Can herbicide resistance move from crops to weeds?

The risks of GMO herbicide-resistant crops as a source for resistance traits in weeds has garnered recent attention in discussions of so-called “superweeds”. [I've commented previously on my general disagreement with the term "superweed" when talking about herbicide resistance].  Some media reports and online sources have suggested that herbicide resistance can be caused by resistance “jumping” from the crops into weeds. In fact, at least one online dictionary defines the problem in these terms:

This topic deserves some scrutiny and clarification to point out areas where this is (or could be) accurate and where it is inaccurate or unlikely.  To start with, though, I would argue that either: 1) the above definition of the problem is simply wrong or 2) we don't have many "super weeds" problems and should not refer to most of our herbicide-resistant weeds using this term.  Here's why:

First, there are very few cases of herbicide-resistant weeds that are resistant because of a trait that originated in a crop (GMO or conventional) - most came about the old-fashioned way through selection of naturally occurring biotypes. Outcrossing species such as creeping bentgrass and canola have been shown to be the source for resistance in a few regionally important weeds.   However, to date no herbicide-resistant weeds in corn, cotton, or soybean production regions appear to have become resistant due to traits moving from the crop.   There are, however, many cases where the adoption of herbicide-tolerant crops (either GMO or developed through conventional breeding) has led to significant changes in herbicide use patterns and repeated use of the same herbicide or mode of action group has led to selection of resistant weeds.   This is not primarily a GMO crop issue; instead this is a lack-of-herbicide-rotation issue.

Second, the idea of resistance traits “jumping” among species is a bit imprecise and sensationalistic. The only known mechanism by which a crop trait could move into weeds (or vice versa) is through pollen-mediated gene flow – basically, sexual crossing between the crop and the weed. This can happen if the crop and the weed are the same species or very close relatives. Hybridization among more distantly related plant species is rarer; often due to one or more physiological barriers such as pollen incompatibility, different number of chromosomes, ploidy differences, etc.  This also is not a GMO crop issue either – sexual compatibility among crop and weed species is not known to depend on whether the crop is a GMO, conventional, or even organic.  

Opportunity for crop-weed gene flow also depends on the proximity of compatible species in a production region. In North America, many of our major field crops do not have significant problems with weedy relatives while others do; these scenarios present vastly different chances for this type of gene flow.   For example, soybean which is primarily inbreeding, does not have compatible relatives in North America thus risk of gene flow is very low. On the other hand, crops like sunflower, rice, and canola have some degree of outcrossing and compatible relatives (eg, wild sunflower, red rice, and a number of Brassica weeds, respectively) in their major production areas. Importantly, gene flow from a crop to a weed is VERY specific to the compatible species only – not to all weeds in a field. A good example here could be rice and red rice (both Oryza sativa) where gene flow among the wild and cultivated types is a real risk but that risk doesn't extend to other non-Oryza spp. weeds that may be in the same field.

Gene flow between crops and related, non-crop plants has been used accidentally or purposely by plant breeders to introduce adaptive traits into crops. Typically, this is done to get traits from wild plants into the crop plant, although the reverse can also happen. Breeding crop cultivars this way is a multi-generation process of crossing and backcrossing that must be conducted in order to get a line that is mostly like the crop parent except for the “new” trait. In the past, “crop” traits were generally considered to provide little benefit to weeds because attributes that are beneficial to crops (large seeds, low seed dormancy, uniform ripening, low shattering) generally are not very good “weed” traits. However, crop traits that could increase the range, reproductive ability, or fitness of a weed (such as tolerance to disease, insects, herbicide, or harsh environmental conditions) could lead to a greater weed problem and should be taken into consideration.

In a few cases where sexually compatible crops and weeds are grown in the same area, there is a possibility of herbicide resistance traits moving from a crop into weeds via hybridization. This phenomena depends on species compatibility and proximity and is not new or specifically related to GMO crops. Generally, the risk of herbicide resistance genes moving from crops to weeds in this way is low and is largely overshadowed by the risks of resistance imposed by selection pressure from non-diverse herbicide programs.

(note: the original draft of the above text was prepared as background information for a Weed Science Society of America press release on the topic of superweeds).

Brad