Millions of fish are farmed in the United States every year. Many of them die from infections. Theoretically, the fish could be protected from many diseases by genetic modifications. This would reduce waste and reduce the negative environmental impact of fish farming. A team of scientists is working on exactly this – and has now inserted an alligator gene into the genome of catfish (Ictalurus punctatus, spotted fork catfish) using CRISPR/Cas9 gene scissors.
In 2021, US catfish farms produced around 139 million kilograms of the edible fish. “Per pound, catfish production accounts for 60 to 70 percent of US aquaculture,” says Rex Dunham, who works on catfish genetic improvement at Auburn University in Alabama. But catfish farming is a good breeding ground for infections. From the time the eggs are laid to when they are “caught,” about 40 percent of animals worldwide die from various diseases, Dunham says.
The alligator gene that Dunham discovered as a possible solution to the problem during his research encodes a protein called cathelicidin. The protein is antimicrobial, says Dunham, and is believed to help protect alligators from infection in the wounds they incur during aggressive fights among themselves. Dunham wondered if animals that had the gene artificially inserted into their genome might be more resistant to disease.
The genetic researcher and his colleagues wanted to go one step further and ensure that the resulting so-called transgenic fish could not reproduce. Because genetically modified animals can cause great damage in the wild if they escape from farms and displace their wild counterparts for food and habitat.
Transgenic Survivors
Dunham, Baofeng Su (also at Auburn University) and their colleagues used the gene-editing tool CRISPR to insert the alligator cathelicidin gene into the part of the fish genome responsible for the production of key reproductive hormones. They wanted to try to “kill two birds with one stone,” according to Dunham. Without the hormone, the fish cannot spawn.
The resulting fish actually appear to be more resistant to infection. When the researchers put two different types of disease-causing bacteria in tanks of water, they found that the genetically engineered fish were more likely to survive than their unedited counterparts. “Depending on the type of infection, the survival rate of the transgenic cathelicidin fish was two to five times higher,” says Dunham.
The transgenic fish are also sterile, as hoped, and unable to reproduce unless subsequently injected with reproductive hormones, according to the researchers, who published their findings online on preprint server bioRxiv. The study has not yet been peer-reviewed.
“When I first heard about the study, I thought: What on earth? Why would you do something like this?” says Greg Lutz of Louisiana State University, who studies the role of genetics in aquaculture . But now Lutz thinks the work shows promise: Resistance can have a big impact on the amount of waste produced in fish farms, and reducing that waste has long been a goal of genome editing in farmed animals, he says.
Breeding disease-resistant fish requires fewer resources and produces less waste overall, says Lutz. While he’s positive about the research, he’s not convinced that CRISPR catfish represent the future of aquaculture. The genome editing process used by the team is comparatively complex and it would probably have to be carried out with every spawning round of the hybrid catfish used in fish farming. “It’s just too difficult to produce enough of these fish to get a viable, genetically healthy line off the ground,” he says.
Ready to eat?
Meanwhile, researchers at Auburn University hope their transgenic catfish will be approved so it can be sold and eaten. However, that could take a while. Only one other type of genetically engineered fish has been approved in the US so far: In April last year, the Food and Drug Administration (FDA) allowed AquAdvantage salmon to be sold – a whopping 26 years after the company AquaBounty first submitted the application. The salmon have an extra gene, taken from the genome of another species of salmon, that makes them much larger than they otherwise would.
But suppose the catfish are eventually approved for sale. Would anyone eat them? Su and Dunham believe so. Once the fish is cooked, the protein made by the alligator gene loses its biological activity, so it’s unlikely to have any consequences for the human who eats the fish, Su says. “So I would eat it right away,” says Dunham.
(jl)
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