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As a molecular carrier and storage protein, apolipoprotein (Apo) mediates the intracellular uptake of lipids, proteins, vitamins and carotenoids. In this study, we identified a novel Apo gene, designated hcApo, from the freshwater pearl mussel Hyriopsis cumingii. The complete hcApo cDNA consists of 4104 nucleotides with an open reading frame encoding 1155 amino acid residues. The hcApo protein contains a conserved lipoprotein N-terminal domain (LPD-N) that is a characteristic of the large lipid transfer protein (LLTP) superfamily. The hcApo mRNA is constitutively expressed in a wide range of tissues with the highest expression level in the liver. Moreover, differential expression analysis revealed that the hcApo gene is more highly expressed in the liver, kidney, mantle and gill of purple line mussels compared to white line mussels. In situ hybridization investigations of the precise expression site of hcApo mRNA in the mantle showed that hcApo mRNA is specifically expressed in the outer epithelial cells of the middle fold and the inner epithelial cells of the outer fold of the mantle, as well as throughout the outer epithelium of the outer fold and ventral mantle. Another very important finding is that significantly positive correlation existed between the hcApo gene expression level and the total carotenoid content in purple line mussels. These findings may provide a better understanding of the roles of hcApo in the molecular mechanisms of shell formation and coloring of H. cumingii., (Copyright © 2014 Elsevier B.V. All rights reserved.)

The freshwater pearl mussel Hyriopsis cumingii is the most important mussel species for freshwater pearl production in China. Mussel shell color is an important indicator of pearl quality. The objective of this study was to assess whether total carotenoid content (TCC) in H. cumingii is related to shell color. TCC of different tissues (gonad, gill, hepatopancreas, kidney, axe foot, mantle, and adductor muscle) of purple and white inner-shell H. cumingii was determined by UV-Vis spectrophotometry. The results revealed that TCC ranged from 12.91 ± 0.78 to 56.30 ± 0.74 μg/g. In general, TCC was higher in the hepatopancreas, followed by the mantle, gonad, gill, kidney, axe foot, and adductor muscle. TCC in gonad, gill, hepatopancreas, kidney, and mantle of purple mussels was significantly higher than that of white mussels. TCC in mussel tissues of H. cumingii was significantly different ( P < 0.001) with respect to shell color. There were significant positive correlations between TCC in mussel mantle and shell color intensity. Future studies will assess the biological roles of carotenoids in shell color formation. [ABSTRACT FROM AUTHOR]