Biology Reference
In-Depth Information
Potential for Exchange of Transgenes between Crops
and Crop Weed Races
In many cases, hybridization can occur between transgenic crops and
weed races of the same species. Such hybridizations are usually very fre-
quent and involve little or no reduction in offspring vigor. Cultivated
sunflower (
Helianthus annuus
) is one such example.Wild sunflowers occur
throughout the United States and Mexico and in some states are desig-
nated as a noxious weed because of their influence on cultivated sun-
flower. Hybridization between crop and wild sunflowers can occur over
distances as great as 1 km, and little or no reduction in fecundity is seen
in hybrids (Arias and Rieseberg 1994). Crop-specific genetic markers
spread to wild sunflower populations have been shown to persist for at
least five generations (Whitton et al. 1997). Gene flow from crop sunflow-
ers to adjacent wild populations is so extensive that the wild populations
actually consist of plants derived from several generations of hybridization
and backcrossing (Linder et al. 1998).
In addition, one of the most widely used transgenes, the
Bacillus
thuringiensis
(Bt) endotoxin gene, not only has no fitness cost to wild sun-
flower plants, but increases their seed production, as well (Snow et al. 2003).
In field situations, transgenic wild sunflowers produced 55% more seeds in
Nebraska and 14% more seeds in Colorado than wild sunflowers lacking
the Bt gene. Thus, genes introduced into crop sunflower for traits such as
insect resistance could easily become established in wild sunflower popula-
tions and are almost certain to confer increased fitness on wild plants.
A second example of hybridization between crops and weed races of
the same species involves radish (
Raphanus sativus
). Radishes are primarily
outcrossing plants pollinated by insects. Klinger et al. (1991) examined
gene flow from crop radish to weed radish planted at distances of 1-1,000
m from crop radish stands in California. A genetic marker in the crop
plants that could be recognized by allozyme analysis permitted the recog-
nition of hybrid offspring. For weed radish planted within 1 m of crop
radishes, hybrids ranged from 14 to 100% of all offspring. Hybrid off-
spring were also noted in the weed plantings 1,000 m from crop radish
stands. In addition, the hybrids themselves showed increased fitness, pro-
ducing more seed than wild plants (Klinger and Ellstrand 1994).The vari-
ability in incidence of hybridization with distance from crop plantings
was very great, so the average distance at which hybridization was noted
was a very poor estimator of the risk of transgene escape (Klinger 2002).
