Correlation analysis between carotenoid lyase BCMO1 and the formation of red shell-color in hard clam Meretrix meretrix

Abstract

Carotenoids, the most widespread biological pigments, are responsible for the coloration of animal tissues and surfaces. Yet, little is known about the mechanism of how carotenoids affect the color of clam shells. The hard clam, Meretrix meretrix, exhibits genetic polymorphism leading to diverse shell colors, making it an ideal model for studying carotenoid-based coloring. Cultured strains of M. meretrix with distinct shell colors (red and white) were selected to explore the relationship between carotenoids and shell color. Raman spectroscopy and pigment content analysis revealed that carotenoids primarily contributed to the red shell. The β-carotene-15,15'-monooxygenase 1 (BCMO1) in M. meretrix was cloned, and a phylogenetic analysis was performed. Expression studies showed that Mm-BCMO1 mRNA was consistently expressed during early development, with a notable increase in umbo larvae. High levels of Mm-BCMO1 expression were observed in the hepatopancreas and mantles, particularly in the red strain compared to the white strain. In addition, protein analysis via western blotting (WB) and immunofluorescence (IF) confirmed that Mm-BCMO1 protein levels corresponded to mRNA levels, mainly localized in epithelial cells of the mantle. Moreover, 12 significant single nucleotide polymorphisms (SNPs) were identified in the promoter and coding sequence (CDS) regions of Mm-BCMO1, potentially explaining the color variation between the two shell color strains. These findings indicate that carotenoids, influenced by Mm-BCMO1, play a crucial role in the pigmentation of red shell color. The study provides insights into carotenoid metabolism in M. meretrix and offers valuable information for molecular marker-assisted breeding in clam aquaculture.