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COPYRIGHT 2006 American Institute of Biological Sciences
Transgenic crop varieties are a rapidly expanding and controversial technology. Their effects on biological and cultural diversity are a key issue in an often polarized debate. Here we provide answers to questions about one important example, that of transgenic maize in Mexico. In situ maize diversity in Mexico is present in traditional varieties in farmers' fields, and in wild and weedy relatives of maize. It is likely that transgenes are present in farmers' local maize varieties, but it is unknown whether they have introgressed. Socioeconomic changes, including migration, trade liberalization, and reduced support for Mexican farmers, may also affect maize diversity. Diversity may increase, decrease, or remain the same, but whether this is viewed as good or bad will depend on subjective values.
Keywords: GE crops, maize, Mexico, risk, gene flow
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The increase in commercial planting of genetically engineered (GE, also known as genetically modified, or GM) crop varieties during the last 10 years has been dramatic, both in the rapidity of its adoption (James 2005) and in the controversy it has sparked (e.g., CEC 2004). But the debate about GE varieties in traditionally based agricultural systems (hereafter "traditional agriculture") did not burst into international headlines until the publication in 2001 of an article reporting evidence of transgenes in farmer varieties (FVs) of maize (Zea mays ssp. mays) in the state of Oaxaca, in southern Mexico (Quist and Chapela 2001). That paper stimulated much discussion and speculation about the implications of the findings for maize diversity in Mexico. Then, almost four years later, Ortiz-Garcia and colleagues (2005) reported that they had found no detectable evidence of transgenes in the same area, reintensifying the debate.
The polarized positions of proponents and opponents of GE crops often conflate facts and values in research and discussions, frequently adding heat, but not light, to the debate. So what do we know about transgenes and maize diversity in Mexico? We offer an overview of the situation through answers to key questions. Our goal is to integrate the genetic, ecological, and social issues at the farm, regional, Mexican, North American, and global levels, emphasizing the distinction between empirical data and speculation based on unexamined assumptions.
Is maize diversity in Mexico important?
Yes. Although any answer to this question can only be value based, most people, regardless of their position on GE crops, agree about the importance of diversity. Maize was domesticated in southern Mexico, and Mexico is the world center of maize diversity (Matsuoka et al. 2002) and also home to that crop's wild relatives, the teosintes (Z. mays sspp.), including the teosinte most dosely related to maize (Z. mays ssp. parviglumis). Much of Mexico's maize diversity is conserved in situ in Mexican farmers' fields as FVs (Aragon-Cuevas et al. 2005), which include landraces, traditional varieties selected by farmers, MVs (modern varieties) adapted to the local environments by farmers and by natural selection, and progeny from crosses among these varieties. This in situ conservation is recognized as an important complement to the Mexican national gene bank's ex situ collection of 10,683 accessions, 83% of which are duplicated in the CIMMYT (Centro Internacional de Mejoramiento de Maiz y Trigo [International Maize and Wheat Improvement Center]) gene bank (Hernandez Casillas 2003). Maize diversity plays a central role in the rich agronomic and cultural diversity of Mexico, its culinary traditions, and its national identity (figure 1; Esteva and Marielle 2003). Maize is the staple of most Mexican diets, with over 12.7 million metric tons (equivalent to 126 kilograms per person) consumed directly as food in 2003 (FAOSTAT 2006). Maize occupies more area than any other Mexican crop (8.0 million hectares [ha]), and FVs account for 79% of maize area in the nation (Aquino et al. 2001) and approximately 90% in Oaxaca, which has high maize diversity (table 1; Aragon-Cuevas et al. 2005).
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Maize is also an important crop globally (first in production and third in consumption, after rice and wheat; FAOSTAT 2006); 56% of global maize production in 2005 occurred in industrial countries using MVs, whose improvement may increasingly depend on the genetic diversity found in FVs (Pollak and Salhuana 2001).
Could transgenic maize affect the genetic diversity of maize in Mexico?
Yes. Transgenic maize could increase or decrease maize diversity, or it could have no lasting effect. This will depend on genetic, ecological, and social processes, which can be empirically understood within the limits of agreement on definitions, methods, and resources (Cleveland and Soleri 2005). We consider the effects of GE varieties per se, as well as effects of transgene flow from those varieties to FVs and to wild and weedy relatives. Seed flow is the first step in (trans)gene flow, followed by pollen flow, hybridization, and introgression (incorporation of the transgene into the host genome with stable inheritance).
Gene flow and its longer-term effects on the diversity of the recipient population depend on a number of variables, including the size of donor and recipient populations, the rate of seed and pollen flow and fertilization, and the relative and absolute fitness of the hybrids, which are determined by the genetic, ecological, and sociocultural processes in specific agricultural systems (Ellstrand 2003). Transgene flow includes not only the gene of interest (e.g., Bacillus thuringiensis, or Bt) but other genetic elements in the transgenic construct, such as promoter, terminator, and marker genes, as well as linked nontransgenic genes of the host genome that "hitchhike" along with the transgenes (Gepts and Papa 2003). In addition to any effects gene flow might have on allele frequencies in the recipient population, it will increase diversity qualitatively because it adds a new gene or genes. Modern GE crop varieties can also affect diversity by reducing the area planted with FVs, or by replacing FVs (see "Are the potential effects of current GE maize varieties on maize diversity and traditional agricultural systems the same as those of conventional MVs?" and "Will economic globalization affect Mexican maize diversity?" below).
Whether any qualitative or quantitative change in diversity will be positive or negative depends entirely on subjective judgments, including those about the role of agriculture in nature and society (Cleveland 2001). For example, conserving the maximum amount of genetic diversity feasible is often the stated goal of conservation. However, maintaining in situ the maximum diversity at the crop species, variety, or population level may not always be beneficial for the agricultural communities managing that diversity (e.g., high levels of heterogeneity within any crop can make field management and selection for desired traits difficult for farmers, as can high heterozygosity in cross-pollinated species). Even if the amount of diversity does not change, there can be disagreement over which type of diversity (e.g., which alleles, at which loci, from which source) is more valuable, a potential issue when considering the effect of transgene flow on other loci in the genome. Agreement about the value of agricultural transgenes will ultimately require a broad benefit--cost analysis comparing current commercial GE varieties, FVs, and MVs with alternatives, including FVs improved conventionally (see "Are there more diversity-friendly alternatives to current GE crops?" below) or by genetic engineering (Cleveland and Soleri 2005).
Concern over the potential effect of GE varieties and transgenes on maize diversity was important in the Mexican government's decision to ban experimental and commercial planting of transgenic maize in 1998 (Alvarez-Morales 2000), which has since been lifted for experimental plantings. This ban was specifically in response to concern for maize diversity; GE varieties of other species are grown commercially in Mexico, including 0.1 million ha of transgenic soybean and cotton in 2005 (James 2005). Potential effects on maize diversity continue to be the focus of debates about transgenic maize in Mexico (CEC 2004).
Where does transgenic maize in Mexico come from?
The United States. The United States is the world leader in developing, promoting, producing, and regulating GE crops, with 49.8 million ha planted in 2005 (55% of global GE crop area) (James 2005). In 2005, 52% of US maize planted was transgenic--26% Bt modified, 17% herbicide resistant, and 9% both Bt modified and herbicide resistant (USDA NASS...
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