Those naughty plants!

Potentially promiscuous pollen from corn tassels by circulating via Flickr.

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

maizevarieties
Image of corn plant by University of Nebraska Lincoln, adapted by Anastasia Bodnar. All other images in this post by Anastasia Bodnar.

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.
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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.
cross2When 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.
cross4Genetic 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.
cross5
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.
Positive
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.
Neutral
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.
Negative
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.
ResearchBlogging.org 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

Anastasia Bodnar

Written by Anastasia Bodnar

Anastasia Bodnar serves as the Policy Director of Biology Fortified, Inc. She is a science communicator and multidisciplinary risk analyst with a career in federal service. She has a PhD in plant genetics and sustainable agriculture from Iowa State University.

5 comments

  1. Isn’t gene flow out of a crop species into a landrace (whether it is transgenic or not) likely to result in an actual increase in biodiversity rather than the reverse -at least on a population level, the individual plant may have a more uniform set of chromosomes than one might expect (from the inbred/hybrid parent) but the addition of information into the overall gene pool surely represents an increase in diversity, unless of course the situation you discuss on farmer selection within the field comes in to play – although arguably if a farmer is selecting the best plants to take seed from every year you’d get a local decrease in biodiversity regardless of proximity to hybrids/inbreds (albeit at a slower pace).
    As far as I see it the main concerns around gene flow should come from those with the “heirloom” obsession – as any gene flow from any source is going to ruin your variety (or the seeds from that season anyway) – and of course plant breeders in general… although I’ll go out on a limb here and assume that plant breeders are well aware of gene flow issues hence the endless hours in the field/greenhouse/growth chamber with paintbrushes, tweezers and all manner of arcance devices involved in planned parenthood for plants.

  2. Ewan, you’re right, of course. I described the extremes here. In real fields, gene flow can positively contribute to genetic diversity, even when the genes are from unwanted sources like modern hybrids or inbreds and weedy relatives. I do worry about places like Mexico, though, where maize landraces are planted nearby hybrid corn year after year. If the landrace population keeps getting pollinated by the hybrids, even if it’s only a small portion of the field, there could very easily be loss of genetic diversity, especially if the alleles confer an advantage. Ah, I wish I remembered my population genetics a little better.
    I’m amazed at the tricks plant breeders use to keep those naughty plants from mating with undesirable partners. I live in Ames, IA near a branch of the USDA Agricultural Research Service and it’s so neat to see things like sunflowers protected from pollinators with netting and cucumbers housed in little mesh cubes to keep the pollinators in. Planning parenthood for plants isn’t easy!
    By the way, have you noticed that even breeders and geneticists, people who know better, often use the word gene when they mean allele? Even though we know what the differene is, it’s just easier to say gene – but the meaning might not be clear to someone who doesn’t know the difference. We say gene flow, but really we often mean allele flow, except in the case of actually new genes like transgenes.

  3. Throughout my college days I used “Allele” as a chracter name on many online games… I suffer worse than most as a result from the allele/gene fallacy.
    I wonder to what extent gene flow from hybrids (or inbreds, or any non-native variety)can be expected to infiltrate a landrace and I’d expect (with absolutely no sound empirical backing) that it would be pretty tough although my understanding of how landraces are utilized/grown in Mexico may be somewhat limited – I’d imagine that introgression of foreign alleles into a low input system would in general tip them away from being as resistant to abiotic stress, less acclimated to the specific soil type and dominant weather etc – if inputs are higher this issue would likely go away, but I think it is rather questionable that anyone would use a landrace in anything other than a low input environment.
    To a certain extent here I can see why Bt for instance, may impact biodiversity (allele diversity?), as I’d guess you may well get a bunch of semi-deleterious alleles (comparitively) hopping over with a Bt gene and then getting stuck there due to the increased fitness provided by the Bt gene (assuming it does anything in the environment it is in – it won’t always) – one can always hope that genetic drift and potential to shift flowering time would take care of things also (which I’m sure everyone would agree is the scientifically sound way to go)

  4. Ewan, I just caught that last statement of yours about shifts in flowering time. I’ve always wondered why researchers in places where there are lots of wild relatives, weedy relatives, or landraces for the crop species of interest (like Mexico) aren’t developing crops that flower earlier or later than most of the wild or landrace ones. Seems like a nice low tech way to keep the plants from being so naughty – so they’ll be more likely to stay with their intended mates. It’s not 100% but it could be quite effective.

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