Comparative single-cell analysis of black rockfish (Sebastes schlegelii) ovarian stroma reveals placenta-like adaptations toward viviparity.

The black rockfish (Sebastes schlegelii) is a viviparous teleost with high fecundity and prolonged female sperm storage for about six months despite lacking a specialized storage organ. During gestation, its ovarian stroma undergoes a functional shift from supporting oocyte growth to maintaining embryo development, resembling the maternal decidua in mammals. Here, we present a single-cell atlas of the ovarian stroma across unfertilized, normal-gestation, and abnormal-gestation states. Cross-species integration with human and mouse datasets reveals convergence on decidualization-like programs during early pregnancy, while highlighting lineage-specific implementation by distinct cell types. We further implicate annexins in ovarian sperm storage and show that abnormal-gestation is associated with lymphatic endothelial skewing and lipid-handling signatures, consistent with stromal lipid recapture. Together, these findings provide a framework for the maternal contribution to teleost placentation and reveal convergent, placenta-like adaptations underlying viviparity.