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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]

Shell coloration in many molluscs is highly variable. A shell color segregation study with progenies from a full factorial cross generated among Pacific oysters exhibiting distinct shell colors (golden, white and black) was conducted to investigate the inheritance of the golden shell color and its correlation with dark pigmentation. Random samples from twenty-three full-sib families were obtained and the shell coloration of offspring within each family was recorded. Results revealed that golden coloration was inherited in a different pattern from dark pigmentation, indicating its different genetic basis. Dark pigmentation was identified as a foreground color while golden or white color were background ones. The locus controlling background colors has two alleles with the allele for golden background being dominant to the allele for white background. In addition, the overlying foreground pigmentation of shells with a golden background was significantly lighter than that of shells with a white background, which suggested an epistatic effect of background color on shell foreground pigmentation. All these findings will facilitate the selection of elite oyster lines with desired shell coloration for aquaculture. [ABSTRACT FROM AUTHOR]

The Pacific oyster ( Crassostrea gigas ) is one of the most important farmed oysters worldwide. To improve breeding progress, research was done to identify molecular markers linked to shell pigmentation, which has been viewed as a key factor for product value. An F 1 -segregating population exhibiting a bimodal distribution of shell pigmentation intensity was obtained by crossing two wild oysters with opposite shell pigmentation. Genomic DNA from nine individuals with lightest shell pigmentation and nine individuals with darkest shell pigmentation were equally pooled for amplified fragment length polymorphism (AFLP) screening. In bulked segregant analysis, six out of 225 selective primer pair combinations produced seven polymorphic fragments tightly associated with shell pigmentation across the segregating population. The seven AFLP markers were mapped onto a single linkage group, and 80% of phenotypic variance could be explained by this locus. In conversion of the seven fragments into sequence-characterized amplified region (SCAR) markers, only one was successfully converted into a co-dominant SCAR marker, named SP-170. The SCAR marker was integrated into a previously constructed linkage map. The SCAR marker obtained in this study will be useful for marker-assisted selection of the Pacific oyster. [ABSTRACT FROM AUTHOR]

Molluscan shellfish have a diverse form of shell coloration and sex determination, understanding the genetic mechanism under these two traits is crucial to their germplasm exploitation and conservation. In our breeding program, a novel orange-shell variant of C. gigas was obtained, and an F2 family was constructed by crossing an orange-shell male with a black-shell female. This F2 family provided an ideal material for QTL mapping of orange shell color and sex traits of C. gigas simultaneously. Here, a high-density genetic linkage map containing 1799 SNP markers was constructed using this F2 family by genotyping-by-sequencing. These markers were distributed on 10 linkage groups and the total map size was 828.31 cM, with an average marker interval of 0.58 cM. Based on this map, six chromosome-level significant QTLs associated with orange shell color were detected, with the phenotypic variance explained ranging from 9.10% to 12.30%. Within the QTL regions, four candidate genes, 3-ketoacyl-CoA thiolase A, fibroblast growth factor receptor 4, poly [ADP-ribose] polymerase tankyrase-1 and tripartite motif-containing protein 45-like, related to the organic matrix of mollusk shells or collagen were identified. Additionally, only one genome-wide significant sex-related QTL was detected, which explained 19.30% of phenotypic variance. Two sex-related genes, proteasome subunit beta type-3 and G-protein coupled estrogen receptor 1 were identified within the QTL region. The QTLs and candidate genes identified in this study provide valuable resources for further investigations on the molecular mechanisms of shell color and sex in C. gigas. • A high-density genetic linkage map containing 1799 SNP markers was constructed for C. gigas using an F2 family. • Six chromosome-level significant QTLs associate with orange shell color were detected in C. gigas. • Only one genome-wide significant sex-related QTL was detected in C. gigas. • Different description methods of phenotype have a great impact on the result of QTL mapping. [ABSTRACT FROM AUTHOR]