GM crops have incurred many criticisms, especially from groups who wish to see GM cultivation stopped completely. One of these criticisms is that the engineered traits might cross over to wild plants growing nearby. The worry is that plants engineered to be resistant to herbicides such as Roundup (glyphosate), might transfer their herbicide resistance causing a “superweed”, incapable of being controlled.
There has been some dispute as to whether this type of hybridization would actually occur in the wild or that if it did, whether the resultant strain would be able to survive. It always seemed to me that some hybridization was bound to occur; the only question was, “how much?” According to a Guardian article dramatically entitled GM crops created superweed, a recent cross fertilization with a GM crop has produced a herbicide resistant weed:
Modified genes from crops in a GM crop trial have transferred into local wild plants, creating a form of herbicide-resistant "superweed", the Guardian can reveal.
The new form of charlock was growing among many others in a field which had been used to grow GM rape. When scientists treated it with lethal herbicide it showed no ill-effects.
OK, let’s examine this. First, it is not totally clear even from the full study whether the herbicide resistance had transferred from the GM crop or if it had evolved independently due to selective pressure. They did perform a PCR to check if the resistance was due to the same gene as in the rapeseed, but it isn’t clear if the gene could be distinguished from a naturally evolved resistance gene. In any case, it is known that selective pressure can cause resistant plants to evolve naturally, so this is not necessarily a problem confined to GM crops. (For example, read how Roundup-Ready cocaine plants evolved naturally in Colombia.) As the head of the land management technologies group at English Nature said:
The glufosinate-ammonium herbicide used in this case put "huge selective pressure likely to cause rapid evolution of resistance".
Still, it is possible that the GM crop could provide an additional route for the resistant weed to develop.
Let’s suppose the resistance did transfer from the GM crop. We know from the full study that out of over 95,000 seedlings of wild relatives collected and grown by the researchers, only two grew into plants with the herbicide resistance: that’s only 0.002%. From a less hyperbolic article on this study from the BBC entitled Scientists play down superweed:
DNA analysis on a leaf sample confirmed the gene trait from the engineered oilseed rape was present, but when the researchers returned the following year to the same field they could find no herbicide tolerance in seedlings of the charlocks growing there.
The herbicide tolerant charlock growing in the field did not reproduce and so was non-existent the following year. So even if a hybrid did once exist it has disappeared. This shouldn’t be too surprising since distant crosses usually result in sterile plants.
But supposing the herbicide resistant weeds were able to reproduce? The solution always seemed to be that you would just spray the field from time to time with another herbicide, and that would kill the so-called superweed. However, according to the Guardian article, even that option would be in doubt, since multiple resistances have been known to arise:
The new plants were dubbed superweeds because they proved resistant to three herbicides while the crops they were growing among had been genetically engineered to be resistant to only one.
Experiments in Germany have shown sugar beets genetically modified to resist one herbicide accidentally acquired the genes to resist another - so called "gene stacking", which has also been observed in oilseed rape grown in Canada.
Gene stacking is where cross-pollination results in genes from different crops occurring together in one individual “volunteer” plant. From this, it is clear that this multiple resistance would only be passed from GM crops if multiple GM plants all with different resistances were all grown nearby. In fact, we know the Canadian example arose from cross pollination of two GM and one conventional crop, each resistant to a different herbicide. Although this is a concern, it should be possible to prevent this happening in future, and farmers still have other herbicides they can use. The problem would be worse if a farmer grew crops resistant to the same herbicide year after year, but this can be avoided if they follow the good practice of crop rotation: in other words, it too can be managed.
As for whether herbicide resistant crops could become weeds in the general environment: crop plants need careful nurturing – they don't survive well in the wild. For example, a 10 year study published in the journal Nature in 2001 showed that nearly all the crops looked at (conventional and GM), died out after four years when not cultivated. And the herbicide tolerance gives no survival advantage unless the plants are regularly sprayed with the herbicide in question, so there is no reason why GM varieties should survive better as weeds in the wild, than conventional varieties.
To quote the reviewer’s comment at the beginning of the study:
Although this study identified hybridisation between oil seed rape and Sinapis arvensis, such a finding needs to be interpreted with caution. The frequency of such an event in the field is likely to be very low, as highlighted by the fact it has never been detected in numerous previous assessments. Furthermore, the conditions where the hybrid was found appear to be quite unusual, restricted as it was to a case where Sinapis was sufficiently abundant in a crop to act as a significant conspecific pollen donor. The consequences of the transfer of the herbicide tolerance trait on the fitness and persistence of Sinapis arvensis were not assessed in this study but are presumed to be negligible. Nevertheless, this unusual occurrence merits further study in order to adequately assess any potential risk of gene transfer.
Of course, this merits further study. But it is still not a reason to stop GM cultivation. The risk appears relatively small, and it can be managed with better separation of different GM crops, use of best practice such as crop rotation, and other herbicides. The risk is clearly not limited to GM crops anyway – resistant strains arise naturally due to competitive pressure. You wouldn’t know this though from articles such as the Guardian, copied verbatim and uncritically here, here, here, here and at numerous other anti-GM sites. Note that article’s provocative and barely true headline GM crops created superweed compared with the much less hysterical BBC’s Scientists play down superweed that has not been reproduced anywhere that I can find.
As usual with anything GM, the facts count less than the conclusion already reached by the anti-GM crowd.