Contents - Faperta

Development of Resistance to Transgenic Plants 385
Larval survival (%)
120
100
80
60
40
20
0
30 Days 50 Days 70 Days 90 Days 110 Days
Mech 12 Mech 162 Mech 184
Non Mech 12 Non Mech 162 Non Mech 184
FIGURE 12.3 Survival of neonate larvae of Helicoverpa armigera on the leaves of transgenic and nontransgenic
cotton hybrids during the crop growing season.
• Expression of the transgene varies across different stages of plant growth, and
generally declines towards the end of the crop-growing season (Figure 12.3). The
variation in expression of the insecticidal protein in transgenic rape is presumed
to be associated with the changes in growing conditions or with integration of the
transgene at different loci (Lin et al., 2001).
High Level of Transgene Expression
Expression of toxins at very high levels is one of the strategies to slow the adaptation by
insects to a toxin and prevent/delay the evolution of biotypes capable of surviving on the
transgenic crops. This strategy can be used effectively if the ecology and genetics of the insect
and cropping system fi t specifi c assumptions (Gould, 1994). These assumptions relate to:
• Pattern of inheritance of resistance;
• Ecological costs of resistance;
• Behavioral responses of larvae and adults to the toxins;
• Movement of larvae, adult dispersal, and mating behavior; and
• Distribution of host plants that do and do not produce the toxin(s).
The U.S. Environmental Protection Agency (EPA, 1998) concluded that for a high-dose
strategy, the Bt transgenic cultivar should produce 25 times the amount of Bt toxin needed
to kill 99% of the susceptible insects. A large number of plants in the fi eld should be surveyed
to make sure that the Bt toxin expression in the transgenic cultivar is at the LD 99 or
higher level. High levels of expression of cry1C protects transgenic broccoli not only from
susceptible or cry1Ab-resistant diamondback moth larvae, but also from those selected for
moderate levels of resistance to Cry1C (Cao et al., 1999; Liu et al., 2000). High levels of
expression of cryV in Phthorimaea operculella (Zeller)-resistant potato lines resulted in
greater mortality of this pest. Transformed tobacco leaves expressing cry2Aa2 protoxin at
2 to 3% of the total soluble protein (20- to 30-fold higher) are highly effective against the
resistant populations of H. zea, H. virescens, and S. exigua (De Cosa et al., 2001).