Genetic introgression of escaped farmed Atlantic salmon (Salmo salar) into wild populations is a major environmental concern for the salmon aquaculture industry. Using sterile fish in commercial aquaculture operations is, therefore, a sustainable strategy for bio-containment. So far, the only commercially used methodology for producing sterile fish is triploidization. However, triploid fish are less robust. A novel approach in which to achieve sterility is to produce germ cell-free salmon, which can be accomplished by knocking out the dead-end (dnd) gene using CRISPR-Cas9. The lack of germ cells in the resulting dnd crispants, thus, prevents reproduction and inhibits subsequent large-scale production of sterile fish. Here, we report a rescue approach for producing germ cells in Atlantic salmon dnd crispants. To achieve this, we co-injected the wild-type (wt) variant of salmon dnd mRNA together with CRISPR-Cas9 constructs targeting dnd into 1-cell stage embryos. We found that rescued one-year-old fish contained germ cells, type A spermatogonia in males and previtellogenic primary oocytes in females. The method presented here opens a possibility for large-scale production of germ-cell free Atlantic salmon offspring through the genetically sterile broodstock which can pass the sterility trait on the next generation.

Precise gene editing such as CRISPR/Cas9-mediated homology directed repair (HDR) can increase our understanding of gene function and improve traits of importance for aquaculture. This fine-tuned technology has not been developed for farmed fish including Atlantic salmon. We performed knock-in (KI) of a FLAG element in the slc45a2 gene in salmon using sense (S), anti-sense (AS) and doublestranded (ds) oligodeoxynucleotide (ODN) templates with short (24/48/84 bp) homology arms. We show in vivo ODN integration in almost all the gene edited animals, and demonstrate perfect HDR rates up to 27% in individual F0 embryos, much higher than reported previously in any fish. HDR efficiency was dependent on template concentration, but not homology arm length. Analysis of imperfect HDR variants suggest that repair occurs by synthesis-dependent strand annealing (SDSA), as we show for the first time in any species that indel location is dependent on template polarity. Correct ODN polarity can be used to avoid 5′-indels interrupting the reading frame of an inserted sequence and be of importance for HDR template design in general. Aquaculture continues to grow faster than any other major food production sector and is quickly becoming the main source of seafood in human diets. In this context, Norway is the largest producer of farmed Atlantic salmon (Salmo salar) worldwide. In later years, the production of salmon in Norway has ceased to grow due to sustainability challenges linked to open sea-cage rearing. Genetic introgression of farmed salmon into wild stocks and the marine parasite, salmon louse, are recognized as the two major concerns 1. The high prevalence of salmon lice in most Norwegian fjords, due to open sea-cage farming, cause high lethality in wild salmonids and is hindering expansion of sea-cage farming. The consequences of genetic introgression caused by escapees remain uncertain, but existing knowledge indicates that it may lead to changes in life-history traits, with potential ecological impacts 2-5. Sequencing of the salmon genome 6 has permitted more detailed studies on the link between genes and key traits, and we and others have shown that single nucleotide polymorphisms (SNPs) to a certain degree can explain the time of maturity 1 and disease resistance 7,8. In this context, New Breeding Technologies (NBTs) by gene editing may offer a solution to some of the problems in salmon farming, with a possible production of salmon displaying traits such as disease resistance and sterility 9-12. We have previously demonstrated the feasibility of double allelic KO in F0 salmon using CRISPR/Cas9, by targeting genes essential for pigmentation 9 , elongation of polyunsaturated fatty acids 13 and reproduction 10. At the same time, CRISPR/Cas9 KO-mutations targeting various phenotypes have been shown by others in several farmed fish species such as tilapia 14-21 , sea bream 22 , sterlet 23 , channel catfish 24,25 , southern catfish 26 , common carp 27 , sturgeon 28 and rainbow trout 29. CRISPR/Cas9 KOs are produced by a Cas9-...

Selective breeding programs in aquaculture are limited by the heritability of the trait and long generation time in most fish species. New breeding technology (NBT) using CRISPR/Cas9-induced homology directed repair (HDR) have the potential to expedite genetic improvement in aquaculture, but the method requires optimization. Here we show that asymmetrical oligonucleotide (ODN) donors induce highly efficient and precise edits in individual Atlantic salmon founder animals. We performed single nucleotide replacement (SNR) in dnd with up to 59.2% efficiency, and inserted FLAG elements into slc45a2 and dnd, with up to 36.7 % and 32.7% efficiency, respectively. We found HDR efficiency to be dependent on template concentration, but a trade-off with respect to toxicity was observed. Using this NBT in salmon we demonstrate that precise modification of the genome can be achieved in a single generation, allowing efficient introgression of favorable alleles and bypassing challenges associated with traditional selective breeding.

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