A loss of genetic diversity of domesticated and wild relatives is cited among the potential drawbacks of the introduction of transgenic crops. Whether this actually is the case depends on various biological and human parameters. In this chapter we present these parameters and describe their effects in the particular case of maize in Mexico. An effect on genetic diversity, if any, will require two essential conditions. Firstly, gene flow has to take place between transgenic and non-transgenic populations. Gene flow is defined here as the transfer of genes from one population or locality to another. Gene flow can take place either via pollen or seeds.1 For example, seeds of a transgenic variety can be planted in a field mixed with seeds of non-transgenic varieties. Alternatively, fields of transgenic and non-transgenic (presumably traditional varieties) are planted within close proximity (several meters to 100s of meters) of each other. Hybridization between these plants (i.e. crossing by pollen transfer) can lead to hybrid seeds, which, if they are part of the seed stock saved by the farmer, can engage in another hybridization cycle in the next season, and so on. Secondly, gene flow has to be of such magnitude as to displace the native diversity. Figure 1 presents a sketch of a local system in which gene flow may take place between a transgenic and a traditional crop. It shows also the parameters that influence the direction and magnitude of this gene flow. In this chapter, we discuss several of these parameters as they may affect gene flow in Mexican maize, the potential role of transgenes in affecting gene flow and its effect on genetic diversity, the available data on the presence of transgene in Mexican maize landraces, and possible solutions to maintaining maize landrace diversity. |
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