Contents - Faperta

Transgenic Resistance to Insects: Gene Flow 421
nontransgenic cultivars is unavoidable, except by fully prohibiting the cultivation of
transgenic crops. A standard of zero adventitious content in a commercial seed line is
unachievable, irrespective of whether the infl ux is from nontransgenic or transgenic
cultivars. Improvements in seed production to minimize the incidence of adventitious
seed content are likely to be at least matched by enhanced sensitivity and precision of the
modern diagnostic tests. While the use of stringent management approaches will minimize
the opportunity for inadvertent admixtures of transgenic with nontransgenic seed, the
issue of transgene fl ow via pollen dispersal presents a more diffi cult problem. Restricting
the movement of pollen between crops is not a new concern, and forms an important basis
of seed quality control.
The production of certifi ed seed of specifi c cultivars requires the maintenance of minimum
isolation distances. There are internationally recognized isolation distances that vary
depending on the crop, and its reproductive characteristics (Briggs and Knowles, 1967;
Simmonds et al., 1999). There may be particular uses of transgenic crops where additional
care may be required, for example, in pharmaceuticals, vaccines, biodegradable plastics,
or speciality biochemicals. Such products should not be mixed with normal food crops.
The environmental release of such transgenic crops will require more stringent levels of
containment for keeping these products out of the food chain. To prevent inadvertent
admixture of such cultivars with those intended for food use presents special challenges.
The economic feasibility and success of molecular farming approaches will depend to a
large extent on the ability to meet these challenges. Recent advances in the genetic engineering
of chloroplasts can be used to limit gene escape through pollen (Daniell et al., 1998).
Gene Flow and the Centers of Genetic Diversity
There is no evidence that a transgene could alter the genetic structure of a crop. Currently
grown transgenic commercial varieties will certainly have a positive effect on agronomic
productivity. However, it is important to develop technology aimed at solving specifi c and
pressing socioeconomic problems. Genetic diversity of crops and their wild relatives is
and will always be susceptible to displacement by high yielding varieties, whether transgenic
or conventional. Therefore, in situ conservation methods must be implemented to
maintain genetic diversity. It is important to reinforce, characterize, and evaluate the present
germplasm collections of landraces, and wild relatives of crops. Information on the
impact of transgenic crops on biodiversity is limited or nonexistent. Therefore, it becomes
necessary to establish sound monitoring methods, develop basic ecological knowledge of
wild relatives of crops, and thoroughly study what has been the effect of hybrids on the
landraces and wild relatives of crops, for example, hybrid corn has been grown in Mexico
over the past 50 years, and some of the landraces have been replaced in favor of high yielding
hybrids. As a result of hybrid production, the identity of landraces has not been lost.
The growers continue to select phenotypes that are preferred by the customers or for their
own use. Monitoring is underway to determine the presence of transgenes in landraces
and test the stability of the transgene in the environment. However, information to date
has suggested that: (1) the transgene does not alter the structure of the maize gene, (2) there
is no transfer of the transgene to other plants, and (3) there is no effect on human health.
There is a better chance of countering the gene fl ow if the transgene is deployed through
hybrids based on male sterility.