Biology Reference
In-Depth Information
When P females are crossed to a strong P line
<
10% of the ovaries are dysgenic,
indicating that P strains strongly repress hybrid dysgenesis. If P males are crossed
to M females (which lack a repression system),
>
90% of the ovaries are dysgenic
in their progeny. P strains are strong inducers of transposition.
M' strains
also contain repressor elements. Transposition repression in M'
strains is due to the KP element (
French et al. 1999
). M' strain females display
intermediate levels of repression of dysgenesis when crossed to P males. Both
males and females from M' strains are able to pass the repressing factor to their
progeny.
Q strains
strongly repress transposition, allowing a low induction of transposi-
tion. Some Q strains show a maternal mode of inheritance of repression while
others have a biparental mode of inheritance. It is thought that a repressor (SR)
results from a 309-bp deletion at the 3
′
end of the
P
element. The SR repres-
sor cannot produce functional transposase but can produce the 66-kDa repressor
and a novel 75-kDa protein, both of which may be involved in Q-type repression
(
French et al. 1999
).
Evolution of resistance to
P
elements can develop rapidly, as demonstrated by
two surveys of
D. melanogaster
along a 2900-km cline along the eastern coast of
Australia. The first occurred in 1983 and the second in 1993. In 1983, P popula-
tions were found in the north, Q populations at central locations, and M' popu-
lations in the south (
French et al. 1999
). After 10 years, Q and M' populations
had increased their range at the expense of P lines.
French et al. (1999)
spec-
ulated that the P and M' mechanisms of repression may be early, emergency,
responses to the harmful effects of transposition by
P
. The surviving
D. melano-
gaster
populations then evolve a superior mechanism by acquiring the biparen-
tally transmitted Q-repression system.
In species of
Drosophila
in which
P
elements have been present for a long
time, no complete functional
P
has been found (
French et al. 1999
). Instead,
many populations contain tandem repeats of elements with degenerate fourth
exons, which might encode some repressor activity. In
D. nebulosa
, a complete
element was isolated but the element contained many base changes in all four
exons and was nonfunctional. These results reinforce the notion that active
transposition of
P
is detrimental to species of
Drosophila
in the wild.
TEs consist of
≈
12% of the genome of
D. melanogaster
and are responsible
for
≈
80% of the spontaneous mutations found (
Guerreiro 2011
). However,
the activity of TEs is not related to their abundance in the genome. Research
to understand the triggers of transposition has focused on biotic and abiotic
