Contents - Faperta

Transgenic Resistance to Insects: Gene Flow 419
for development of resistance in the pest populations also need to be assessed, and strategies
devised to use different genes in different crops in the same environment. Transgenic
crops containing protease inhibitors may pose similar problems, and their use needs to be
carefully planned to avoid the evolution of pest populations capable of withstanding
the transgene. Resistance to abiotic stress factors may present additional challenges, as
this would enable the plants to grow in environments where they were unable to do well
earlier (Fraley, 1992). This confers additional advantage to the transgenic plant, and there
are chances for gene transfer through cross-pollination. The exotic species model can be
used to assess the risk of introducing transgenic plants with resistance to abiotic stress
factors. The risk assessment in such cases requires more information, and should take
into account the nature of competitive advantage conferred by the transgene under
specifi c conditions.
Gene Flow and Enhanced Fitness of Herbivores
Widespread cultivation of transgenic crops with resistance to insect pests and diseases
will impose intense selection pressure on pest populations to adapt to the transgene.
Development of insect pest and disease-resistant cultivars has been one of the primary
objectives of plant breeding for many years (Simmonds et al., 1999). The history of plant
breeding has clearly established that insect pest and pathogen populations can quickly
adapt to crop cultivars with new resistance genes (Bonman, Khush, and Nelson, 1992;
McIntosh and Brown, 1997). However, crop improvement is an ongoing process, and plant
breeders have not stopped breeding for insect pest and disease resistance simply because
the target pest or disease might overcome the resistance. Development of management
strategies to minimize the evolution of insect and pathogen populations that might overcome
the resistance genes has been ongoing for many years. In order to establish an
appropriate resistance management plan, it is important to understand the nature of interactions
between the host crop, the insect pest/pathogen population(s), and the mechanisms
involved. The key to maintaining an effective management plan is regular monitoring of
the response of the insect pest/pathogen populations to the cultivars grown by the farmers.
Although many resistance genes have been identifi ed in crop germplasm, there is no
easy way to predict the quality or durability of these resistance genes. The “breakdown”
of resistance is usually associated with qualitative resistance conferred by major genes
(R genes), where resistance versus susceptibility results from a gene-for-gene interaction
between the R genes in the host and avirulence genes in the pathogen (Flor, 1971). The
resistance conferred by many R genes has not been durable as a consequence of rapid
changes in pathogen populations (Leach et al., 2001). The most widely cited examples of
durable resistance against bacterial or fungal pathogens involve multigenic quantitative
traits (Johnson, 1984; Parlevliet, 2002). However, there are examples where single R genes
have conferred highly durable resistance, for example, the Lr34 gene conferring resistance
to leaf rust in wheat (Kolmer, 1996) and the Xa4 gene conferring bacterial blight resistance
in rice (Bonman, Khush, and Nelson, 1992). The experience gained from plant breeding
over many years will help defi ne the appropriate management approaches for insect pest-
and disease-resistant transgenic crops to prevent or minimize the establishment of insect
pests and pathogen populations that may overcome the resistance mechanism underpinning
the resistance genes. It is not the transgenic versus nontransgenic status of the
crop plants that may result in projected problems, but the way the crops are grown and
managed will have a greater infl uence. The widespread cultivation of genetically modifi ed
crops with insect or disease resistance is no more likely to result in the development of