Biology Reference
In-Depth Information
1982
) and many laboratory studies have shown that colonies can change rapidly
from M to P strains. The cut-and-paste model has been the conceptual basis of
targeted gene replacement
(see Section 9.8.4) in
Drosophila
(
Engels et al. 1990,
Gloor et al. 1991, Sentry and Kaiser 1992, Rong and Golic 2000
).
Guerreiro (2011)
reviewed factors that make TEs move in the
Drosophila
genome, which is important if transformations obtained by the use of TE vectors
are to be stable. Activation of TEs by both biotic and abiotic stresses occurs in
many organisms. In
Drosophila
, thermal stresses are associated with movement
of TEs, as are dysgenic crosses, hybrid crosses, and colonization events. However,
many TE movements occur without any obvious cause. Clearly, there are mecha-
nisms in the genome that subdue the movement of TEs because TE movement
rates typically are low. Furthermore, although heat shocks may initiate activity,
in other cases it does not; likewise, X-irradiation may activate
P
elements, and
gamma irradiation may activate
412
but not
hobo
. Injection of viruses may have
an effect or not, depending on the TE tested. The host genetic background and
environmental conditions seem to be important in activating TEs in
Drosophila
(and probably in other arthropods).
Guerreiro (2011)
suggests that TEs increase
the evolutionary potential of species, leading to new phenotypes on which
selection can act and suggests, “The coevolution of TEs and host genomes may
constitute a way to diminish the detrimental effect of transpositions by the
silencing mechanisms. In this way, bursts of TE activity followed by calm periods
could occur during evolution
…
These alternating periods of low and high rate
of transposition may be crucial for the generation of genetic variability, and as
an effective way to avoid the complete elimination of TEs from host genomes.”
9.6 Origin of
P
Elements in
D. melanogaster
P
elements are relatively new in the genome of
D. melanogaster
. Laboratory
strains of
D. melanogaster
collected before 1950 lacked
P
, but most colonies col-
lected from the wild within approximately the past 60 years have
P
elements
(
Anxolabehere et al. 1988, Engels 1989, 1992, Powell and Gleason 1996
).
P
elements are relatively common in other species of
Drosophila
. Surveys indi-
cated that closely related, full-sized, and potentially active
P
elements occur
in
D. willistoni
,
guanche
,
bifasciata
, and
Scaptomyza pallida
(
Hagemann et al.
1996
). A
P
element isolated from
S. pallida
, a drosophilid distantly related to
D.
melanogaster
, is able to transpose in
D. melanogaster
and to mobilize a defec-
tive
melanogaster P
element (
Simonelig and Anxolabehere 1991
).
Phylogenetic analyses of DNA sequences from
P
elements in 17
Drosophila
species in the
melanogaster
species group within the subgenus
Sophophora
