Contents - Faperta

Transgenic Resistance to Insects: Gene Flow 411
particular fi tness components of the weed (Ellstrand, Prentice, and Hancock, 1999).
However, plant breeders have released many cultivars with new genes for resistance to
insect pests, diseases, and environmental stress over many years. Any impacts resulting
from the introgression of such traits into weedy species are equally likely for the products
of plant breeding and genetic modifi cation. The risks are no different and the use of resistance
genes in cultivars from traditional breeding has not enhanced the survival and
spread of weeds in the past. When serious weeds have arisen following hybridization of
crops and wild species, their aggressive nature has arisen from a coupling of morphological
traits conferring weedy attributes and the synchrony of development with the crop
rather than a gain in fi tness from resistance genes (de Wet and Harlan, 1975).
Another problem highlighted by natural hybridization between crops and their wild
relatives is the increased potential of extinction of wild taxa. Some “genetically aggressive”
species, referred to as compilospecies (Harlan and de Wet, 1963), may completely
assimilate another locally rare species through repeated cycles of hybridization and introgression,
causing it to become extinct. The highly successful wheat crop is considered to
have assimilated genes from more than one species of Aegilops (Harlan and de Wet, 1963).
Extinction by hybridization does not depend on relative fi tness, but on patterns of mating
(Ellstrand, Prentice, and Hancock, 1999). The impact of the release of transgenic crops will
be no different than the impact of existing nontransgenic crops. Interspecifi c hybridization
is a common process, but hybrids are rare, and most are sterile, and there is a very low
chance of gene introgression into the wild relatives (Fitter, Perrins, and Williamson, 1990).
Transgenic plants may also become weeds, except in the context of their normal agricultural
environment. Gene escape may occur when a plant species invades a seminatural
habitat or the gene is transferred into the wild relative, and persists in uncultivated land.
Its spread can be checked by methods similar to any other single trait. There are differences
among plant species to disperse from the environment other than the one in which
they are released, and their ability to establish feral populations. Such an event has to be
compared with that of the original plant.
The process of introgression between a transgenic crop modifi ed for better agronomic
characters and a wild relative could potentially lead to increased weediness and adaptation
to the environment (Gueritaine et al., 2002). However, formation of a hybrid and hybrid
progeny could be associated with functional imbalance and low fi tness, which reduces the
risk of gene escape and establishment of the wild species in the fi eld. Inter- and intraspecifi
c gene fl ow between transgenic and nontransgenic plants leads to development of
new genetic materials in cultivated or wild populations. Introgression of novel genes into
wild relatives of crops may have a substantial impact on crop evolution or wild populations
leading either to more desirable cultivars, more aggressive weeds, and/or extinction
of rare and endangered species. A better understanding of gene fl ow is therefore essential
for deployment of transgenic plants. Such information would be helpful in assessing the
ecological risk of transgene escape from cultivated germplasm to closely related wild
relatives. The consequences of gene fl ow from crop to wild relatives include genetic assimilation,
wherein alien genes replace host genes, and demographic swamping, wherein
hybrids are less fertile than their wild parents and as a result the wild populations shrink
(Haygood, Ives, and Andow, 2003). Factors infl uencing gene fl ow include mating system,
mode of pollination, mode of seed dispersal, and the habitat characteristics where the
crops are grown (Messeguer, 2003). Hybridization is perhaps the most serious genetic
threat to endangered species, with extinction often taking place in less than fi ve generations
(Wolf, Takebayashi, and Rieseberg, 2000). Pollen can function as a vehicle to disseminate
introduced, genetically engineered genes throughout a plant population or into a