Genetically Modified Fish Could Wipe Out Natural Species

by Chris Sigurdson, Purdue News Service

A transgenic organism is one that contains genes from another species. The research is part of an effort to assess the risks and benefits of biotechnology and its products, such as genetically modified fish.

Purdue animal scientist Bill Muir and biologist Rick Howard used minute Japanese fish called medaka to examine what would happen if male medakas genetically modified with growth hormone from Atlantic salmon were introduced to a population of unmodified fish. The research was conducted in banks of aquariums in a laboratory setting.

The results warn that transgenic fish could present a significant threat to native wildlife. "Transgenic fish are typically larger than the native stock, and that can confer an advantage in attracting mates" Muir says. "If, as in our experiments, the genetic change also reduces the offspring's ability to survive, a transgenic animal could bring a wild population to extinction in 40 generations."

Extinction results from a phenomenon that Muir and Howard call the "Trojan gene hypothesis." By basing their mate selection on size rather than fitness, medaka females choose the larger, genetically modified but genetically inferior medaka, thus inviting the hidden risk of extinction.

The transgenic medaka were produced by inserting a gene construct consisting of the human growth hormone driven by the salmon growth promoter into medaka. The viability of groups of modified and conventional fish were measured at three days of age, and 30 percent fewer transgenic fish survived to that age. The researchers calculated that large males had a four-fold mating advantage, based on observations of wild-type medaka. Computer models then were used to predict the consequences of the transgenic mating advantage combined with the reduced viability of the young.

The study represents scientists policing science, Muir says. "I hope people understand that scientists are investigating the risks of biotechnology as well as the benefits, so decisions can be made with as much information as possible. It's important to understand the risks so they can be addressed."

Muir also cautions that the results of his laboratory study should be interpreted conservatively. "The study does confirm there are significant risks to natural animal populations associated with the release of transgenic animals. We assumed a consistent environment with only one variable Ð sexual preference for size coupled with low life expectancy for the transgenic. The natural world is not nearly as orderly, and genetic background changes could negate the Trojan gene," he says.

The dominance of sexual preference over Charles Darwin's classic theory of survival of the fittest is not unknown to wildlife specialists and geneticists. Muir likes to use the example of the male bird of paradise with its long swells of gloriously colored plumage as an example: "The male bird of paradise with the longest, thickest tail attracts the most females. Subsequent offspring also exhibit the long tail and also compete well for females. Unfortunately, the birds with the biggest tails also have the biggest problem escaping predators who appreciate large birds pinned in place by their plumage. Obviously the bird with the most sex appeal is the also the worst choice as a fit mate. Not unlike high school, some might say."

The researchers' next goal is to replicate the study with larger fish of economic importance in a bigger environment. They're looking for an indoor swimming pool where they can raise tilapia and check the results of the medaka study.

The study was published in November in the Proceedings of the National Academy of Science.