Are Mexico's
Indigenous
Maize
Varieties at Risk?
Mexican farmers safeguard some of the world's most important maize
biodiversity. What do we know about how they maintain landraces?
What might happen
if transgenic maize finds its way into their fields?
Maize Landraces:
Always Evolving
A widely held misconception about maize landraces is that they do not change. In fact, the landraces found even in remote areas of Mexico today are not the same as the maize found in the same location hundreds of years ago. Maize is an open-pollinating species. Individual maize plants readily exchange genes with other maize plants growing nearby, a characteristic that farmers recognized long ago as a way to adapt varieties to their own needs. Today's farmers in Oaxaca, Mexico, for example, readily notice when their maize has been inbred over too many generations and lost vigor. Some will say the maize "gets tired" ("se cansa" ) and will seek other varieties to mix with it.
In short, diversity in farmers' fields is not a static condition, but a dynamic process maintained by an influx of new genes, together with farmer selection. Likewise, landraces themselves are constantly evolving, while farmers maintain the traits that they desire.
Do Single-Gene
Traits Displace Genetic Diversity?
What happens when a characteristic controlled by a single gene, such as transgenic, Bt-based insect resistance, is introduced into the genetic background of an established variety?
Current knowledge and theory in maize genetics suggest that there should be little impact on genetic diversity. Most genes in maize are independent, meaning that they will diffuse independently through a maize population rather than remain linked to other genes in that population. Suppose a modern yellow-grained variety carrying a transgene, such as Bt, is planted in a field in Mexico with a traditional white-grained landrace. After a few generations, there would be plants with yellow grain and the transgene, white grain and the transgene, yellow grain and no transgene, and white grain and no transgene.
Although the gene would have introgressed into some plants, diversity would not decrease. In fact, one could argue that overall genetic diversity would increase. Whether this increased diversity is desirable is a very different issue.
What Could
Happen in Real Maize Fields?
What actually happens in maize fields in Oaxaca and other Mexican states? It is critical to remember that maize varieties are subject both to environmental selection and human management practices, which greatly influence whether a gene (and trait) is lost or fixed and at what frequency it occurs.
Tracking the effects of environmental selection is relatively straightforward compared to assessing the impact of farmers' management practices. If a transgene confers a trait that works against a plant's survival, plants carrying that gene will be eliminated from the gene pool through natural selection. If no environmental selection pressure acts on the gene, population genetics models indicate that the gene will be fixed at the frequency at which it was introduced, or it will be lost over time. Finally, if the gene confers a selective advantage, it will increase and spread through the population. Again, since the transgenic maize varieties now being commercially grown use single-gene traits, in none of these cases should overall genetic diversity be decreased. There are implications, however, for the rate of diffusion (or conversely, containment) of transgenes.
Perhaps the most influential and least understood influence on genetic diversity and the "maintenance" of landraces is farmers' management practices, particularly the practices farmers use to choose seed for planting. The ancestors of today's Oaxacan farmers, who developed maize from a weedy grass to a robust food crop, probably used these practices, which encourage the flow of genes among different varieties of maize. If today's smallholders had access to transgenic varieties, and if they perceived those varieties to be valuable, they might foster their diffusion into their local maize populations. Clearly this is a complex process that merits much research.
What Could Happen to
Wild Relatives of Maize?
Finally, there is the question of potential impacts on the wild relatives of maize, Tripsacum and teosinte. It is very difficult to produce maize x Tripsacum hybrids, although CIMMYT has produced some using sophisticated laboratory techniques. The only known naturally occurring maize x Tripsacum hybrid is "Guatemala grass," a vigorous but sterile forage that can be propagated only vegetatively.
Mexican annual teosintes are the closest relatives of maize. Maize genes can flow easily into teosinte, but the long history of maize and teosinte sharing the same fields in Mesoamerica has not produced a "swamping" of the teosinte by maize, suggesting that some genetic mechanisms may be at work to maintain the genetic integrity of teosinte.
Given the difficulty of creating maize x Tripsacum hybrids, it seems extremely unlikely that transgenes would introgress into the Tripsacum genus. Introgression into teosinte would be much more likely, and the same principles related to natural and farmer selection cited earlier should apply. In short, one would not expect to see a negative impact on diversity per se, but only limited research has been conducted to date on this aspect of gene flow.
Validating the
Hypotheses
This brief look at some of the underlying issues related to transgenes and Mexican landraces has focused mostly on potential impacts on genetic diversity. The observations are drawn from basic models and will need to be validated through targeted experiments. Clearly the potential impacts of an introgression of a transgene would also extend to the environment, farmers' welfare, marketplace concerns such as consumer acceptance, intellectual property considerations, and the regulatory sphere. These issues should be taken up in appropriate fora.
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For more information: Mauricio Bellon (m.bellon@cgiar.org) Julien Berthaud (j.berthaud@cgiar.org) David Hoisington (d.hoisington@cgiar.org) Masa Iwanaga (m.iwanaga@cgiar.org) Suketoshi Taba (s.taba@cgiar.org) |
August, 2004