Comparative transcriptomics reveals the molecular genetic basis of pigmentation loss in Sinocyclocheilus cavefishes

<p><span><span><span>Cave-dwelling animals evolve distinct troglomorphic traits, such as loss of eyes, skin pigmentation, and augmentation of senses following long-term adaptation to the perpetual darkness. However, the molecular genetic mechanisms underlying these phenotypic variations remain unclear. In this study, we conducted comparative histology and comparative transcriptomics study of the skin of eight <i>Sinocyclocheilus</i> species (Cypriniformes: Cyprinidae) that included surface and cave-dwelling species. We analyzed four surface and four cavefish species by using next-generation sequencing, and a total of 802,798,907 clean reads were generated and assembled into 505,495,009 transcripts, which contributed to 1,037,334 unigenes. Bioinformatics comparisons of four different surface-cave fish groups revealed between 10,629 and 6442 significantly differentially expressed unigenes. Further, tens of differentially expressed genes (DEGs) potentially related to skin pigmentation were identified. Most of these DEGs (including <i>GNAQ</i>, <i>PKA</i>, <i>NRAS</i>, and <i>p38</i>) are downregulated in cavefish species. They are involved in key signaling pathways of pigment synthesis, such as the melanogenesis, Wnt, and MAPK pathways. This trend of downregulation was confirmed through qPCR experiments. This study will deepen our understanding of the formation of troglomorphic traits in cavefishes.</span></span></span></p>