Germ cell markers in fishes - A review

Characterization of potential spermatogonia biomarker genes in the European eel (Anguilla anguilla)

Identification of specific molecular markers for spermatogonial stem cells in teleost is crucial for enhancing the efficacy of reproductive biotechnologies in aquaculture, such as transplantation and surrogate production in fishes. Since it is not yet possible to distinguish spermatogonial stem cells of European eel (Anguilla anguilla) using specific molecular markers, we isolated spermatogonial cells from immature European eels to find these potential markers. We attempted this by studying three candidate genes: vasa, nanos2, and dnd1. Two vasa (vasa1 and vasa2) genes, nanos2, and dnd1 were identified, characterized, and studied in the muscle, testis, and isolated spermatogonia. Our results showed that vasa1 and vasa2 had the highest levels of expression when measured by qPCR. In situ hybridization and immunochemistry assays showed that the four genes were localized explicitly in type A spermatogonia. However, vasa1 and vasa2 exhibited stronger signals in the immature testicular tissue than the other two potential markers. According to this, vasa1 and vasa2 were found to be the most effective markers for spermatogonial cells in the European eel.

Effects of CRISPR/Cas9-mediated dnd1 knockout impairs gonadal development in striped catfish

Clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 technology allows for the generation of loss-of-function mutations to enable efficient gene targeting to produce desired phenotypes, such as the production of germ cell-free fish. This technology could provide several applications for aquaculture and conservation of fisheries resources, such as the prevention of overpopulation in fish culture and gene flow from escaped farmed fish into wild populations and the production of germ cell-free recipient larvae for germ cell transplantation. This study aimed to develop CRISPR/Cas9 mediated dead-end 1 (dnd1) knockout techniques for striped catfish (Pangasianodon hypophthalmus). To optimise CRISPR/Cas9-induced dnd1 knockout, three single-guide RNAs (sgRNAs) were designed to target upstream sequences of start codon of the dnd1 gene. A combination of two concentrations of each sgRNA (100 and 200 ng/µl) and three concentrations of Cas9 (100, 250, and 500 ng/µl) was microinjected into fertilised striped catfish eggs. These sgRNAs/Cas9 could induce indel mutations and lower the primordial germ cell (PGC) numbers. Histological analyses indicated that sgRNA3 targeting upstream and nearest to the start codon at 200 ng/µL and Cas9 at 500 ng/µL showed the lowest PGC number. The reduction in PGC number was confirmed by in situ hybridisation using antisense dnd1 and vasa probes. All sgRNA/Cas9 combinations reduced the expression of dnd1, cxcr4b, dazl, nanos1, nanos2, and vasa, and the lowest expression levels were observed in gonads obtained from fish injected with 200 ng/µL sgRNA3 and 500 ng/µL Cas9 (P < 0.05). In addition, at 1 year of age, a significantly lower gonadosomatic index was observed in fish injected with all sgRNA and Cas9 at 500 ng/µL. Moreover, compared to the control fish, the ovaries and testes presented different morphologies in the sgRNA/Cas9-injected fish, that is, few previtellogenic oocytes in the ovary and spermatogonial cell-less testes. In conclusion, CRISPR/Cas 9 targeting dnd1 knockout at the upstream sequences of start codon was achieved, which resulted in the downregulation of dnd1 and lowered PGC number.

Rxrs and their partner receptor genes inducing masculinization plausibly mediated by endocrine disruption in Paralichthys olivaceus

Retinoid X receptors (RXRs) can form homo- or heterodimers with orphan receptors involved in multiple intertwined signaling pathways. However, there is limited study on the formation of sex phenotypes and the regulation of steroidogenesis by RXRs in fish. Here, in Paralichthys olivaceus, we first indicated that PPARγ::RXRα was predictably a transcription factor for steroidogenesis genes, and Foxl2 and Dmrt1 were also transcription factors for rxrs and their partner receptor genes. When the flounder fry were exposed to LG100268 (LG, RXRs agonist, 50 mg/kg diet), the percentage of males increased from 50% to 71.4%. Before histological differentiation of the flounder ovary (3 cm TL) and testis (6 cm TL), the transcripts of rar β and rar γ (P < 0.05) were activated, and the steroidogenesis gene Hsd3b1 was down-regulated (P < 0.05). The ratios of testosterone (T)/17β-estradiol (E2) were all greatly increased (P < 0.05), and the ratio of 11-ketotestosterone (11-KT)/E2 was elevated at 3 cm TL. Moreover, LG was used to treat the cultured gonads in vitro (10 μM) and the fish with intraperitoneal injection in vivo (12 mg/kg body weight), respectively. LG was able to up-regulate rxr γ, rar γ, and ppar δ, and Hsd3b1 was significantly up-regulated (P < 0.05). The ratios of 11-KT/E2 in the culture medium and in the ovaries of the fish were decreased. Furthermore, the recombinant flounder Foxl2 protein was able to significantly down-regulate ppar γ (P < 0.05) and tr β (P < 0.01) in the ovaries in vitro, and the result of the Dmrt1 in the testes was opposite to that of the Foxl2, probably indicating a feedback loop between RXRs' partner receptors and Foxl2/Dmrt1. These findings introduce for the first time the mode of action of RXRs on the flounder steroidogenesis and provide important data to learn the potential function of RXRs in fish sex differentiation and the potential role of RXRs in aquatic animals in the presence of water pollutants.