Gonadal Sex and CgSox-like Orchestrate Germ Cell Differentiation in Crassostrea gigas

Sex determination and germ cell differentiation are fundamental processes that define reproductive fate across metazoans, yet how somatic cues shape germline sexual identity remains unclear in species with flexible sex systems. Here, we investigate gonadal sex differentiation in the Pacific oyster (Crassostrea gigas), a species exhibiting both dioecy and hermaphroditism, using single-nucleus transcriptomic profiling. Our analyses uncover a coordinated regulatory network in which the CTNNB1/TCF7L2 complex promotes female germline specification, while male-biased CgSox-like gene acts in male germ cells to repress WNT and NOTCH signaling and suppress Foxl2l expression in surrounding female niche cells. Comparative analyses reveal that both meiosis-promoting and meiosis-inhibiting factors are required to establish germline sexual identity. Cross-species comparisons further demonstrate that oyster female niche cells share strong transcriptional homology with vertebrate granulosa cells, whereas male niche cells display distinct developmental trajectories. Together, these findings reveal a unique plasticity of sex determination in hermaphroditic species and provide mechanistic insight into the evolution of germline sexual fate.