Biology Reference
In-Depth Information
This chapter describes
P
elements and hybrid dysgenesis, and the methods
used in introducing
P
-element vectors into the germ line of
D. melanogaster
. This
approach to germ-line transformation inspired entomologists to attempt to engi-
neer insects other than
Drosophila
using TE vectors with a broader host range.
An analysis of the spread of
P
elements into natural populations of
D. melano-
gaster
previously lacking these TEs allows us to learn more about the evolution of
P
and other TEs. Because TEs have been proposed as possible “drive mechanisms” for
the genetic modification of wild insect populations, the study of
P
-element invasion
into
D. melanogaster
may serve as a model to understand the potential for using
this type of drive mechanism for other insects. Analyses of
P
-element invasions also
may provide clues to the evolution of resistance (suppressive factors) to TEs.
Finally, several new tools have been developed that allow genetic modifi-
cation of
Drosophila
and other insects (see Section 9.19). These methods may
result in higher rates of transformation, increased stability and more effective
levels of gene expression.
9.3
P
Elements and Hybrid Dysgenesis
Intact
P
elements are 2907bp long and encode a single polypeptide that has
transposase activity (
Figure 9.1
). There are four exons (numbered from 0 to 3)
flanked by inverted repeats that are 31 bp long. The presence of
intact inverted
repeats
is required if the
P
element is to transpose (move).
Multiple copies of
P
(30-60) are dispersed throughout the genome of P strains
of
D. melanogaster
, but their activity is suppressed by multiple mechanisms.
Many
P
elements in
D. melanogaster
, and other
Drosophila
species, have some
sequences deleted, which makes them incapable of transposing.
Figure 9.1
Structure of an intact
P
element. There are four exons (0-3), separated by short introns
(thin line). The 31-bp inverted terminal repeats (sequences 1-31 and 2877-2907) are indicated by
the filled arrows. There are also inverted repeats at sequences 126-136 and 2763-2773. (Modified
from
Engels 1989
.)