Biology Reference
In-Depth Information
weeds that carry transgenes for various characteristics. So far, major crops
that carry genes for Bt endotoxins or resistance to herbicides do not
appear to be more invasive into natural communities than their conven-
tional varieties (Crawley et al. 2001). Most geneticists believe that species
that acquire extra genes will experience a fitness cost except in situations
in which the added gene exerts a strong beneficial effect (Tiedje et al.
1989).This suggests that escaping transgenes are not likely to benefit most
weed species in nonagricultural settings. Many studies, however, have not
shown such reductions in fitness (see, e.g., Snow et al. 1999).The persist-
ence of crop sunflower alleles in wild sunflower populations over many
generations suggests that genes that have little fitness cost can persist in
wild populations indefinitely (Whitton et al. 1997). Studies with
Arabidop-
sis thaliana
, discussed earlier, show that fitness of transgenic lines carrying
herbicide resistance is reduced considerably in herbicide-free settings
(Bergelson and Purrington 2002). Much more research is needed on this
question, and it appears likely that the answer will vary, depending on
many aspects of the ecological situation.
Transgenetic crop plants carrying herbicide resistance genes are
designed to tolerate the use of heavier applications of the herbicides in
question.Thus, although this use will immediately control certain weeds,
weedy plants not inhibited by the particular herbicide will gain greater
ecological and evolutionary potential to exploit what amounts to an
empty niche. Shifting patterns of weed infestation are likely to result.
More intense use of chemical herbicides will also select for increased
herbicide resistance in weedy plants. A worldwide survey of herbicide-
resistant weeds in 1995-96 revealed 183 resistant varieties of 124 weed
species (Heap 1997). Resistance has evolved to all major chemical groups
of herbicides, with many weeds showing multiple resistance patterns.
Although the number of new varieties showing herbicide resistance has
been roughly constant at about nine per year in recent years, major inten-
sification of herbicide use would likely speed the evolution of resistance.
Transgenes that escape from crop plants to weed races or wild relatives
of the crop may affect many nontarget organisms. The Bt endotoxin,
which has been introduced to a number of major crop plants, might affect
many nontarget species of lepidoptera that feed on crop relatives
(Letourneau et al. 2002). Information on the feeding habits of such
potential nontarget species is very incomplete, so the potential of such
impacts is very uncertain. Benefits of transgenic insecticidal crop varieties
may also be offset by declines in populations of natural enemies of crop
pests induced by the transgenic crop itself (Obrycki et al. 2001).
