Genetically modified corn and soybeans get all the press, but since as far back as 1996, transgenic squash, engineered to include resistance to three of the most deadly squash viruses, has been farmed in both the U.S. and Mexico. Since gene flow from GM crops into their wild counterparts is inevitable, some scientists worry that farmers may be inadvertently creating a race of super squash weeds. This would be a problem, because some varieties of wild squash — specifically, Texana gourds — are considered serious weed threats in cotton and soybean fields. This is what happens when you mess with Mother Nature — your transgenic squash genetically contaminates wild squash which then proliferates through your genetically modified corn and soybean fields.
Or at least that’s one theory. Some researchers at Penn State, led by biologist Andrew Stephenson, set out to find out exactly what we can expect to happen as transgenic squash genes migrate into the wild. They planted mixed populations of transgenic and wild squash and recorded their susceptibility to various viral and bacterial pathogens over a three-year period. (Abstract of paper in the Proceedings of the National Academy of Science here. Layman-friendly summary of research here.)
As expected, the GM squash did a good job of resisting the viral pathogens, which instead hit the wild squash hard, resulting in stunted, slower growth and fewer flowers. But then an interesting thing happened. Squash have evolved in conjunction with their own specialist herbivores, cucumber beetles. Most herbivores are repelled by bitter compounds produced in squash leaves called cucurbitacins. But not tcucumber beetles; they just can’t get enough of the stuff. Unfortunately for the squash, the cucumber beetles are a transmission agent for a bacterial squash killer — wilt disease — which spreads via beetle poop that falls into the “open wounds” of chewed-upon squash leaves.
Cucumber beetles, quite understandably, don’t enjoy feasting on stunted, droopy squash plants that have been hammered by viruses. Instead, they seek out the biggest, healthiest, most abundantly flowering squash plants they can find. Which means the Penn State researchers discovered that the genetically modified squash that had most successfully resisted viral infection perversely attracted hordes of beetles, and thus were more likely to contract the deadly wilt disease than their wild counterparts. Built-in resistance to one set of mortal threats unexpectedly set them up to be vulnerable to an entirely different doom.
The moral of the story? In this case, the dangers of viral resistance from genetically modified squash escaping to the wild and creating a race of super-squash weeds that would rampage through the corn fields might be a little overblown. That does not mean, however, that Mother Nature will automatically maintain the status quo no matter how dramatically humans tinker with the structure of living things. It just tells us that figuring out how ecosystems will react to the introduction of new species with new traits is a complex business, with a host of unexpected variables lying in wait to trip us up.