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The Pacific oyster (Crassostrea gigas), as the most productive cultured shellfish in the world, has considerable economic importance and value. Yet research on its sex determination remains lacking. Here, a novel HMG-box-containing homolog of the sox gene was uncovered in C. gigas (named CgSox-like). Alignment with the HMG-box of sox genes from other animals revealed that CgSox-like had a high similarity to the Sox gene of scallops. Among different adult tissues, CgSox-like was mainly expressed in the gonads. qPCR analysis revealed that CgSox-like and CgSox8 , a sox gene family gene mainly expressed in male gonads, shared similar expression patterns, with elevated expression as the male gonads developed. RNA interference and dual luciferase reporter assays demonstrated that CgSox-like was located upstream of CgSox8 and promoted the expression of CgSox8. In addition, analysis of CgSox-like and CgSox8 methylation patterns indicated that both genes were in hypomethylation state during the gonadal development. Finally, demethylase inhibitors resulted in elevated expression levels of CgSox-like and CgSox8 genes, suggesting that epigenetics was engaged in modulating the expression of sex-related genes in the Pacific oyster. Taken together, the novel gene CgSox-like was implicated in the sex determination or gonadal development process in male oysters. These results will provide valuable information for the investigation of sex determination mechanism and sex control breeding. • CgSox-like is involved in sex determination or gonadal development in oysters. • CgSox-like was located upstream of CgSox8 and promoted the expression of CgSox8. • DNA methylation participates in the regulation of sex determination in Crassostrea gigas. [ABSTRACT FROM AUTHOR]

The Pacific oyster (Crassostrea gigas) is one of the most important species which has the largest production among all cultured shellfish. Recently, several shell color strains of C. gigas have been developed to improve the commercial values, however the inheritance of shell pigmentation is still unclear. A total of 29 families cross-mated between different pigmented oysters were produced to investigate the inheritance of the shell pigmentation and purple pigmentation. Random samples from each family were obtained and their shell pigmentation were recorded. Results revealed that shell pigmentation was controlled by two genetic locus, among which one is responsible for secretion of pigmentation and the other is responsible for distribution mode of pigmentation. The locus controlling the secretion of pigmentation has two alleles with the allele for the presence of pigmentation being dominant to the allele for the absence of pigmentation. Similarly, another locus controlling the distribution mode of pigmentation has two alleles with the allele for striped distribution being dominant to the allele for solid distribution. In addition, one independent locus with two alleles was suggested to control the purple-striped pigmentation, which one allele for shell devoid of purple pigmentation is dominant to another allele for shell with purple pigmentation. The findings will provide valuable information for the efficient selective breeding of shell color strains in the Pacific oyster. • A two-locus model for inheritance of shell pigmentation were demonstrated. • The allele for secretion of pigmentation is dominant to the other allele for absence of pigmentation. • The allele for striped distribution of pigmentation is dominant to the allele for solid distribution of pigmentation. • An indenpendent loci was found to control the inheritance of purple striped pigmentation. [ABSTRACT FROM AUTHOR]

Crassostrea nippnoa is a potential aquaculture species because of its unique flavor, delicious taste and edibility during the summer, while C. gigas is a major global aquaculture species due to the fast growth rate, high disease resistance and strong environmental adaptability. This study firstly described the reciprocal hybridization between the two oyster species, and four crosses were produced: C. gigas ♀ × C. gigas ♂ (GG), C. gigas ♀ × C. nippona ♂ (GN), C. nippona ♀ × C. gigas ♂ (NG) and C. nippona ♀ × C. nippona ♂ (NN). The embryonic development of reciprocal crosses was similar to that of the pure crosses. However, fertilization rates were obviously different between the reciprocal hybrids and parental species, where the fertilization rates of reciprocal hybrids were significantly lower than those of parental species. At larval stages, the survival rates of the hybrid cohorts were significantly lower than those of inbred cohorts, and the critical stage of reciprocal hybrids where highest mortality occurred was the umbo period. The growth rate of GN progeny was not different from the inbred cohorts, while the growth rate of the NG progeny was higher than that of inbred cohorts. Furthermore, both hybrid cohorts had normal metamorphosis and produced viable spats. During the spat stage, the survival rates of reciprocal hybrids were not different from that of GG spat but were much higher than that of the NN spat. The growth rates of reciprocal hybrids were comparable to that of GG spat but significantly higher than that of NN spat. The shell height of the reciprocal crosses GN and NG were increased by 46.66% and 60.75% compared with the NN cross at day 360, respectively. The reciprocal hybrids had significant survival advantage and growth heterosis compared to NN cohort. The results demonstrated that the cross NG was deemed to be the better hybridization combination for genetic improvement, which could have a potential utilization for oyster aquaculture. • C. nippona was successfully hybridized with Pacific oyster. • Reciprocal hybrids exhibited larger growth heterosis and survival advantage at spat stage. • The cross NG was deemed to be the better hybridization combination in oyster genetic improvement. [ABSTRACT FROM AUTHOR]

Crassostrea nippona has recently been identified as a potential aquaculture species for its high glycogen content and delicate flavor in summer when Crassostrea gigas suffer from low meat quality. In 2014, we initiated the selective breeding program to improve growth traits of C. nippona through successive generations of mass selection, yet the genetic impact of an intense artificial selection on the genetic variability of C. nippona was not fully understood. In this study, the genetic diversity and genetic structure of three generations of mass-selected lines (G1-G3) and three wild populations were investigated using both 15 microsatellite loci and mitochondrial COI sequences (mtCOI). The selected lines exhibited no significant decrease in the average number of alleles (N a : 7.53–9.87), observed heterozygosity (H o : 0.58–0.64), expected heterozygosity (H e : 0.67–0.71) and alleles richness (A r : 6.35–7.75), compared with those of the wild populations. The abundant genetic diversity of selected lines was successfully maintained during three generations of mass selection due to no detectable loss. The effective population sizes (N e − lin) estimated by linkage disequilibrium methods for G1, G2 and G3 were 64.1, 25.3 and 47.4, respectively. Moreover, little genetic differentiation within the selected lines was observed in AMOVA analysis (global F ST : 0.005, P > 0.05) and significant genetic differentiation among the wild populations was revealed by global F ST (0.15, P < 0.01), pairwise F ST (0.095–0.211), Nei's D (0.260–0.730) and clustering results, which might suggest genetically isolated populations occurring in these sampling locations. To maximize future selective breeding efforts, a larger scale of broodstock and multi-line breeding strategy along with alleviating selection intensity is recommended against the reduction in the genetic diversity and effective population size in subsequent breeding practices. This study contributes to an increasing understanding of the efficiency of current breeding procedures in maintaining genetic variation and provides insight into future genetic improvement programs of C. nippona. • This study is the first to investigate the genetic diversity of C. nippona using both microsatellites and mtCOI. • Microsatellite analysis revealed no significant difference in genetic diversity between selected and wild populations. • The genetic diversity of the selected lines was successfully maintained over successive generations of mass selection. • Little genetic differentiation among the selected lines was observed based on the microsatellite result. [ABSTRACT FROM AUTHOR]

The Iwagaki oyster (Crassostrea nippona) is a potential aquaculture species with delicious taste and edibility in summer when the Pacific oyster (Crassostrea gigas) suffer from low meat quality. Although there is an increasing interest in hatchery mass production and field grow-out among farmers, large-scale expansion of C. nippona aquaculture has been compromised by unimproved growth performance. To improve the growth rate of C. nippona , we initiated the mass selective breeding program for fast growth in 2014 and estimated the genetic parameters of growth traits in the first three successive mass-selected generations. Each selected line exhibited better growth performance than the corresponding control group at all sampling dates. At grow-out stage, the average selection response (SR), realized heritability (h R 2) and genetic gain (GG) for shell height was 0.45–0.56, 0.27–0.42 and 9.20–11.12% in the three mass selected lines, respectively. A significant increase in GG for shell height was observed in the third-generation selected line compared with that in the first-generation (P < 0.05). Meanwhile, the average SR (0.44–0.49) and GG (11.07–12.46%) for body weight of C. nippona were observed in the selective breeding program after three successive generations of mass selection. The encouraging results obtained in this study suggest that genetic variances still remain at a relatively high level in the breeding populations of C. nippona , and demonstrate the feasibility of improving the growth traits by selective breeding for C. nippona. • Mass selection for fast growth of Crassostrea nippona obtained considerable selection responses. • Continuous and stable genetic gains for growth were observed during an intense mass selection. • Realized heritability of growth traits proved to possess great potential for genetic improvements. [ABSTRACT FROM AUTHOR]

Comparative genomics has become an important strategy for transferring genetic information from a well-characterized model species to one that is less described and can facilitate in functional gene mining and genomic evolution. Crassostrea angulata and Crassostrea gigas are closed related species that they can hybridize to produce fertile offspring, and C. gigas has accumulated a considerable number of genetic and genomic resources while C. angulata lags far behind. In the current study, 684 single nucleotide polymorphism (SNP) markers developed from C. gigas expressed sequence tags (ESTs) were used for transferability test in C. angulata , and 635 loci were successfully amplified. Then a genetic linkage map of C. angulata was for the first time constructed using these transferable SNP markers. A total of 988 transferable SNP markers were used for polymorphism screening in a mapping family, and 273 loci were distributed on the genetic map which was composed of 10 linkage groups and estimated 1040.29 cM in total length. The average spacing between markers was 3.96 cM. By comparing the shared EST-SNP marker locations on homologous linkage groups, comparative mapping analysis between the C. angulata genetic map and an integrated map of C. gigas revealed putative macro-collinearity in the two oyster genomes with little difference in markers order. These transferable SNP markers could be applied in genetic diversity and population structure studies, and the genetic map would be useful for quantitative trait loci mapping and genetic improvement through marker-assisted selection for C. angulata. • A total of 635 transferable SNP markers were developed for C. angulata from EST sequences of C. gigas. • The first genetic linkage map of C. angulata was constructed composing of 273 transferable EST-SNPs. • High macro-collinearity was revealed by the shared markers on C. gigas and C. angulata genetic maps. [ABSTRACT FROM AUTHOR]