R-spondin1 plays an indispensable role in ovarian development of Qi River crucian carp (Carassius auratus) by regulating estrogen synthesis

R-spondin1 (Rspo1) is a member of the secreted furin-like domain-containing protein family, and it is recognized for its significance in mammalian ovarian development. However, its role in teleost ovarian development remains largely uninvestigated. The Qi River crucian carp (Carassius auratus) is a species capable of gynogenesis, and it encounters challenges of premature ovarian maturation in aquaculture settings. Previous research established the essential involvement of Rspo1 in oocyte growth in Qi River crucian carp, but the precise molecular mechanisms underlying its role remain poorly understood. In this study, we categorized the pre-spawning ovarian development process of premature Qi River crucian carp into five stages through meticulous examination of morphology and histology. Immunofluorescence analysis revealed colocalization of Rspo1 with Vasa protein in oogonia, primary growth stage, and cortical vacuolar stage oocytes, and it was also detected in somatic cells. After a 60-day period of RNA interference via injection of Rspo1 double-stranded RNA into late-previtellogenesis stage ovaries, a substantial proportion of oocytes were arrested in the primary growth stage and exhibited a marked reduction in the expression of germ cell marker genes and an increase in apoptosis signaling. RNA-sequencing and real-time PCR analyses indicated a potential association between genes involved in hormone synthesis, lipid storage, and cell proliferation with ovary development in Qi River crucian carp. Furthermore, a significant decrease in levels of serum estrogens and vitellogenin was observed after Rspo1 knockdown. Dual-fluorescence in situ hybridization analysis demonstrated co-expression of Rspo1 with cyp19a1a in ovarian germ and surrounding somatic cells. Furthermore, results of a promoter assay indicated that Rspo1 can dose-dependently activate cyp19a1a expression. Collectively, these findings suggest that Rspo1 plays a role in ovarian development and oocyte growth by modulating cyp19a1a expression and influencing estrogen synthesis. These results provide valuable insights into the molecular mechanisms underlying the involvement of Rspo1 in ovarian development in Qi River crucian carp.