Gonadal transcriptome analysis of Opsariichthys bidens reveals sex-associated genes

Opsariichthys bidens is a unique economically important freshwater fish in China. Male O. bidens grow faster than females, and male fish have attractive blue-green stripes on the body surface during the breeding period. The breeding of all-male stocks can significantly improve the efficiency of breeding. To accelerate the breeding of all-male stocks, additional studies of the mechanism regulating sex differentiation and gonad development are needed. In this study, transcriptome sequencing of the ovaries and testes of O. bidens was performed using Illumina high-throughput sequencing. The results revealed a total of 21,703 differentially expressed genes, including 8645 up-regulated genes and 5880 down-regulated genes expressed in the ovary compared with the testis. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes analyses revealed multiple differentially expressed genes involved in sex differentiation and gonad development. Eight differentially expressed genes (zp3, cyp19a, hsd17b1, msh4, dmrt1, rspo2, kif23, and gdf9) that play a key role in sex differentiation and gonadal development were selected for RT-qPCR validation. The expression levels of selected genes in the testes and ovaries were significantly different (P < 0.05). zp3, cyp19a, hsd17b1, and gdf9 were female-biased genes, and msh4, dmrt1, rspo2, and kif23 were male-biased genes. zp3, cyp19a, hsd17b, and msh4 were only slightly expressed in the kidney and liver, and dmrt1, rspo2, kif23, and gdf9 were all expressed in gill, kidney, and liver tissue. None of the genes were expressed in the heart or muscle. In this study, differentially expressed genes related to the sex determination and differentiation of O. bidens were identified. Overall, our findings provide important data for future studies of the molecular mechanisms of sex differentiation and gonad development of O. bidens and will aid the breeding of all-male species.