Insect Control: Biological and Synthetic Agents - Index of

442 12: Insect Transformation for Use in Control
proportion of the females mate more than once
so the net effect on Ro is a function of sperm competitiveness
in multiple-mated females. Successive
releases in multiple generations continues to drive
R o down until either the population size is below its
economic threshold (at which point it does not
impact upon the economy of the relevant commodity)
or until it is eradicated. The advantages of using
this method of control have been discussed elsewhere
(Hendrichs, 2000) but clearly rest on the
species specificity of the method and its low environmental
impact. Nonetheless implementing this
‘‘clean’’ control method faces some constraints
including fitness costs associated with the development
of specific strains and the effects of mass
rearing, radiation, handling, and release on many
aspects of field behavior (Robinson and Franz,
2000). Transgenic technology has the potential to
positively impact existing SIT programs by improving
aspects of insect production and monitoring and
also has the potential to increase the use of SIT to
include species that have otherwise been intractable
to this technology.
Perhaps the biggest impact transgenic insect technology
could have on any SIT program is to create
strains of insects that permit the removal or elimination
of females. The most significant effect that
this would have on an SIT program is the reduction
of the costs of mass-rearing and release because
fewer insects need to be reared and released. In
other programs, for example those that might
involve vectors of human disease, removal of females
prior to release may be essential since females would
still be capable of biting and transmitting disease.
Transgenic technology offers numerous options for
creating strains that permit the selective removal of
females prior to release. Conceptually the problem is
one of expressing genes in one sex or the other that
can be easily selected for or against. Using transgenic
technology specificity of expression can be achieved
by a variety of ways. First, expression can be controlled
through the use of sex-specific promoters in
combination with genes resulting in lethality. Sexspecific
promoters from the vitellogenin and yolk
protein genes have been characterized, and are examples
of female-specific promoters that have been tested
in transgenic insects with respect to possible
applications in insect control programs (Heinrich
and Scott, 2000; Kokoza et al., 2000; Thomas et al.,
2000). These promoters are active in adult stages and
would have limited application in SIT programs.
They would not be useful for eliminating females at
the larval stage and so any cost savings resulting from
rearing both sexes would still be accrued. For eliminating
females prior to mass-rearing, the challenge
remains to identify promoters or other regulatory
sequences that selectively eliminate females during,
or immediately following, embryogenesis.
12.2.2. Challenges of Long-Term Gene
Introduction into Natural Populations
Transgenic strategies relying on load imposition to
reduce or eliminate a population involve the shortterm
introduction of ‘‘effector’’ genes (usually dominant
or conditionally dominant lethals) into the
gene pool of native populations. As described
above for the SIT, approaches relying on inundative
releases of nongenetically engineered mass-reared
insects carrying dominant lethal mutations have
been successful. More subtle approaches have been
proposed whereby deleterious genes can be transmitted
over the course of a few generations before
the load effects are encountered (Schliekelman and
Gould, 2000a, 2000b). The most ambitious plans
involve the permanent and stable alteration of the
genotypes of wild insects. These ideas present enormous
and novel challenges to insect geneticists.
Introducing new laboratory-produced genotypes
into populations is relatively simple (e.g., inundative
releases of radiation-sterilized flies), but maintaining
those genotypes in the population and, in fact, having
them increase in frequency is an unprecedented
undertaking in applied insect genetics.
An allele can increase in frequency in populations
for a variety of reasons. For example, if there is a
fitness advantage associated with a genotype then
over time this allele is expected to become more
abundant. If selection pressures are sufficiently
high then the forces of natural selection can result
in relatively rapid changes in allele frequencies. The
global spread of insecticide resistance in Culex mosquitoes
is one of many such examples of selection
driven increase in gene frequencies (Raymond,
1991). The fitness costs associated with transgenic
mosquitoes are largely unknown and is an area of
research in need of attention. A few reports on the
ability of transgenics to compete with wild conspecifics
or on fitness estimates based on life table
analyses, consistently indicate that the process of
transgenesis decreases the fitness of the host insect
(Catteruccia et al., 2003; Irvin et al., 2004). The
sources of these fitness costs have not been precisely
determined but are expected to be partitioned
between costs associated with inbreeding during
the process of creating a transgenic line of insects,
transgene expression, and mutagenesis associated
with transgene integration. It should be remembered,
however, that genetic sexing strains generated
through standard Mendelian genetics are less fit than