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PIWI-interacting RNAs (piRNAs) are crucial for silencing transposable elements, germ cell development, and gametogenesis. Triploid Pacific oysters (Crassostrea gigas) are vital in the oyster aquaculture industry due to reduced fertility and rapid growth. This study integrates piRNA and mRNA expression analyses to elucidate their potential contributions to the sterility of triploid C. gigas. Bioinformatics analysis reveals a distinct U-bias at the 5′ terminal of oyster piRNAs. The abundance of piRNA clusters is reduced in triploid gonads compared to diploid gonads, particularly in sterile gonads, with a significant decrease in piRNA numbers. A specific piRNA cluster is annotated with the PPP4R1 gene, which is downregulated in infertile female triploids and exhibits a negative correlation with three piRNAs within the cluster. Differential expression analysis identified 46 and 88 piRNAs in female and male comparison groups, respectively. In female sterile triploids, the expression of three target genes of differentially expressed piRNAs associated with cell division showed downregulation, suggesting the potential roles of piRNAs in the regulation of cell division-related genes, contributing to the gonad arrest observed in female triploid oysters. In male triploid oysters, piRNAs potentially interact with the target genes associated with spermatogenesis, including TSSK4, SPAG17, and CCDC81. This study provides a concise overview of piRNAs expression in oyster gonads, offering insights into the regulatory role of piRNAs in triploid sterility. [ABSTRACT FROM AUTHOR]

Tetraploids play a crucial role in the large-scale production of all-triploid Pacific oyster, Crassostrea gigas by mating with diploids. For nursery farms, research related to larval tolerance to the environment appears to be extremely important. For this goal, fertilization, embryonic development, growth, and settlement from fertilization stage to metamorphosis stage of the diploid, triploid, and tetraploid larvae of C. gigas "Haida No. 2" strain were studied under controlled conditions of temperature (20, 24, 28, and 32 ℃) and salinity (18, 21, 24, 27, and 30 psu). The results showed that increasing temperature and lowering salinity reduced survival time of sperm while increasing temperature and raising salinity accelerated the germinal vesicle breakdown (GVBD) ratio of stripped eggs in diploids and tetraploids for the first time. The appropriate condition for hatching of four crosses (DD, DT, TD, and TT, males were listed first, D for diploid, T for tetraploid) were listed as follows: 20–28 ℃/27–30 psu for DD, 24 ℃/27–30 psu for DT, 24–28 ℃/27–30 psu for TD, and 24 ℃/27–30 psu for TT. Diploids and triploids had similar performance but were all better than tetraploids under different temperature and salinity. The settlement rate increased with rising temperature while salinity and ploidy had little influence on settlement. The information obtained in this study can contribute to increasing the yield of triploid and tetraploid "Haida No. 2" of C. gigas. [ABSTRACT FROM AUTHOR]

Tetraploid oysters are valuable germplasm resources for the production of triploid oysters. However, the broodstock condition and gonadal development of tetraploid oysters are poorly understood. To assess the effects of temperature on gamete quantity and quality, tetraploid and diploid oysters were held at controlled temperatures of 18, 20 and 22 °C for 120 days. The gametogenesis process in tetraploid and diploid oysters showed an accelerated trend in response to increased conditioning temperatures. Nonetheless, the gonadal development of tetraploid oysters was slower than that of diploid oysters. Histomorphometric analysis of gonadal tissues showed variations in observed sex ratio under different conditioning temperatures. The proportions of tetraploid and diploid males increased with elevated temperature. However, survival rates of tetraploid and diploid oysters at 22 °C were significantly lower than those at 18 and 20 °C (P < 0.05). In terms of the number of oocytes and spermatozoa, tetraploid oysters were fewer than diploid oysters. Overall, these results demonstrated the differences in broodstock conditioning and gonadal development between tetraploid and diploid oysters, suggesting the need for specialized broodstock cultivation strategies for tetraploid oysters. [ABSTRACT FROM AUTHOR]

Triploid Pacific oyster Crassostrea gigas is increasingly important in aquaculture due to its improved growth and meat quality. However, adaptability differences between diploid and triploid oysters in varying environments are inconclusive. To address this concern, we compared the growth, physiological parameters (clearance rate, CR; oxygen consumption rate, OCR; ammonia excretion rate, AER; the Arrhenius break temperature (ABT) based on heart rate), and metabolism-related gene expression (HK, PK, and PEPCK) in diploid and triploid C. gigas at various temperatures (17 ℃, 20 ℃, 23 ℃, 26 ℃, and 29 ℃) and salinities (18 psu, 22 psu, 26 psu, 30 psu, and 34 psu). Triploids exhibited higher shell heights than diploids across various temperature and salinity treatments. No significant difference in CR, OCR, or AER was observed between diploids and triploids. Compared to diploids, triploids had higher O: N ratios at 29 ℃ but lower O: N ratios at 18 and 22 psu. Except for the 23–26 ℃ range, diploids had lower Q10 values, suggesting that they are less sensitive to respiration changes within these temperature ranges. Additionally, triploids demonstrated higher thermal adaptation, as evidenced by a higher ABT value (triploids: 26.52 ℃ > diploids: 25.71 ℃). The PEPCK/PK and PEPCK/HK ratios indicated that triploids have lower anaerobic metabolism levels than diploids at 17 ℃, 23 ℃, and 26 ℃, but higher levels at salinities of 18 psu, 22 psu, and 26 psu. Overall, triploids showed greater adaptability at 17 ℃, 23 ℃, 26 ℃ and 29 ℃, while lower adaptability at salinities of 18 psu, 22 psu, and 26 psu. Our findings provide insights into the physiological metabolism underlying temperature and salinity adaptation in diploid and triploid oysters. [ABSTRACT FROM AUTHOR]

The Pacific oyster, Crassostrea gigas, is an important cultured species in the northern coastal areas of China. In recent years, the triploid oyster was popular worldwide because of its fast growth and high meat quality and has rapidly occupied a large portion of the oyster market. As a multi-generation selection strain, "Haida No.2" of C. gigas has good growth performance. At the same time, we have successfully obtained the triploid and tetraploid "Haida No.2." This study was conducted to evaluate the performance of the triploid of "Haida No.2" under different rearing environments. The results showed that there was no significant difference in fertilization rate and hatching rate between diploids and triploids, and so did the survival rate at the larval stage, while the cumulative survival rate of triploids was significantly lower than that of diploids. At grow-out stage, the growth and survival rate of triploids were significantly higher than diploids on day 420 at two sites; meanwhile, the triploid rate was slight decline but still kept a high level (96–98%) on 420 days at two sites. However, high gametogenesis and subsequent mass spawning of triploid females were observed at two sites (32% at Rongcheng and 54% at Rushan), and the fecundity of triploid females relative to diploid females was high (23.73% at Rongcheng and 41.15% at Rushan) under different rearing environments. Our results showed that the triploid of "Haida No.2" had better performance at grow-out stage than diploids. However, the phenomenon of high gametogenesis of triploid and spawning is not favorable for the industrialization of triploid oysters, and the mechanism of high gametogenesis of triploids needs further study. [ABSTRACT FROM AUTHOR]

The molluscan family Ostreidae, commonly known as oysters, is an important molluscan group due to its economic and ecological importance. In recent years, an abundance of genomic data of Ostreidae species has been generated and available in public domain. However, there is still a lack of a high-efficiency database platform to store and distribute these data with comprehensive tools. In this study, we developed an oyster genome database (OysterDB) to consolidate oyster genomic data. This database includes eight oyster genomes and 208,923 protein-coding gene annotations. Bioinformatic tools, such as BLAST and JBrowse, are integrated into the database to provide a user-friendly platform for homologous sequence searching, visualization of genomes, and screen for candidate gene information. Moreover, OysterDB will be continuously updated with ever-growing oyster genomic resources and facilitate future studies for comparative and functional genomic analysis of oysters (http://oysterdb.com.cn/). [ABSTRACT FROM AUTHOR]

Large-scale production of triploid oysters requires tetraploids as broodstock; hence, tetraploidy is a key asset for the triploid oyster industry. To explore the possibility of obtaining new allotetraploid strains with improved traits through hybridization and polyploidization, we conducted a comparative study for growth, survival, and gonadal development of tetraploid Crassostrea gigas, tetraploid C. angulata, and four allotetraploids. Firstly, allotriploid GGA (C. gigas ♀ × C. angulata ♂) and AAG (C. angulata ♀ × C. gigas ♂) were obtained from the hybridization between diploid C. gigas and diploid C. angulata by inhibiting the polar body II. Subsequently, allotetraploid GGGA (triploid C. gigas ♀ × diploid C. angulata ♂), allotetraploid AAAG (triploid C. angulata ♀ × diploid C. gigas ♂), allotetraploid GGAG (allotriploid GGA ♀ × diploid C. gigas ♂), allotetraploid AAGG (allotriploid AAG ♀ × diploid C. gigas ♂), autotetraploid GGGG (triploid C. gigas ♀ × diploid C. gigas ♂), and autotetraploid AAAA (triploid C. angulata ♀ × diploid C. angulata ♂) were obtained by inhibiting polar body I. The growth, survival, and gonad development of six groups (GGGA, AAAG, GGAG, AAGG, GGGG, and AAAA) were assessed. There were no detectable differences in fertilization and hatching rates among the six groups. Flow cytometry analysis showed that the tetraploid ratios of the four allotetraploid groups exceeded 88% on day 20. At larval stage, the allotetraploids GGGA and GGAG groups had higher growth rates and cumulative survival rates than the autotetraploid groups. Field-based phenotyping at three sites indicated that certain allotetraploid groups exhibited higher growth and survival compared to the autotetraploid groups. In particular, the best group over 9 months of testing was the allotetraploid GGAG, which significantly outperformed the autotetraploid groups in growth and survival. Furthermore, most of the individuals from the four allotetraploids were fully mature, with morphologically normal oocytes or sperm. The establishment of these four allotetraploids offers new germplasm resources for oyster genetic improvement and commercial production. [ABSTRACT FROM AUTHOR]

Tetraploid oysters are of rising aquacultural importance because they can be used to produce 100% triploid seeds. Nevertheless, notable gaps in the understanding of the environmental preferences of tetraploid oyster larvae still exist. To investigate the combined effects of temperature and salinity on the growth, survival and metamorphosis of larvae of tetraploid Crassostrea gigas (GGGG), tetraploid C. angulata (AAAA) and two allotetraploid oysters (GGAG and AAGG), a total of 192 independent experiments were established, including 4 temperatures (21, 24, 27 and 30 ℃) × 4 salinities (22, 25, 28 and 31) × 4 populations (GGGG, AAAA, GGAG and AAGG) treatment combinations. Results showed that both temperature and salinity significantly affected growth, survival and metamorphosis of tetraploid oyster larvae and there was a significant interaction between salinity and temperature on survival and metamorphosis. The highest specific growth rate (SGR) of AAAA (7.61%), GGAG (8.47%) and AAGG (8.33%) occurred at a salinity-temperature combination of 25/27 °C, and that of GGGG (8.07%) occurred at a salinity-temperature combination of 25/24 °C. The highest accumulative survival rate (ASR) of AAAA (22.72%), GGAG (33.88%) and AAGG (31.10%) occurred at a salinity-temperature combination of 25/24 °C, and that of GGGG (27.51%) occurred at a salinity-temperature combination of 25/21 °C. The largest metamorphosis rate (MR) of AAAA (25.50%), GGAG (33.27%) and AAGG (32.73%) were found at a salinity-temperature combination of 28/27 °C, and that of GGGG (27.60%) were found at a salinity-temperature combination of 28/24 °C. In general, the larval performance of allotetraploid oysters was superior to that of autotetraploid oysters, with an allotetraploid superiority ranging from 3.57 to 9.39% in SGR, 32.82 to 103.26% in ASR and − 12.08 to 28.79% in MR. The optimum larval rearing salinity-temperature condition recommended for GGGG, GGAG and AAGG is 25–28 at 21–24 °C, while for AAAA is 25–28 at 24–27 °C; a temperature range of 24–27 °C and a salinity range of 28–31 are suggested to improve spat production of these four tetraploid oysters. [ABSTRACT FROM AUTHOR]

Shell color is one of the shell traits of molluscs, which has been regarded as an economic trait in some bivalves. Pacific oysters (Crassostrea gigas) are important aquaculture shellfish worldwide. In the past decade, several shell color strains of C. gigas were developed through selective breeding, which provides valuable materials for research on the inheritance pattern and regulation mechanisms of shell color. The inheritance patterns of different shell colors in C. gigas have been identified in certain research; however, the regulation mechanism of oyster pigmentation and shell color formation remains unclear. In this study, we performed transcriptomic and physiological analyses using black and white shell oysters to investigate the molecular mechanism of melanin synthesis in C. gigas. Several pigmentation-related pathways, such as cytochrome P450, melanogenesis, tyrosine metabolism, and the cAMP signaling pathway were found. The majority of differentially expressed genes and some signaling molecules from these pathways exhibited a higher level in the black shell oysters than in the white, especially after l-tyrosine feeding, suggesting that those differences may cause a variation of tyrosine metabolism and melanin synthesis. In addition, the in vitro assay using primary cells from mantle tissue showed that l-tyrosine incubation increased cAMP level, gene and protein expression, and melanin content. This study reveals the difference in tyrosine metabolism and melanin synthesis in black and white shell oysters and provides evidence for the potential regulatory mechanism of shell color in oysters. [ABSTRACT FROM AUTHOR]

The Pacific oyster Crassostrea gigas is one of the most commercially important aquaculture species worldwide. The occurrence of inbreeding is common in the breeding practice of oysters due to limited population size, successive high selection pressure, and genetic drift. To determine the potential effects of inbreeding on the potential of genetic improvement for fast growth of oysters, genetic parameters of growth traits in the 7th to 9th generation of the orange-shell strain (a typical inbred population) were estimated. At the grow-out stage, the average genetic gain (GG), selection response (SR), and realized heritability ( h R 2 ) for shell height were 6.06–7.06%, 0.43–0.50, and 0.24–0.27, respectively. Meanwhile, the average GG, SR, and h R 2 for body weight were 6.03–7.36%, 0.38–0.41, and 0.21–0.22. There were no significant differences in the corresponding genetic parameters among the three generations (P > 0.05). Genetic parameters were estimated for an unselected population of C. gigas that served as a control group and were significantly higher than those of the orange-shell strain (P < 0.05). The results of this study suggest that inbreeding reduced the potential for genetic improvement of the rapid growth of oysters. However, some easy-to-operate and low-cost measures including the large size of broodstock, balanced sex ratio, and artificial spawning have been applied to the mass selection of the orange-shell strain, which prevents the situation from getting worse. These results will contribute to a better understanding of the negative effects of inbreeding on C. gigas and the management of inbreeding in shellfish breeding programs. [ABSTRACT FROM AUTHOR]

Oysters are one of the most commercially important shellfish species and have been cultured for thousands of years. Oyster aquaculture supports the major aquaculture industries in many countries. Over the last few decades, the oyster breeding and aquaculture industries have developed rapidly to meet the continually growing demand. Many researchers have made significant efforts toward the genetic improvement of commercially important traits in oysters. Some strains with fast‐growing, disease‐resistant, and stable shell‐colours have been developed through selective breeding. Some hybrid varieties have been developed by crossing different geographical populations or cultivated strains. Several hybrids exhibit considerable genetic variation and improved productive performance. Additionally, polyploid induction technologies have been applied in the oyster aquaculture industry, which provides a useful tool for performance improvement and genetic containment of cultured stocks. At present, the development of molecular breeding also provides a great opportunity for oyster genetic improvement. These advances in oyster breeding have improved the quality of oysters, brought great economic benefits, and been conducive to the sustainability of oyster production. Nonetheless, there are still some limitations and obstacles in oyster breeding, such as infectious diseases, summer mortality, conservation of germplasm resources, environmental contamination, and climate change. The present review provides an overview of the current status, challenges, and prospects in oyster breeding. [ABSTRACT FROM AUTHOR]

The increasing seawater temperature during summer months frequently results in severe mortalities in the Pacific oyster Crassostrea gigas around the world, becoming one of the most significant problems challenging the oyster farming industry. In northern China, significant recurrent summer losses of C. gigas have occurred, and its impact on oyster aquaculture has increased in recent years. Selective breeding for improved oyster resistance to high temperature could help to reduce this massive mortality, but the extent of genetic variation underlying this trait is currently unknown. In this study, we constructed 38 full-sib families using the wild C. gigas and estimated the genetic parameters by performing two month-long high-temperature challenge experiments (30 ℃). Experiment 1 was performed in March 2022 followed by experiment 2 in June 2022 (spawning season). In both challenge experiments, there were significant differences in survival among families, suggesting that C. gigas has a different ability to survive under heat stress. Notably, significantly greater mortality was observed for experiment 2, which related to reproductive status and may contribute to additional stress. Thermal tolerance was defined using both binary test survival and time of death traits. Heritability estimates for thermal tolerance were low to moderate (0.16–0.36 for experiment 1 and 0.16–0.33 for experiment 2) using both a Bayesian (MCMCglmm) and a likelihood-based (ASReml-R) approach and estimated heritability of the threshold animal model using ASReml-R (0.16) appeared to be lower compared to MCMCglmm (0.31–0.32). Notably, the genetic and phenotypic correlations for thermal tolerance between two experiments were 0.463 (BS) to 0.491 (TD) and 0.510 (family survival), respectively, which suggested a significant re-ranking of the family breeding values in different time periods. Finally, the genetic and phenotypic correlations were low between growth traits (shell height, shell length, and shell width) and thermal tolerance, suggesting that selection for these traits should be conducted separately. This study reports the first estimation of genetic parameters for chronic thermal tolerance in C. gigas and indicates that this trait is heritable and selective breeding for thermal tolerance is a feasible and promising approach to reduce summer mortality. [ABSTRACT FROM AUTHOR]

The sterility advantages of triploids make them paramount for oyster aquaculture. Tetraploids are always used as male parents for triploid industrial breeding in current hatcheries, thus they are the core asset for triploid oyster culture. The oyster Crassostrea angulata is the most productive oyster species in China. However, the optimal conditions for tetraploid induction have never been examined. In this study, we investigated the effectiveness of three induction methods (Method I, 2n ♀ × 2n ♂ + inducer abbreviated as DD + ind; Method II, 3n ♀ × 2n ♂ + inducer abbreviated as TrD + ind; Method III, 2n ♀ × 4n ♂ + inducer abbreviated as DT + ind) and three inducers (salinity; cytochalasin B, CB; and 6-dimethylaminopurine, 6-DMAP) for tetraploid induction in C. angulata. The optimal induction conditions for the three induction methods were a CB concentration of 0.5 mg/L treated for 20 min, 0.75 mg/L CB for a constant induction treatment of 20 min and 6-DMAP at a concentration of 100 mg/L for 20 min, respectively. When the optimal induction conditions were applied to a single pair of oysters, the D-larval tetraploid rate for the three methods were 18.93 ± 1.46%, 58.32 ± 1.03% and 51.61 ± 1.37%, respectively. After 30 days, the survival rates of the experimental groups were only 10.36 ± 2.51%, 28.07 ± 2.03% and 21.99 ± 1.27%, respectively, at day 30. The tetraploid rates in DD + ind and DT + ind were both decreased significantly over time, but they still reached 11.81% and 21.99% at 30 days, and 8% and 16% at day 180, respectively. Tetraploid rates in TrD + ind were 58.32 − 71.28% at the larval stage and 80% at day 180. In summary, despite very different induction rates, surviving tetraploids of C. angulata could be obtained in all three methods. Method II (TrD + inducer) was regarded as the optimal method to induce C. angulata tetraploids, and the optimal induction condition was a CB concentration of 0.75 mg/L and treated for 20 min under this method. [ABSTRACT FROM AUTHOR]

A key aspect of selective breeding programs is whether the traits of interest respond similarly in different environments. The magnitude of the genotype by environment (G × E) interaction and heritability are responsible for this aspect. Commercial hatcheries in northern China supply batches of seed to oyster farmers each year, which are cultivated in a wide range of growing sites. However, the extent to which G × E interactions affect the ability to identify quality oyster lines has not been measured. Therefore, the objective of this study was to estimate the genetic parameters and G × E interactions for harvest traits in C. gigas cultured in northern China. Oysters from 40 mostly full-sib families (the offspring of 36 sires and 40 dams) were tested at three major commercial cultivation sites: Rushan, Hungdao and Rongchceng. We recorded shell height (SH), shell length (SL), shell width (SW), individual weight (IW), survival rate (SR), wet meat weight (WMW), meat ratio (MR) and shell shape (SS) at harvest. Compared with control line, selected line exhibited high superiority in survival rate at three sites. Heritability estimates for the harvest traits were low to high across different sites (0.55–0.75 for SH, 0.25–0.49 for SL, 0.20–0.33 for SW, 0.37–0.71 for IW, 0.10–0.12 for SR, 0.40–0.52 for WMW, 0.14–0.34 for MR and 0.18–0.30 for SS). Genetic correlations among growth traits were medium to high and positive, ranging between 0.52 ± 0.15 (between SH and SW at Huangdao) to 0.90 ± 0.04 (between SH and IW at Rongcheng). Nevertheless, we found significantly negative genetic correlations between growth traits and shell shape, an index that takes into account growth traits and shell shape should be developed to improve both simultaneously. Since it is not possible to disentangle this result from the correlation that mathematically determined alone, this result requires further study. Notably, genetic correlations between growth traits and survival were low, ranging from − 0.31 ± 0.17 (between SL and SS at Huangdao) to 0.14 ± 0.19 (between SW and SS at Rongcheng). G × E interactions were estimated as genetic correlations between the same trait measured on different oysters at three sites. Moderate G × E interactions were found for growth traits (0.45–0.87), meat yield traits (0.40–0.73) and shell shape (0.22–0.69), and weak G × E interaction was found for survival at harvest (0.80–0.86). This study reveals the presence of weak to high re-ranking of genotypes for harvest traits in C. gigas cultured in different commercial sites, suggesting that G × E interactions should be considered in oyster breeding program servicing multiple sites. [ABSTRACT FROM AUTHOR]

Parallel diversification provides a proper framework for studying the role of natural selection in evolution. Yet, empirical studies from ecological 'non‐model' species of invertebrates are limited at the whole genome level. Here, we presented a chromosome‐scale genome assembly for Crassostrea angulata and investigated the parallel genomic evolution in oysters. Specifically, we used population genomics approaches to compare two southern–northern oyster species pairs (C. angulata–C. gigas and southern–northern C. ariakensis) along the coast of China. The estimated divergence time of C. angulata and C. gigas is earlier than that of southern and northern C. ariakensis, which aligns with the overall elevated genome‐wide divergence. However, the southern–northern C. ariakensis FST profile represented more extremely divergent "islands". Combined with recent reciprocal hybridization studies, we proposed that they are currently at an early stage of speciation. These two southern–northern oyster species pairs exhibited significant repeatability in patterns of genome‐wide differentiation, especially in genomic regions with extremely high and low divergence. This suggested that divergent and purifying selection has contributed to the genomic parallelism between southern and northern latitudes. Top differentiated genomic regions shared in these two oyster species pairs contained candidate genes enriched for functions in energy metabolism, especially adipogenesis, which are closely related to reproductive behaviours. These genes might be good candidates for further investigation in vivo. In conclusion, our results suggest that similar divergent selection and shared genomic features could predictably transform standing genetic variation within one species pair into differences in another. [ABSTRACT FROM AUTHOR]

Background: Molluscan shell, composed of a diverse range of architectures and microstructures, is a classic model system to study the relationships between molecular evolution and biomineralized structure formation. The shells of oysters differ from those of other molluscs by possessing a novel microstructure, chalky calcite, which facilitates adaptation to the sessile lifestyle. However, the genetic basis and evolutionary origin of this adaptive innovation remain largely unexplored. Results: We report the first whole-genome assembly and shell proteomes of the Iwagaki oyster Crassostrea nippona. Multi-omic integrative analyses revealed that independently expanded and co-opted tyrosinase, peroxidase, TIMP genes may contribute to the chalky layer formation in oysters. Comparisons with other molluscan shell proteomes imply that von Willebrand factor type A and chitin-binding domains are basic members of molluscan biomineralization toolkit. Genome-wide identification and analyses of these two domains in 19 metazoans enabled us to propose that the well-known Pif may share a common origin in the last common ancestor of Bilateria. Furthermore, Pif and LamG3 genes acquire new genetic function for shell mineralization in bivalves and the chalky calcite formation in oysters likely through a combination of gene duplication and domain reorganization. Conclusions: The spatial expression of SMP genes in the mantle and molecular evolution of Pif are potentially involved in regulation of the chalky calcite deposition, thereby shaping the high plasticity of the oyster shell to adapt to a sessile lifestyle. This study further highlights neo-functionalization as a crucial mechanism for the diversification of shell mineralization and microstructures in molluscs, which may be applied more widely for studies on the evolution of metazoan biomineralization. [ABSTRACT FROM AUTHOR]

Melanogenesis is a multistep process to produce melanin for dark pigmentation in skin coloration. Previous studies in vertebrates demonstrated that cystine and tyrosine amino acids are involved in the melanin synthesis. However, very little is known about the melanogenesis in bivalve. In this study, cystine supplementation for 30 days significantly upregulated the expression of CgB-aat1, CgCbs and CgTyr and pheomelanin content in the Pacific oyster Crassostrea gigas. Transmission electron microscope (TEM) results revealed more melanosomes in the connective tissue and melanin granules were secreted in epithelium of mantle. In contrast, tyrosine supplementation had no clear effect on melanogenesis except the gene expression changes of CgB-aat1 and CgCbs. In addition, prolonged supplementation of cystine or tyrosine for 60 days had a negative impact on melanogenesis. Indeed, after 60 days, expression of most of the melanin synthesis-related genes under study was decreased, and melanin content was significantly reduced, indicating that cystine and tyrosine might inhibit production of eumelanin and pheomelanin, respectively. In addition, in vitro analysis using primary cell culture from mantle tissue indicated that incubation with cystine, tyrosine, or B-AAT1 polypeptide, CBS/TYR recombinant proteins induced the increase of CgB-aat1 and CgCbs expression in a dose-dependent manner, suggesting the presence of a regulatory network in response to cystine and tyrosine amino acids intakes in pheomelanin synthesis-related gene expression. Taken together, these data indicate that cystine-CgB-aat1-CgCbs-CgTyr axis is a potential regulator of the pheomelanin biosynthesis pathway, and thus plays an important role in the mantle pigmentation in C. gigas. This work provides a new clue for selective cultivation of oyster strains with specific shell colors in bivalve breeding. [ABSTRACT FROM AUTHOR]

Heterosis, also known as hybrid vigor, is widely used in aquaculture, but the molecular causes for this phenomenon remain obscure. Here, we conducted a transcriptome analysis to unveil the gene expression patterns and molecular bases underlying thermo-resistant heterosis in Crassostrea gigas ♀ × Crassostrea angulata ♂ (GA) and C. angulata ♀ × C. gigas ♂ (AG). About 505 million clean reads were obtained, and 38,210 genes were identified, of which 3779 genes were differentially expressed between the reciprocal hybrids and purebreds. The global gene expression levels were toward the C. gigas genome in the reciprocal hybrids. In GA and AG, 95.69% and 92.00% of the differentially expressed genes (DEGs) exhibited a non-additive expression pattern, respectively. We observed all gene expression modes, including additive, partial dominance, high and low dominance, and under- and over-dominance. Of these, 77.52% and 50.00% of the DEGs exhibited under- or over-dominance in GA and AG, respectively. The over-dominance DEGs common to reciprocal hybrids were significantly enriched in protein folding, protein refolding, and intrinsic apoptotic signaling pathway, while the under-dominance DEGs were significantly enriched in cell cycle. As possible candidate genes for thermo-resistant heterosis, GRP78, major egg antigen, BAG, Hsp70, and Hsp27 were over-dominantly expressed, while MCM6 and ANAPC4 were under-dominantly expressed. This study extends our understanding of the thermo-resistant heterosis in oysters. [ABSTRACT FROM AUTHOR]

Colorful shell of bivalve is mainly because of the biological pigments, of which melanin plays an important role in shell color formation. More and more studies focus on the genes function involved in melanin synthesis, but relatively few studies address the biochemical character and ultrastructure of melanin in bivalve from microscopic perspective. Here, we investigated the histological structure of mantle of Crassostrea gigas with orange shell color. Distribution of melanin in mantle was verified with histochemical staining. In addition, immunofluorescence technique showed that strongly positive signal of CgTYR was specific to the mantle margin, which is consistence with the location of brown granules in H&E staining. The further result of elementary composition of melanin displayed that metal Ca, Fe, and Zn were detected using scanning transmission electron microscope and energy dispersive spectroscopy mapping methods. Next, based on TEM observations, it was speculated that the series of cellular events leading to the formation and release of melanin. Melanocyte in the primary stage showed many mitochondria and rough endoplasmic reticulum as well as an extensive Golgi complex with numerous vesicles intermingled with melanosome. Subsequently, melanosome was expended and their hue gradually intensified, and Golgi complex and mitochondria were still observed in the cytoplasm. Finally, after melanosome was discharged into intercellular spaces, the disintegration of membranes in some cells, and severe cellular vacuolization. These data enrich the understanding of ultrastructural characteristic and formation of melanin in mantle of bivalve and pave the way for further investigating shell coloration at the cellular level. [ABSTRACT FROM AUTHOR]

In order to evaluate the effects of triploidy induction on a selected strain 'Haida No. 2' of the Pacific oyster Crassostrea gigas, which is characterized with golden shell color and high growth rate, the growth, survival rate and stability of triploid rate were analyzed at different development stages in the present study. Three different conditions inhibiting the release of polar body II or polar body I were tested: (A) Cytochalasin-B (CB), 0.5 mg L−1 at 10 min post-insemination for 15 min; (B) CB, 0.5 mg L−1at 15 min post-insemination for 20 min; and (C) CB, 0.7 mg L−1, at 15 min post-insemination for 20 min. The triploidy induction treatments significantly reduced the D-larvae and survival rates at the larvae stage but not at the juvenile and adult stages. Triploid rate dramatically decreased at the larval stage and did not significantly change at the juvenile and adult stages. Regarding the stability of the triploid rate, there was a significant difference between the three treatment groups. Larvae from the treatment A and control groups exhibited higher growth rates in shell height than those from the other two treatment groups at day 27. Triploid juveniles and adults from the treatment A group exhibited a higher wet weight than diploids from the control group and triploids from the other treatment groups. Triploidy induction did not affect the shell color of the progeny. The results obtained in the study demonstrate that triploidy induction has the potential to be used to increase the production of C. gigas variety 'Haida No. 2' without modifying its golden shell color. [ABSTRACT FROM AUTHOR]

Although Crassostrea nippona and C. gigas are occasionally found to be sympatric, little is known about the differences in tolerance against environmental stresses between species, which may potentially result in severe economic losses due to the neglect of species-specific biological characteristics in farming practices. Therefore, two independent and consecutive experiments were performed to evaluate the differences in temperature and salinity tolerance between C. nippona and C. gigas spat and determine the optimal environmental conditions for the aquaculture of C. nippona spat. The experimental results of dynamic treatment showed that the accumulative survival rate (ASR) of C. nippona spat was generally lower than that of C. gigas throughout temperature changes and salinity decreases, while the ASR of C. nippona spat was superior to that of C. gigas with increasing salinity. In addition, the daily growth rate (DGR) of both species was significantly inhibited at 18 °C and peaked at 25 °C with increasing temperature (p < 0.05) at each experimental salinity. At 32 °C, the survival rate (SR) of C. nippona spat was significantly lower than that of C. gigas on day 10 at all salinities tested (p < 0.05), whereas the difference in the SR between species at 35 psu gradually disappeared with prolonged stress time. The data indicated that C. gigas spat was suitable for rearing at temperatures of 25 and 32 °C at all salinity levels, except the combination of 32 °C and 35 psu. By contrast, combinations of 25 °C and salinities of 21–35 psu were considered as the optimal environmental conditions for the long-term culture of C. nippona spat. These present findings contribute to a better understanding of the tolerance of C. nippona spat to environmental stresses and suggest that particular attention should be paid to the inferior adaptability of C. nippona when farming in locations outside their natural habitats. [ABSTRACT FROM AUTHOR]

The Pacific oyster Crassostrea gigas is the most commonly farmed mollusks worldwide, while its production has been hindered by massive mortalities linked to high temperatures. Selective breeding focusing on thermal tolerance is an attractive option for reducing the impact of massive mortalities, but the genetic basis underlying this trait is currently unknown. Hybridization between C. gigas and C. angulata was conducted and followed by one generation of family selection for thermal tolerance. The genetic parameters for thermal resistance, summer survival and growth and selection response were investigated by using 50 full-sibling families. The mean survival of all families ranged from 30.2 to 69.5%, reflecting the large variation of thermal tolerance in the hybrid population. The estimates of heritability for thermal tolerance were low to moderate, ranging from 0.19 ± 0.03 to 0.27 ± 0.05, confirmed that there is genetic basis for thermal tolerance. The phenotypic (r = 0.537, P < 0.01) and genetic correlations (r = 0.546, P < 0.01) between thermal tolerance and summer survival were positive and significant, while the phenotypic and genetic correlations between growth and survival (including thermal tolerance and summer survival) were positive but low (P > 0.05). High response to selection (ΔG = 36.33%) was observed after one generation of selection for thermal tolerance, and there was also corresponding response (ΔG = 14.46%) for summer survival. These results demonstrated that genetic selection to improve summer survival of oyster may be facilitated by selecting highly heat-tolerant lines. [ABSTRACT FROM AUTHOR]

High temperature is considered to be one of the important causes of mass mortality of diploid and triploid oysters in summer. In order to compare the tolerance adaptability of diploid and triploid oysters to heat stress, the activities of superoxide dismutase activity (SOD), catalase activity (CAT) and contents of malondialdehyde (MDA) and the expression of Hsp70 and metallothionein (MT) genes in diploid and triploid oysters under acute and chronic heat stress were studied. The results showed that the survival rate of diploid oysters was significantly higher than that of triploid oysters under acute and chronic heat stress (p < 0.05). Under acute heat stress, the SOD levels in gills of both diploid and triploid oysters increased significantly from hour 3 (p < 0.05), but there was no significant difference in SOD levels between diploid and triploid oysters at all time points. The SOD level in hepatopancreas of triploid oysters was significantly higher than that of diploid oysters at all time points except 48 h (p < 0.05). The CAT level of diploid and triploid oysters decreased sharply at 48 h and that of diploid oysters was significantly higher than that of triploid oysters (p < 0.05). Under acute heat stress, the increase in MDA content in triploid oysters was significantly higher than that in diploid oysters (p < 0.05). Under acute heat stress, the expression levels of Hsp70 in the gills and hepatopancreas were significantly increased in diploid and triploid oysters (p < 0.05), but increased varies between gills and hepatopancreas. The expression levels of MT in the gills and hepatopancreas were significantly decreased in diploid and triploid oysters (p < 0.05). The results obtained in this study provide physiological and immunological evidence to explain differences in high‐temperature tolerance between diploid and triploid oysters and help us better understand the mass mortality of oysters that occurs during high temperatures in summer. [ABSTRACT FROM AUTHOR]

The Pacific oyster Crassostrea gigas is one of the most widely cultivated aquaculture species and contributes significantly to total seafood production for human beings. However, mass mortality occurred frequently, and in some regions almost all oysters died during seed production and grow-out stage. In order to explore whether hybridization breeding can improve its growth and survival, a complete diallel cross between a selected strain 'Haida No. 1' (S) and an orange shell variant (O) of C. gigas was carried out. The larval growth and survival were compared among hybrids and purebred strains at temperatures of 16, 20, 24, 28 and 32°C; salinities of 15, 20, 25, 30 and 35; and stocking densities of 0.5, 1, 2, 4 and 8 larvae mL−1. Under different environments, the hybridization between two strains of C. gigas showed the heterosis of growth and survival. The mean shell height and survival rate of the two reciprocal crosses (OS, SO) were significantly higher than those of the two purebred strains (SS, OO) under all environmental conditions. In particular, OS showed greater heterosis than the purebred strains and SO progeny. The results showed that the productive traits of the 'Haida No. 1' could be improved by crossing with the orange shell line. Meanwhile, the results from this study also indicated that hybridization between the two strains of C. gigas may be a promising way for breeding new variety with high survival rate. [ABSTRACT FROM AUTHOR]

Crassostrea gigas is a commercially important species which is the mostly widely cultured in the world. However, most of the oyster broodstock in China remains unselected. Through a 7-generation of selection on the shell color and growth traits of adult oysters, an excellent strain of C. gigas with black shell coloration and mantle has been developed. In order to facilitate the industrialized breeding, it is necessary to explore the growth performance of the black shell strain in larval stage and find out the optimal conditions for larval development. In this study, the accumulated growth rate and survival rate of the black shell strain and wild population of C. gigas larvae were measured respectively, and a central composite design as well as a response surface method was used to investigate the combined effect of temperature, salinity, and rearing density on the growth of both the two populations. No significant differences were found in survival rate between the two populations, and two model equations for the growth of the two populations were established. The optimizations of accumulated growth rate for two populations were explored. When the temperature, salinity, and rearing density was 25.14 °C, 30.28 psu, and 1.00 ind. ml−1, respectively, the accumulated growth rate for the black shell strain maximized 15.40 μm day−1. With a combination of temperature of 25.06 °C, salinity of 29.27 psu and rearing density of 1.00 ind. ml−1, the maximum value of accumulated growth rate for wild population was 13.20 μm day−1. Furthermore, the larval growth rates were compared between the black shell strain and wild population, and the larvae of the black shell strain significantly grew faster than those of wild population when temperature, salinity and rearing density in a suitable range. [ABSTRACT FROM AUTHOR]

The Pacific oyster, Crassostrea gigas, naturally distributing along the coast of northwest Pacific, is one of the most important bivalve species due to its high economic value and fecundity. In China, we have initiated a selective breeding program on both shell color and growth rate of C. gigas since 2010. A black shell line was obtained through four-generation family selection. In this study, mass selection for growth improvement was conducted in the sixth generation and seventh generation of black shell lines. To assess the progress of potential genetic improvement, the progeny of two generations of black shell lines were selected to evaluate their shell heights via a 450-day farming experiment. As the results, after growing for 450 days, the sixth generation and seventh generation of selected lines were 9.03% and 11.42% larger than the control lines, respectively. During the grow-out stage, the genetic gain of two generations was 8.82%±0.18% and 11.54%±0.43%, respectively; and the corresponding realized heritability was 0.45± 0.04 and 0.41 ±0.04, respectively. These results indicated that the mass selection for shell height achieved steady progress in the two generations of C. gigas. [ABSTRACT FROM AUTHOR]

Seasonal variations in the biochemical composition and the reproductive cycle of pen shell Atrina pectinata in Bohai Sea were investigated from May 2013 to April 2014. Histological analysis indicated that the reproductive cycle of A. pectinata can be divided into two phases, inactive stage and gametogenesis, which were equally and significantly influenced by seawater temperature and food availability. Gametogenesis began in late autumn (October), and completed in June and July. Spawning took place in August, coinciding with the highest water temperature and the richest phytoplankton. The significantly high glycogen content in adductor muscles sustained throughout the late active and ripeness stages, but plummeted during spawning and inactive stages. The protein content in female gonads exhibited a synchronous increase along with oocyte diameter and lipid content, suggesting that the female gonads could accumulate protein and lipid for vitelline in A. pectinata. Furthermore, the RNA/DNA ratio was found to be a useful index to indicate the level of gonad maturation in both males and females. The findings of the present study provided a foundation for the fishery resource administration and the aquaculture development of this species. [ABSTRACT FROM AUTHOR]