Solving one problem can cause another. And we bring you an example of that truism today from the world of agricultural biotechnology.

NPR's Joe Palca has this example of the law of unintended consequences.

JOE PALCA: When the biotech world was young, scientists and regulators struggled with a problem. They could put things called transgenes into seeds that would give crop plants desirable properties such as resistance to disease. But ecologists worried what would happen if these transgenes spread from cultivated plants to their wild neighbors growing nearby.

Professor ANDREW STEPHENSON (Biology, Penn State University; Plant Ecologist): People are concerned that when a transgene escapes into the wild populations that it will provide a fitness advantage.

PALCA: That's Andrew Stephenson. He's a plant ecologist at Penn State. A fitness advantage means the wild plants, with this advantageous transgene, might grow out of control, mucking up the ecosystem. Stephenson studies a wild squash plant called the Texas gourd.

Prof. STEPHENSON: The fruits are about the size of a baseball to the size of a softball, and they're round or slightly pear-shaped.

PALCA: Tastes terrible. It shows up throughout the American Southwest and Mexico. Stephenson wanted to know what would happen when his wild gourd picked up a transgene that's used in cultivated squash to protect the crop from plant viruses. So he purposely created a strain of Texas gourd with the transgene and planted it next to the same gourd that didn't have the transgene. Then he waited for spring when aphids would bring a plant virus to the field.

Prof. STEPHENSON: As the virus spread through our field, it only affected the Texas gourd.

PALCA: That is the gourd without the transgene. As they report in the journal PNAS, at first, it looked like what scientists feared was happening. The plants that had the transgene did have that fitness advantage. They stayed healthy while their unmodified cousins got sick. But then something strange happened. After the aphids arrived in the field, along came another pest: the cucumber beetle. The beetles carry a different plant disease, not a virus, but a bacteria.

Prof. STEPHENSON: And these bacteria secrete an exopolysaccharide, like a snot-like stuff.

PALCA: And when the beetles eat the gourd's leaves and flowers, this snot-like stuff gets into the plant's plumbing, first causing the leaves to wilt and ultimately killing the plant. But here's the funny thing. The cucumber beetle will eat plants that are sick with a virus�

Prof. STEPHENSON: But they tend not to like them. So they fly off to someplace else.

PALCA: And that someplace else is the healthy plant protected from the virus by the transgene. So�

Prof. STEPHENSON: As the virus spreads through the susceptible plants in the field, the beetles become increasingly concentrated onto the healthy transgenic plants.

PALCA: Yes, the plants are protected from the virus, but that makes them more vulnerable to the bacteria. Stephenson says no one, no one suspected that would happen.

Prof. STEPHENSON: And these indirect ecological effects were not predictable based on any sort of laboratory study.

PALCA: Now in this particular case, the unintended consequences aren't bad. The wild gourd apparently won't grow out of control. But Norman Ellstrand says that's not the point. Ellstrand is a plant geneticist at the University of California, Riverside.

Professor NORMAN ELLSTRAND (Genetics, University of California, Riverside; Plant Geneticist): We know the transgene sometimes end up giving us unexpected effects. And when you do an experiment like Stephenson's group did, you see how complicated ecology actually is.

PALCA: Because basically you say, oh, never thought of that.

Prof. ELLSTRAND: Exactly.

PALCA: In other words, you don't know what you don't know.

Joe Palca, NPR News, Washington.

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