Zebrafish Foxl2l suppresses stemness of germline progenitors and directs feminization

Zebrafish is an important organism for genetic studies, but its germ cell types and the mechanism of sex differentiation remain elusive. Here, we conducted a single-cell transcriptomic profiling and charted a developmental trajectory going from germline stem cells, through early, committed, and late progenitors, to pre-meiotic and meiotic cells. A transcription factor, Foxl2l, is expressed in the progenitors committed to the ovary fate. CRISPR-Cas9-mediated mutation of foxl2l produced 100% male fish with normal fertility. Another single-cell profiling of foxl2l ^-/- germ cells reveals the arrest of early progenitors. Concomitantly the expression of nanos2 (stem cell marker) and id1 (transcription repressor in stem cells) was elevated together with an increase of nanos2 ⁺ germ cell in foxl2l mutants, indicating the reversion to the stem cell state. Thus, we have identified developmental stages of germ cells in juvenile zebrafish and demonstrated that Foxl2l drives zebrafish germ cell progenitors toward feminization and prevents them from reverting back to the stem cell state.