Sequential hermaphroditism, whereby the sex of one individual changes during its development, has been described in many fish species. Yet, the genetic regulation of this dynamic process of sexual differentiation is not well understood. Here, we report the characterization of intersexual differences in an aquacultural fish, the zig-zag eel (Mastacembelus armatus), based on 4 years of artificial breeding. All M. armatus individuals initially developed as females, with approximately half of them then becoming males after going through an intersex phase. A DM-domain candidate gene, dmrtb1, was identified from gonadal transcriptome data screening. The expression of dmrtb1 was restricted to testis, similar to dmrt1, which is one of the conserved sex-determining genes in vertebrates. Interestingly, dmrtb1 was expressed at the onset of sex change (early stage), and gradually increased in expression in the intersex gonads until the late stage of differentiation. Dmrtb1 was also expressed in the seminiferous lobules containing cysts with germ cells. A mixed cell population from ovotestis was identified by single-cell RNA sequencing, and included mitotic fetal germ cells, meiotic prophase fetal germ cells, and gonad endothelial cells. Spermatocyte and oocyte marker genes were both enriched in the same gonadal cell population, indicating this cell population might have the potential of bisexual differentiation. The correlation between dmrtb1 expression and sex-change development suggests M. armatus as an emerging model to study the molecular mechanism of sexual differentiation in fish. Such information will be vital for promoting the conservation of this species in the wild as well as for developing a sustainable aquacultural industry.

在许多鱼类中已经描述了连续雌雄同体，即个体的性别在其发育过程中发生变化。然而，这种性别分化动态过程的遗传调控尚不清楚。在这里，我们报告了基于 4 年人工养殖的水产养殖鱼类锯齿鳗 ( Mastacembelus armatus )的两性差异特征。所有M. armatus个体最初都发育为女性，其中大约一半在经历双性人阶段后成为男性。从性腺转录组数据筛选中鉴定了DM 域候选基因dmrtb1。dmrtb1的表达仅限于睾丸，类似于dmrt1，这是脊椎动物中保守的性别决定基因之一。有趣的是，dmrtb1在性别变化开始时（早期）表达，并在雌雄同体的性腺中表达逐渐增加，直到分化后期。Dmrtb1也在含有生殖细胞包囊的生精小叶中表达。通过单细胞 RNA 测序鉴定了来自卵睾的混合细胞群，包括有丝分裂胎儿生殖细胞、减数分裂前期胎儿生殖细胞和性腺内皮细胞。精母细胞和卵母细胞标记基因都富集在同一性腺细胞群中，表明该细胞群可能具有双性分化的潜力。dmrtb1之间的相关性表达和性别变化发展表明M. armatus作为研究鱼类性别分化分子机制的新兴模型。这些信息对于促进该物种在野外的保护以及发展可持续的水产养殖业至关重要。