Absence of detectable transgenes in local landraces of maize in Oaxaca, Mexico (2003–2004)

Abstract

In 2000, transgenes were detected in local maize varieties (landraces) in the mountains of Oaxaca, Mexico [Quist, D. & Chapela, I. H. (2001) Nature 414, 541–543]. This region is part of the Mesoamerican center of origin for maize ( Zea mays L.), and the genetic diversity that is maintained in open-pollinated landraces is recognized as an important genetic resource of great cultural value. The presence of transgenes in landraces was significant because transgenic maize has never been approved for cultivation in Mexico. Here we provide a systematic survey of the frequency of transgenes in currently grown landraces. We sampled maize seeds from 870 plants in 125 fields and 18 localities in the state of Oaxaca during 2003 and 2004. We then screened 153,746 sampled seeds for the presence of two transgene elements from the 35S promoter of the cauliflower mosaic virus and the nopaline synthase gene (nopaline synthase terminator) from Agrobacterium tumefaciens . One or both of these transgene elements are present in all transgenic commercial varieties of maize. No transgenic sequences were detected with highly sensitive PCR-based markers, appropriate positive and negative controls, and duplicate samples for DNA extraction. We conclude that transgenic maize seeds were absent or extremely rare in the sampled fields. This study provides a much-needed preliminary baseline for understanding the biological, socioeconomic, and ethical implications of the inadvertent dispersal of transgenes from the United States and elsewhere to local landraces of maize in Mexico.

Transgene flow in Mexican maize revisited: Socio‐biological analysis across two contrasting farmer communities and seed management systems

“…Transgenes were already found absent and present at different frequencies in several regions across Mexico and also within the Oaxaca region, where our study was conducted (Dyer et al., 2009; Ortiz‐García et al., 2005a; Piñeyro‐Nelson et al., 2009a, 2009b; Quist & Chapela, 2001). Each of these studies used a different method to either sample maize, to conduct transgene detection analysis, and/or to interpret their results.…”

“…This is a kind of “population genetics approach,” in which the presence of a gene or transgene is the only aspect under analysis and very little can be inferred and estimated about the spread of transgenes into other communities and regions. Even when the same fields were sampled over years using the same method, researchers obtained different results for transgene frequency in these localities (Ortiz‐García et al., 2005a; Piñeyro‐Nelson et al., 2009a). Moreover, even when large sample sizes were used (about 9,000 leaf samples per community in Piñeyro‐Nelson et al., 2009a or 50,126 kernels analyzed in one growing season in Ortiz‐García et al., 2005a), they did not yield conclusive results either.…”

“…Even when the same fields were sampled over years using the same method, researchers obtained different results for transgene frequency in these localities (Ortiz‐García et al., 2005a; Piñeyro‐Nelson et al., 2009a). Moreover, even when large sample sizes were used (about 9,000 leaf samples per community in Piñeyro‐Nelson et al., 2009a or 50,126 kernels analyzed in one growing season in Ortiz‐García et al., 2005a), they did not yield conclusive results either. This is because seed saving and sharing practices can vary each year, even for the same farmer.…”

“…In previous studies, LOD levels were established by visual inspection of a dilution series in an agarose gel (Ortiz‐García et al., 2005a; Piñeyro‐Nelson et al., 2009a, 2009b; Quist & Chapela, 2001) and, therefore, it is not possible to verify whether this was also the case for the inconsistent results reported earlier. In fact, when transgene flow or introgression has taken place in a landrace or wild relative, the copy numbers of PCR targets (i.e., transgene elements) will most likely differ depending on both the number of transgene events that have occurred and the number of crossings.…”

“…The only previous study reporting no detection of transgenes in communities of Oaxaca was criticized using the argument that the results obtained could have involved false negatives due to the expression of a variety of secondary metabolites in landrace samples inhibiting PCR amplification (Ortiz‐García et al., 2005a; Piñeyro‐Nelson et al., 2009a, 2009b). However, the detection service provider for that study analysis confirmed that they had tested for PCR inhibitors through inhibition tests and did not find any such molecules (Schoel & Fagan, 2009).…”