GENETIC engineering may soon be helping to protect wild salmon stocks, by preventing escaped farmed fish from interbreeding with them. Six teams from five countries 鈥� Belgium, France, Ireland, Britain and Norway 鈥� are attempting to produce salmon that can be bred in captivity, but which will be sterile if they escape.
Although farmers continue to improve security, many salmon still escape. In Norway, for example, the Directorate of Fisheries estimates that between 500 000 and 700 000 farmed salmon escaped last year. Around 34 per cent of the fish caught in coastal fisheries are believed to come from farmed stock. And scientists at the Scottish Office鈥檚 Freshwater Fisheries Laboratory in Pitlochry have even discovered hybrids of farmed salmon and wild trout. This is raising fears that wild varieties may be pushed into extinction (鈥淪aving Snake River鈥檚 Wild Salmon鈥�, New 杏吧原创, 22 April).
Terry Smith heads the team working on the project to breed sterile fish at the National Diagnostic Centre in Galway, Ireland. 鈥淭here is no point in making a fish sterile unless the process can be reversed鈥� he says. The researchers plan to introduce a gene to prevent the expression of a hormone called gonadatropin releasing hormone (GnRH), without which both male and female salmon are sterile. The fish could then be made fertile by giving them a dose of purified GnRH, either by injection or by mixing it with food pellets.
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Under normal circumstances, the instructions from the gene for GnRH are ferried from the nucleus to the cell鈥檚 protein factories 鈥� called ribosomes 鈥� by messenger RNA. The ribosomes then manufacture the GnRH protein. The researchers aim to design a gene which will produce antisense-mRNA, which neutralises the normal mRNA. The instructions to produce GnRH would never reach the ribosomes and the hormone would not be manufactured.
Genes rely on neighbouring pieces of DNA called promoters to tell them when to start working. This normally ensures that particular genes only come into play in the correct parts of the body. The role of Smith鈥檚 team is to find a suitable promoter to fit onto the antisense-mRNA gene.
Salmon GnRH has so far only been detected in brain tissue, but researchers believe it may also be produced in tiny amounts elsewhere in the fish, so a 鈥渦niversal鈥� promoter will be needed.
The project is still at an early stage and will run for at least another two years. The French team will insert the modified genes into salmon eggs. The Norwegians have already cloned the GnRH gene, and Belgian and British researchers are looking at other species 鈥� tilapia and zebra-fish 鈥� in an attempt to find a promoter suitable for all farmed fish.
Canadian researchers demonstrated last year that they could engineer salmon that grow to Il times the size of their wild cousins (Nature, vol 371, p 209). But these fish could have worrying implications for the wild population if they were to escape and breed. Malcolm Windsor, secretary of the North Atlantic Salmon Conservation Organisation (NASCO) in Edinburgh, says: 鈥淥ne day, one could imagine that only sterile fish will be allowed in aquaculture. But until then, we need to know how they would behave in the wild.鈥�
