Reduced Vitellogenin Expression is Associated with Impaired Oogenesis in Female Triploid Pacific Oysters (Crassostrea gigas)

Triploidy is widely applied in oyster aquaculture because of its association with reduced gametogenesis, yet female triploid Pacific oyster (Crassostrea gigas) exhibit pronounced variability in ovarian development. The mechanisms underlying ovarian impairment in triploid females remain poorly understood. In this study, we investigated the role of vitellogenin (VTG) in oogenesis and gonadal development by comparing diploid females with β triploid females exhibiting severe gonadal arrest. Quantitative PCR and in situ hybridization analyses revealed that the expression of VTG was significantly reduced in the gonads of β triploid females compared with diploids, with VTG mRNA predominantly localized in the follicle cells. Functional suppression of VTG expression in diploid females using RNA interference resulted in abnormal germ cell development, reduced gonadal coverage area, and impaired oocyte maturation, demonstrating that adequate VTG expression is important for normal ovarian development. Targeted steroid profiling by LC-MS/MS demonstrated significant alterations in the gonadal endocrine environment of triploids, including elevated testosterone levels. In vivo hormone administration further showed that 17β-estradiol significantly upregulated VTG expression, while testosterone markedly suppressed it. Together, these findings indicate that reduced VTG expression, potentially arising from disrupted endocrine regulation, is closely associated with impaired oogenesis in female triploid oysters. This study provides mechanistic insight into triploid-associated ovarian dysgenesis and contributes to a better understanding of reproductive regulation in bivalve aquaculture species.