Abstract

The initiation of meiosis in germ cells is largely regulated by extrinsic cues from the gonadal environment, but the logic of these signals remains poorly understood in non-mammalian vertebrates. Retinoic acid has long been considered a principal meiosis-inducing signal, yet recent genetic and reconstitution studies indicate that retinoic acid alone is insufficient. In mice, bone morphogenetic protein 2 cooperates with retinoic acid to establish the oogenic program through the bone morphogenetic protein-responsive transcriptional regulator Zglp1, but whether this regulatory logic is conserved beyond mammals is unknown. Here, using cultured chicken (Gallus gallus) primordial germ cells, we show that bone morphogenetic protein 2 cooperates with retinoic acid to promote a meiotic-entry transcriptional program. Retinoic acid alone induced a limited retinoic acid-responsive state, whereas combined treatment reduced the primordial germ cell program and activated early meiotic genes. The resulting transcriptome matched the premeiotic-to-meiotic-entry transition of the embryonic ovary in a single-cell atlas of chicken germ cells. We also generated a genome-edited primordial germ cell line carrying an SYCP3 promotergreen fluorescent protein reporter as a platform for dissecting meiotic-entry signals in culture. Comparative genomic analysis revealed Gallus-specific pseudogenization of ZGLP1, which is intact in closely related galliform species. Retinoic acid and bone morphogenetic protein may therefore act through a different downstream regulator in chicken.