Characterization of ddx4 and dnd Homologs in Snakeskin Gourami (Trichopodus pectoralis) and Their Expression Levels during Larval Development and in Gonads of Males and Females

Traceability of primordial germ cells in three neotropical fish species aiming genetic conservation actions

Primordial germ cells (PGCs) are embryonic pluripotent cells that can differentiate into spermatogonia and oogonia, and therefore, PGCs are a genetic source for germplasm conservation through cryobanking and the generation of germline chimeras. The knowledge of PGC migration routes is essential for transplantation studies. In this work, the mRNA synthesized from the ddx4 3'UTR sequence of Pseudopimelodus mangurus, in fusion with gfp or dsred, was microinjected into zygotes of three neotropical species (P. mangurus, Astyanax altiparanae, and Prochilodus lineatus) for PGC labeling. Visualization of labeled PGCs was achieved by fluorescence microscopy during embryonic development. In addition, ddx4 and dnd1 expressions were evaluated during embryonic development, larvae, and adult tissues of P. mangurus, to validate their use as a PGC marker. As a result, the effective identification of presumptive PGCs was obtained. DsRed-positive PGC of P. mangurus was observed in the hatching stage, GFP-positive PGC of A. altiparanae in the gastrula stage, and GFP-positive PGCs from P. lineatus were identified at the segmentation stage, with representative labeling percentages of 29% and 16% in A. altiparanae and P. lineatus, respectively. The expression of ddx4 and dnd1 of P. mangurus confirmed the specificity of these genes in germ cells. These results point to the functionality of the P. mangurus ddx4 3'UTR sequence as a PGC marker, demonstrating that PGC labeling was more efficient in A. altiparanae and P. lineatus. The procedures used to identify PGCs in P. mangurus consolidate the first step for generating germinal chimeras as a conservation action of P. mangurus.

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.