Biologist Rick Howard and his colleagues have discovered a paradox that crops up when new genes are deliberately inserted into a fish's chromosomes to make the animal grow larger. While the genetically modified fish will be bigger and have more success at attracting mates, they may also produce offspring that are less likely to survive to adulthood. If this occurs, as generations pass, a population could dwindle in size and, potentially, disappear entirely.
"Ours is the first demonstration that a genetically modified organism has a reproductive advantage over its natural counterpart," said Howard, a professor of biological sciences in Purdue's School of Science. "Though altering animals' genes can be good for humans in the short run, it may prove catastrophic for nature in the long run if not done with care. And we don't know just what kind of care is necessary yet, or how much."
This research, which Howard conducted with William Muir of the animal sciences department and Andrew DeWoody of the forestry and natural resources department, appears in this week's (Feb. 17) online issue of the journal Proceedings of the National Academy of Sciences. Howard and Muir published a related article in the same journal in December 1999 that showed larger animals had a mating advantage, but their previous article did not relate mating advantage to genetic modification (see below URL for related news release).
The most common question posed about genetically modified organisms - GMOs for short, and also called transgenics - is whether they are safe for people to eat. When GMOs were first made commercially available in 1996, many food crops, such as corn and soybeans, were altered to produce substantially more yield than they do in nature. The debate on GMOs in supermarkets has not yet been resolved, and Howard said it could be drawing attention from an equally important issue: whether GMOs are safe for the planet.
"With all the concern over whether transgenic food is safe for humans, the environment has been more or less left out of the picture," Howard said. "Plenty of laboratories are studying whether GMOs are safe for human consumption, but to my knowledge, ours is the only one that looks at whether they will be safe for the Earth."
As a step toward resolving this issue, Howard and his colleagues set out to examine the risk transgenic fish might pose to natural-bred populations. They chose to examine the Japanese medaka, a small fish that breeds daily, because it would reproduce often enough for trends to emerge relatively quickly.
"We took several tanks and put a female into each one with two males - one natural, and one transgenic," Howard said. "The transgenic males were 83 percent heavier than the natural-bred males, so it was easy to distinguish which male was mating with the female."
Howard found that the larger transgenic males mated three times for every time the natural-bred males did - not surprising, considering the premium that female medakas place on male size. But though the transgenic males mated more often, fewer of their young survived to adulthood.
"What surprised us was how fast a GMO mating advantage could cause a transgene to spread in a population," Howard said. "Additionally, we were intrigued at the outcome predicted if this advantage was combined with the disadvantage of young with reduced survival ability."
Howard observed that for every 100 offspring sired by a natural male that survived to adulthood, only 70 of the young produced by a transgenic male survived.
"Putting both of these things together, a population invaded by a few genetically modified individuals would become more and more transgenic, and as it did the population would get smaller and smaller," he said. "We call this the 'Trojan gene effect.'"
Over time, continued Howard, this effect could continue to multiply itself over generations, eventually decimating a population.
"Imagine a pie, and you eat 30 percent of it every day," he said. "Half of it is gone in two days, and within a week less than one-tenth of it remains. It is conceivable that a similar effect could occur among fish populations if GMOs with 'Trojan genes' escape into the wild."
Though the report's conclusions are sobering, Howard cautions that the group's research is based on computer models as well as observation of actual fish mating behavior - behavior that took place within a controlled laboratory setting rather than in nature, where other factors could influence the outcome. Consequently, these ideas should not be taken as gospel under all circumstances.
"We were aiming to detect the potential risk a GMO might pose to the environment," he said. "The protocols our group developed with this report could be used as a tool for agencies and food producers for that purpose."
This research has been funded in part by the U.S. Department of Agriculture.
Source: Rick Howard, (765) 494-8136, rhoward@bilbo.bio.purdue.edu
Related Web sites:
Writer: Chad Boutin, (765) 494-2081, cboutin@purdue.edu
Rick Howard home page: http://www.bio.purdue.edu/about/faculty/howard/index.html
Previous Purdue release on Howard's transgenic fish research: http://news.uns.purdue.edu/UNS/html4ever/0002.Muir.trojan.html
Transgenic male mating advantage provides opportunity for Trojan gene effect in a fish
Richard D. Howard, J. Andrew DeWoody and William M. Muir
Genetically modified (GM) strains now exist for many organisms, producing significant promise for agricultural production. However, if these organisms have some fitness advantage, they may also pose an environmental harm when released. High mating success of GM males relative to wild-type males provides such an important fitness advantage. Here, we provide the first documentation that GM male medaka fish modified with salmon growth hormone possess an overwhelming mating advantage. GM medaka offspring possess a survival disadvantage relative to wild-type, however. When both of these fitness components are included in our model, the transgene is predicted to spread if GM individuals enter wild populations (because of the mating advantage) and ultimately lead to population extinction (because of the viability disadvantage). Mating trials indicate that wild-type males use alternative mating tactics in an effort to counter the mating advantage of GM males, and we use genetic markers to ascertain the success of these alternative strategies. Finally, we model the impact of alternative mating tactics by wild-type males on transgene spread. Such tactics may reduce the rate of transgene spread, but not the outcome.
Journal
Proceedings of the National Academy of Sciences