Fishy Alpha Males

With the ocean’s natural bounty declining around the world, an increasing amount of fish consumed today, such as most Atlantic salmon, is farmed. To further boost the efficiency of fish farms, companies are looking to raise fish genetically modified to have an extra growth hormone gene that makes the fish grow faster. Indeed, raising fast-growing fish inside pens—whether kept inland or in ocean bays—can reduce harvesting pressure on the wild fish populations that are threatened. However, new research suggests that as a way to protect wild fish stocks, such efforts may be futile should some of these modified fish escape into the environment.

Biologist Richard Howard and his colleagues at Purdue University recently showed that genetically engineered fish have the potential to replace some wild fish populations. What’s more, because the offspring of modified fish are less viable, their replacement of native fish could eventually drive entire species to extinction.

To investigate the effects of genetically engineered fish on wild populations, the biologists looked at how well males from both groups competed with each other for the same mate. In this experiment, the researchers used as their model Japanese medaka, a relatively small fish that reproduces quickly.

To create the genetically modified fish, the researchers inserted into fertilized medaka eggs copies of a salmon gene that codes for a growth hormone. As a result, the modified fish grew to be 83 percent larger than their wild counterparts.

Then the Purdue team conducted a series of mating experiments in the lab. The researchers placed one genetically modified male, one wild male, and a single wild female in a 4.5-liter fish tank and then monitored the fishes’ mating behavior. Presumably because of its larger size, the modified male was typically more successful than the wild male at attracting the female and fertilizing her eggs.

Even though the genetically modified fish regularly chased the wild ones away from the females, the wild fish weren’t entirely deterred from mating. They quickly adopted new strategies. For instance, while the modified male courted the female, the wild male would sneak in and try to fertilize the female’s eggs. “At first, I thought it was an aberration,” says Howard. “But then it kept on happening. That literally knocked my socks off.”

Despite their relentlessness, the wild fishes’ hard work resulted in little payoff. When the researchers analyzed the DNA of eggs fertilized during the trials, the results showed that 75 percent of all matings went to the genetically modified males. The researchers describe their results in the March 2 Proceedings of the National Academy of Sciences.

“This is a real concrete example of how [genetically engineered] fish will have a mating advantage,” says Allison Snow, an evolutionary biologist at Ohio State University. Should such modified fish escape into the environment, these results suggest that they could wipe out native fish, she says.

Yet, another factor could lead to a species’ demise altogether. In a separate experiment, the Purdue researchers observed that the offspring of genetically modified medaka were less likely to survive into adulthood than those sired by wild fish. Based on a mathematical model, the researchers predict that as transgenic fish would spread their genes throughout the population, the medaka species would go extinct after 50 generations. The researchers call this the Trojan-gene effect.

To what extent the results of these laboratory experiments can predict what will happen in the wild is unclear, says Howard. In certain salmon species, he says, small males are prevalent and can compete successfully with larger male fish by sneaking in during competitive matings. Therefore, should a genetically modified male find its way into the environment, the large number of sneaking males already in the population “should stop a Trojan gene,” says Howard.

However, whether a male fish becomes a large, dominant fish or a small sneaker is determined very early in the animal’s development: Fish that have a rapid growth rate early in life become sneakers instead of dominant males. Because genetically modified fish also grow rapidly early in life, “if they get out into the wild, it’s not really clear to me whether these guys will all be sneakers or really large dominant males,” says Howard.

Still, the findings highlight the potential environmental risks posed by the release, deliberate or not, of genetically modified fish into the environment. Over the past couple of decades, researchers have genetically modified a number of commercial crop plants, such as pest-resistant cotton and herbicide-resistant corn. More recently, researchers have been increasingly engineering animals, such as mosquitoes that are poor transmitters of malaria (SN: 5/25/02, p. 324: Available to subscribers at Better Mosquito: Transgenic versions spread less malaria) and chickens that produce pharmaceuticals in their eggs (SN: 4/6/02, p. 213: Scrambled Drugs: Transgenic chickens could lay golden eggs).

In response to the expanding practice, the Ecological Society of America recently released a position paper on genetically engineered organisms. While citing the many benefits of such plants and animals, the report calls for more rigorous risk assessment studies before the organisms are released into the environment. The report also recommends that genetically engineered organisms be modified to prevent the transfer of foreign genes to species in the wild.

The firm Aqua Bounty is currently seeking approval from the Food and Drug Administration to commercialize the company’s genetically engineered salmon. Aware of the environmental concerns, the company’s researchers are developing ways of genetically modifying the fish to make them sterile, as well as fast growers.

Snow commends the company’s efforts to prevent its fish from reproducing in the wild but wonders whether the technique will be 100 percent effective. “Can you guarantee that all the fish are going to be sterile?” says Snow, suggesting that it might not be possible. Still, she says, it may be possible to keep these genetically engineered organisms at “manageable and very low levels that would cause no problems” in the environment.