Contents - Faperta

328 Biotechnological Approaches for Pest Management and Ecological Sustainability
crops can contribute to increased yields and agricultural growth in such situations.
Adopting transgenic crops offers the additional advantage of controlling insect pests that
have become resistant to commonly used insecticides (Sharma et al., 2003). For rational
pest management, transgenic cultivars have to be deployed in combination with low
dosages of insecticides (Schell, 1997), and other methods of pest control.
Developments in plant biotechnology offer both promises and challenges. Close proximity
of transgenic crops to the sprayed fi elds of nontransgenic crops may result in greater
damage in transgenic crops because of insect migration from sprayed fi elds to the transgenic
crops, and the resultant increase in pest pressure may reduce the benefi ts of transgenic
crops. Toxins from Bt have been widely used as “natural” insecticides for many
years. However, an increase in the presence of Bt toxins in the ecosystem (via transgenic
crops) may signifi cantly increase the pressure on insect populations to evolve into resistant
biotypes. The evidence on these issues is still inconclusive, and there is a need for careful
monitoring before the transgenic crops are deployed on a large scale (Sharma and Ortiz,
2000; Sharma et al., 2002b). One of the approaches to overcome these problems is to develop
a new generation of transgenic crops with better genes, and use combinations of genes to
delay the development of resistance in insect populations. Transgenic crops at times fail to
provide adequate control of insect pests due to environmental infl uences on gene expression
or development of resistance, ineffectiveness against secondary pests, and adverse
effects on nontarget organisms. The major limitations of using transgenic plants are:
• Secondary pests are not controlled in the absence of sprays for the major pests.
• The need to control the secondary pests through chemical sprays will kill the
natural enemies and thus offset one of the advantages of transgenics.
• Proximity to sprayed fi elds and insect migration may reduce the benefi ts of
transgenics.
• Development of resistance in insect populations may limit the usefulness of
transgenics.
• Effects on nontarget organisms.
• Gene escape into the environment.
• Social and ethical issues.
Stability of Transgene Expression
Stable expression of the transgene is an important component for commercial deployment
and success of transgenic crops. Impairment of transgene expression may occur due to
mutations, deletions, and DNA rearrangement. In general, transgenes are not predisposed
for mutations and recombinations, and are known to occur at a frequency of 10 4 to 10 6 as
in endogenous genes (Peterhans et al., 1990). Reduced expression can occur via cis- or transacting
regulatory elements present in the environment of the transgene. Changes in transgene
expression occur at the transcriptional and posttranscriptional level that affect the
stability, processing, or transport of m-RNA, and the resultant proteins. Homology in DNA
sequence can occur due to multiple homologous copies (multiple direct or inverted repeats)
and endogenous genes sharing sequences with the transgene. Inactivation can be effected
in the cis- (same locus) or trans- (allelic positions) modes. Transgenes containing multiple,
complete, or partial repeats at one locus may be silenced. In many cases, silencing does not
occur in the primary transformants, but in the subsequent generations (Kilby, Leyser, and