in most of the lophotrochozoan phyla as well as in copepods (Crustacea). We have traced one intronless DCE/yellow gene through most of the analyzed lophotrochozoan genomes and we could show that it was subjected to genomic diversification in some species, while it is conserved in other species. DCE/yellow was expressed in most phyla, although it showed tissue specific expression patterns. In the parasitic copepod Mytilicola intestinalis DCE/yellow even belonged to the 100 most expressed genes. Both tissue specificity and high expression suggests that diverse functions of this gene family also evolved in other phyla apart from insects.

A Kunitz proteinase inhibitor (HcKuPI) participated in antimicrobial process during pearl sac formation and induced the overgrowth of calcium carbonate in Hyriopsis cumingii

Proteinase inhibitors with the ability to inhibit specific proteinases are usually closely connected with the immune system. Interestingly, proteinase inhibitors are also a common ingredient in the organic matrix of mollusk shells. However, the molecular mechanism that underlies the role of proteinase inhibitors in immune system and shell mineralization is poorly known. In this study, a Kunitz serine proteinase inhibitor (HcKuPI) was isolated from the mussel Hyriopsis cumingii. HcKuPI was specifically expressed in dorsal epithelial cells of the mantle pallium and HcKuPI dsRNA injection caused an irregular surface and disordered deposition on the aragonite tablets of the nacreous layer. These results indicated that HcKuPI plays a vital role in shell nacreous layer biomineralization. Moreover, the expression pattern of HcKuPI during LPS challenge and pearl formation indicated its involvement in the antimicrobial process during pearl sac formation and nacre tablets accumulation during pearl formation. In the in vitro calcium carbonate crystallization assay, the addition of GST-HcKuPI increased the precipitation rate of calcium carbonate and induced the crystal overgrowth of calcium carbonate. Taken together, these results indicate that HcKuPI is involved in antimicrobial process during pearl formation, and participates in calcium carbonate deposition acceleration and morphological regulation of the crystals during nacreous layer formation. These findings extend our knowledge of the role of proteinase inhibitors in immune system and shell biomineralization.

Gene expression profiles at different stages for formation of pearl sac and pearl in the pearl oyster Pinctada fucata

Background

The most critical step in the pearl formation during aquaculture is issued to the proliferation and differentiation of outer epithelial cells of mantle graft into pearl sac. This pearl sac secretes various matrix proteins to produce pearls by a complex physiological process which has not been well-understood yet. Here, we aimed to unravel the genes involved in the development of pearl sac and pearl, and the sequential expression patterns of different shell matrix proteins secreted from the pearl sac during pearl formation by pearl oyster Pinctada fucata using high-throughput transcriptome profiling.

Results

Principal component analysis (PCA) showed clearly different gene expression profiles between earlier (before 1 week) and later stages (1 week to 3 months) of grafting. Immune-related genes were highly expressed between 0 h – 24 h (donor dependent) and 48 h – 1 w (host dependent), and in the course of wound healing process pearl sac was developed by two weeks of graft transplantation. Moreover, for the first time, we identified some stem cell marker genes including ABCG2, SOX2, MEF2A, HES1, MET, NRP1, ESR1, STAT6, PAX2, FZD1 and PROM1 that were expressed differentially during the formation of pearl sac. The expression profiling of 192 biomineralization-related genes demonstrated that most of the shell matrix proteins (SMPs) involved in prismatic layer formation were first up-regulated and then gradually down-regulated indicating their involvement in the development of pearl sac and the onset of pearl mineralization. Most of the nacreous layer forming SMPs were up-regulated at 2 weeks after the maturation of pearl sac. Nacrein, MSI7 and shematrin involved in both layer formation were highly expressed during 0 h – 24 h, down-regulated up to 1 week and then up-regulated again after accomplishment of pearl sac formation.

Conclusions

Using an RNA-seq approach we unraveled the expression pattern of the key genes involved in the development of pearl sac and pearl as a result of host immune response after grafting. These findings provide valuable information in understanding the molecular mechanism of pearl formation and immune response in P. fucata.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5579-3) contains supplementary material, which is available to authorized users.

From the raw bar to the bench: Bivalves as models for human health

Bivalves, from raw oysters to steamed clams, are popular choices among seafood lovers and once limited to the coastal areas. The rapid growth of the aquaculture industry and improvement in the preservation and transport of seafood have enabled them to be readily available anywhere in the world. Over the years, oysters, mussels, scallops, and clams have been the focus of research for improving the production, managing resources, and investigating basic biological and ecological questions. During this decade, an impressive amount of information using high-throughput genomic, transcriptomic and proteomic technologies has been produced in various classes of the Mollusca group, and it is anticipated that basic and applied research will significantly benefit from this resource. One aspect that is also taking momentum is the use of bivalves as a model system for human health. In this review, we highlight some of the aspects of the biology of bivalves that have direct implications in human health including the shell formation, stem cells and cell differentiation, the ability to fight opportunistic and specific pathogens in the absence of adaptive immunity, as source of alternative drugs, mucosal immunity and, microbiome turnover, toxicology, and cancer research. There is still a long way to go; however, the next time you order a dozen oysters at your favorite raw bar, think about a tasty model organism that will not only please your palate but also help unlock multiple aspects of molluscan biology and improve human health.

Developmental characteristics of pearl oyster Pinctada fucata martensii: insight into key molecular events related to shell formation, settlement and metamorphosis

Background

Marine bivalves undergo complex development processes, such as shell morphology conversion and changes of anatomy and life habits. In this study, the transcriptomes of pearl oyster Pinctada fucata martensii and Pacific oyster Crassostrea gigas at different development stages were analyzed to determine the key molecular events related to shell formation, settlement and metamorphosis.

Result

According to the shell matrix proteome, biomineralization-related genes exhibited a consensus expression model with the critical stages of shell formation. Differential expression analysis of P. f. martensii, revealed the negative regulation and feedback of extracellular matrixs as well as growth factor pathways involved in shell formation of larvae, similar to that in C. gigas. Furthermore, neuroendocrine pathways in hormone receptors, neurotransmitters and neuropeptide receptors were involved in shell formation, settlement and metamorphosis.

Conclusion

Our research demonstrated the main clusters of regulation elements related to shell formation, settlement and metamorphosis. The regulation of shell formation and metamorphosis could be coupled forming the neuroendocrine-biomineralization crosstalk in metamorphosis. These findings could provide new insights into the regulation in bivalve development.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5505-8) contains supplementary material, which is available to authorized users.

Transcriptional changes in the Japanese scallop (Mizuhopecten yessoensis) shellinfested by Polydora provide insights into the molecular mechanism of shell formation and immunomodulation

The Japanese scallop (Mizuhopecten yessoensis) is one of the most important aquaculture species in Asian countries; however, it has suffered severe infection by Polydora in northern China in recent years, causing great economic losses. The Polydora parasitizes the shell of scallops, badly destroying the shell's structure. To investigate the molecular response mechanism of M. yessoensis to Polydora infestion, a comprehensive and niche-targeted cDNA sequence database for diseased scallops was constructed. Additionally, the transcriptional changes in the edge mantle, central mantle and hemocytes, tissues directly related to the disease, were first described in this study. The results showed that genes involved in shell formation and immunomodulation were significantly differentially expressed due to the infestation. Different transcriptional changes existed between the two mantle regions, indicating the different molecular functions likely responsible for the formation of different shell layers. The differential expression of genes for immune recognition, signal transduction and pathogen elimination presented an integrated immune response process in scallops. Moreover, neuromodulation and glycometabolism involved in the regulation process with relevant function significantly enriched. The study provides valuable information for mechanism study of shell formation and immunomodulation in scallops.

Potential trade-offs between biomineralization and immunity revealed by shell properties and gene expression profiles of two closely related Crassostrea species

Species of the Ostreidae family are key ecosystem engineers and many of them - including Crassostrea gigas and Crassostreavirginica - are commercially important aquaculture species. Despite similarities in their morphology and ecology, these two species differ in their ability to defend against pathogens, potentially reflecting species-specific differential specialization of hemocytes on immune defense versus biomineralization. To test this hypothesis, we investigated the expression levels of immune- and biomineralization-related genes as well as mineralogical and mechanical properties of the shells and the calcium sequestration ability of the hemocytes of C. gigas and C. virginica The expression of biomineralization-related genes was higher in C. virginica than in C. gigas in multiple tissues including the mantle edge and hemocytes, while the expression of immune genes was higher in the hemocytes of C. gigas Hemocytes of C. virginica contained more calcium (stored intracellularly as calcium carbonate mineral) compared with those of C. gigas Analysis of the adult shells showed that the crystallinity of calcite was higher and the laths of the foliated layer of the shell were thicker in C. virginica than in C. gigas Mechanically, the shells of C. virginica were stiffer, harder and stronger than those of C. gigas Taken together, our results show that the species-specific differences in physiology (such as disease resistance and exoskeleton properties) are reflected at the cellular and molecular levels in the differential specialization of hemocytes on potentially competing functions (immunity and biomineralization) as well as different expression profiles of other tissues involved in biomineralization (such as the mantle edge).