WWW.WAS.ORG • WORLD AQUACULTURE • DECEMBER 2023 27 (CONTINUED ON PAGE 28) early 80’s. This means only a fraction of the escapees need to survive to make a significant contribution to the wild breeding pool. The most recent surveys have found evidence of genetic introgression in 150 (63 percent) of the 239 rivers monitored in Norway, of which 68 were considered severely impacted (The Scientific Council for Salmon Management 2021, Figure 2). Recent advances in gene editing technology have also led to other concerns around escapees. We are able to remove or insert DNA into the salmon genome to alter their biology. This has enormous potential for making salmon grow faster, be more resistant to disease, require less feed, or prefer more sustainable diets. It can also be used to enhance our basic understanding of salmon biology. But this technology can also be highly contentious, especially as it is possible to insert DNA from other species into the salmon genome to attain desirable traits. This has already been done in the US where AquaAdvantage salmon have had DNA from two other species (chinook salmon Oncorhynchus tshawytscha and ocean pout Macrozoarces americanus) inserted into their genome (Yaskowiak et al. 2006). A solution to the problem of farm escapees, whether gene edited or not, would be to farm sterile salmon. Then in the event they do escape, their environmental impact would be limited only to their remaining lifespan. Where gene edited salmon are commercially produced, they are already required to be sterile (AquAdvantage Salmon must be sterile, US Food and Drug Administration 2022). But this is not the case for salmon which have undergone selective breeding. Today, we only have one realistic option to mass produce sterile fish, which is triploidy. This is a form of genetic manipulation that essentially increases the DNA content of the individual by 50 percent. Sterile Triploid Salmon Wild salmon are diploids, meaning they have two complete chromosome sets with one inherited from each parent. This is the same in humans, where half of your DNA comes from your mother and the other half from your father. A chromosome is just the name given to the structure which contains DNA. Triploids are individuals that have three complete chromosome sets. The additional set can theoretically come from either parent, but in the case of farmed triploid salmonids, it comes from the maternal side. In humans, triploidy is lethal and leads to the death of the embryo. In fishes, it is generally not lethal, has occurred naturally in some species, and has been artificially induced in many others. Although inducing triploidy is a genetic manipulation, it does not trigger regulation under the Gene Technology Act. In other words, triploids are not considered to be genetically modified organisms (GMOs). Arguments for this are that polyploidization, whereby an individual has more than two chromosome sets, has played a natural role in fish evolution. Salmonids have gone through at least four rounds of genome duplication in the past whereby the amount of genetic material they contain doubled. The last round was 80 million years ago. So somewhat confusingly, what we call a diploid today would have been considered a polyploid 80 million years ago (Houston and Macqueen 2019). There are also some natural self-sustaining species of triploid fish although they have unusual modes of reproduction such as by cloning themselves. In contrast, a recent survey of 6000 wild Norwegian and Russian salmon found 1 triploid (Jørgensen et al. 2018) suggesting it is not naturally self-sustaining in this species. We have known salmonids can be efficiently triploidized since the mid-1970’s. The early interest in doing so was because they were expected to grow faster than regular diploid salmon. This is because triploids have bigger cells to accommodate their extra DNA and they were not expected to sexually mature, a process that is energetically expensive and slows down growth. However, the early results were disappointing. Although triploids do have bigger cells, they have fewer of them than diploids so they gain no immediate size advantage. In addition, triploid males still reach sexual maturity but their sperm is non-functional. This means the males are technically sterile, but they still spend energy on testicular development which slows down growth. Triploidization does prevent ovary development and female maturation, but this has not been linked with any obvious improvement in growth. And as the final nail in the early evaluations coffin, farm trials in Scotland and Canada during the 1990’s tended to find triploids struggled with bone deformities, ocular cataracts, and with illnesses during the summer months (Fraser et al. 2012). Because of their poor reputation the industry never continued with triploid Atlantic salmon apart from in regions with extreme levels of pre-harvest sexual maturation which reduces the quality of the fillet. This is mainly the Tasmanian industry in Australia, where the warm sea temperatures mean all-female populations of either diploids or triploids are normally used. Females are less likely to mature before harvest size than males, and early maturation is much more common when it is warmer. All-female triploids have also been proposed for land-based salmon farming using recirculation aquaculture systems (RAS), as this approach also struggles with high incidences of pre-harvest maturation due to the relatively warm FIGURE 2. Genetic status in 239 salmon stocks in Norway based on a measure of genetic integrity. Eleven of the 239 stocks are not defined as salmon stocks and they are shown with a colored ring instead of a filled circle. The level of genetic introgression within each stock has been determined as none (green), low (yellow), moderate (orange), or high (red). Figure adapted from Diserud et al. 2020.
RkJQdWJsaXNoZXIy MjExNDY=