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• Carotenoid metabolism of Crassostrea gigas was studies for the first time with a beta-carotene feeding method. • The function of two apolipoprotein genes in carotenoid storage was validated by RNAi in the oyster. • A negative feedback regulation mechanism was proposed to regulate carotenoid metabolism in the oyster. Carotenoids are essential micronutrients for animals, and they can only be obtained from the diet for mollusk as well as other animals. In the body, carotenoids undergo processes including absorption, transport, deposition, and metabolic conversion; however, knowledge of the involved genes is still limited. To elucidate the molecular mechanisms of carotenoid processing and identify the related genes in Pacific oyster (Crassostrea gigas), we performed a comparative transcriptome analysis using digestive gland tissues of oysters on a beta-carotene supplemented diet or a normal diet. A total of 718 differentially expressed genes were obtained, including 505 upregulated and 213 downregulated genes in the beta-carotene supplemented group. Function Annotation and enrichment analyses revealed enrichment in genes possibly involved in carotenoid transport and storage (e.g., LOC105342035), carotenoid cleavage (e.g., LOC105341121), retinoid homeostasis (e.g., LOC105339597) and PPAR signaling pathway (e.g., LOC105323212). Notably, down-regulation of mRNA expressions of two apolipoprotein genes (LOC105342035 and LOC105342186) by RNA interference significantly decreased the carotenoid level in the digestive gland, supporting their role in carotenoid transport and storage. Based on these differentially expressed genes, we propose that there may be a negative feedback mechanism regulated by nuclear receptor transcription factors controlling carotenoid oxygenases. Our findings provide useful hints for elucidating the molecular basis of carotenoid metabolism and functions of carotenoid-related genes in the oyster. [ABSTRACT FROM AUTHOR]

One of the most important physiological process in the life cycle of most bivalves is the sexual reproduction required for species propagation. Understanding the molecular regulatory mechanisms of gender determination and gonad development have potential application in bivalve aquaculture, but there is still limited knowledge about this issue. The Pacific oyster Crassostrea gigas is a commercially important bivalve in aquaculture worldwide, as well as a representative specie to study the sexual development in bivalves for its complicated sexuality, including dioecy, sex change and rare hermaphroditism. In this study, an integrated analysis of miRNA and mRNA expression profiles of oyster gonads at different gametogenesis stages was performed to explore their possibly regulatory patterns in sexual development of C. gigas. Based on small RNA sequencing, a total of 122 miRNAs were identified, including 53 conserved and 69 novel miRNAs. Six miRNAs exhibited sex-specific pattern that expressed exclusively from either male or female gonads with increasing abundance throughout the gonadal development. Moreover, 287 putative targets of the six sex-biased miRNAs were predicted by combining previous transcriptome data. GO and KEGG enrichment analysis of target genes revealed that these sex-biased miRNAs might participate in the regulation of gametogenesis. Further, potential miRNA-mRNA interactions based on six sex-biased miRNAs were collected. The homolog of a key participator in testis development, meiotic recombination protein SPO11 in C. gigas (CgSPO11), was predicted as a target of female-biased miRNA lgi-miR-96a. This negative miRNA-mRNA expression relationship was validated in additional samples by quantitative real-time PCR (qRT-PCR) and the regulation of CgSPO11 by lgi-miR-96a was preliminarily confirmed by a dual-luciferase reporter assay. This study provides information regarding the gonad miRNAs of C. gigas and leads to the discovery of miRNA-mRNA interactions in gonad development, which will facilitate further investigations in molecular mechanisms of bivalve sex differentiation. • A total of 122 miRNAs were identified during the Pacific oyster gonad development. • Six sex-biased miRNAs might participate in the regulation of oyster gametogenesis. • The CgSPO11 gene is a target of lgi-miR-96a. [ABSTRACT FROM AUTHOR]