A lot of people (including me!) are concerned with the possibility of genetically engineered crops spreading their pollen to nearby fields and to wild relatives. I covered some physical and genetic ways to prevent this in Gene flow, IP, and the terminator, but we all know that 100% exclusion of unwanted pollen is impossible (at least for now).
So, what happens when a farmer’s field or wild plant population is “contaminated” with a transgene? Can they be decontaminated? What about gene flow from non-transgenic crops? Strangely, none of the people concerned with transgenic gene flow seem to be concerned about non-transgenic pollen from modern cultivars, which is a much bigger problem. I use “contaminated” in quotes because nature doesn’t see the distinctions we see. Transgene or not, wild or cultivated, all go into a big mixing pot to be stirred by random mating and natural selection.
If we are to be concerned about transgenes, we must consider the actual effects that those genes might have, and how those genes might act within a population. There are three types of effects an “escaped” transgene could have on wild populations:
1) Some transgenes are expected to have a negative effect on the fitness of wild plants. For example, a gene that dramatically increases the size and number of fruits produced by a plant is desirable from an agricultural perspective, but will likely have detrimental effects on a wild plant, because the plant would have less resources to devote to other needs like herbivore defense and drought tolerance. These types of genes will not persist in a wild population.
2) In contrast, some transgenes are expected to have a positive effect on the fitness of wild plants. For example, a gene for herbivore resistance, such as that found in crops engineered to produce the insecticidal Bt toxin, would help cultivated and non-cultivated plants escape damage from susceptible herbivores. These types of genes will be selected for and thus persist in a wild population.
3) Finally, some transgenes are neutral, expected to have no effect on fitness of wild plants. For example, if a wild plant acquires the gene for glyphosate tolerance, but is never sprayed with this chemical, its fitness will presumably be unaffected. These genes may persist in a wild population at low levels. Since we are talking about one gene, it’s actually very easy to breed it out of a population. We just need to know which plants have the gene and which ones don’t, then keep only seeds from plants that don’t have the gene. This will take one generation if the plants are tested before fertilization, or a few generations if the seeds are tested. Some transgenes are easy to see, while others require a DNA test. For example, plants “contaminated” with a hypothetical transgene encoding for color could be identified by sight and removed. Plants “contaminated” with a transgene that isn’t easy to see can be screened for the suspected gene with PCR, a relatively easy process.
Gene flow from all cultivated plants to wild relatives is a much larger problem than transgenes. Wild populations are generally in genetic equilibrium, such that the population has just the right balance of alleles for each of its genes to ensure maximum survival and the ability to adapt to changes in the environment. When pollen from cultivated plants fertilizes plants in wild populations or landraces, only 1/2 of the genes in the resulting plants are wild. If enough of the plants are fertilized with cultivated pollen, genetic diversity in future generations can be seriously decreased. With a decrease in diversity, the population is less able to survive changes in the environment. Rice in Asia has been severely affected by non-transgenic gene flow, so much so that it is difficult if not impossible to find wild rice plants that do not contain some genes from modern cultivars.
A specific example of confusion about the problem of gene flow can be found in the movie “The World According to Monsanto” which can be found on YouTube. Starting at the end of part 6 of 8 and through half of part 7 of 8, the movie discusses contamination of maize landraces in mexico with transgenes. They say that farmer’s plants are becoming “monsterous” due to transgenes. They say that “transgenes will insert themselves into different places in the genomes of the farmer’s corn”, which is less than scientifically accurate, to say the least.
Natural transposons found in many organisms can “jump”, inserting themselves into new places in the genome, but transgenes don’t have this ability any more than normal genes do (they don’t). It’s hard to say what is causing the “monstrosities” but I’d wager it has a lot more to do with the combination of highly different genomes that haven’t had contact for decades or possibly even hundreds of years. I don’t know what else to say, except that the interpretation of gene flow presented in the movie is overly simplistic and ignores a lot of the bigger issues associated with loss of biodiversity. Blaming genetic engineering is easy, but doesn’t help solve any problems.
Note – I’m only discussing the gene flow part of the movie in this post, so please hold your horses if you have comments on the rest of it. If you have particular parts of the movie you’d like to discuss, let me know in a comment.