Biology Reference
In-Depth Information
desired trait is selected for and the resultant population contains the desired
gene (introgression model).
14.11 Potential Risks Associated with Releases of Genetically
Modified Arthropods
Risk
equals the potential for damage
and
the likelihood of its occurrence. Risk
estimates may be different for pest and beneficial arthropods and may depend
on whether the modified strain is expected to persist in the environment or is
unable to reproduce and cannot persist (
Table 14.7
). Risks also will vary with
the specific transgene(s) inserted, or other genetic modification obtained (para-
transgenesis or insertion of a novel symbiont). It is easier to suggest
potential
types
of harm than to
quantify the likelihood
of its occurrence. Each release
should be considered on a case-by-case basis.
14.11.1 Could Gene Silencing Reduce Program Effectiveness?
There is always the risk that a genetically modified arthropod population could
be released into the field and fail to function as expected due to
gene silenc-
ing
or
RNAi
. Transgenic plants and mammals often inactivate multiple copies
of transgenes that overexpress proteins or are otherwise abnormal (
Dorer and
Henikoff 1997, Wolffe 1997, Henikoff 1998, Birchler et al. 2000, Sijen and Kooter
2000
). Gene silencing is due to systems that have evolved as a means to prevent
high levels of expression of TEs or viruses that can cause genetic damage to their
hosts. In fungi and plants, gene silencing is associated with several mechanisms,
including methylation of DNA, as well as posttranscriptional and transcriptional
processes involving RNAi.
Multiple mechanisms of transgene silencing occur in
D. melanogaster
(
Dorer
and Henikoff 1994, 1997, Pal-Bhadra et al. 1999, Jensen et al. 1999
). Methods
may have to be developed to manage transgene silencing in arthropods or this
phenomenon could reduce the effectiveness of released strains over time after
release into the field. The use of insulators or boundary elements may limit gene
silencing (
Bell et al. 2001
) and genetic elements such as histone deacetylase
RPD3, which can counteract gene silencing in both
Drosophila
and yeast, may be
useful in counteracting gene silencing (
De Rubertis et al. 1996
).
Gene silencing could be a positive attribute if specific genes in insects could
be turned off. Gene silencing was purposefully induced in
D. melanogas-
ter
by introducing a sequence that codes for an extended hairpin-loop RNA
by
P
-mediated transformation (
Kennerdell and Carthew 2000
). As discussed in
Chapter 9,
Table 14.4
, and Section 14.5.7, gene silencing by RNAi methods could
be used to manage pest arthropods.
