Sterility in hybrid animals is widely known to be due to a cytological mechanism of aberrant homologous chromosome pairing during meiosis in hybrid germ cells. In this study, the gametes of four marine fish species belonging to the Sciaenid family were artificially fertilized, and germ cell development was examined at the cellular and molecular levels. One of the intergeneric hybrids had gonads that were testis-like in structure, small in size, and lacked germ cells. Specification of primordial germ cells (PGCs) and their migration toward genital ridges occurred normally in hybrid embryos, but these PGCs did not proliferate in the hybrid gonads. By germ cell transplantation assay, we showed that the gonadal microenvironment in hybrid recipients produced functional donor-derived gametes, suggesting that the germ cell-less phenotype was caused by cell autonomous proliferative defects of hybrid PGCs. This is the first evidence of mitotic arrest of germ cells causing hybrid sterility in animals.

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Tuna (genus Thunnus), particularly Pacific bluefin tuna (T. orientalis; PBT), are commercially important fish in the aquaculture industry worldwide. The objective of this study was to investigate sexual dimorphism in the growth performance of aquaculture-produced PBT and develop techniques for its sex manipulation, for the first time in tuna. A comparison of the body size between sexes revealed that male-cultured PBTs were larger than females at harvest. We also confirmed that cyp19a1a (encoding a gonadal aromatase) expression increased specifically in the genotypic female PBT gonads during sex differentiation. This suggested that aromatase plays an important role in ovarian differentiation and that the suppression of aromatase activity may effectively induce masculinization in genotypic females. Therefore, we administered letrozole—an aromatase inhibitor (AI)—into sexually undifferentiated PBT through the oral route. AI administration resulted in a 100% sex reversal of genotypic females into phenotypic males at the molecular level. Our results provide the basis for future studies on the establishment of mono-sex male production technology in PBT, which would help improve the productivity of closed-cycle PBT aquaculture. Furthermore, this study offers important insights into the understanding of the sex-wise growth of tuna species in aquacultural settings, and developing sex manipulation techniques.