Carotenoids, instead of pteridines, determine color of xanthophores and erythrophores in tilapia

There has been controversy over whether the chromogenic substances of xanthophores and erythrophores are pteridines or carotenoids in fish. In this study, we first extracted and quantified carotenoids and pteridines from erythrophores and xanthophores in tilapia, respectively. The results showed that the carotenoids and pteridines in erythrophores were significantly higher than those of xanthophores, and the carotenoids were significantly higher than the pteridines in both types of cells. Then, we established homozygous mutant lines of the key genes for pteridine synthesis (xdh) and carotenoid metabolism (plin6) in Nile tilapia. Compared with the wild type (WT), there were no significant changes in the body color of xdh-/-, while the pigmentation, fluorescence intensity, and size of both xanthophores and erythrophores were significantly reduced in plin6-/-. The pteridines were significantly decreased in xdh-/-, but not in plin6-/-, while the carotenoids were significantly decreased in plin6-/-, but not in xdh-/-. To confirm these results, we further established homozygous mutant lines for another two key genes involved in pteridine synthesis (gch2) and carotenoid absorption (scarb1). Consistently, no significant body color changes were observed in gch2-/-, while no pigmented xanthophores and erythrophores were observed in scarb1-/-. The pteridines were significantly reduced in gch2-/-, but not in scarb1-/-, while almost no carotenoids were detected in scarb1-/-, but carotenoids remained unchanged in gch2-/- compared with WT. Taken together, these results, combined with existing findings in cyprinid fish, demonstrate that the color of xanthophores and erythrophores is determined by carotenoids rather than pteridines in fish.