Cutaneous transcriptomic profiling and candidate pigment genes in the wild discus ( Symphysodon spp.)

Discus (Symphysodon spp.) is a popular freshwater ornamental fish with high market value due to its extraordinary skin pigmentation. The molecular mechanisms that govern skin colouration have yet to be identified. Six skin samples from wild discus were collected in the D1 (dorsal intense black bar stripe), D2 (ventral lighter bar stripes and orange carotenoid spots) and D3 (posterior without stripe nor spot) regions of wild discus for transcriptomic profiling and analysis. Differential analysis was conducted in 3 pair-wise comparisons, D1vsD3, D2vsD3 and D2vsD1. The results demonstrated that the melanin synthesis-associated genes (ERBB4, ADCY9, WNT2, 14-3-3 proteins and TSPAN genes) were upregulated in D1 and D2 regions, whereas carotenoids metabolism-associated genes (PLIN2, AHCY, RETSAT, STARD10, TTC39B and RDH1 genes) were uniquely expressed in D2 region. Amino acid metabolism-related pathways were enriched in both the D1 and D2 regions. MAPK pathway enrichment was detected merely in the D1 region, while various transporter activities were enriched in the D2 region. Further investigations speculated the key regulator roles of GNAQ, ADCY9, 14-3-3 and TSPAN genes in black vertical bar formation. This is the first fundamental report on discus skin pigment genes that could be extended into more detailed studies in the future.