Gene flow is a potential concern associated with the use of transgenic crops because
it could affect genetic diversity of related landraces and wild relatives. This concern has
taken on added importance with the looming introduction of transgenic crops in centers
of crop domestication (Mexico, China) and those producing pharmaceutical compounds.
For gene flow to take place among cultivars and their wild relatives, several steps
have to be fulfilled, including the presence of cultivars or wild relatives within pollen
or seed dispersal range, the ability to produce viable and fertile hybrids, at least
partial overlap in flowering time, actual gene flow by pollen or seed, and the establishment
of crop genes in the domesticated or wild recipient populations. In contrast with
domestication genes, which often make crops less adapted to natural ecosystems, transgenes
frequently represent gains of function, which might release wild relatives from constraints
that limit their fitness. In most sexually reproducing organisms, the chromosomal region
affected by selection of a single gene amounts to a small percentage of the total
genome size. Because of gene flow, the level of genetic diversity present in the
domesticated gene pool becomes a crucial factor affecting the genetic diversity of the
wild gene pool. For some crops, such as cotton and maize, the introduction of transgenic
technologies has led to a consolidation of the seed industry and a reduction in the
diversity of the elite crop gene pool. Thus, diversity in improved varieties grown by
farmers needs to be monitored. Several areas deserve further study, such as the actual
magnitude of gene flow and its determinants in different agroecosystems, the long-term
effects of gene flow on genetic diversity both across gene pools and within genomes,
the expression of transgenes in new genetic backgrounds, and the effects of socio-economic
factors on genetic diversity.