Biology Reference
In-Depth Information
into an active TE through a three-step process. The defective
P
(unable to move)
used a remote template (another
P
that was itself unable to transpose because
it lacked 21bp at its 5
′
end) for part of the template for the new element.
The new element had a restored 5
′
end that allowed it to transpose, which it
obtained from a third element. This provides strong evidence that the search for
homology occurs during the DNA repair process after a ds break (
Peronnet et al.
2000
). Conversion to activity could, in some cases, make a transgene unstable
within the transgenic arthropod's genome and could pose a potential risk for
horizontal gene transfer.
Schetelig et al. (2011)
review methods currently available for enhancing trans-
gene stability. Several different transgene stabilization methods were developed
for
Drosophila
. The use of site-specific recombination methods make it possible
to integrate DNA cassettes into a specific genomic DNA locus and the elimina-
tion of the transposon sequences subsequently should increase their stability
because they are unable to move by transposition (
Handler et al. 2004, Tkachuk
et al. 2011
).
9.18 Suppression of Transgene Expression
Transgenic plants and mammals are known to inactivate multiple copies of
genes if they overexpress a gene or exhibit abnormal transcription (
Henikoff
1998
). The inactivation phenomenon is thought to be due to a cellular defense
mechanism that prevents high levels of expression of TEs or of viruses. In fungi
and plants,
gene silencing
is associated with methylation of the DNA, or post-
transcriptional and transcriptional processes. Transgene silencing has been
described in
D. melanogaster
for the
white-alcohol dehydrogenase
transgenes
(
Pal-Bhadra et al. 1999
). Transgene silencing in
Drosophila
also is associated with
the production of heterochromatin (
Dorer and Henikoff 1994, 1997
).
Pal-Bhadra
et al. (2002)
reported that RNAi mechanisms could affect transcriptional and
posttranscriptional transgene silencing in
Drosophila
, especially when the trans-
genes are present in high copy number. Transgenes that are introduced into
different locations in the genome may be silenced along with the endogenous
genes. Methods to eliminate transgene silencing will be necessary, or this phe-
nomenon could reduce the effectiveness of transgenic insects released for pest-
management programs (
Hoy 2000
).
9.19 Other Transformation Methods
TE vectors have disadvantages. First, they usually insert randomly, which can
result in position effects in the expression of the inserted transgene. Second,
they have a relatively low insertion rate (often much
<
10%), requiring the
