Mantle margin morphogenesis in Nodipecten nodosus (Mollusca: Bivalvia): new insights into the development and the roles of bivalve pallial folds

Transcriptional and post-translational regulation of MITF mediated by bHLH domain during the melanogenesis and melanocyte proliferation in Crassostrea gigas

Melanocyte differentiation is orchestrated by the master regulator transcription factor MITF. However, its ability to discern distinct binding sites linked to effective gene regulation remains poorly understood. This study aims to assess how co-activator acetyltransferase interacts with MITF to modulate their related lysine action, thereby mediating downstream gene regulation, including DNA affinity, stability, transcriptional activity, particularly in the process of shell pigmentation. Here, we have demonstrated that the CgMITF protein can be acetylated, further enabling selective amplification of the melanocyte maturation program. Collaboration with transcriptional co-regulator p300 advances MITF dynamically interplay with downstream targeted gene promoters. We have established that MITF activation was partially dependent on the bHLH domain, which was well conserved across species. The bHLH domain contained conserved lysine residues, including K6 and K43, which interacted with the E-box motif of downstream targeted-genes. Mutations at K6 and K43 lead to a decrease in the binding affinity of the E-box motif. CgMITF protein bound to the E-box motif within the promoter regions of the tyrosinase-related genes, contributing to melanogenesis, and also interacted with the E-box motif within the TBX2 promoter regions, associated with melanocyte proliferation. We elucidated how the bHLH domain links the transcriptional regulation and acetylation modifications in the melanocyte development in C. gigas.

Physical Phenomena Governing Mineral Morphogenesis in Molluscan Nacre

Mollusks, as well as many other living organisms, have the ability to shape mineral crystals into unconventional morphologies and to assemble them into complex functional mineral-organic structures, an observation that inspired tremendous research efforts in scientific and technological domains. Despite these, a biochemical toolkit that accounts for the formation of the vast variety of the observed mineral morphologies cannot be identified yet. Herein, phase-field modeling of molluscan nacre formation, an intensively studied biomineralization process, is used to identify key physical parameters that govern mineral morphogenesis. Manipulating such parameters, various nacre properties ranging from the morphology of a single mineral building block to that of the entire nacreous assembly are reproduced. The results support the hypothesis that the control over mineral morphogenesis in mineralized tissues happens via regulating the physico-chemical environment, in which biomineralization occurs: the organic content manipulates the geometric and thermodynamic boundary conditions, which in turn, determine the process of growth and the form of the biomineral phase. The approach developed here has the potential of providing explicit guidelines for the morphogenetic control of synthetically formed composite materials.

Metabolic profiling of Mytilus coruscus mantle in response of shell repairing under acute acidification

Mytilus coruscus is an economically important marine bivalve mollusk found in the Yangtze River estuary, which experiences dramatic pH fluctuations due to seasonal freshwater input and suffer from shell fracture or injury in the natural environment. In this study, we used intact-shell and damaged-shell M. coruscus and performed metabolomic analysis, free amino acids analysis, calcium-positive staining, and intracellular calcium level tests in the mantle to investigate whether the mantle-specific metabolites can be induced by acute sea-water acidification and understand how the mantle responds to acute acidification during the shell repair process. We observed that both shell damage and acute acidification induced alterations in phospholipids, amino acids, nucleotides, organic acids, benzenoids, and their analogs and derivatives. Glycylproline, spicamycin, and 2-aminoheptanoic acid (2-AHA) are explicitly induced by shell damage. Betaine, aspartate, and oxidized glutathione are specifically induced by acute acidification. Our results show different metabolic patterns in the mussel mantle in response to different stressors, which can help elucidate the shell repair process under ocean acidification. furthermore, metabolic processes related to energy supply, cell function, signal transduction, and amino acid synthesis are disturbed by shell damage and/or acute acidification, indicating that both shell damage and acute acidification increased energy consumption, and disturb phospholipid synthesis, osmotic regulation, and redox balance. Free amino acid analysis and enzymatic activity assays partially confirmed our findings, highlighting the adaptation of M. coruscus to dramatic pH fluctuations in the Yangtze River estuary.

Ca2+ addition facilitates the shell repair with eggs production of Pomacea canaliculata through biomineralization and food intaking regulation

Pomacea canaliculata was by far one of the most harmful invasive organisms in the world, causing serious harm to aquatic crops and ecosystem. Calcium carbonate is a common component of aquatic environment, which is important for the growth of Pomacea canaliculata. Therefore, the objective of this study was to investigate the response characteristics of P. canaliculata suffered shell breakage to the addition of calcium carbonate in water environment. In this experiment, we explored the effects of calcium carbonate addition on the P. canaliculata shell repair rate, food intake, egg production, shell strength, and calcium content through breaking the snails shell and the addition of calcium carbonate treatment. The results showed that snail broken-shell repaired mostly within 21 days. The snails experienced a significant increase in shell repair rates during earlier days of the treatment, especially for female snails. Food intake of snails exhibited different patterns when their shells were broken and calcium carbonate was added. Shell breakage treatment combined with calcium carbonate addition significantly increased the diameter of snail eggs compared with the control and the calcium carbonate addition treatment without shell-broken snail group. There was no significant difference in shell strength or calcium content of male snails between the treatments. The study suggests that P. canaliculata exhibits a sex-dependent response pattern when subjected to shell damage and calcium carbonate addition. The findings can provide some references to better understand the invasion mechanism and survival strategy of the P. canaliculata.

Development of Nanopore sequencing-based full-length transcriptome database toward functional genome annotation of the Pacific oyster, Crassostrea gigas

The Pacific oyster (Crassostrea gigas) is a widely cultivated shellfish in the world, while its transcriptome diversity remains less unexplored due to the limitation of short reads. In this study, we used Oxford Nanopore sequencing to develop the full-length transcriptome database of C. gigas. We identified 77,920 full-length transcripts from 21,523 genes, and uncovered 9668 alternative splicing events and 87,468 alternative polyadenylation sites. Notably, a total of 16,721 novel transcripts were annotated in this work. Furthermore, integrative analysis of 25 publicly available RNA-seq datasets revealed the transcriptome diversity involved in post-transcriptional regulation in C. gigas. We further developed a Drupal based webserver, Cgtdb, which can be used for transcriptome visualization, sequence alignment, and functional genome annotation analyses. This work provides valuable resources and a useful tool for integrative analysis of various transcriptome datasets in C. gigas, which will serve as an essential reference for functional annotation of the oyster genome.

Critical Evaluation of Embedding Media for Histological Studies of Early Stages of Chick Embryo Development

A histological examination is an important tool in embryology, developmental biology, and correlated areas. Despite the amount of information available about tissue embedding and different media, there is a lack of information regarding best practices for embryonic tissues. Embryonic tissues are considered fragile structures, usually small in size, and frequently challenging to position correctly in media for the subsequent histological steps. Here, we discuss the embedding media and procedures that provided us with appropriate preservation of tissue and easier orientation of embryos at early development. Fertilized Gallus gallus eggs were incubated for 72 h, collected, fixed, processed, and embedded with paraplast, polyethylene glycol (PEG), or historesin. These resins were compared by the precision of tissue orientation, the preview of the embryos in the blocks, microtomy, contrast in staining, preservation, average time, and cost. Paraplast and PEG did not allow correct embryo orientation, even with agar–gelatin pre-embedded samples. Additionally, structural maintenance was hindered and did not allow detailed morphological assessment, presenting tissue shrinkage and disruption. Historesin provided precise tissue orientation and excellent preservation of structures. Assessing the performance of the embedding media contributes significantly to future developmental research, optimizing the processing of embryo specimens and improving results.

Bioaccumulation Pattern of the SARS-CoV-2 Spike Proteins in Pacific Oyster Tissues

There is mounting evidence of the contamination of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in the sewage, surface water, and even marine environment. Various studies have confirmed that bivalve mollusks can bioaccumulate SARS-CoV-2 RNA to detectable levels. However, these results do not provide sufficient evidence for the presence of infectious viral particles. To verify whether oysters can bind the viral capsid and bioaccumulate the viral particles, Pacific oysters were artificially contaminated with the recombinant SARS-CoV-2 spike protein S1 subunit (rS1). The bioaccumulation pattern of the rS1 in different tissues was investigated by immunohistological assays. The results revealed that the rS1 was bioaccumulated predominately in the digestive diverticula. The rS1 was also present in the epithelium of the nondigestive tract tissues, including the gills, mantle, and heart. In addition, three potential binding ligands, including angiotensin-converting enzyme 2 (ACE 2)-like substances, A-type histo-blood group antigen (HBGA)-like substances, and oyster heat shock protein 70 (oHSP 70), were confirmed to bind rS1 and were distributed in tissues with various patterns. The colocalization analysis of rS1 and those potential ligands indicated that the distributions of rS1 are highly consistent with those of ACE 2-like substances and oHSP 70. Both ligands are distributed predominantly in the secretory absorptive cells of the digestive diverticula and may serve as the primary ligands to bind rS1. Therefore, oysters are capable of bioaccumulating the SARS-CoV-2 capsid readily by filter-feeding behavior assisted by specific binding ligands, especially in digestive diverticula. IMPORTANCE This is the first article to investigate the SARS-CoV-2 spike protein bioaccumulation pattern and mechanism in Pacific oysters by the histochemical method. Oysters can bioaccumulate SARS-CoV-2 capsid readily by filter-feeding behavior assisted by specific binding ligands. The new possible foodborne transmission route may change the epidemic prevention strategies and reveal some outbreaks that current conventional epidemic transmission routes cannot explain. This original and interdisciplinary paper advances a mechanistic understanding of the bioaccumulation of SARS-CoV-2 in oysters inhabiting contaminated surface water.

Form and function of the mantle edge in Protobranchia (Mollusca: Bivalvia)

We analyzed, by optical and transmission electron microscopy, the morphology and function of the mantle edge, including the formation of the periostracum, of ten species of protobranchs. Five species from the order Nuculida, four species from the order Nuculanida and one species from the order Solemyida were studied. A second outer fold, which seems to function as a template for the internal marginal crenulations of the valves, is present in the crenulated species of Nucula. The minute non-crenulated Ennucula aegeensis shows the glandular basal cells displaced toward the periostracal groove, resembling a minute additional fold between the outer and middle folds. Intense secretion of glycocalyx, together with active uptake of particles, have been observed in the inner epithelium of the middle mantle fold and the whole epithelium of the inner mantle fold in all the studied species. Contrary to the rest of the bivalves, all the protobranchs analyzed have two basal cells involved in the formation of the external nanometric pellicle of the periostracum, a character that would support the monophyly of protobranchs. A three-layered pattern is the general rule for the periostracum in protobranchs, like for other bivalves. The presence of pouches of translucent layer inside the tanned dark layer under periostracal folds is characteristic of the species with a folded periostracum; its function is unclear but could give flexibility to the periostracum. The non-nacreous internal shell layer and the presence of translucent pouches under periostracal folds in Sarepta speciosa resemble those found in nuculanids. However, the free periostracum is rather similar to those of N. hanleyi and E. aegeensis, with a continuous vesicular layer. All the latter supports the inclusion of Sarepta in the order Nuculanida but could indicate either a basal lineage or that the translucent vesicular layer is an adaptive trait.

Heme-Peroxidase 2 Modulated by POU2F1 and SOX5 is Involved in Pigmentation in Pacific Oyster (Crassostrea gigas)

Color polymorphism is frequently observed in molluscan shellfish, while the molecular regulation of shell pigmentation is not well understood. Peroxidase is a key enzyme involved in melanogenesis. Here, we identified a heme-peroxidase 2 gene (CgHPX2), and characterized the expression patterns and transcriptional regulation of CgHPX2 in the Pacific oyster Crassostrea gigas. Tissues expression analysis showed that CgHPX2 was a mantle-specific gene and primarily expressed in the edge mantle in black shell color oyster compared with white shell oyster. In situ hybridization showed that strong signals for CgHPX2 were detected in the both inner and outer surface of the outer fold of mantle in the black shell color oyster, whereas positive signals in white shell oyster were mainly localized in the outer surface of the outer fold of mantle. In the embryos and larvae, a high expression level of CgHPX2 was detected in the trochophore stage in both black and white shell color oysters. The temporal localization of CgHPX2 was mainly detected in the shell gland and edge mantle of trochophore and calcified shell larvae, respectively. In addition, a 2227 bp of 5' flanking region sequence of CgHPX2 was cloned, which contained a presumed core promoter region and many potential transcription factor binding sites. Further luciferase assay experiment confirmed that POU domain, class 2, transcription factor 1 (POU2F1), and SRY-box transcription factor 5 (SOX5) were involved in transcriptional regulation of CgHPX2 gene through binding to its specific promoter region. After CgPOU2F1 and CgSOX5 RNA interference, the CgHPX2 gene expression was significantly decreased. These results suggested that CgPOU2F1 and CgSOX5 might be two important transcription factors that positively regulated the expression of CgHPX2 gene, improving our understanding of the transcriptional regulation of molluscan shell pigmentation.

Nonmuscle Myosin II is Required for Larval Shell Formation in a Patellogastropod

The molecular mechanisms underlying larval shell development in mollusks remain largely elusive. We previously found evident filamentous actin (F-actin) aggregations in the developing shell field of the patellogastropod Lottia goshimai, indicating roles of actomyosin networks in the process. In the present study, we functionally characterized nonmuscle myosin II (NM II), the key molecule in actomyosin networks, in the larval shell development of L. goshimai. Immunostaining revealed general colocalization of phosphorylated NM II and F-actin in the shell field. When inhibiting the phosphorylation of NM II using the specific inhibitor blebbistatin in one- or 2-h periods during shell field morphogenesis (6–8 h post-fertilization, hpf), the larval shell plate was completely lost in the veliger larva (24 hpf). Scanning electron microscopy revealed that the nascent larval shell plate could not be developed in the manipulated larvae (10 hpf). Further investigations revealed that key events in shell field morphogenesis were inhibited by blebbistatin pulses, including invagination of the shell field and cell shape changes and cell rearrangements during shell field morphogenesis. These factors caused the changed morphology of the shell field, despite the roughly retained “rosette” organization. To explore whether the specification of related cells was affected by blebbistatin treatments, we investigated the expression of four potential shell formation genes (bmp2/4, gata2/3, hox1 and engrailed). The four genes did not show evident changes in expression level, indicating unaffected cell specification in the shell field, while the gene expression patterns showed variations according to the altered morphology of the shell field. Together, our results reveal that NM II contributes to the morphogenesis of the shell field and is crucial for the formation of the larval shell plate in L. goshimai. These results add to the knowledge of the mechanisms of molluskan shell development.

Quantitative proteomic analysis reveals the molecular mechanism of the Yesso scallop (Patinopecten yessoensis) in response to Polydora infection

The Yesso scallop is a large and ancient molluscan group with great economic value; however, it has recently suffered severe cases of Polydora infection. Polydora parasitizes the shells of scallops, badly damaging shell structures and affecting growth and mortality. To investigate the molecular mechanism of Yesso scallops’ response to Polydora infection, proteomic profiling changes in the mantle tissues of Polydora-infected (diseased) and healthy scallops were systematically analysed by tandem mass tags (TMT) labelling technology in this study. A total of 519 differentially expressed proteins (DEPs) were identified. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis revealed most innated immune-related functions and pathways were significantly downregulated in diseased scallops, except the phagocytosis pathway, indicating an important role of phagocytosis in response to Polydora infection. DEPs involved in the phagocytosis pathway were associated with phagocytic receptor recognition, phagosome biogenesis and pathogen degradation, and they were further verified by quantitative real-time PCR. The results elucidate the molecular components of phagocytosis in molluscs for the first time. Polydora can be encapsulated by melanization with an obvious appearance in shells; indeed, melanization-related DEPs were upregulated in diseased scallops. Inhibition of apoptosis and nervous modulation may be also involved in the response mechanism, with some highly associated proteins significantly differentially expressed. Finally, a protein–protein interaction network was constructed to provide a global view of the interaction relationships of the DEPs. The study predicts the molecular response mechanism of Yesso scallops to Polydora infection, and lays a theoretical foundation for Polydora disease control.

The temptin gene of the clade Lophotrochozoa is involved in formation of the prismatic layer during biomineralization in molluscs

The biomineralization mechanism of mollusc shell has been studied for a long time, but there is a lack of understanding about the relationship between the shell formation in vitro and the signaling system in vivo. In this study, we cloned a novel shell matrix protein gene (hc-temptin), which only be characterized as a water-borne protein pheromone of molluscs in previous studies, from the freshwater mussel Hyriopsis cumingii. By bioinformatics analysis we found that temptin was a gene unique to the clade Lophotrochozoa, and it exists in all mollusc taxa except Cephalopoda. The current data supported the premise that temptin was generated in the early emergence of molluscs and that it maintained a high mutation rate to evolve relative independently. The specificity of hc-temptin expression in the mantle tissue suggests its potential to participate in biomineralization. Its sequence contained typical Ca²⁺ binding sites. Our experiments involving the pearl formation process, damaged shell repair process, and RNAi experiment showed that hc-temptin was a shell matrix protein that plays an important role in formation of the prismatic layer. The results of this study provided new insights about the origin of the temptin gene and its role in molluscs.

Sticky problems: extraction of nucleic acids from molluscs

Traditional molecular methods and omics-techniques across molluscan taxonomy increasingly inform biology of Mollusca. Recovery of DNA and RNA for such studies is challenged by common biological properties of the highly diverse molluscs. Molluscan biomineralization, adhesive structures and mucus involve polyphenolic proteins and mucopolysaccharides that hinder DNA extraction or copurify to inhibit enzyme-catalysed molecular procedures. DNA extraction methods that employ the detergent hexadecyltrimethylammoniumbromide (CTAB) to remove these contaminants importantly facilitate molecular-level study of molluscs. Molluscan pigments may stain DNA samples and interfere with spectrophotometry, necessitating gel electrophoresis or fluorometry for accurate quantification. RNA can reliably be extracted but the 'hidden break' in 28S rRNA of molluscs (like most protostomes) causes 18S and 28S rRNA fragments to co-migrate electrophoretically. This challenges the standard quality control based on the ratio of 18S and 28S rRNA, developed for deuterostome animals. High-AT content in molluscan rRNA prevents the effective purification of polyadenylated mRNA. Awareness of these matters aids the continuous expansion of molecular malacology, enabling work also with museum specimens and next-generation sequencing, with the latter imposing unprecedented demands on DNA quality. Alternative methods to extract nucleic acids from molluscs are available from literature and, importantly, from communications with others who study the molecular biology of molluscs. This article is part of the Theo Murphy meeting issue 'Molluscan genomics: broad insights and future directions for a neglected phylum'.

Pearl Sac Gene Expression Profiles Associated With Pearl Attributes in the Silver-Lip Pearl Oyster, Pinctada maxima

Pearls are highly prized biomineralized gemstones produced by molluscs. The appearance and mineralogy of cultured pearls can vary markedly, greatly affecting their commercial value. To begin to understand the role of pearl sacs—organs that form in host oysters from explanted mantle tissues that surround and synthesize pearls—we undertook transcriptomic analyses to identify genes that are differentially expressed in sacs producing pearls with different surface and structural characteristics. Our results indicate that gene expression profiles correlate with different pearl defects, suggesting that gene regulation in the pearl sac contributes to pearl appearance and quality. For instance, pearl sacs that produced pearls with surface non-lustrous calcification significantly down-regulate genes associated with cilia and microtubule function compared to pearl sacs giving rise to lustrous pearls. These results suggest that gene expression profiling can advance our understanding of processes that control biomineralization, which may be of direct value to the pearl industry, particularly in relation to defects that result in low value pearls.

Bivalves are NO different: nitric oxide as negative regulator of metamorphosis in the Pacific oyster, Crassostrea gigas

BACKGROUND

Nitric oxide (NO) is presumed to be a regulator of metamorphosis in many invertebrate species, and although NO pathways have been comparatively well-investigated in gastropods, annelids and crustaceans, there has been very limited research on the effects of NO on metamorphosis in bivalve shellfish.

RESULTS

In this paper, we investigate the effects of NO pathway inhibitors and NO donors on metamorphosis induction in larvae of the Pacific oyster, Crassostrea gigas. The nitric oxides synthase (NOS) inhibitors s-methylisothiourea hemisulfate salt (SMIS), aminoguanidine hemisulfate salt (AGH) and 7-nitroindazole (7-NI) induced metamorphosis at 75, 76 and 83% respectively, and operating in a concentration-dependent manner. Additional induction of up to 54% resulted from exposures to 1H-[1,2,4]Oxadiazole[4,3-a]quinoxalin-1-one (ODQ), an inhibitor of soluble guanylyl cyclase, with which NO interacts to catalyse the synthesis of cyclic guanosine monophosphate (cGMP). Conversely, high concentrations of the NO donor sodium nitroprusside dihydrate in combination with metamorphosis inducers epinephrine, MK-801 or SMIS, significantly decreased metamorphosis, although a potential harmful effect of excessive NO unrelated to metamorphosis pathway cannot be excluded. Expression of CgNOS also decreased in larvae after metamorphosis regardless of the inducers used, but intensified again post-metamorphosis in spat. Fluorescent detection of NO in competent larvae with DAF-FM diacetate and localisation of the oyster nitric oxide synthase CgNOS expression by in-situ hybridisation showed that NO occurs primarily in two key larval structures, the velum and foot. cGMP was also detected in the foot using immunofluorescent assays, and is potentially involved in the foot's smooth muscle relaxation.

CONCLUSION

Together, these results suggest that the NO pathway acts as a negative regulator of metamorphosis in Pacific oyster larvae, and that NO reduction induces metamorphosis by inhibiting swimming or crawling behaviour, in conjunction with a cascade of additional neuroendocrine downstream responses.

Early shell field morphogenesis of a patellogastropod mollusk predominantly relies on cell movement and F-actin dynamics

Background

The morphogenesis of the shell field is an essential step of molluscan shell formation, which exhibits both conserved features and interlineage variations. As one major gastropod lineage, the patellogastropods show different characters in its shell field morphogenesis compared to other gastropods (e.g., the pulmonate gastropod Lymnaea stagnalis), likely related to its epibolic gastrulation. The investigation on the shell field morphogenesis of patellogastropods would be useful to reveal the lineage-specific characters in the process and explore the deep conservation among different molluscan lineages.

Results

We investigated the early shell field morphogenesis in the patellogastropod Lottia goshimai using multiple techniques. Electron microscopy revealed distinct morphological characters for the central and peripheral cells of the characteristic rosette-like shell field. Gene expression analysis and F-actin staining suggested that the shell field morphogenesis in this species predominantly relied on cell movement and F-actin dynamics, while BrdU assay revealed that cell proliferation contributed little to the process. We found constant contacts between ectodermal and meso/endodermal tissues during the early stages of shell field morphogenesis, which did not support the induction of shell field by endodermal tissues in general, but a potential stage-specific induction was indicated.

Conclusions

Our results emphasize the roles of cell movement and F-actin dynamics during the morphogenesis of the shell field in Lo. goshimai, and suggest potential regulators such as diffusible factors and F-actin modulators. These findings reflect the differences in shell field morphogenesis of different gastropods, and add to the knowledge of molluscan larval shell formation.

Functional morphology of the sperm-containing chambers of the sea slug Okenia polycerelloides in the context of sexual selection

Sea slugs are interesting models to study post-copulatory sexual selection in simultaneous hermaphrodites due to the enormous variation of their reproductive systems. However, the knowledge of the functional morphology of their reproductive system is limited to few species, and it is rarely discussed in the context of sexual selection theory. In this study, we investigated the functional morphology of the sperm-containing chambers (i.e., ampulla, seminal receptacle, and bursa copulatrix) of the reproductive system of Okenia polycerelloides (Ortea & Bouchet, 1983), based on light, confocal, and electron microscopy. Although the morphology of the ampulla is similar to other species, indicating that it is a site for autosperm storage, we found some sperm facing the ampullar epithelium, a feature commonly regarded as characteristic of the seminal receptacle of sea slugs. The seminal receptacle of O. polycerelloides showed secretory activity and contained sperm with distribution and orientation suggestive of stratification of allosperm from distinct mating events, a feature that would affect sperm competition. The bursa copulatrix had epithelial cells with secretory and absorptive characteristics, and contained degraded sperm and yolk granules within its lumen. Comparative analyses of the contents of each organ demonstrated that sperm digestion occurs in the bursa copulatrix and affects sperm heads first, changing their morphology from slender and curved to shorter and ellipsoid before complete lysis. Although digestion and absorption of surplus sperm are currently the main hypothesized functions for the bursa copulatrix, its role in cryptic female choice should not be ruled out. The close structural connection between the seminal receptacle and bursa copulatrix, as well as their muscular walls, would enable control over the fate of the sperm received in each mating event, that is, storage or digestion.

The evo-devo of molluscs: Insights from a genomic perspective

Molluscs represent one of ancient and evolutionarily most successful groups of marine invertebrates, with a tremendous diversity of morphology, behavior, and lifestyle. Molluscs are excellent subjects for evo-devo studies; however, understanding of the evo-devo of molluscs has been largely hampered by incomplete fossil records and limited molecular data. Recent advancement of genomics and other technologies has greatly fueled the molluscan "evo-devo" field, and decoding of several molluscan genomes provides unprecedented insights into molluscan biology and evolution. Here, we review the recent progress of molluscan genome sequencing as well as novel insights gained from their genomes, by emphasizing how molluscan genomics enhances our understanding of the evo-devo of molluscs.

Identification of three cell populations from the shell gland of a bivalve mollusc

The molluscan larval shell formation is a complicated process. There is evidence that the mantle of the primary larva (trochophore) contains functionally different cell populations with distinct gene expression profiles. However, it remains unclear how these cells are specified. In the present study, we identified three cell populations from the shell gland in earlier stages (gastrula) from the bivalve mollusc Crassostrea gigas. These cell populations were determined by analyzing the co-expression relationships among six potential shell formation (pSF) genes using two-color hybridization. The three cell populations, which we designated as SGCPs (shell gland cell populations), formed a concentric-circle pattern from outside to inside of the shell gland. SGCP I was located in the outer edge of the shell gland and the cells expressed pax2/5/8, gata2/3, and bmp2/4. SGCP II was located more internally and the cells expressed two engrailed genes. The last population, SGCP III, was located in the central region of the shell gland and the cells expressed lox4. Determination of the gene expression profiles of SGCPs would help trace their origins and fates and elucidate how these cell populations are specified. Moreover, potential roles of the SGCPs, e.g., development of sensory cells and shell biogenesis, are suggested. Our results reveal the internal organization of the embryonic shell gland at the molecular level and add to the knowledge of larval shell formation.

Early stages of Margaritifera margaritifera glochidiosis in Atlantic salmon: Morphopathological characterization

Freshwater mussels of the order Unionida encyst into the fish mucosa to metamorphose and complete their life cycle, causing a parasitic disease known as glochidiosis. This parasitic stage represents a bottleneck for the survival of naiads, particularly for critically endangered species as Margaritifera margaritifera; however, little is known about the events occurring during this critical stage. Therefore, this study aimed to histologically characterize the development of M. margaritifera glochidiosis in Atlantic salmon to get insight into the pathogenesis of this interaction. Fish exposed to glochidia were sampled during the first 44 days post-exposure, and organs were observed by stereomicroscopy and light microscopy. Glochidia attached to the gills by pinching the lamellar epithelium, whereupon an acute proliferative branchitis engulfed most of the larvae. However, during the first 14 days, a severe detachment of unviable glochidia occurred, associated with the presence of pleomorphic inflammatory infiltrate and epithelial degeneration. In the cases where larvae remained attached, a chronification of the lesions with none to scarce inflammation was observed. These results provide key information to better understand the complex host-parasite interaction during the early stages of glochidiosis and provide valuable information to optimize artificial rearing of naiads in conservation of threatened freshwater mussel populations.

Histological and Expression Differences Among Different Mantle Regions of the Yesso Scallop (Patinopecten yessoensis) Provide Insights into the Molecular Mechanisms of Biomineralization and Pigmentation

The molecular mechanisms of shell formation and pigmentation are issues of great interest in molluscan studies due to the unique physical and biological properties of shells. The Yesso scallop, Patinopecten yessoensis, is one of the most important maricultural bivalves in Asian countries, and its shell color shows polymorphism. To gain more information about the underlying mechanisms of shell formation and pigmentation, this study presents the first analyses of histological and transcriptional differences between different mantle regions of the Yesso scallop, which are thought to be responsible for the formation of different shell layers. The results showed major microstructural differences between the edge and central mantles, which were closely associated with their functions. Different biomineralization-related GO functions, which might participate in the formation of different shell layers, were significantly enriched in the different mantle regions, indicating the different molecular functions of the two mantle regions in shell formation. The melanogenesis pathway, which controls melanin biosynthesis, was the most significantly enriched pathway in the DEGs between the two mantle regions, indicating its important role in shell pigmentation. Tyr, the key and rate-limiting gene in melanogenesis, was expressed at a remarkably high level in the central mantle, while the upstream regulatory genes included in melanogenesis were mainly upregulated in the edge mantle, suggesting the different molecular functions of the two mantle regions in shell pigmentation.

A computational framework for the morpho-elastic development of molluskan shells by surface and volume growth

Mollusk shells are an ideal model system for understanding the morpho-elastic basis of morphological evolution of invertebrates’ exoskeletons. During the formation of the shell, the mantle tissue secretes proteins and minerals that calcify to form a new incremental layer of the exoskeleton. Most of the existing literature on the morphology of mollusks is descriptive. The mathematical understanding of the underlying coupling between pre-existing shell morphology, de novo surface deposition and morpho-elastic volume growth is at a nascent stage, primarily limited to reduced geometric representations. Here, we propose a general, three-dimensional computational framework coupling pre-existing morphology, incremental surface growth by accretion, and morpho-elastic volume growth. We exercise this framework by applying it to explain the stepwise morphogenesis of seashells during growth: new material surfaces are laid down by accretive growth on the mantle whose form is determined by its morpho-elastic growth. Calcification of the newest surfaces extends the shell as well as creates a new scaffold that constrains the next growth step. We study the effects of surface and volumetric growth rates, and of previously deposited shell geometries on the resulting modes of mantle deformation, and therefore of the developing shell’s morphology. Connections are made to a range of complex shells ornamentations.

Molluska are the second most diversified phylum of the animal kingdom, and their evolutionary success can be partly attributed to the hard shell that provides both protection and support to the soft body. The distinctive anatomical features of these hard shells are their rich pigmentation patterns and complex structural ornamentations. While the pigmentation patterns are primarily of biochemical origin, the ornamentations result from mechanical deformation of the mantle due to growth induced forces. This mechanical basis of “growth and form” has been previously investigated using simplified morpho-mechanical models, but restricted to reduced geometric representations. Here we propose a three-dimensional computational framework coupling morphology, incremental surface growth by accretion, and morpho-elastic volume growth, to enable an improved representation of the growth and structural parameters controlling the evolution of these ornamentations. We study the effects of growth rates, and of previously deposited shell geometries on the resulting modes of mantle deformation, and present a “phase diagram” of morphogenesis in molluskan shells. Our main motivation for focusing on generic physical processes involved in development is that they may shape living beings in a predictive way and partly determine the spectrum of forms that have been and could have been generated during evolution.

Terebratulide brachiopod shell biomineralization by mantle epithelial cells

To understand mineral transport pathways for shell secretion and to assess differences in cellular activity during mineralization, we imaged with TEM and FE-SEM ultrastructural characteristics of outer mantle epithelium (OME) cells. Imaging was carried out on Magellania venosa shells embedded/etched, chemically fixed/decalcified and high-pressure frozen/freeze-substituted samples from the commissure, central shell portions and from puncta. Imaging results are complemented with morphometric evaluations of volume fractions of membrane-bound organelles. At the commissure the OME consists of several layers of cells. These cells form oblique extensions that, in cross-section, are round below the primary layer and flat underneath fibres. At the commissure the OME is multi-cell layered, in central shell regions it is single-cell layered. When actively secreting shell carbonate extrapallial space is lacking, because OME cells are in direct contact with the calcite of the forming fibres. Upon termination of secretion, OME cells attach via apical hemidesmosomes to extracellular matrix membranes that line the proximal surface of fibres. At the commissure volume fractions for vesicles, mitochondria and lysosomes are higher relative to single-cell layered regions, whereas for endoplasmic-reticulum and Golgi apparatus there is no difference. FE-SEM, TEM imaging reveals the lack of extrapallial space between OME cells and developing fibres. In addition, there is no indication for an amorphous precursor within fibres when these are in active secretion mode. Accordingly, our results do not support transport of minerals by vesicles from cells to sites of mineralization, rather by transfer of carbonate ions via transport mechanisms associated with OME cell membranes.

Genome-wide identification, characterization and expression analysis of the MITF gene in Yesso scallops (Patinopecten yessoensis) with different shell colors

The microphthalmia-associated transcription factor (MITF) is the center of the regulator network of melanin synthesis in vertebrates. However, the role of MITF in shell color formation is poorly studied in mollusks. In the present study, an MITF gene, PyMITF, was first identified at the whole-genome level in Yesso scallop (Patinopecten yessoensis), an evolutionarily and economically important species, the shell color of which shows polymorphism. The PyMITF is a large gene spanning ~37 kb in the genome with 7 introns and 8 exons. A basic helix-loop-helix leucine zipper (bHLH-LZ) domain was detected in the PyMITF protein sequence, which can bind the canonical E-box sequence in the promoter region of the downstream genes. Phylogenetic analysis of the MITFs among vertebrates and invertebrates revealed that the molecular evolution of MITFs was consistent with the species taxonomy. Different expression levels of PyMITF were detected among different shell color strains, indicating the important role of PyMITF involved in shell pigmentation. Besides, PyMITF was expressed at a significantly higher level in the central mantle than that in the edge mantle, proving the participation of the central mantle in shell color formation in molecular level for the first time. The work provides valuable information for the molecular mechanism study of shell color formation.

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.

Towards a ground pattern reconstruction of bivalve nervous systems: neurogenesis in the zebra mussel Dreissena polymorpha

Bivalvia is a taxon of aquatic mollusks that includes clams, oysters, mussels, and scallops. Within heterodont bivalves, Dreissena polymorpha is a small, mytiliform, freshwater mussel that develops indirectly via a planktotrophic veliger larva. Currently, only a few studies on bivalve neurogenesis are available, impeding the reconstruction of a ground pattern in Bivalvia. In order to inject novel data into this discussion, we describe herein the development of the serotonin-like and α-tubulin-like immunoreactive (lir) neuronal components of D. polymorpha from the early trochophore to the late veliger stage. Neurogenesis starts in the early trochophore stage at the apical pole with the appearance of one flask-shaped serotonin-lir cell. When larvae reach the veliger stage, four flask-shaped serotonin-lir cells are present in the apical organ. At the same time, the anlagen of the cerebral ganglia start to form at the base of the apical organ. From the apical organ, one pair of cerebro-visceral connectives projects posteriorly and connects to a posterior larval sensory organ that contains serotonin- and α-tubulin-like flask-shaped cells. Additional, paired serotonin-lir neurites originate from the apical organ and project into the velum. One unpaired stomatogastric serotonin-lir cell develops ventrally to the stomach at the veliger stage. The low number of serotonin-lir cells in the apical organ of bivalve veligers is shared with larvae of basally branching gastropods and scaphopods and is thus considered a feature of the last common ancestor of Conchifera, while the overall simplicity of the larval neural architecture appears to be a specific trait of Bivalvia.

A SoxC gene related to larval shell development and co-expression analysis of different shell formation genes in early larvae of oyster

Among the potential larval shell formation genes in mollusks, most are expressed in cells surrounding the shell field during the early phase of shell formation. The only exception (cgi-tyr1) is expressed in the whole larval mantle and thus represents a novel type of expression pattern. This study reports another gene with such an expression pattern. The gene encoded a SoxC homolog of the Pacific oyster Crassostrea gigas and was named cgi-soxc. Whole-mount in situ hybridization revealed that the gene was highly expressed in the whole larval mantle of early larvae. Based on its spatiotemporal expression, cgi-soxc is hypothesized to be involved in periostracum biogenesis, biomineralization, and regulation of cell proliferation. Furthermore, we investigated the interrelationship between cgi-soxc expression and two additional potential shell formation genes, cgi-tyr1 and cgi-gata2/3. The results confirmed co-expression of the three genes in the larval mantle of early D-veliger. Nevertheless, cgi-gata2/3 was only expressed in the mantle edge, and the other two genes were expressed in all mantle cells. Based on the spatial expression patterns of the three genes, two cell groups were identified from the larval mantle (tyr1 ⁺/soxc ⁺/gata2/3 ⁺ cells and tyr1 ⁺/soxc ⁺/gata2/3 ^- cells) and are important to study the differentiation and function of this tissue. The results of this study enrich our knowledge on the structure and function of larval mantle and provide important information to understand the molecular mechanisms of larval shell formation.

Hemocytes participate in calcium carbonate crystal formation, transportation and shell regeneration in the pearl oyster Pinctada fucata

In this study, light microscope, scanning and transmission electron microscope, hematoxylin-eosin and fluorescent staining, and mass spectrometry methods were employed to observe the calcium carbonate (CaCO3) crystal formation, hemocyte release and transportation, and hemocyte distribution at the shell regeneration area and to analyse the proteome of hemocytes in the pearl oyster, Pinctada fucata. The results indicated that intracellular CaCO3 crystals were observed in circulating hemocytes in P. fucata, implying that there was a suitable microenvironment for crystal formation in the hemocytes. This conclusion was further supported by the proteome analysis, in which various biomineralization-related proteins were detected. The crystal-bearing hemocytes, mainly granulocytes, may be released to extrapallial fluid (EPF) by the secretory cavities distributed on the outer surface of the mantle centre. These granulocytes in the EPF and between the regenerated shells were abundant and free. In the regenerated prismatic layer, the granulocytes were fused into each column and fragmented with the duration of shell maturation, suggesting the direct involvement of hemocytes in shell regeneration. Overall, this study provided evidence that hemocytes participated in CaCO3 crystal formation, transportation and shell regeneration in the pearl oyster. These results are helpful to further understand the exact mechanism of hemocyte-mediated biomineralization in shelled molluscs.

Transcriptomics provides insight into Mytilus galloprovincialis (Mollusca: Bivalvia) mantle function and its role in biomineralisation

The mantle is an organ common to all molluscs and is at the forefront of the biomineralisation process. The present study used the Mediterranean mussel (Mytilus galloprovincialis) as a model species to investigate the structural and functional role of the mantle in shell formation. The transcriptomes of three regions of the mantle edge (umbo to posterior edge) were sequenced using Illumina technology which yielded a total of 61,674,325 reads after adapter trimming and filtering. The raw reads assembled into 179,879 transcripts with an N50 value of 1086bp. A total of 1363 transcripts (321, 223 and 816 in regions 1, 2 and 3, respectively) that differed in abundance in the three mantle regions were identified and putative function was assigned to 54% using BLAST sequence similarity searches (cut-off less than 1e(-10)). Morphological differences detected by histology of the three mantle regions was linked to functional heterogeneity by selecting the top five most abundant Pfam domains in the annotated 1363 differentially abundant transcripts across the three mantle regions. Calcium binding domains dominated region two (middle segment of the mantle edge). Candidate biomineralisation genes were mined and tested by qPCR. This revealed that Flp-like, a penicillin binding protein potentially involved in shell matrix maintenance of the Pacific oyster (Crassostrea gigas), had significantly higher expression in the posterior end of the mantle edge (region one). Our findings are intriguing as they indicate that the mantle edge appears to be a heterogeneous tissue, displaying structural and functional bias.

Inferring muscular ground patterns in Bivalvia: Myogenesis in the scallop Nodipecten nodosus

Background

Myogenesis is currently investigated in a number of invertebrate taxa using combined techniques, including fluorescence labeling, confocal microscopy, and 3D imaging, in order to understand anatomical and functional issues and to contribute to evolutionary questions. Although developmental studies on the gross morphology of bivalves have been extensively pursued, organogenesis including muscle development has been scarcely investigated so far.

Results

The present study describes in detail myogenesis in the scallop Nodipecten nodosus (Linnaeus, 1758) during larval and postmetamorphic stages by means of light, electron, and confocal microscopy. The veliger muscle system consists of an anterior adductor muscle, as well as four branched pairs of striated velum retractors and two pairs of striated ventral larval retractors. The pediveliger stage exhibits a considerably elaborated musculature comprising the velum retractors, the future adult foot retractor, mantle (pallial) muscles, and the anterior and posterior adductors, both composed of smooth and striated portions. During metamorphosis, all larval retractors together with the anterior adductor degenerate, resulting in the adult monomyarian condition, whereby the posterior adductor retains both myofiber types. Three muscle groups, i.e., the posterior adductor, foot retractor, and pallial muscles, have their origin prior to metamorphosis and are subsequently remodeled.

Conclusions

Our data suggest a dimyarian condition (i.e., the presence of an anterior and a posterior adductor in the adult) as the basal condition for pectinids. Comparative analysis of myogenesis across Bivalvia strongly argues for ontogenetic and evolutionary independence of larval retractors from the adult musculature, as well as a complex set of larval retractor muscles in the last common bivalve ancestor.

Electronic supplementary material

The online version of this article (doi:10.1186/s12983-015-0125-x) contains supplementary material, which is available to authorized users.

Mantle margin morphogenesis in Nodipecten nodosus (Mollusca: Bivalvia): new insights into the development and the roles of bivalve pallial folds

BACKGROUND

Despite extensive knowledge on bivalve anatomy and development, the formation and differentiation of the mantle margin and its associated organs remain largely unclear. Bivalves from the family Pectinidae (scallops) are particularly promising to cast some light on these issues, because they exhibit a complex mantle margin and their developmental stages are easily obtained from scallop farms. We investigated the mantle margin of the scallop Nodipecten nodosus (L. 1758) during larval and postmetamorphic development.

METHODS

A thorough analysis of the mantle margin development in Nodipecten nodosus, from veliger larvae to mature adults, was conducted by means of integrative microscopy techniques, i.e., light, electron, and confocal microscopy.

RESULTS

Initially unfolded, the pallial margin is divided into distal and proximal regions by the periostracum-forming zone. The emergence of the pallial musculature and its neural innervation are crucial steps during bivalve larval development. By the late pediveliger stage, the margin becomes folded, resulting in a bilobed condition (i.e., outer and inner folds), a periostracal groove, and the development of different types of cilia. After metamorphosis, a second outgrowth process is responsible for emergence of the middle mantle fold from the outer surface of the inner fold. Once the three-folded condition is established, the general adult features are rapidly formed.

CONCLUSIONS

Our data show that the middle mantle fold forms from the outer surface of the inner fold after metamorphosis and that the initial unfolded mantle margin may represent a common condition among bivalves. The first outgrowth process, which gives rise to the outer and inner folds, and the emergence of the pallial musculature and innervation occur during larval stages, highlighting the importance of the larval period for mantle margin morphogenesis in Bivalvia.