Pest Management Science Pest Manag Sci (2008) Review Gene flow from glyphosate-resistant crops Carol Mallory-Smith ∗ and Maria Zapiola Oregon State University, Crop and Soil Science, 107 Crop Science, Corvallis, Oregon 97331, USA Abstract: Gene flow from transgenic glyphosate-resistant crops can result in the adventitious presence of the transgene, which may negatively impact markets. Gene flow can also produce glyphosate-resistant plants that may interfere with weed management systems. The objective of this article is to review the gene flow literature as it pertains to glyphosate-resistant crops. Gene flow is a natural phenomenon not unique to transgenic crops and can occur via pollen, seed and, in some cases, vegetative propagules. Gene flow via pollen can occur in all crops, even those that are considered to be self-pollinated, because all have low levels of outcrossing. Gene flow via seed or vegetative propagules occurs when they are moved naturally or by humans during crop production and commercialization. There are many factors that influence gene flow; therefore, it is difficult to prevent or predict. Gene flow via pollen and seed from glyphosate-resistant canola and creeping bentgrass fields has been documented. The adventitious presence of the transgene responsible for glyphosate resistance has been found in commercial seed lots of canola, corn and soybeans. In general, the glyphosate-resistant trait is not considered to provide an ecological advantage. However, regulators should consider the examples of gene flow from glyphosate-resistant crops when formulating rules for the release of crops with traits that could negatively impact the environment or human health.  2008 Society of Chemical Industry Keywords: glyphosate resistance; gene flow; environmental impact 1 INTRODUCTION Gene flow, defined as the change in gene frequency in a population due to movement of gametes, individuals or groups of individuals from one place to another, 1 has been raised consistently and repeatedly as a concern related to the introduction of genetically engineered (GE) crops. 2–4 Gene flow is a natural phenomenon that is not unique to GE crops. The concerns raised relative to gene flow from GE glyphosate-resistant (GR) (Roundup Ready  ) crops include: the emergence of volunteer crops that are more difficult or more expensive to control, the transfer of the transgene to wild or weedy relatives and transfer of the transgene to conventional and organic crops. Gene flow to non-GE cultivars also may cause marketing issues because of the adventitious presence of a transgene. Gene flow can occur via pollen and seed (Fig. 1) and, for some species, may also occur via vegetative propagules. More emphasis has been placed on the potential for pollen to move transgenes. Although pollen is an important means of gene flow, the intentional movement of seed during commerce may be of greater importance for the long-distance dispersal of transgenes. 5 Gene flow via vegetative propagules has rarely been addressed, but it could be an important avenue for transgene movement. The authors recognize that there are concerns in addition to gene flow from GR crops, such as the evolution of glyphosate-resistant weeds, or species shifts to more tolerant weeds because of the increased selection pressure from repeated glyphosate applications. Glyphosate-resistant volunteers also are an issue, especially in cropping systems with multiple GR crops. Because glyphosate is the herbicide most often used in no-till and minimum-till systems, GR volunteer crop plants and glyphosate-resistant or tolerant weeds will jeopardize the sustainability of those systems. Although the main focus of this article is gene flow as it pertains to GR crops in the USA, some references to management issues are included. The review will address crops that are commercially available or are in the process of deregulation in the USA. 1.1 Status of GR crops Genetically engineered GR crops have been sold in the USA since 1994. The glyphosate resistance trait allows glyphosate, a non-selective herbicide, to be used on crops that would otherwise be killed by the herbicide. Glyphosate-resistant crops are resistant because they have a transgene that produces a herbicide-insensitive target-site enzyme, EPSPS (5-enol-pyruvylshikimate-3-phosphate synthase). The gene (CP4 EPSPS) was originally isolated from Agrobacterium sp. strain CP4. 6 Canola (Brassica napus L.) contains a glyphosate oxidoreductase (GOX ) gene (goxv247 ) from Ochrobactrum anthropi strain LBAA in addition to the CP4 EPSPS gene. 7 The ∗ Correspondence to: Carol Mallory-Smith, Oregon State University, Crop and Soil Science, 107 Crop Science, Corvallis, Oregon 97331, USA E-mail: Carol.Mallory-Smith@oregonstate.edu (Received 17 April 2007; revised version received 6 June 2007; accepted 15 June 2007) DOI: 10.1002/ps.1517  2008 Society of Chemical Industry. Pest Manag Sci 1526–498X/2008/$30.00