Contents - Faperta

Transgenic Resistance to Insects: Nature of Risk and Risk Management 435
given harm is not zero, for worst-case scenarios, the probability of occurrence could be taken
as one and the focus should be on the potential detrimental consequences of the event.
One of the concerns related to transgenic plants is the potential environmental harm if
these organisms escape or are released into the environment. Harm can take many different
forms from transient to permanent in time and space, and from local to global in scope.
To defi ne harm, it is necessary to distinguish between the terms risk and hazard, which
are often confused. Risk is the probability that a transgene will spread into natural populations
once released, and hazard is the probability of species extinction, displacement, or
ecosystem disruption given that the transgene will spread into the population (Muir and
Howard, 2001a). Long-term hazards to the ecosystem are diffi cult to predict because not
all nontarget organisms may be identifi ed, and the species can evolve in response to the
hazard. An infi nite number of direct and indirect biotic interactions can occur in nature.
The only way to ensure that there is no harm to the environment is to release only those
transgenic organisms whose fi tness is such that the transgene will not spread, in which
case the hazard, becomes irrelevant because the transgene is lost from the population.
Hazard to the Environment
Hazards posed by genetically modifi ed plants include the tendency of a self-pollinated
crop to outcross because of self-sterility or any other factors. Or a plant may have a tendency
to become a weed, yield toxic substances in the product, or change in the toxin(s) produced
by the plant. Any of these attributes may pose a risk to people consuming the product,
working with it, or to the environment. In the case of toxin genes inserted into crop plants,
the range of the expressed toxin may be much wider than expected, with adverse consequences
for nontarget organisms and the environment. Plants may also display a change
in appearance, reaction to other biotic and abiotic stress factors, or the end use characteristics.
However, it is not easy to measure weediness, as such characteristics are not easily
defi ned. The ecological consequences in most cases are only qualitative and, therefore,
risk assessment for genetically modifi ed plants requires a detailed assessment of the modifi
ed plant in comparison to the plant from which it has been derived. The procedures
adopted should also take cognizance of the environment where the plant is to be released.
Pleiotropic effects of a transgene that have antagonistic effects on net fi tness can result in
unexpected hazards, such as local extinction of the species containing the transgene (Muir
and Howard, 2001b). If a transgene enhances mating success while reducing juvenile viability,
the least fi t individuals obtain the majority of the matings, while the resulting transgenic
offspring have low probability of survival, resulting in a gradual spiraling down of
population size until both wild-type and transgenic genotypes become extinct (Hedrick,
2001). Transgenes for pest resistance or stress tolerance can increase offspring viability and
can also reduce male fertility (Rahman and Maclean, 1999). Extinction hazards in such
cases are similar to the use of sterile males to eradicate insect pests. However, in the sterile
insect technique, the sterile males have to be introduced repeatedly to cause extinction.
Antagonistic pleiotropic effects of transgenes on viability and fertility represent a new
class of “Trojan Genes,” which persist in the environment. Attempts to reduce transgenic
male fertility that do not result in complete male sterility may increase hazard rather than
reduce it. If any of the net fi tness components are improved by the transgene, while having
no adverse side effects, the transgene will invade a population (Muir and Howard, 2001a).
However, advantages in one fi tness component can offset disadvantages in another and
may still increase invasion risk.