Biology Reference
In-Depth Information
unlikely if the process were dependent upon
random
collisions between homol-
ogous DNA sequences.
Targeted-gene insertion makes it possible to introduce new genetic infor-
mation into specific chromosomal sites, or to modify existing genes in directed
ways. Random insertions can cause lethality if they insert into essential genes,
or can result in poor levels of expression if insertion occurs into heterochromatic
regions.
9.9 Transformation of Other Insects with
P
Vectors
DNA from
Drosophila melanogaster
has been introduced into
D. simulans
and the more distantly related
D. hawaiiensis
with
P
-element vectors, produc-
ing hybrid dysgenesis similar to that found in
D. melanogaster
(
Brennan et al.
1984, Daniels et al. 1989
). Thus,
P
vectors can integrate and transpose in sev-
eral
Drosophila
species.
Rio et al. (1988)
suggested that
P
transposase was active
in mammalian cells and yeast, which elicited optimism about the possibility of
using
P
elements for genetically engineering other arthropods. Unfortunately,
efforts to use
P
vectors to transform arthropod species outside the genus
Drosophila
failed (
O'Brochta and Handler 1988, Handler and O'Brochta 1991,
Handler et al. 1993a, Handler and James 2000
). However, transformation of
insects other than
Drosophila
has been accomplished with other vectors (
Fraser
2012
), as described in Section 9.15.
9.10 Evolution of Resistance to
P
Elements
The spread of
P
elements into populations of
D. melanogaster
occurred world-
wide since the 1930s. This invasion was remarkable because intact autonomous
P
elements induce severe disadvantages in the newly invaded populations. If
P
elements invade a small population, that population usually is lost (
Engels 1997
).
If evolution of repression systems (resistance to transposition) fails to occur
quickly enough, the invaded populations go extinct (
Corish et al. 1996
).
In fact, several
P
repressor systems (resistance mechanisms) have been identi-
fied. The repressors either are transmitted cytoplasmically (maternally inherited)
or through the nuclear genome, in which case the transmission is biparental.
The repressor systems have been classified as P, M', or Q (
Corish et al. 1996,
Badge and Brookfield 1998, French et al. 1999
).
P fly strains have a strong maternally inherited repression system called P
cytotype (
Engels 1989
).
P cytotype
is mediated by a 66-kDa protein produced
by differential splicing of the complete element's transcript (
Laski et al. 1986
).
