Contents - Faperta

Transgenic Resistance to Insects: Gene Flow 415
Sunflower
Wild sunfl ower occurs in disturbed habitats such as roadsides and agricultural areas, often
reaching heights of 2 to 3 m in optimal growing conditions. In contrast to wild populations,
commercial varieties lack several fi tness-related traits such as branching, extended fl owering
period, self-incompatibility, and seed dormancy, and often exhibit greater resistance to
diseases (Seiler, 1992). Isozyme and molecular studies indicated that gene fl ow among
populations has been extensive (Rieseberg and Seiler, 1990; Arias and Rieseberg, 1995),
and populations near commercially grown sunfl ower harbor crop-specifi c DNA markers
due to crop-wild hybridization. The marginal wild populations (3 m from the cultivar)
showed the highest percentage (27%) of gene fl ow (Arias and Rieseberg, 1994). Cultivated
sunfl ower hybridizes spontaneously with wild/weedy populations. F1 wild-crop hybrids
had lower fi tness than wild progeny, especially when grown under favorable conditions,
but the F1 barrier to the introgression of crop genes into wild populations is quite “permeable.”
Therefore, episodes of crop-to-wild hybridization are likely to lead to long-term
persistence of nondeleterious crop genes in wild populations.
Vegetables
Most domesticated species are associated with a closely related wild or weedy species,
for example, Cucurbita pepo (L.) with C. texana (Scheele) Decker, with which they are fully
compatible, indicating the possibility of introgressive hybridization. Evidence for this is
based on genetic homogeneity [as shown by electrophoretic data from C. pepo and its associated
forms C. texana and C. fraterna (Bailey) Andres] and introgressive hybridization
(Wilson, 1990).
Beets
Weed beets pose a serious problem for sugarbeet, Beta vulgaris var. saccharifera Alef.
(Desplanque, Hautekeete, and van Dijk, 2002). Spreading of transgenic traits into wild beet
can occur if genetically engineered biennial plants survive in the winter, fl ower in spring,
and spread their pollen (Pohl Orf et al., 1999). Survival of sugar beet, transgenic as well as
conventional beets, in Germany and along the Dutch border is possible. Survival rates
were well correlated with temperature. Differences between sugar beet hybrids and breeding
lines have been detected, but not within different breeding lines or hybrids. There
were no detectable differences between transgenic and nontransgenic plants. Traditionally,
the only effi cient method of weed control has been manual removal, but the introduction
of transgenic herbicide- tolerant sugarbeets may provide an alternative solution because
nontolerant weed beets can be destroyed by herbicide. In northern France, weed beets are
present in variable densities in sugarbeet fi elds of up to 80 weed beet plants m 2 . Diploid
F 1 crop-wild hybrids and triploid variety bolters (individuals with a low vernalization
requirement) have been found in low densities in virtually all sugarbeet fi elds. Gene fl ow
is possible between all forms, as illustrated by overlapping fl owering periods in the fi eld
and successful controlled cross-pollination. The F 1 wild hybrids result from pollination in
the seed-production region by wild plants possessing the dominant bolting allele B for
fl owering without experiencing a period of cold. In the case of a transgene for herbicide
tolerance incorporated into male-sterile seed-bearer plants, such hybrids will contain both
the herbicide tolerance and the bolting allele. Appearance of transgenic weed beets is
possible, but can be retarded if the transgene for herbicide tolerance is incorporated into
the tetraploid pollinator breeding line.