A helix-loop-helix gene orchestrates male fate in common carp.

Fish show exceptional diversity in sex-determination mechanisms, making them a powerful system for studying how these pathways evolve. However, no definitive master sex-determining gene has yet been identified in Cyprinidae, a widely distributed and species-rich fish family. Here, we identified a male-specific gene in common carp encoding Pangu, a helix-loop-helix (HLH) domain peptide localized to Sertoli cells. Loss- and gain-of-function analyses demonstrate that pangu is both necessary and sufficient for male sex determination. Expression and reporter assays indicate that pangu represses cyp19a1a, a key regulator of ovarian differentiation, and disruption of cyp19a1a restores testicular development in the feminized phenotype of XY pangu mutants. These findings establish pangu as the master sex-determining gene in common carp and link an HLH domain peptide to sex differentiation.