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Keywords: Dead end; Pacific bluefin tuna; Primordial germ cell; Sterile; Thunnus orientalis EN Dead end Pacific bluefin tuna Primordial germ cell Sterile Thunnus orientalis 3872 3876 5 06/14/22 20220701 NES 220701 The I dead-end i ( I dnd i ) gene was first identified in zebrafish, I Danio rerio i , as a germ plasm component encoding an RNA-binding protein (Weidinger et al., 2003). To examine germ cell deficiency in addition to observations by GFP labeling, we conducted whole-mount immunostaining using anti-germ cell marker Vasa antibodies, as described previously (Hayashida, Higuchi, Hashimoto, et al., 2021), and histological analysis. In fact, inhibiting I dnd i expression causes faulty migration and ablation of zebrafish PGCs, resulting in germ cell deficiency (Weidinger et al., 2003). The zebrafish Dnd protein binds to 3' untranslated regions (3'UTR) of germline RNAs, such as I nanos i mRNAs, thereby protecting them against microRNA-mediated degradation, so they can contribute to the migration and survival of primordial germ cells (PGCs) (Kedde et al., 2007; Weidinger et al., 2003). [Extracted from the article]

Triploidization influences various biological characteristics of fish, which is associated with reductions in the number of multiple cell types in different tissues/organs. Our behavioral analyses revealed that triploid Pacific bluefin tuna (Thunnus orientalis) larvae exhibit lower sensitivity to light compared to diploids. Furthermore, histological analyses revealed a reduction in the number of ganglion cells and an increase in their size in the retinas of triploid T. orientalis larvae. Our findings provide the first evidence indicating that triploidization reduces sensory perception during the larval stage of fish. [ABSTRACT FROM AUTHOR]

Pacific bluefin tuna (PBT), Thunnus orientalis , is one of the most important species for aquaculture in Japan. Recently, the reduction in muscle fat content associated with sexual maturation in farmed PBT has become a serious problem. To develop technologies for inducing sterility, detailed and reliable data on gonadal development in PBT are needed. Here, we demonstrated the process of gonadal sex differentiation, and of early ovarian and testicular development during the immature stages in PBT. Gonadal sex differentiation was first characterized by the formation of the ovarian cavity in female and of the efferent ducts in male 57 days post hatching (dph). The gonads then differentiated into ovaries or testes according to the genotypic sex until 83 dph. During this period, primordial germ cells, oogonia, and type-A spermatogonia were solitarily distributed in the gonads, and the number of germ cells did not differ between sexes. After gonadal sex differentiation, gonads of PBTs developed in a sexually dimorphic manner: proliferation and differentiation of germ cells occurred earlier in the ovaries than in the testes. The oogonia in ovaries formed cysts at 185 dph, but the type-A spermatogonia were solitarily distributed in testes at this stage, and cysts of type-A spermatogonia were first observed at 247 dph. Moreover, the oogonia entered meiosis and differentiated into chromatin-nucleolus stage oocytes until 247 dph, and subsequently into peri-nucleolus stage oocytes until 285 dph, whereas the type-A spermatogonia differentiated into type-B spermatogonia, spermatocytes, spermatids, and spermatozoa from 446 dph onwards. We believe the results of this study provide the necessary basis for future studies on sterile PBT production. • To develop technologies for producing sterile fish, detailed and reliable data on gonadal development are needed. • We demonstrated the morphological changes and timing of gonadal sex differentiation in Pacific bluefin tuna (PBT). • We also showed the developmental process of the ovary and testis during the immature stages of PBT. • This study will provide the necessary basis for future studies on sterile PBT production. [ABSTRACT FROM AUTHOR]

Pacific bluefin tuna (Thunnus orientalis) is one of the most important species for aquaculture in Japan. In recent years, the reduction in muscle fat content during sexual maturation in farmed T. orientalis has become a serious problem. Herein, as a first step toward sterilization to avoid the deterioration of flesh quality associated with sexual maturation, we examined cold-shock conditions for inducing triploidy in T. orientalis. In this study, three treatment variables (initiation timing, duration, and water temperature of the cold-shock treatment) were considered, using the fertilized eggs collected immediately after spontaneous spawning in a land-based tank. We found that the optimal cold-shock conditions for T. orientalis were 5 °C for 15 min starting from 5 min after fertilization. Under these optimal conditions, we obtained high triploidization (96.6 ± 3.3%) and total hatching (64.5 ± 8.7%) rates. Therefore, using eggs obtained by spontaneous spawning in a land-based tank and the triploidization method established in this study, it is possible to repeatedly and efficiently produce triploid T. orientalis. In recent years, deterioration of flesh quality associated with sexual maturation in aquaculture-produced Thunnus orientalis has become a serious problem. To avoid this problem by sterilization technique, in this study, we have developed an efficient method for the induction of triploidy in T. orientalis , using the fertilized eggs collected immediately after the spontaneous spawning in a land-based tank. We believed that the results of present study are a valuable first step toward the sterilization of T. orientalis. • We established a technique for the mass production of triploid Pacific blue tuna. • The optimal condition was 5 °C for 15 min starting from 5 min after fertilization. • High triploidization (96.6%) and hatching rates (64.5%) were obtained. [ABSTRACT FROM AUTHOR]