Insect Control: Biological and Synthetic Agents - Index of

444 12: Insect Transformation for Use in Control
insertion sites relative to the donor site is also an
important factor in determining the ability of the
transposable element to spread. A highly active
element that moves only, or even predominately, to
tightly linked sites would not be a suitable agent to
spread genes through an insect population, while
an element with a reduced transposition rate that
moved to unlinked loci either on the same or different
chromosomes would be a viable spreading
agent.
As mentioned above (see Section 12.1), at present
little or no information exists as to the mobility
properties of the four transposable elements used
to genetically transform nondrosophilid insect species.
Indeed, even for the relatively well-characterized
P element of D. melanogaster, little is known about
its mode of movement within and between chromosomes.
This element does show a tendency to insert in
or near the 5 0 ends of genes, perhaps due to a relaxation
of the DNA double helix during gene transcription.
It has also been suggested that this element
recognizes a structural feature at the insertion site
rather than a strict canonical motif (Liao et al.,
2000). This may well be true of other transposable
elements and may well be an important factor in
determining transposable element spread, but this
remains an underexplored area of research.
12.2.3. Engineering of Beneficial Insects
Progress in this area has been limited to the stable
introduction of genes, using the piggyBac transposable
element, into the silkworm, Bombyx mori
(Tamurua et al., 2000). Initially these experiments
have been proof of principle experiments in which
enchanced green fluorescent protein (EGFP) was
used as a genetic marker to demonstrate that transformation
could be achieved. Recently, Imamura
et al. (2003) demonstrated that the GAL4/UAS
system functions sufficiently in transgenic B. mori
to enable tissue-specific expression of a reporter
gene to occur. Transformation frequencies using
the piggyBac transposable element as the gene vector
were in the order of several percent. These
experiments pave the way for gene identification
using enhancer trapping in Bombyx which will further
elevate the use of this species as a model system
for other lepidopteran species. The extension of
these techniques into practical benefits of silk production
remains a challenge. While there have been
reports of sperm-mediated transformation of the
honeybee, Apis mellifera (Robinson et al., 2000),
this technology has not yet been exploited by the
honey industry and, since honey is a food, genetic
engineering of its source may encounter public
resistance. Similarly, the initial report of genetic
transformation of the predatory mite Metaseiulus
occidentalis, which is used as a biological control
agent, has not been pursued in field applications
(Presnail and Hoy, 1992).
12.3. Conclusion
Genetic transformation technologies have been successfully
extended into selected nondrosophilid
species using transposable elements. This significant
and exciting success has, however, being confined to
the laboratory where it has enabled novel genotypes
to be constructed and tested. These technologies
have yet to be extended to the field, despite many
years elapsing since genetic transformation protocols
were first established for key pest species such
as C. capitata and A. aegypti. If the full potential
and benefits of genetic modification of medically
and agriculturally significant insect species is to be
realized then there must be a stronger linkage between
the formulation of ideas and the subsequent
timely and safe testing of these in transgenic insect
strains in the laboratory, and in the field. Key to this
is improving the robustness of transgenic technology
in these insect species. Alternatively, the wisdom
of establishing a handful of insect transformation
centers that would provide this service to the community
needs to be explored. This may be particularly
attractive for species, such as A. gambiae, that
remain difficult to transform. Providing a central
transformation center may encourage researchers
to develop and test new concepts, confident that at
least the transgenic insects containing the desired
transgenes will be routinely produced in a timely
manner. It is critical to demonstrate in the laboratory
and then in field cage experiments clear and concrete
examples of how transgenic insect technology is
beneficial to the general public so that arguments
about the benefits of these new approaches can be
clearly made to this interested and undoubtedly
concerned audience.
References
Ashburner, M., Hoy, M.A., Peloquin, J.J., 1998. Prospects
for the genetic transformation of arthropods. Insect
Mol. Biol. 7, 201–213.
Atkinson, P.W., 2002. Genetic engineering in insects of
agricultural importance. Insect Biochem. Mol. Biol.
32, 1237–1242.
Billingsley, P.F., 2003. Environmental constraints on the
physiology of transgenic mosquitoes. In: Takken, W.,
Scott, T.W. (Eds.), Ecological Aspects for Application of