Contents - Faperta

384 Biotechnological Approaches for Pest Management and Ecological Sustainability
sunfl ower moth, Homoeosoma electellum (Hulst.) populations (Brewer, 1991). Ives and Andow
(2002) suggested that at least three processes are involved in explaining the effectiveness
of the high-dose/refuge strategy, which depends on: (1) the intensity of selection, (2) assortative
(nonrandom) mating due to spatial subdivision, and (3) variation in male mating
success due to spatial subdivision. Understanding these processes will lead to a greater
range of possible resistance management tactics. For example, efforts to encourage adults
to leave their natal fi elds may have the unwanted effect of speeding rather than slowing
resistance evolution. Furthermore, when Bt maize causes high mortality of susceptible
target pests, spraying insecticides in refuges to reduce pest populations may not greatly
disrupt resistance management. The following tactics have been accepted for resistance
management in Bt transgenic crops.
• Stability of transgene expression
• High level of transgene expression
• Gene pyramiding
• Planting refuge crops
• Destruction of carryover population
• Control of alternate hosts
• Use of planting window
• Following crop rotations
• Use of pesticide application based on ETLs
• Use of IPM from the beginning
Stable Transgene Expression
For effi cient pest control, it is important to achieve optimum and stable expression of the
transgene that exercises a constant pressure on the pest populations. Selective removal
of nucleotides has been used to improve transgene expression (F.J. Perlak et al., 1991).
Transgene expression is also infl uenced by the region in which the transgene integrates in
different genotypes. Breeding programs should take cognizance of this fact and design
screening procedures to reject plants with unstable transgene expression. Based on the
information available on molecular mechanisms of transgene silencing, a few precautions
need to be observed in genetic transformation of plants to minimize transgene silencing in
the transformants. These include:
• The transformation vector should not contain duplicated sequences that might
serve as initiation sites for de novo methylation.
• Different genes in the same construct should not contain identical promoters or
termination signals, and the transgene should be codon optimized to match the
composition of the host genome.
• Bacterial vector sequences should not fi nd a place within the segment that will be
integrated into the plant genome.
• If the insert contains two transgenes, the promoters should read in different directions.
If the insert contains more than two genes, all the promoters should read in
the same direction, and should be separated by appropriate stuffer elements to
prevent read through.