Transcriptomic analysis of colour dimorphism of Opsariichthys bidens provides insights into the mechanism of male colour

Opsariichthys bidens (O. bidens) is a fish species native to China. It has become a popular ornamental fish because of its interesting colour dimorphism. Its colour pattern is unique; however, with the exception of model species, such as zebrafish, the molecular mechanisms of most fish colour patterns remain unclear. In the present study, we compared and analysed the transcriptomic data of the skin and anal fins of male and female O. bidens. A total of 9028 differentially expressed genes (DEGs) were identified, of which 5259 were upregulated and 3769 were downregulated in males. Compared with known pigmentation genes, Mitf, Tyr, Tyrp1, Dct, Pnp4a, Alx4a, Gch1, and Csf1r were identified as potential genes related to colour dimorphism. Second, we tracked the process of colour development in male O. bidens and found that there were three stages in the formation of body colour. We explored the distribution, migration, and expression patterns of genes related to pigment cells at each stage using microscopic imaging, transmission electron microscopy, enzyme-linked immunosorbent assay, and high-performance liquid chromatography. Melanophores, xanthophores, and iridophores were found to be distributed on the skin or anal fins of O. bidens. It was inferred that this unique body colour pattern is caused by the differentiation and migration of pigment cells at specific stages. This discovery provides a basis for studying the molecular mechanisms of pigmentation in ornamental fish and will contribute to the genetic selection and breeding of O. bidens in ornamental fish cultures in the future. •The molecular mechanism of colour dimorphism of Opsariichthys bidens was proposed.•It is inferred that this unique body colour pattern is caused by the differentiation and migration of pigment cells at a specific stage.•Providing basic data for selection and breeding of native ornamental fish and regulation of body colour