The breeding of new strains in economically fish species is often a decades-long process due to prolonged sexual maturation cycles. For example, grass carp require approximately five years to reach sexual maturity, and breeding a new strain typically takes more than 20 years, making the process highly time- and labor-intensive.

A major challenge in aquaculture is how to exploit small fish species with short maturation cycles, high reproductive efficiency, and strong controllability to establish cross-species germline stem cell transplantation-based surrogate reproduction systems. Such strategies could significantly shorten the gamete maturation period of species like grass carp and greatly improve breeding efficiency.

Recently, a research group led by Prof. SUN Yonghua from the Institute of Hydrobiology (IHB) of the Chinese Academy of Sciences used zebrafish as surrogate hosts with a maturation cycle of only three months, and successfully generated all-X sperm and all-female grass carp offspring within six months via surrogate reproduction. This work was published as a cover article in Science China Life Sciences.

In this study, the researchers first analyzed the gonads of 3-month-old grass carp and observed that the primordial ovaries were significantly larger than the testes at this developmental stage. Using single-cell RNA sequencing combined with established molecular criteria for fish germline stem cells, they successfully identified Nanos2-positive female germline stem cells within the enlarged gonads.

These cells were then isolated from juvenile grass carp ovaries and transplanted into zebrafish larvae that had been depleted of endogenous germ cells. Strikingly, within the zebrafish gonadal microenvironment, the transplanted grass carp germ cells rapidly initiated spermatogenesis and matured into functional sperm within just three months.

Notably, all resulting sperm were of the all‑X genotype. Fertilization of wild-type grass carp eggs with these sperm yielded all‑female offspring, enabling the generation of an all‑female grass carp strain in only 6 months. This represents a dramatic acceleration compared to conventional sex-control breeding strategies, which typically require at least two generations and over a decade to achieve similar outcomes.

Furthermore, this study also provides the first definitive identification of female germline stem cells in a commercially fish species and demonstrates their capacity to differentiate into sperm. These findings challenge the traditional designation of such cells as “oogonial stem cells,” suggesting that this term may be conceptually inaccurate.

In summary, by employing zebrafish as surrogate hosts, the researchers successfully achieved surrogate reproduction across species with an approximately 20-fold difference in sexual maturation time and a nearly 20,000-fold disparity in adult body size. This work establishes a paradigm for ultra-fast, sex-controlled breeding in grass carp and holds strong potential for extension to other major aquaculture species, thereby accelerating genetic improvement in aquaculture.

(Editor: MA Yun)