World Aquaculture Magazine - June 2020

58 JUNE 2020 • WORLD AQUACULTURE • WWW.WA S.ORG the gonads of channel catfish was greatestt when introduced 0, 1 and 5 days after the host triploid channel catfish hatched compared to 2-4 days after hatch (Gurbatow 2020). There have been great genetic improvements made in growth, disease resistance, body composition and carcass yield of aquatic organisms through gene transfer and gene editing. However, neither of these technologies have been applied on a large scale because of government regulation and the fear of environmental risk. Absolute reproductive control, both the ability to sterilize the organism with absolute certainty, coupled with the ability to restore fertility on demand, obviates all long-term ecological impact of transgenic and gene-edited aquatic organisms. Such a technology could be used in a variety of fish, including any domestic genotype, interspecific hybrid, transgenic or exotic, to minimize impacts on the natural environment, protect genetic biodiversity and ecosystems, increasing environmental friendliness of aquaculture and transgenic fish. Two technologies that might achieve this objective are repressible transgenic sterilization and gene editing of reproductive genes followed by hormone therapy and these can overcome the shortcomings of current sterilization breeding programs. Channel catfish were repressibly sterilized by knocking down primordial germ cell genes with shRNAi and cDNA overexpression constructs and repressors such as copper sulfate and sodium chloride used to produce fertile broodstock (Su et al. 2015, Li et al. 2017, 2018). However, not only was gamete production eliminated but gonadal development was also knocked out, resulting in a 25 percent reduction in growth rate and survival. Alternatively, knockout of the reproductive hormone genes FSH , GnRH or LH resulted in infertility in channel catfish (Qin et al. 2016, Qin 2019), which was restored with hormone therapy (Qin 2019). At this time the gene editing approach appears best as no negative pleiotropic effects were observed for traits such as growth rate and survival. Virgin Salmon — A Sustainable Solution for Coexistence of Farmed and Wild Salmon Strains Without Mixing Hilal Güralp, Kai O. Skaftnesmo, Erik Kjærner-Semb, Fernanda Almeida, Anne Hege Straume, Rüdiger Schülz, Eva Andersson, Per Gunnar Fjelldal, Lene Kleppe, Rolf B. Edvardsen and Anna Wargelius 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 biocontainment. So far the only methodology used commercially for producing sterile salmon is triploidization; however, triploid fish are less robust (Leclercq et al. 2011, Fraser et al. 2014, Sambraus et al. 2017). A novel approach in which to achieve sterility is to produce germ cell-free salmon, which can be accomplished by knocking out the dnd (dead end) gene using CRISPR- Cas9 (Wargelius et al. 2016). To explore the potential use of germ cell free salmon in aquaculture, we have monitored the welfare of the germ cell-free salmon through a production cycle, and so far no negative effect of lacking germ cells has been detected on growth, smoltification, lethality, bone deformities and flesh quality (Kleppe et al. 2017a). The lack of germ cells in the resulting dnd crispants thus prevents reproduction and subsequent large-scale production of the sterile fish. We continue to search for factors that can be used to develop protocols for sterility (Kleppe et al . 2015, Kleppe et al. 2017b, Kleppe et al. 2020) and we have also explored a method for inherited sterility in dnd knockout broodstock (Patent WO2020/070105 ). Inherited sterility in broodstock salmon may be achieved by rescue of dnd crispant salmon embryos. In rescued dnd crispant one-year old salmon we found germ cells, type A spermatogonia in males and previtellogenic primary oocytes in females. This method opens a possibility for large-scale production of Atlantic salmon broodstock that can inherit the sterility trait, while at the same time being able to produce 100 percent germ-cell free offspring. This approach can potentially solve the problem of genetic introgression and premature maturation in farmed salmon and ensure a stable production of 100 percent sterile fish, and thus represents significant commercial potential. Use of this sterility technology may also pave the way for genome editing for other traits, such as disease resistance, with negligible risk of escaped fish interbreeding and passing edited alleles on to wild stocks. Gene Editing to Induce Sterility in Fish Farming Xavier Lauth, John Buchanan, Takeshi Umazume, Melissa Hoffman, Valerie Williams, Michelato Kawakami and Edgar Hidalgo Two strategies were developed to generate, breed and mass produce infertile fish. These solutions rely on precise genetic modifications to create broodstock lines that can be seamlessly incorporated into breeding programs. The approaches were validated in tilapia but should be transferrable to other fish species. Adoption of these technologies should result in broad economic and environmental benefits for aquaculture. FIGURE 14. Channel catfish female x blue catfish male hybrid fingerlings produced from mating a xenogenic male and a channel catfish female (left) compared morphologically to control channel catfish fingerlings (right). The channel catfish female x blue catfish male hybrid has characteristics of the blue catfish (paternal predominance), a steep slope from the snout to the insertion of the dorsal spine, dorsal hump, a bluish color, very few spots and a relatively straight anal fin. In contrast, the channel catfish has a flatter head, no pronounced hump, a greyish color, more spots and a rounded anal fin. The reciprocal hybrid, blue catfish female x channel catfish male has an almost identical appearance to channel catfish (Perera et al. 2017).