A history of the cultured pearl industry

Freshwater pearl culture in Bangladesh: Current status and prospects

Freshwater pearl farming is an emerging sector of aquaculture in Bangladesh which plays a growing role at major jewelry markets. With some improved techniques, high quality image or designer pearls are now produced from freshwater mussels Lamellidens marginalis. Yet it is difficult to reach in conclusion as the quantities produced, culture techniques used, and the upgrading of the existing culture technique are not well documented. Furthermore, many obstacles such as proper dissemination of culture technologies among the interested peoples, optimization of the culture environment and culture methods, standardization of breeding protocol and so on need to be addressed by the scientific community. This review article reports for the first time about the status of freshwater pearl culture in Bangladesh highlighting the fundamentals of pearl production, culture techniques used in farms, challenges, and prospects for upgradation of current culture principles in Bangladesh.

Direct control of shell regeneration by the mantle tissue in the pearl oyster Pinctada fucata

Molluscs rapidly repair the damaged shells to prevent further injury, which is vital for their survival after physical or biological aggression. However, it remains unclear how this process is precisely controlled. In this study, we applied scanning electronic microscopy and histochemical analysis to examine the detailed shell regeneration process in the pearl oyster Pinctada fucata. It was found that the shell damage caused the mantle tissue to retract, which resulted in relocation of the partitioned mantle zones with respect to their correspondingly secreting shell layers. As a result, the relocated mantle tissue dramatically altered the shell morphology by initiating de novo precipitation of prismatic layers on the former nacreous layers, leading to the formation of sandwich-like "prism-nacre-prism-nacre" structure. Real-time PCR revealed the up-regulation of the shell matrix protein genes, which was confirmed by the thermal gravimetric analysis of the newly formed shell. The increased matrix secretion might have led to the change of CaCO₃ precipitation dynamics which altered the mineral morphology and promoted shell formation. Taken together, our study revealed the close relationship between the physiological activities of the mantle tissue and the morphological change of the regenerated shells.

A freshwater mussel species reflects a Miocene stream capture between the Mekong Basin and East Asian rivers

Freshwater mussels belonging to the genus Cristaria Schumacher, 1817 (Bivalvia: Unionidae) are widespread from Mongolia to Indochina while the range of one species, C. plicata (Leach, 1814), covers two biogeographic subregions, i.e., East Asian (Amur River to Vietnam) and Sundaland (Mekong River basin). We present here a taxonomic revision of the nominal taxon Anodonta bellua Morelet, 1866 which was described from the Mekong (Lake Tonle-Sap, Cambodia) but is currently considered a synonym of C. plicata. We obtained molecular data for newly collected Cristaria representatives from the Mekong’s tributaries in Laos, which were found as a divergent species-level phylogenetic clade within the genus that is distant from C. plicata. Nevertheless, comparative morphological and morphometric studies did not reveal any significant differences between these two congeners. Our time-calibrated biogeographic modeling reveals that the split between Cristaria bellua (Mekong) and C. clessini (East Asia) probably occurred in the mid-Miocene (15.8 Ma) and may reflect an ancient stream capture between the Mekong Basin and East Asian rivers.

A high-quality, haplotype-phased genome reconstruction reveals unexpected haplotype diversity in a pearl oyster

Homologous chromosomes in the diploid genome are thought to contain equivalent genetic information, but this common concept has not been fully verified in animal genomes with high heterozygosity. Here we report a near-complete, haplotype-phased, genome assembly of the pearl oyster, Pinctada fucata, using hi-fidelity (HiFi) long reads and chromosome conformation capture data. This assembly includes 14 pairs of long scaffolds (>38 Mb) corresponding to chromosomes (2n = 28). The accuracy of the assembly, as measured by an analysis of k-mers, is estimated to be 99.99997%. Moreover, the haplotypes contain 95.2% and 95.9%, respectively, complete and single-copy BUSCO genes, demonstrating the high quality of the assembly. Transposons comprise 53.3% of the assembly and are a major contributor to structural variations. Despite overall collinearity between haplotypes, one of the chromosomal scaffolds contains megabase-scale non-syntenic regions, which necessarily have never been detected and resolved in conventional haplotype-merged assemblies. These regions encode expanded gene families of NACHT, DZIP3/hRUL138-like HEPN, and immunoglobulin domains, multiplying the immunity gene repertoire, which we hypothesize is important for the innate immune capability of pearl oysters. The pearl oyster genome provides insight into remarkable haplotype diversity in animals.

X-ray Micro-Tomography as a Method to Distinguish and Characterize Natural and Cultivated Pearls

Digital radiography and computed tomography are two fundamental diagnostic techniques in different fields of research, including cultural heritage studies and gemmology. The application of these physical methods of investigation has gained considerable importance as they are non-invasive techniques. The presented work has been mainly focused on micro-tomographic analysis. The project is concerned with the study of natural and cultivated pearls in order to develop an investigation methodology for the analysis, distinction and characterization of different types of pearls, some of them belonging to different precious jewels from private collections. The investigations, carried out on a total of 22 heterogeneous types of pearls, allowed us to establish their origin (natural or cultivated) or to confirm/deny if a hypothesis was already expressed, and as well to highlight the cultivation methodology used case by case. Furthermore, it was possible to ascertain how large and varied the market for cultured pearls is nowadays and how difficult is, in some particular cases, to ascertain their attribution to a certain origin.

Isolation and characterization of a Vibrio sp. strain MA3 associated with mass mortalities of the pearl oyster Pinctada fucata

In the summers of 2019 and 2020, a previously undescribed disease occurred in both juvenile and adult shellfish, causing mass mortalities in cultured pearl production, characterized by the major symptom of extreme atrophy of the soft tissues, including the mantle. However, the causative organism was uncertain. We isolated Vibrio sp. strain MA3 from the mantles of diseased pearl oysters Pinctada fucata. Analyses of 16S rRNA gene and DNA gyrase sequence homologies and its biochemical and morphological characteristics suggested that strain MA3 is a new strain of Vibrio alginolyticus. In addition, a hemolysin gene (Vhe1) of strain MA3 was detected as one of the virulence factors, and the complete sequence was determined. BLAST searches showed that Vhe1 shares 99.8% nucleotide sequence identity with Vibrio alginolyticus strain A056 lecithin-dependent hemolysin (ldh) gene, complete cds. Experimental infection of healthy oysters via injection with strain MA3 indicated it could cause high mortalities of the typically affected oysters from which the strain was isolated. These results suggest that the newly isolated Vibrio sp. strain MA3 is a putative causal agent of the recent disease outbreaks in Akoya pearl oysters.

Pearl Powder-An Emerging Material for Biomedical Applications: A Review

Pearl powder is a well-known traditional Chinese medicine for a variety of indications from beauty care to healthcare. While used for over a thousand years, there has yet to be an in-depth understanding and review in this area. The use of pearl powder is particularly growing in the biomedical area with various benefits reported due to the active ingredients within the pearl matrix itself. In this review, we focus on the emerging biomedical applications of pearl powder, touching on applications of pearl powder in wound healing, bone repairing, treatment of skin conditions, and other health indications.

Trivalent Iron Is Responsible for the Yellow Color Development in the Nacre of Akoya Pearl Oyster Shells

The gold and cream colors of cultured Akoya pearls, as well as natural yellow nacre of pearl oyster shells, are thought to arise from intrinsic yellow pigments. While the isolation of the yellow pigments has been attempted using a large amount of gold pearls, the substance concerned is still unknown. We report here on the purification and characterization of yellow pigments from the nacre of Akoya pearl oyster shells. Two yellow components, YC1 and YC2, were isolated from the HCl-methanol (HCl-MeOH) extract from nacreous organic matrices obtained by decalcification of the shells with ethylenediaminetetraacetic acid (EDTA). Energy-dispersive X-ray and infrared spectroscopy analyses suggested that YC1 and YC2 precipitated under basic conditions are composed of Fe-containing inorganic and polyamide-containing organic compounds, respectively. YC1 solubilized under acidic conditions exhibited positive reactions to KSCN and K₄[Fe(CN)₆] reagents, showing the same ultraviolet-visible absorption spectrum as those of Fe(III)-containing compounds. In addition, X-ray absorption fine structure analysis supported the compound in the form of Fe(III). The total amount of Fe was approximately 2.6 times higher in the yellow than white nacre, and most Fe was fractionated into the EDTA-decalcifying and HCl-MeOH extracts. These results suggest that Fe(III) coordinated to EDTA-soluble and insoluble matrix compounds are mainly associated with yellow color development not only in the Akoya pearl oyster shells but also in the cultured Akoya pearls.

Evaluation of Small Molecular Polypeptides from the Mantle of Pinctada Martensii on Promoting Skin Wound Healing in Mice

Skin wound healing, especially chronic wound healing, is a common challenging clinical problem. It is urgent to broaden the sources of bioactive substances that can safely and efficiently promote skin wound healing. This study aimed to observe the effects of small molecular peptides (SMPs) of the mantle of Pinctada martensii on wound healing. After physicochemical analysis of amino acids and mass spectrometry of SMPs, the effect of SMPs on promoting healing was studied through a whole cortex wound model on the back of mice for 18 consecutive days. The results showed that SMPs consisted of polypeptides with a molecular weight of 302.17-2936.43 Da. The content of polypeptides containing 2-15 amino acids accounted for 73.87%, and the hydrophobic amino acids accounted for 56.51%. Results of in vitro experimentation showed that SMPs possess a procoagulant effect, but no antibacterial activity. Results of in vivo experiments indicated that SMPs inhibit inflammatory response by secretion of anti-inflammatory factor IL-10 during the inflammatory phase; during the proliferative phase, SMPs promote the proliferation of fibroblasts and keratinocytes. The secretion of transforming growth factor-β1 and cyclin D1 accelerates the epithelialization and contraction of wounds. In the proliferative phase, SMPs effectively promote collagen deposition and partially inhibit superficial scar hyperplasia. These results show that SMPs promotes dermal wound healing in mice and have a tremendous potential for development and utilization in skin wound healing.

Electroextraction of Insoluble Proteins from the Organic Matrix of the Nacreous Layer of the Japanese Pearl Oyster, Pinctada fucata

The nacreous layer of shells and pearls is composed of aragonite crystals arranged in an organic matrix. The organic matrix contains chitin and several proteins that regulate the formation of the nacreous layer. Owing to their strong interactions in the organic matrix, the current method for extraction of insoluble proteins from the pre-powdered nacreous layer involves heating to high temperatures in the presence of a detergent (e.g., sodium dodecyl sulfate, SDS) and reductant (e.g., dithiothreitol, DTT), which is likely to induce protein degradation. Therefore, we have developed an electroextraction method to isolate proteins from the organic matrix of a nacreous organic sheet, that was obtained following the decalcification of shells in their original shape. Our electroextraction method employs milder conditions without heating or detergent. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) patterns of the electro-extracted proteins (EEPs) under non-reduced and reduced conditions revealed that this method yielded a greater number of different proteins compared with the conventional extraction method and the isolated EEPs retained their disulfide bonds. Our method is able to easily extract insoluble proteins from the nacreous layer under mild conditions and will undoubtedly aid future analyses into the functions of the nacreous layer proteins.

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.

Shell repair and the potential microbial causal in a shell disease of the pearl oyster Pinctada fucata

The pearl oyster Pinctada fucata is famous for producing luxurious pearls. As filter feeders, they are confronted with various infectious microorganisms. Despite a long history of aquaculture, diseases in P. fucata are not well studied, which limits the development of the pearl industry. We report here a shell disease in P. fucata and a study of the shell repair processes. Scanning electron microscopy (SEM) revealed that the nacreous layer gradually recovered from disordered CaCO₃ deposition, accompanied by a polymorphic transition from a calcite-aragonite mixture to an aragonite-dominant composition, as revealed by X-ray diffraction analysis. SEM also showed that numerous microbes were embedded in the abnormal shell layers. Similar indications were induced by a high concentration of microbes injected into the extrapallial space, suggesting the potential pathogenic effect of uncontrolled microbes. Furthermore, hemocytes were found to participate in pathogens resistance and might promote shell repair. These results further our understanding of pathogen-host interactions in pearl oysters and have implications for biotic control in pearl aquaculture.

Comparison of Two Pearl Sacs Formed in the Same Recipient Oyster with Different Genetic Background Involved in Yellow Pigmentation in Pinctada fucata

Color is one of the most important factors determining the commercial value of pearls. Pinctada fucata is a well-known pearl oyster producing high-quality Akoya pearls. Phenotypic variation in amount of yellow pigmentation produces white and yellowish pearls. It has been reported that polymorphism of yellow pigmentation of Akoya pearls is genetically regulated, but the responsible gene(s) has remained unknown. Here, we prepared pearl sac pairs formed in the same recipient oyster but coming from donor oysters that differ in their color. These two pearl sacs produced pearls with different yellowness even in the same recipient oyster. Yellow tone of produced pearls was consistent with shell nacre color of donor oysters from which mantle grafts were prepared, indicating that donor oysters strongly contribute to the yellow coloration of Akoya pearls. We also conducted comparative RNA-seq analysis and retrieved several candidate genes involved in the pearl coloration. Whole gene expression patterns of pair sacs were not grouped by pearl color they produced, but grouped by recipient oysters in which they were grown, suggesting that the number of genes involved in the yellow coloration is quite small, and that recipient oyster affects gene expression of the majority of genes in the pearl sac.

Influence of temperature and pearl rotation on biomineralization in the pearl oyster, Pinctada margaritifera

The objective of this study was to observe the impact of temperature on pearl formation using an integrative approach describing the rotation of the pearls, the rate of nacre deposition, the thickness of the aragonite tablets and the biomineralizing potential of the pearl sac tissue though the expression level of some key genes. Fifty pearl oysters were grafted with magnetized nuclei to allow the rotation of the pearls to be described. Four months later, 32 of these pearl oysters were exposed to four temperatures (22, 26, 30 and 34°C) for 2 weeks. Results showed that the rotation speed differed according to the movement direction: pearls with axial movement had a significantly higher rotation speed than those with random movement. Pearl growth rate was influenced by temperature, with a maximum between 26 and 30°C but almost no growth at 34°C. Lastly, among the nine genes implicated in the biomineralization process, only Pmarg-Pif177 expression was significantly modified by temperature. These results showed that the rotation speed of the pearls was not linked to pearl growth or to the expression profiles of biomineralizing genes targeted in this study. On the basis of our results, we consider that pearl rotation is a more complex process than formerly thought. Mechanisms involved could include a strong environmental forcing in immediate proximity to the pearl. Another implication of our findings is that, in the context of ocean warming, pearl growth and quality can be expected to decrease in pearl oysters exposed to temperatures above 30°C.

A Spirochaete is suggested as the causative agent of Akoya oyster disease by metagenomic analysis

Mass mortality that is acompanied by reddish browning of the soft tissues has been occurring in cultured pearl oyster, Pinctada fucata martensii. The disease is called Akoya oyster disease (AOD). Although spreading pattern of the disease and transmission experiments suggest that the disease is infectious, the causative agent has not yet been identified. We used shotgun and 16S rRNA-based metagenomic analysis to identify genes that are present specifically in affected oysters. The genes found only in diseased oysters were mostly bacterial origin, suggesting that the causative agent was a bacterial pathogen. This hypothesis was supported by the inhibition of AOD development in naïve oysters injected with the hemolymph of diseased animals followed immediately with penicillin bath-administration. Further analyses of the hemolymph and mantle specifically and universally detected genes of bacteria that belong to phylum Spirochaetes in diseased pearl oysters but not in healthy oysters. By in situ hybridization or immunostaining, a Brachyspira-like bacterium was observed in the smears of hemolymph from affected oysters, but not from healthy oysters. Phylogenetic analysis using 16S rRNA sequences showed that the presumptive causative bacterium was outside of but most closely related to family Brachyspiraceae. We propose ‘Candidatus Maribrachyspira akoyae’ gen. nov, sp nov., for this bacterium.

Differential mantle transcriptomics and characterization of growth-related genes in the diploid and triploid pearl oyster Pinctada fucata

To explore the molecular mechanism of triploidy effect in the pearl oyster Pinctada fucata, two RNA-seq libraries were constructed from the mantle tissue of diploids and triploids by Roche-454 massive parallel pyrosequencing. The identification of differential expressed genes (DEGs) between diploid and triploid may reveal the molecular mechanism of triploidy effect. In this study, 230 down-regulated and 259 up-regulated DEGs were obtained by comparison between diploid and triploid libraries. The gene ontology and KEGG pathway analysis revealed more functional activation in triploids and it may due to the duplicated gene expression in transcriptional level during whole genome duplication (WGD). To confirm the sequencing data, a set of 11 up-regulated genes related to growth and development control and regulation were analyzed by RT-qPCR in independent experiment. According to the validation and annotation of these genes, it is hypothesized that the set of up-regulated expressed genes had the correlated expression pattern involved in shell building or other interactive probable functions during triploidization. The up- regulation of growth-related genes may support the classic hypotheses of 'energy redistribution' from early research. The results provide valuable resources to understand the molecular mechanism of triploidy effect in both shell building and producing high-quality seawater pearls.

Bivalve-specific gene expansion in the pearl oyster genome: implications of adaptation to a sessile lifestyle

INTRODUCTION

Bivalve molluscs have flourished in marine environments, and many species constitute important aquatic resources. Recently, whole genome sequences from two bivalves, the pearl oyster, Pinctada fucata, and the Pacific oyster, Crassostrea gigas, have been decoded, making it possible to compare genomic sequences among molluscs, and to explore general and lineage-specific genetic features and trends in bivalves. In order to improve the quality of sequence data for these purposes, we have updated the entire P. fucata genome assembly.

RESULTS

We present a new genome assembly of the pearl oyster, Pinctada fucata (version 2.0). To update the assembly, we conducted additional sequencing, obtaining accumulated sequence data amounting to 193× the P. fucata genome. Sequence redundancy in contigs that was caused by heterozygosity was removed in silico, which significantly improved subsequent scaffolding. Gene model version 2.0 was generated with the aid of manual gene annotations supplied by the P. fucata research community. Comparison of mollusc and other bilaterian genomes shows that gene arrangements of Hox, ParaHox, and Wnt clusters in the P. fucata genome are similar to those of other molluscs. Like the Pacific oyster, P. fucata possesses many genes involved in environmental responses and in immune defense. Phylogenetic analyses of heat shock protein70 and C1q domain-containing protein families indicate that extensive expansion of genes occurred independently in each lineage. Several gene duplication events prior to the split between the pearl oyster and the Pacific oyster are also evident. In addition, a number of tandem duplications of genes that encode shell matrix proteins are also well characterized in the P. fucata genome.

CONCLUSIONS

Both the Pinctada and Crassostrea lineages have expanded specific gene families in a lineage-specific manner. Frequent duplication of genes responsible for shell formation in the P. fucata genome explains the diversity of mollusc shell structures. These duplications reveal dynamic genome evolution to forge the complex physiology that enables bivalves to employ a sessile lifestyle in the intertidal zone.