The skin-color is associated with its physiological state: A case study on a colorful variety, hybrid grouper (Epinephelus fuscoguttatus x Epinephelus lanceolatus)

The hybrid grouper (Epinephelus fuscoguttatus female x Epinephelus lanceolatus male) named Hulong grouper is a vital species variety in the grouper farming industry. During the breeding process, the hybrid grouper gradually showed the skin color variation phenomenon, including normal (N), white (W), secondary color (SC), and yellow (Y). We utilized a comparative transcriptome analysis approach via high throughput sequencing in different skin color grouper groups to understand the underlying molecular mechanism. Quantitative RT-PCR validated 20 differentially expressed genes consistent with the RNA-seq trend, indicating that our sequencing results are stable and reliable. Four skin transcriptomes were compared pairwise. The results revealed that four immune response-related genes (tlr4, ddit4, gpcr, and hsd3b) were the common DEGs in the different groups suggesting that there were some relations between pigmentation variance and immune difference. Therefore, further analysis of SOD, CAT, complement C3 and C4, and other immune-related genes showed that the normal group's immune status and anti-oxidative stress ability are stronger than other skin color groups. Moreover, the KEGG pathway analysis results showed that some DEGs were associated with the MAPK signal pathway, so we compared the proteic level differences of erk1/2, p38mapk, and jnk1 among different skin-color groups. We found the MAPK is a potential pathway involved in fish skin color regulation and plays an essential role in physiological and biochemical processes. In addition, expression pattern analysis showed that the mRNA expression of most melanin synthesis-related genes have the highest level in normal color individuals, consistent with the section analysis. These results shed light on the regulatory mechanism of different skin colors transformation in hybrid Hulong grouper. Understanding fish skin color genetics will facilitate future molecular selection of the fish skin colors with high market values.