What Happens When Pollen Moves From Genetically Engineered Crops to Wild Relatives or Non-Genetically Engineered Varieties? In Areas of Genetic Diversity?

Most plants reproduce via self-fertilization or movement of genes from one parent to another via pollen. In fact, this process is an essential tenet of genetic diversity. But movement of unwanted genes, naturally occurring or engineered, may result in adventitious presence (AP), a situation where unwanted substances unavoidably are present in production and marketing of agricultural products. AP can occur for a variety of reasons, including gene flow, and sometimes results in economic consequences for commercial GE crops (1).

Generalizations about whether gene flow presents significant economic or environmental risks cannot be made for either conventionally bred or GE crops; case-by-case evaluation is required. Many major agricultural crops are sexually compatible with wild and/or weedy relatives, and, if the plants grow in overlapping regions, crop-to-weed or crop-to-wild relative gene flow could result (2; for a review see Reference 3). This outcrossing to wild populations can result in new combinations of genes that can improve, harm, or have no effect on the fitness of recipient plants. Genes can also flow from wild relatives to cultivated crops, introducing new traits into next generation seed, but only affect the crop if it is replanted. Gene transfer among plants may be a larger containment issue than unwanted pesticides, because genes reproduce in the recipient plant (4).

Pollen drift is a major, although not the only, conduit through which unwanted genes end up in crops. Numerous factors affect the frequency of gene flow resulting from pollen drift, i.e., biology of the species, the environment, and production practices, and these should be considered in developing strategies to minimize gene flow. Successful cross-pollination requires that parental plants (a) flower at the same time; (b) be close enough to allow a vector (insect, wind, or animal) to transfer pollen to receptive females; and (c) produce pollen that can result in embryos developing into viable seeds and germinating (for a review see Reference 5). Successful pollination also depends on the longevity of pollen viability and the distance it must travel (3, 6). Also important is whether the plant self-pollinates, as is the case for tomatoes, soybeans, and most cereal crops, or is open-pollinated, as in the case for corn and canola, where pollen from one plant fertilizes another. Gene flow is more frequent with the latter.

There are wild, weedy species in the United States compatible with some existing or anticipated commercialized GE crops. The first GE trait in a commercial crop with wild relatives in the United States was virus-resistant squash (7). USDA APHIS determined the impact of gene flow of the GE trait to wild varieties; the squash received nonregulated status (see "Can Federal Regulatory Agencies Stop Planting of Genetically Engineered Crops That Pose Environmental Risks?“) and was grown commercially after it was shown that viruses against which resistance was directed did not infect wild varieties or increase their competitiveness (8).

HT traits have been engineered into major U.S. commercial crops such as canola, corn, cotton, and soybean. Whether gene flow of HTtraits leads to more competitive, herbicideresistant weeds depends on factors such as species, location, and trait. One crop for which this might be a concern in the United States is cultivated rice, which outcrosses with perennial, wild red rice (Oryza rufipogon Griff.), considered a noxious weed in the United States. Red rice is sexually compatible with cultivated rice, grows in many of the same regions, often has overlapping flowering times, and thus is a prime candidate for gene flow with cultivated rice. Breeders generally try to avoid gene movement from red rice to cultivated varieties because of its undesirable traits, e.g., awned seeds and red pericarp. When GE HT traits were introduced into cultivated rice, attention shifted to the impact of genes moving into red rice. To study this, experiments were conducted to determine gene flow rates under natural field conditions from cultivated rice to wild red rice and weedy rice (O. sativa f. spontanea) in China and Korea, respectively (9). An HT gene and simple sequence repeat (SSR) fingerprinting were used to monitor gene flow, which ranged from 0.01 to 0.05% for weedy rice and from 1.21 to 2.19% for wild red rice. Although frequencies were low, gene flow did occur, emphasizing the need to avoid outcrossing when genes could enhance the ecological fitness of weedy species. In another study, resistance to imidazolinone herbicides, created by mutagenesis, not by engineering, was used to assess gene flow and fitness of the recipient (10), a reminder that gene flow is not limited to GE varieties and its impact is dependent on the trait, not the means by which the gene was created.

Numerous studies have evaluated pollenmediated, intraspecies gene flow from canola to its wild relatives. One study evaluated the outcrossing of B. napus with wild relatives, including B. rapa L. (rapeseed), Raphanus raphanistrum L., Sinapis arvensis L., and Erucastrum gallicum (11). Hybridization between B. napus and B. rapa in two field experiments was ∼7% in commercial fields and ∼13.6% in the wild. Gene flow from GE B. napus to the other three wild varieties was shown to be low (<2 to 5 x 10⁻⁵); however, genes could move into the environment via wild B. rapa or commercial B. rapa volunteers. Analysis of 16 of these types of studies identified major factors affecting pollen-mediated gene flow from B. napus (12), using either a donor plot surrounded by receptor plants (continuous design) or a receptor field only on one side of the donor plot (discontinuous design). With continuous designs, cross-fertilization averaged 1.78% ± 2.48% immediately adjacent to the donor plot and was fairly constant at 0.05% ± 0.05% at distances over ten meters (1 meter = 3.3 feet). With discontinuous designs, outcrossing rates were 0.94% (±0.51) next to donors and 0.1% (±0.11) at distances over 100 meters. Thus, most outcrossing occurred in the first ten meters from the field, although numerous factors relating to the field, plant, pollen, and environment influenced the rate. Aside from pollen flow, volunteer HT populations can also arise via seed-mediated flow (13) and from feral populations (12).

An example of a trait moving from a commercial GE crop to a non-GE variety is tripleresistant canola (14) (see “Could the Use of Genetically Engineered Crops Result in a Loss of Plant Biodiversity?”); this outcome was predicted prior to release of the GE variety because of canola’s tendency to outcross (15). The multiply resistant volunteers, with two GE traits and one mutant HT trait, could still be controlled with other herbicides; however, their presence has decreased the utility of HT canola (16). Movement of HT genes could have been monitored more closely to prolong the effectiveness of the HT varieties.

A well-publicized study of pollen-mediated gene flow involved precommercial GE HT creeping bentgrass (Agrostis stolonifera L.), a wind-pollinated, highly outcrossing, perennial grass (17) (see "Can Federal Regulatory Agencies Stop Planting of Genetically Engineered Crops That Pose Environmental Risks?“). Because bentgrass has native, weedy relatives in the United States with which it outcrosses (18), transgene movement to related Agrostis species and dissemination of seeds and vegetative propagules were examined. Following a single growing season of GE bentgrass, most transgene flow was found within 1.2 mi (2 km) in the direction of prevailing winds; limited gene flow was found to 13 mi (21 km). Further study showed that nine HT creeping bentgrass plants (0.04% of samples) grew ∼2.4 mi (3.8 km) beyond the control area—six from pollen-mediated gene flow in the direction of prevailing winds and three from dispersed GE seeds (19). Three years after production halted in HT bentgrass fields, 62% of 585 bentgrass plants tested positive for the HT gene; 0.012% of seedlings from seed of HT plants were HT positive (20), suggesting that under some conditions transgenes can establish in wild populations after short exposures. Although no long-term ecological studies were done, it was suggested that herbicide application or drift could lead to persistence of the HT trait in wild plants (21).

Prior to commercial release of HT alfalfa (see "Can Federal Regulatory Agencies Stop Planting of Genetically Engineered Crops That Pose Environmental Risks?“). studies were done to assess gene flow in fields grown for seed and for forage (for review, see Reference 22). Under intentionally poorly managed fields (20 to 50% bloom), gene flow from a forage field to a seed field was <0.5% at 165 ft and 0.01% at 350 to 600 ft (23). This type of information can be used to establish distances and practices to minimize gene flow (22). Gene flow also occurs to feral alfalfa, frequently found growing outside cultivation areas, and this type of gene flow is affected by the same barriers as other alfalfa gene flow, i.e., flowering synchrony, presence of pollinators, and distances between alfalfa fields and feral plants. Gene flow to feral alfalfa, which is less abundant and less conducive to seed set, can be reduced by decreasing feral flowers through frequent mowing or animal predation.  

In areas of genetic diversity of plants related toGEvarieties, additional precautions are needed to reduce possible impacts of introgression of GE traits, when potential significant environmental consequences could occur, and to minimize this occurrence. For example, outcrossing of GE HT rice with wild rice varieties has potentially significant environmental such impacts, whereas gene flow of the vitamin A trait from GE Golden Rice is less likely to have such impacts. Where possible impact is significant, planting of GE crops near wild species should be avoided or GURT-type technologies could be used to prevent gene(s) from moving to wild varieties (see “Will Plants with Terminator-Type Genes Prevent Replanting of Genetically Engineered Crops?”, 2).

On the basis of published studies, gene flow will occur when compatible plants are present and thus GE traits can move and persist in unintended plants. Even in the absence of gene flow, GEvarieties can persist in the agricultural environment. For example, in Sweden, GE volunteer oilseed rape plants (0.01 plant perm2) were observed ten years after a trial of GE HT oilseed rape (24). Farmers need to be cognizant of gene movement from GE crops and the possible persistence of GE varieties. For organic farmers the presence of GE traits in their crops could create a problem if a contract was signed limiting the presence of GE traits in their organic products (see “What Happens When Pollen Moves from Genetically Engineered Crops to Organic Crops?”). Conventional farmers should also be aware of transgene movement to a non-GE crop if it is intended for export or other sensitive markets (see “Does the Export Market Affect Decisions by Farmers to Grow Genetically Engineered Crops?”).

References:

1. Counc. Agric. Sci. Technol. (CAST). 2007. Implications of gene flow in the scale-up and commercial use of biotechnology-derived crops: Economic and policy considerations. Counc. Agric. Sci. Technol. Issue Pap. 37. CAST, Ames, IA
Summary of gene flow implications of current commercialized GE crops, how gene flow affects adventitious presence and its mitigation, regulatory and risk assessment approaches, and economic implications.

2. Arriola PE, Ellstrand N. 1996. Crop-to-weed gene flow in the genus Sorghum (Poaceae): spontaneous interspecific hybridization between johnsongrass, Sorghum halepense, and crop sorghum, S. bicolor. Am. J. Bot. 83:1153–60

3. Ellstrand NC. 2003. Dangerous Liaisons? When Cultivated Plants Mate with their Wild Relatives. Baltimore, MD: Johns Hopkins Univ. Press
Classic and current knowledge about crop genetics, hybridization, and evolutionary ecology with regard to gene flow and hybridization between crops (including GE) and native species.

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7. ISB (Inf. Syst. Biotechnol.). 1995. Genetically engineered virus resistant squash approved for sale. NBIAP News Rep., Jan. http://www.isb.vt.edu/news/1995/news95.Jan.txt. Last accessed 2011-12-12. PDF

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17. Watrud LS, Lee EH, Fairbrother A, Burdick C, Reichman JR, et al. 2004. Evidence for landscape-level, pollen-mediated gene flow from genetically modified creeping bentgrass with CP4 EPSPS as a marker. Proc. Natl. Acad. Sci. USA 101:14533–38

18. Belanger FC, Meagher TR, Day PR, Plumley K, MeyerWA. 2003. Interspecific hybridization between Agrostis stolonifera and related Agrostis species under field conditions. Crop Sci. 43:240–46

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20. Zapiola ML, Mallory-Smith CA, Thompson JH, Rue LJ, Campbell CK, Butler MD. 2007. Gene escape from glyphosate-resistant creeping bentgrass fields: past present and future. Presented at 60th Meet. West. Soc.Weed Sci., Portland, OR, March 13–15

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22. van Deynze A, Fitzpatrick S, Hammon B, McCaslin MH, Putnam DH, et al. 2008. Gene flow in alfalfa: biology, mitigation, and potential impact on production. CAST Special Publ. 28

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Updated 2/16/12