sealPurdue News

April 2000

Transgenic fish could threaten wild populations

WEST LAFAYETTE, Ind. – Purdue University researchers have found that releasing a transgenic fish to the wild could damage native populations even to the point of extinction.

A transgenic organism is one that contains genes from another species. The Purdue research is part of an effort by Purdue and the U.S. Department of Agriculture to assess the risks and benefits of biotechnology and its products, such as genetically modified fish. The study was published in November in the Proceedings of the National Academy of Science.

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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 – 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.

Sources: Bill Muir, (765) 494-8032;

Rick Howard, (765) 494-8136

Writer: Chris Sigurdson, (765) 494-8396;

Purdue News Service: (765) 494-2096;

NOTE TO JOURNALISTS:  Copies of the journal article are available from Bill Muir, (765) 494-8032;

Purdue animal scientist Bill Muir and colleagues hope to extend their research on bioengineered fish to species that may be used in fish farming, such as this tilapia. (Purdue Ag Communications Photo by Tom Campbell)

A publication-quality photograph is available at the News Service Web site and at the ftp site. Photo ID: Muir.trojan

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Possible ecological risks of transgenic organism release when transgenes affect mating success: Sexual selection and the Trojan gene hypothesis

William M. Muir and Richard D. Howard

Widespread interest in producing transgenic organisms is balanced by concern over ecological hazards, such as species extinction if such organisms were to be released into nature. An ecological risk associated with the introduction of a transgenic organism is that the transgene, though rare, can spread in a natural population. An increase in transgene frequency is often assumed to be unlikely because transgenic organisms typically have some viability disadvantage. Reduced viability is assumed to be common because transgenic individuals are best viewed as macromutants that lack any history of selection that could reduce negative fitness effects. However, these arguments ignore the potential advantageous effects of transgenes on some aspect of fitness such as mating success. Here, we examine the risk to a natural population after release of a few transgenic individuals when the transgene trait simultaneously increases transgenic male mating success and lowers the viability of transgenic offspring. We obtained relevant life history data by using the small cyprinodont fish, Japanese medaka (Oryzias latipes) as a model. Our deterministic equations predict that a transgene introduced into a natural population by a small number of transgenic fish will spread as a result of enhanced mating advantage, but the reduced viability of offspring will cause eventual local extinction of both populations. Such risks should be evaluated with each new transgenic animal before release.

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