Sequence characteristics, evolutionary history and expression pattern of BCO2 in Chinese mitten crab Eriocheir sinensis.

β-carotene 9', 10'-oxygenase (BCO2) is a pivotal enzyme in the carotenoid cleavage. To fill the research gap of BCO2 in Chinese mitten crab Eriocheir sinensis, we first investigated ESIN_BCO2 gene from its genome, revealing its evolutionary history, gene structure, and expression patterns. The results showed that ESIN_BCO2 gene has a full-length open reading frame (ORF) of 1572 bp, encoding a protein of 523 amino acids. BCO2 was characterized by ten conserved motifs and an RPE65 domain, belonging to carotenoid cleavage oxygenase (CCO) family. Phylogenetic analysis revealed that BCO1 was the ancestral gene, from which BCO2 and NinaB diverged during evolution. Notably, the Ka/Ks ratios for BCO2 in Decapoda were approximately lower than BCO2 from other crustaceans (0.014 to 0.045 vs 0.112 to 0.185). ESIN_BCO2 was predominantly expressed in the hindgut, with significantly higher expression levels in females than in males. It was predominantly localized near the nuclei (N) of epithelial cells (epi) and basal cells (bc) in the hindgut. Moreover, dietary β-carotene supplementation significantly upregulated BCO2 expression in the female hindgut. These findings provide valuable insights into the evolution and function of BCO2 in E. sinensis as well as the other crustaceans, potentially shedding light on the conservation and divergence of carotenoid metabolism mechanisms across diverse crustaceans.