The image of a super-race of salmon, standing fin and flipper above their ordinary brethren, has been conjured up by scientists.
By giving the fish a hyperactive gene for producing growth hormone, they report in this week's "Nature", they have persuaded them to grow at up to 37 times the normal rate. The success if that is how it should be regarded raises the startling prospect of oysters as big as dinner plates in our larders, and pork chops that would feed a family.
The idea isn't terribly new. More than ten years ago, "Nature" ran a picture on its cover of the supermouse, a turbocharged rodent fuelled with added growth hormone by genetic engineering. So far, however, neither butchers nor fishmongers have had to get to grips with the huge creatures that were then predicted.
In the case of the salmon, the growth produced by the added gene is spectacular but short-lived. The salmon reach sexual maturity sooner, and then die. Dr Robert Devlin, one of those involved, believes that they might finish twice as large as conventional fish.
This is still significant, and the faster growth could offer advantages to fish farmers by bringing their salmon to market sooner. But success would depend on the extra gene changing only the growth rate, and no other characteristics of the fish and that, experiments in other animals suggest, is unlikely.
Fish offer the best prospects for the technique because, unlike higher species, they grow continuously throughout life. They also have greater unexploited potential than, say, domesticated cattle or pigs. But growing faster could make the fish very fatty, or give them circulatory or other problems. As for taste, nobody yet knows: "These fish are too precious to be consumed," Dr Devlin says.
Dr John Clark, a specialist in animal genetics from the Roslin Research Station in Midlothian, says that mammals present an altogether tougher prospect than fish. He points out that the human disease called acromegaly, caused by a tumour in the pituitary gland that makes it secrete extra doses of growth hormone, has effects that go far beyond increased size. If the condition occurs in adolescence, it does indeed produce a giant: but if it occurs in adult life, the changes are limited to the hands, feet, sinuses and the jaw, producing a striking change in physical appearance. "The result is not an improvement," Dr Clark remarks.
Early experiments have alerted animal rights campaigners and given scientists pause. At the US Department of Agriculture's research station in Beltsville, Maryland, pigs given added growth hormone suffered from arthritis, impotence and muscle weakness. Experiments with sheep in Australia produced animals with diabetes, abnormal kidneys and malformed bones. They lived for less than a year.
In any case, the gene alone is not enough. In any animal there are also sections of DNA that regulate the behaviour of the gene, and of these we know relatively little.
That the approach might work is shown by the effects of the hormone bovine somatotrophin (BST) on cows. This increases milk yields because, Dr Clark says, the hormone is released in the right dose and at the right time. But this is done not by genetic engineering within the cow, but by producing BST synthetically and giving it by injection or slow-release capsule.
All this suggests that the era of cows as big as camels is some way off, if it ever arrives at all. The prospect of bigger or at least faster-growing fish appears more plausible, but they would still have to overcome the consumer resistance that appears likely to scupper BST milk. Ultimately it is the buyer who will decide whether any genetically-engineered food has a future.
