Insect Control: Biological and Synthetic Agents - Index of

366 10: Genetically Modified Baculoviruses for Pest Insect Control
to consider as suggested by Hammock (1992) that
genetically modified baculoviruses are designed to
be biological insecticides and not biological control
agents that will become permanently established.
Thus, a recombinant baculovirus should by its very
design show reduced fitness in comparison to the
wild-type.
10.5.3. Movement of the Introduced Gene to
Another Organism
The insecticidal efficacy of natural baculoviruses is
dramatically improved by insertion of a foreign gene
into the genome or inactivation (deletion) of an
endogenous gene from the genome or a combination
of both. The insertion strategy generally involves the
insertion of an effector gene that encodes a protein
that is detrimental to the target insect, alters its life
cycle or stops it from feeding. The deletion strategy
generally involves the inactivation of an endogenous
gene (e.g., egt or orf603) by inserting another gene
into its coding sequence. This other gene can be a
marker gene such as lacZ or an effector gene as
described above. In both cases, the genes are placed
under a baculoviral or insect promoter. Several critical
points should be kept in mind with these strategies.
First, the genes are placed under promoters
that are active only in insect cells (and in the case
of late/very late baculoviral promoters, these promoters
also require the products of baculoviral early
genes for activity). Thus, should the effector gene
and its promoter somehow jump to the genome of
a noninsect cell, the gene will not be expressed.
Second, the proteins encoded by the effector genes
are chosen because they target some critical aspect
of the pest insect life cycle or body. The proteins are
not biologically active in the noninsects (although it
is possible that they may induce an immunological
response). Thus, if the effector gene somehow
jumps to the genome of a noninsect cell, and if
this gene is somehow expressed, detrimental effects
will not result.
Genomic variants of baculoviruses are often
found in individual field collected insects (Cherry
and Summers, 1985; Maeda et al., 1990; Shapiro
et al., 1991; Hodgson et al., 2001); this suggests that
recombination and/or transposition events commonly
occur between baculovirus genomes. Extensive
homology between the donor and recipient
DNA molecules and replication of both DNA molecules
is required for high-frequency recombination
to occur (Kamita et al., 2003b). Such conditions
may occur when two heterologous viruses (that
share some genomic homology) infect the same cell
within the same insect. This scenario is the most
likely one in which an effector gene of a GM baculovirus
will jump to another organism (i.e., another
insect virus). Should the effector gene jump to another
virus under these conditions, the fitness of the
new recombinant virus will be reduced in comparison
to the original GM baculovirus and it too should
be rapidly eliminated from the environment. In a
second scenario in which the effector gene jumps
from the GM baculovirus to the genome of the
insect host, the effector gene could cause an adverse
effect. However, these effects should be limited to a
single individual because once this individual dies
the effector gene will also ‘‘die.’’ It is also possible
that heterologous or random recombination events
may lead to the movement of an effector gene to
another organism. The same arguments that were
made in regard to the homologous recombination
based movement can be made in this case. However,
the likelihood of heterologous recombination is
much lower than the likelihood of homologous
recombination.
10.6. Field Testing and Practical
Considerations
Laboratory and greenhouse testing has generated a
great deal of knowledge about the efficacy, safety,
and environmental fate of GM baculovirus pesticides
as described above. Mathematical models
have also been generated to evaluate the effectiveness
and ecological consequences of the release of
GM baculoviruses (Dwyer and Elkinton, 1993;
Dwyer et al., 1997; Dushoff and Dwyer, 2001).
Field testing, however, over both the short-term
(e.g., single growing season) and long-term (e.g.,
multiple seasons and years) is still necessary to confirm
the findings of laboratory and greenhouse tests
and the accuracy of mathematical models. The commercial
potential of GM baculoviruses and practical
considerations regarding their use can also be determined
by field testing. Issues regarding the commercialization
of GM baculovirus insecticides including
marketing, in vivo and in vitro production, formulation,
storage, and public acceptance are discussed
in detail by Black et al. (1997).
Some of the earliest field trials of GM baculoviruses
(occlusion-negative AcMNPVs carrying
junk DNA or lacZ marker gene) were performed
in England during the mid to late 1980s (Levidow,
1995; Black et al., 1997). In the USA, the first field
trial (a 3-year study) of a GM baculovirus (a polh
gene deleted AcMNPV that was co-occluded with
wild-type AcMNPV) was begun in 1989 (Wood
et al., 1994). These early field trials were performed