Isolation Distances for Transgenic Alfalfa Seed Production in the Pacific Northwest

Abstract

Alfalfa (Medicago sativa L.) was the first perennial crop genetically engineered (GE) to resist glyphosate herbicide. Since alfalfa is insect pollinated, the potential for transgene flow and occurrence of adventitious presence (AP) in conventional varieties is a concern, especially for organic and export producers. In response, the industry developed seed production isolation distances. However, no landscape‐scale studies have been conducted to validate industry standards. Our objective was to quantify the extent of gene movement from GE seed fields to commercial conventional seed fields on a landscape level to provide the industry with information to support coexistence strategies. Alfalfa seed fields were mapped in the Touchet area, Walla Walla Valley, Washington, during spring 2013. Fourteen conventional seed fields located at various distances from GE sources were identified, harvested, and tested for the presence of the GE trait. Although other variables affect gene flow, this article focuses on geographic distance, since this variable can be easily controlled by growers and industry. Using distance as the predictor variable, we modeled the relationship between AP proportion and distance to GE source fields at 0.1, 0.5, and 0.9% AP threshold levels using quadratic regression. As distance increased, transgene flow generally decreased. At a threshold level of 0.9% AP, an isolation distance of 330 m between conventional and GE fields is needed to ensure that at least 95% of the samples fall below this threshold level. Spatial prediction maps were consistent with our statistical analysis. At an average distance of 2.2 km, AP was 0.05%, and this decreased to 0.03% at an average distance of 8.8 km from GE seed fields.

Gene flow in commercial alfalfa (Medicago sativa subsp. sativa L.) seed production fields: Distance is the primary but not the sole influence on adventitious presence

“…Distance from GE fields strongly influences AP in conventional fields although other factors, such as the area of GE fields in the vicinity, also played a role. Decreases in AP with increasing distances between GE and non-GE test plots have been reported in previous gene flow studies in alfalfa [7][8][9][10]18]. Similar declines in AP with increasing distances from the GE source have also been found in crops and fruit trees [13,23,42,49,[61][62][63].…”

“…We used preliminary data from three fields to estimate the seed sample size needed to PLOS ONE quantify AP. Based on these data, 7200 seeds were tested per harvested seed sample and adjusted to germination [7]. The phenotypic assay (seedling germination assay) was used to identify putative glyphosate resistant seedlings, which were confirmed using TraitChek test strips.…”

“…Since the introduction of genetically engineered (GE) crops, the acreage and the types of GE crops planted have been increasing. An important concern with the introduction of GE crops is the movement of GE genes to non-GE or conventional fields of the same crop or to wild populations of a close relative [1][2][3][4][5][6][7]. At issue is the occurrence of adventitious presence (AP), the unwanted presence of GE genes in non-GE products, which can negatively impact farmers that grow products for low AP tolerance markets, such as the export and organic markets.…”

“…At issue is the occurrence of adventitious presence (AP), the unwanted presence of GE genes in non-GE products, which can negatively impact farmers that grow products for low AP tolerance markets, such as the export and organic markets. One strategy to limit AP in non-GE fields is to establish minimum isolation distances between GE and non-GE fields [7][8][9][10][11][12][13][14].…”

“…Pollinator-mediated gene flow occurs among alfalfa seed production fields [ 7 , 8 , 11 , 18 ], thus it is important to understand the variables that influence the movement of GE genes. Previous studies have emphasized the importance of distance between GE and non-GE fields on pollen mediated gene flow [ 7 – 12 , 18 ]. Besides physical distance, there are likely other variables that affect the probability of a gene flow event.…”