Transcriptomic and proteomic analyses reveal the common and unique pathway(s) underlying different skin colors of leopard coral grouper (Plectropomus leopardus)

To gain a comprehensive and unbiased molecular understanding of the different skin colors of P. leopardus, we used Illumina HiSeq 2500 and TMT (Tandem Mass Tag) to compare transcription and protein levels between red and black skin of P. leopardus. We identified 797 upregulated and 314 downregulated genes (differentially expressed genes; DEGs) in red (RG) compared with black (BG) skin of P. leopardus. We also identified 377 differentially abundant proteins (DAPs), including 314 upregulated and 63 downregulated proteins. These DEGs and DAPs were significantly enriched in melanin synthesis (e.g., pyrimidine metabolism, Phenylalanine, tyrosine, and tryptophan biosynthesis, melanogenesis, phenylalanine metabolism, and tyrosine metabolism), oxidative phosphorylation (e.g., phosphonate and phosphinate metabolism, and oxidative phosphorylation), energy metabolism (e.g., HIF-1, glycolysis/gluconeogenesis, fatty acid biosynthesis, and fatty acid degradation), and signal transduction (e.g., Wnt, calcium, MAPK, and cGMP-PKG signaling pathways), etc. Further analysis of MAPKs showed that the activation levels of its main members JNK1 and ERK1/2 differed significantly between red and black skin colors. After RNAi was used to interfere with ERK1/2, it was found that the local skin of the tail of P. leopardus would turn black. Combined transcriptome and proteome analysis showed that most DEGs-DAPs in red skin were higher than in black skin (58 were upregulated, 1 was downregulated, and 4 were opposite). These DEGs-DAPs showed that the differences between red and black skin tissues of P. leopardus were related primarily to energy metabolism, signal transduction and cytoskeleton. These findings are not only conducive to understand the skin color regulation mechanism of P. leopardus and other coral reef fish, but also provide an important descriptive to the breeding of color strains. The skin color of P. leopardus gradually darkens or blackens due to environmental factors such as changes in light intensity and human activities, and this directly affects its ornamental and economic value. In this study, RNAseq and TMT were used to conduct comparative quantitative transcriptomics and proteomics and analyze differences between red and black P. leopardus skin. The results showed that energy metabolism, signal transduction and cytoskeleton were the main metabolic pathways causing their skin color differences. These findings contribute to existing data describing fish skin color, and provide information about protein levels, which are of great significance to a deeper understanding of the skin color regulation mechanism in P. leopardus and other coral reef fishes.