Contents - Faperta

Development of Resistance to Transgenic Plants 379
gene by retrotransposon-mediated insertion was linked to high levels of resistance to the
Bt toxin Cry1Ac in H. virescens.
In the susceptible strain of P. gossypiella, Cry1Aa, Cry1Ab, Cry1Ac, and Cry1Ja bound to
a common binding site that was not shared by the other toxins tested (Gonzalez et al.,
2003). Reciprocal competition experiments with Cry1Ab, Cry1Ac, and Cry1Ja showed that
these toxins do not bind to any additional binding sites. In the resistant strain, binding of
125I-Cry1Ac was not signifi cantly affected. However, 125I-Cry1Ab did not bind to the
BBMV. The results indicated that resistance fi ts the “mode 1” pattern of resistance described
previously in P. xylostella, P. interpunctella, and H. virescens. Griffi tts et al. (2001) reported
the cloning of a Bt toxin resistance gene, Caenorhabditis elegans bre-5, which encodes a putative
beta-1,3-galactosyltransferase. The lack of bre-5 in the intestine led to development of
resistance to Bt toxin Cry5B.
Feeding Behavior
Feeding behavior, survival, and development of insects on transgenic crops will have a
major bearing on development of resistance to transgenic crops. Gore et al. (2002) observed
a change in behavior of H. zea larvae infesting Bt cotton (Bollgard ® , NuCOTN33B). More
larvae moved from the terminal parts of Bt cotton than in non-Bt cotton within one hour
of infestation. Greater infestation was recorded on white fl owers and small bolls, necessitating
scouting of these plant parts in addition to terminals and squares. The fi rst- and
second-instar larvae damage squares and fl owers, but mostly feed on cotton bolls from
third-instar onwards, and there were no differences in feeding behavior on transgenic and
nontransgenic cotton (Wu, Guo, and Wang, 2000). However, Zhao et al. (2000b) suggested
that fl owers of Bt cotton were more preferred by the third- and fourth-instars, since they
did not express high doses of toxin. The H. armigera larvae have the ability to compensate
for food intake, digestibility, and utilization of transgenic crops (Gujar, Kalia, and Kumari,
2001). Neonates of H. armigera have the ability to detect and avoid transgenic Bt cotton
Zhong 30 and transgenic CpTI-Bt cotton SGK 321, as compared to the nontransgenic cotton
Shiyuan 321 (Zhang et al., 2004). The larvae consumed more food on CpTI-Bt transgenic
cotton than on Bt transgenic cotton, possibly to compensate for reduced nutritional quality
of the food. Fourth-instars were found in equal numbers on transgenic and nontransgenic
cottons, but food consumption on transgenic cotton was lower than on the nontransgenic
cotton. In no-choice tests involving fi fth-instars, signifi cantly less time was spent in feeding
on the two transgenic cottons. The neonates selectively feed on the nontransgenic cotton
or the preferred plant parts. Diamondback moth, P. xylostella, resistant to Bt toxins may be
able to use Cry1Ac as a supplementary food protein (Sayyed, Cerda, and Wright, 2003).
Bt transgenic crops could therefore have unanticipated nutritionally favorable effects,
increasing the fi tness of resistant populations, and resulting in evolution of resistance to
Bt transgenic crops.
Cross Resistance
Akhurst and Liao (1996) studied the susceptibility of H. armigera to Cry1Ab, Cry1Ac,
Cry2Aa, and Cry2Ab. The Cry1Ac selected insects with 188-fold resistance (in the 18th
generation) showed 69-fold resistance to Cry1Ac of MVP formulation, and fi ve-fold resistance
to Dipel ® that contains Cry1Ab, Cry1Ac, and Cry2Aa. The Cry1Ac selected insects
with 82-fold resistance (in the 28th generation) showed as high as 157-fold resistance to
Cry1Ab. Cry1Ac selected insects with 204-fold resistance (in the 19th generation) showed