Biology Reference
In-Depth Information
Movements of
P
elements cause mutations by inactivating genes, alter-
ing rates of transcription or developmental or tissue-specific gene expression.
P
-element movements break chromosomes and cause nondisjunction during
meiosis that can lead to chromosome rearrangements and germ-cell death.
Transposition of
P
elements in somatic cells reduces the life span of
D. mela-
nogaster
males, as well as reducing fitness, mating activity, and locomotion
(
Woodruff et al. 1999
).
A syndrome called
hybrid dysgenesis
is induced in
D. melanogaster
when
males from a strain that contains
P
(P males) are mated with females lacking
P
(M females) (
Bingham et al. 1982
). Their F
1
progeny exhibit a high rate of muta-
tion, chromosomal aberrations and, sometimes, complete sterility, caused by
transposition of
P
in their germ-line chromosomes. The reciprocal cross does not
exhibit these negative effects because the P female's cytotype suppresses move-
ment of
P
. P-M hybrid dysgenesis disrupts piwi-interacting RNAs (piRNAs), allow-
ing the
P
elements to function (
Khurana et al. 2011
). However, as dysgenic flies
age, fertility is restored,
P
elements are silenced and
P
-element piRNAs are pro-
duced, which also silence other resident TEs.
When transposition of
P
elements occurs in germ-line tissues in
D. melano-
gaster
, three short introns have to be cut out of the original RNA transcript
to produce the mRNA that codes for a functional 87-kilodalton (kDa) trans-
posase (
Laski et al. 1986, Rio et al. 1986, Kobayashi et al. 1993
). In somatic
cells, a 66-kDa protein is produced that can function as a repressor of
P
activity
(
Lemaitre et al. 1993
).
9.4
P
-Element Structure Varies
Many
P
elements in the
D. melanogaster
genome are defective. Some have
internal deletions and are unable to produce their own transposase but if they
retain their 31-bp terminal repeats, they can move if supplied with transposase
by intact elements. An early report by
Eggleston et al. (1988)
suggested that
P
elements are unable to mobilize other TE families in
D. melanogaster
. However,
that is not true because
P
elements mobilize the
Tc1
element from the nema-
tode
Caenorhabditis elegans
(
Szekely et al. 1994
).
Cross mobilization may be common. For example, four different TEs,
Ulysses
,
Penelope
,
Paris
and
Helena
, can be mobilized in dysgenic crosses between
strains of
D. virilis
crosses despite the fact that the TEs are structurally diverse
(
Petrov et al. 1995
).
Ulysses
is a retroelement related to the
Ty3-gypsy
super-
family and
Penelope
is similar to another class of retroelements.
Paris
is in the
mariner/Tc1
superfamily that transposes without an RNA intermediate, whereas
