Abstract:Hypermelanosis is an abnormal coloration that commonly occurs on the blind-side of half-smooth tongue sole Cynoglossus semilaevis in captivity, resulting in deterioration in flesh quality and reduced market price. This problem has become the bottleneck for sustainable development of flatfish farming industry. Melanin-concentrating hormone (MCH) is produced in pituitary acting as an inhibitor for hypermelanosis in fish. The present study aims to identify the role of MCH in regulating the blind-side hypermelanosis in C. semilaevis. cDNAs encoding two MCH alleles (pMCH1 and pMCH2) were cloned using RACE method and their structures were analyzed. The spatial and temporal expression patterns of MCH mRNA were measured using the quantitative real-time PCR. Results showed that C. semilaevis pMCH1 cDNA sequence was 476 bp in length and encoded 134 amino acids, which shares high identity (66.9%) with Paralichthys olivaceus. C. semilaevis pMCH1 was clustered with the Pleuronectiformes, Tetraodontiformes and Perciformes species based on the phylogenetic analysis. By contrast, C. semilaevis pMCH2 cDNA sequence was 626 bp in length and encoded 147 amino acids. The transcript levels of both MCH1 and MCH2 had the highest expression level in pituitary compared to other tissues. MCH1 mRNA was also detected among other tissues. Furthermore, MCH2 mRNA was highly expressed in the brain, eye-side skin, blind-side skin, gonad and gill, whereas the peripheral tissue had relatively low expression level. Correlating the MCH mRNA expression levels and the degree of the blind-side hypermelanosis showed that, MCH1 mRNA expression levels in the pituitary and skin had similar trends, which peaked when fish had about 10% blind-side hyperpigmentation, and then significantly reduced as the blind-side hypermelanosis level increased. For fish with normal blind-side coloration and 50% blind-side hyperpigmentation, their pituitary MCH2 mRNA levels were significantly higher than those from fish with 10% and 80% blind-side hyperpigmentation. The skin MCH2 mRNA level significantly increased with the increased hyperpigmentation degree on the blind-side of fish. The present study provides new insights into the mechanisms underlying the regulation of blind-side hypermelanosis in C. semilaevis.
