Many people, including me, are concerned about potential harm to crop biodiversity from gene flow. Most people’s concern focuses on transgenics. There is a certain probability, albeit small, that transgenes will end up in the progeny of non-transgenic plants, weedy relatives of the crop, or wild relatives that grow nearby due to pollen flow. Transgenes can also be moved from place to place by accidental or purposeful movement of seeds.
How much transgene flow is actually happening is a subject of some controversy, but what about gene flow between non-transgenic plants?
There is potential for problems whenever plants that aren’t supposed to cross stray from their intended mates. Some things to think about include how gene flow happens at the field and genetic levels and what characteristics of the genes themselves can affect permanence of contaminating genes once they get into a variety they shouldn’t be in.
Gene flow with transgenes can help us to think about gene flow of “regular” genes
In the 2004 paper Gene Flow from Cultivated Rice (Oryza sativa) to its Weedy and Wild Relatives, Li Juan Chen showed that a marker gene “flowed” in their test field from transgenic cultivated rice to weedy rice at rates between 0.011 and 0.046 % and to wild rice at rates between 1.21 and 2.19 %. The marker gene Chen used is called bar, which is easy to screen for because it makes plants resistant to the antibiotic and herbicide biaphalos. Just spray the progeny and you’ll know if they’ve got the gene. Chen confirmed presence of the bar gene with PCR. These rates seem pretty low, but rice is mostly a self-pollinator, and the pollen is very short lived. If out-cross rates in rice reach 2.19 % we could expect to see rates even higher in other species. This tells us that transgenes can be passed to weeds, but also, more broadly, tells us that any gene can be passed from cultivated rice to weed rice.
Gene flow could be a problem in the opposite direction as well. In the 2009 paper Gene flow from weedy red rice (Oryza sativa L.) to cultivated rice and fitness of hybrids, Vinod Shivrain showed that progeny of a cross between weedy red rice and cultivated rice were more successful if their mother was the cultivated plant. These hybrid grains can fall back to the field on accident or be collected and planted the following year with the regular seed. Either way, the rice farmer now has rice plants that don’t have all of the desired characteristics of the cultivated rice. The plants will have at least some genes from the weedy rice that could help it out compete the desired rice plants but produce less grain. This paper shows that gene flow from weeds to crops can happen, and that it can be a problem.
Maize, unlike rice, is a promiscuous out-crosser. The pollen is heavy and still fairly short lived, so mostly pollinates plants that are nearby, but wind-carried pollen and stray seed can carry transgenes away from their intended fields. The story of transgenes in landraces of maize is summed up beautifully in the 2007 paper Gene flow from transgenic maize to landraces in Mexico: An analysis (pdf). Kristin Mercer tells us that research on the subject has had mixed results. Transgenes likely do exist in landraces in Mexico, but the extent of the “contamination” is not as wide as some researchers have proposed. Some of Kristen’s other research focuses on how crop alleles move in wild sunflower populations. The sum of her research is that we can expect gene flow back and forth between any compatible plants: wild, weedy, cultivated, transgenic, landrace.
Gene flow’s effect on biodiversity
Understanding the impact of gene flow on biodiversity (or more appropriately, crop diversity) requires some understanding of what happens at the genetic level. I like to sit down and draw pictures to help me think about genetics. I hope it helps some Biofortified readers as well!
The image to the right shows two hypothetical varieties of corn. On the left is a modern inbred variety. All the plants are identical. There is no or low genetic variability within the inbred, because there is only one version of each gene present in the variety. On the right is a landrace or heirloom variety. All the plants are different from each other to some degree. There is high genetic variability within the landrace because there can be many versions of each gene present in the variety.
Below is a (very) simplified view of what happens at the chromosomal level when an inbred is crossed with a landrace (in a hypothetical crop with one chromosome). Note: a hybrid or even an open pollinated variety could be substituted for inbred here, it was just easier to use an inbred. Similarly, a wild variety could contaminate a landrace. One landrace can contaminate another. One inbred could contaminate another. Weedy relatives can contaminate crops. Crops can contaminate wild varieties… you get the idea.
In the inbred (red), the two sister chromatids for each chromosome are identical to each other. There is only one version of each gene in the inbred, also known as two copies of the same allele. In the landrace (blue), the two sister chromatids are different from each other. For each gene in the landrace, there can be two different alleles. The different shades of blue indicate different alleles for some genes on the sister chromatids.
Imagine a situation where a field with the inbred is right next to a field with the landrace. Pollen will flow between the fields (to some degree – depending on weather conditions, pollen size, and tons of other factors). If the inbred and the landrace are crossed (whether pollen from the inbred fertilizes the landrace or vice versa), each of the offspring will have about half of the genetic information from the inbred and half from the landrace. Since the two chromatids are the same for the inbred, none of the information from the inbred is lost in any individual. Since the two chromatids in the landrace individual are different, each of the offspring only receive half of the genetic information from the landrace.
When those offspring make gametes, recombination often occurs which results in chromatids that contain some alleles from each grandparent. Crossing over, shown here, is one type of recombination. If those gametes then combine with the inbred, their progeny will only have about 1/4 of its genes from the landrace grandparent.
Genetic diversity can be lost in certain situations. For example, if a farmer growing a landrace finds plants in the field that have positive traits, the farmer will choose to plant those seeds for the next year. If those beneficial traits are due to genes from the inbred, the farmer could effectively select for plants with one or more genes with the inbred and against plants that don’t contain any genes from the inbred. If pollen from the inbred is reintroduced year after year, the farmer could plant seeds from those plants that contain more and more alleles from the inbred variety, and alleles from the landrace could be lost over time.
On the other hand, if the farmer chooses seeds from plants that look more like the landrace, then alleles from the inbred could be lost fairly quickly. If pollen or seeds from the inbred are introduced infrequently, the landrace would maintain a low level of alleles from the inbred, with those alleles eventually disapearing.
Of course there are many situations in between, and those depend greatly on what effect each gene or allele has on the plants they have contaminated.
Once it’s in there, how long will it stay?
Transgene or not, wild or cultivated, all of the genetic material goes into a big mixing pot to be stirred by random mating and natural selection in the case of wild plants or by breeding and artificial selection in the case of cultivated plants. One of Kristen’s points in Gene flow from transgenic maize to landraces in Mexico: An analysis (pdf) is that the permanence of transgenes in a non-transgenic population depends a lot on what the transgene is exactly, and the same idea applies to non-transgenic alleles.
To break it down: Any given transgene or any allele of a gene can have one of three effects on the plant: positive, neutral, and negative. The effect depends on what plant the allele is contaminating and what trait is conferred by the allele. Finally, how long a contaminating allele stays in a population depends on all of these factors.
Some alleles would be beneficial in almost any situation. Herbivore resistance, including genetically engineered Bt toxin and increased expression of non-transgenic chemical defenses, would help both cultivated and non-cultivated plants escape damage from susceptible herbivores. These types of transgenes and alleles would be likely to persist in any population they contaminated. These would definitely be bad traits to have in weeds. They could be desirable in a landrace from a farmer’s point of view.
A gene that increases the size and number of fruits produced by a plant is desirable from an agricultural perspective, but could have a negative effect a wild plant, because the plant would have less resources to devote to other needs like herbivore defense and drought tolerance. These types of alleles will not persist in a wild population, but could persist in a landrace if it is seen as desirable to farmers.
Alleles or genes that are specific for certain farming systems won’t persist in wild populations, weeds, or landraces unless they are exposed to those farming conditions. These include genetically engineered genes like glyphosate tolerance and the non-transgenic allele for Clearfield tolerance. If these alleles or genes contaminate a population but that population is never sprayed with the chemical, there is no selection pressure to keep the trait.
Of course these are just three examples of different traits and there are thousands if not millions of traits out there that might have different effects, but you get the idea.
Every day, pollen blows and seed is moved. Every day, genes and alleles are transferred from one plant population to another, no matter if they are transgenes or not. Those naughty plants just won’t keep to themselves! If we are truly concerned about gene flow, we really should be considering gene flow from all sources, not just transgenic crops.
Chen LJ, Lee DS, Song ZP, Suh HS, & Lu BR (2004). Gene flow from cultivated rice (Oryza sativa) to its weedy and wild relatives. Annals of botany, 93 (1), 67-73 PMID: 14602665
Shivrain VK, Burgos NR, Gealy DR, Sales MA, & Smith KL (2009). Gene flow from weedy red rice (Oryza sativa L.) to cultivated rice and fitness of hybrids. Pest management science, 65 (10), 1124-9 PMID: 19530257
Mercer, K., & Wainright, J. (2008). Gene flow from transgenic maize to landraces in Mexico: An analysis Agriculture, Ecosystems & Environment, 123 (1-3), 109-115 DOI: 10.1016/j.agee.2007.05.007