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When it comes to plants, there can be no gene flow without two compatible partners. And most plants are quite choosy, preferring a close relative rather than someone outside its family. Pollen travels in gusts of wind, on the pollen basket of bees, as cargo of flies or in the hands of human plant breeders. If the pollen alights upon a compatible mate, there will be fertilization and the resulting seed will carry the genes of the parents. What will happen then if a transgene from a genetically engineered crop plant cross-pollinates with wild relatives?
A new study in PNAS suggests that we cannot always anticipate the consequences.
In the Central Valley of California where we live, the crops plants are mostly exotic, imported over from afar- tomatoes and corn from Central and South Americas, cotton from Pakistan, safflower and alfalfa from the Near and Middle East, and rice from China. This means that genes from the crops grown in the valley will not transfer to wild relatives in the foothills nearby because the species are not compatible. It is as if California were a large, oval-shaped, flat-bottomed platter with steep, slippery sides holding all the crop plants, and their genes, at the bottom.
In other parts of the world, gene flow between crops and their wild relatives is common. Spontaneous hybridization between transgenic cultivars and wild relatives occurs for 12 of the world’s 13 most important crops. Consequently, there are concerns that genes from crop plants could escape and enhance the fitness of wild species, altering native populations.
There is renewed interest in this subject lately because, increasingly, many of crop plants carry “transgenes”- that is they were introduced through genetic engineering.
Most ecologists believe that if pollen carries a gene that confers a “fitness” advantage (e.g., enhancing resistance to a virus for example), and it has wild relatives nearby, hybrid progeny could potentially survive and establish new populations carrying that trait.
It is important to note that the theoretical fitness advantage is not specific to transgenes- it can occur with genes introduced by conventional breeding approaches as well. Virtually all the food that we eat today carries resistance genes introduced by conventional breeding (yes that includes organic produce).
In this 3-year study, Sasu and coworkers examined the consequences of cross-pollination of the Texas gourd,
a native to Texas and states along the lower Mississippi River, with cultivated pumpkins and squashes. They used a crookneck squash
that was engineered with a virus resistant transgene (VRT). The Vrt squashu is resistant to several related viruses including the watermelon mosaic virus shown here.
The researchers found that viral diseases (mostly Zucchini yellow mosaic virus) colonized and spread rapidly through fields during the period of peak gourd reproduction. The VRT plants were highly productive, as measured by fruit, flower and pollen production in comparison to the non-transgenic plants. These data suggest that the VRT trait would confer a fitness advantage when introduced into a population of wild texana gourds.
Surprisingly, the fitness advantage of the VRT plants came with a cost. When the non-VRT plants were infected with the virus, the plants became less attractive to beetles that carried a disease-causing bacteria. That is, even though the VRT plants were resistant to the virus, they were more susceptible to the bacteria transmitted by the beetle. Thus, there is an indirect, ecological cost associated with the VRT when the beetle and the bacteria are also present in the population. Whereas a simple ecological model would have predicted that the VRT would increase in the wild populations, this is not the case when other pathogens are present.
Sasu MA, Ferrari MJ, Du D, Winsor JA, & Stephenson AG (2009). Indirect costs of a nontarget pathogen mitigate the direct benefits of a virus-resistant transgene in wild Cucurbita. Proceedings of the National Academy of Sciences of the United States of America PMID: 19858473
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