Cyprinid Juji (Gobiocypris rarus) as a model fish to study germ cell development and gonadal differentiation

Although the cyprinid zebrafish has been widely used to study gametogenesis and gonadal differentiation, it has remarkable limitations, such as the loss of sex determinants in laboratory strains, a juvenile ovary stage in both sexes, and oocyte defects generally leading to sex reversal. Here, we develop a novel small cyprinid model for studying gonadal differentiation and gametogenesis, Juji (Gobiocypris rarus), which possesses a genetic sex determination system. We generate a transgenic line of Juji, Tg(ddx4:EGFP-UTRddx4), enabling lifetime visualization of germline development, and find that the dimorphic expression of ddx4:EGFP, rather than primordial germ cell number, at early stages determines the sexual fate of adults. RNA-seq analysis of juvenile ovaries and testes prior to germ cell differentiation identifies sexually dimorphic genes and signaling pathways. Several key factors, such as foxl2l and dmrt1, which drive gonadal differentiation, are among the sexually dimorphic genes. Knockout of dmrt1 leads to sex reversal in juvenile males, with all mutants developing into females. Furthermore, transplanting germline stem cells from genetic males into dmrt1 mutants results in all transplants developing into females, highlighting the essential Sertoli-cell-specific role of dmrt1 in triggering spermatogenesis. Our study establishes Juji as a promising model for investigating sex differentiation, gametogenesis, and gonadal development.