Gonadal Steroid and Transcriptome Profiles Reveal Sexual Dimorphism in the Freshwater Bivalve Sinanodonta woodiana

Sinanodonta woodiana is one of the most widely distributed freshwater bivalves in the world. However, the sexual dimorphism of this species remains poorly understood. In this study, gonads of clearly sexed S. woodiana were used to analyze steroid hormones, including estrone (E1), estradiol-17β (E2), estriol (E3), and testosterone (T), by liquid chromatography-mass spectrometry, followed by comparative transcriptome analysis. Compared with males, females showed no significant differences in E1 (p = 0.251), E3 (p = 0.136), or T (p = 0.917), whereas E2 concentration in females (15.2 ± 1.4 ng/g wet weight) was significantly higher than that in males (3.2 ± 1.7 ng/g wet weight) (p = 0.009). In addition, a total of 6716 differentially expressed genes (DEGs) were identified, including several sex-related genes such as forkhead box L2 (FOXL2), upregulated in females, and splicing factor 1 (SF1), upregulated in males. All DEGs were mainly enriched in biological processes such as metabolic process, cellular process, and single-organism process, as well as signaling pathways including neuroactive ligand–receptor interaction, glycosphingolipid biosynthesis—lacto and neolacto series, and oocyte meiosis. Our findings provide valuable information for sex identification of adult S. woodiana and offer candidate markers for future testing in juveniles, while also enhancing our understanding of its sex-specific gene-expression diversity.