Biology Reference
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show that sequences from the
P
family fall into distinct subfamilies or clades
that are characteristic for particular species subgroups (
Clark and Kidwell 1997,
Clark et al. 1998
). These clades indicate that vertical transmission of
P
elements
has occurred, but in some cases the
P
phylogeny is
not
congruent with species
phylogeny, indicating horizontal movement has occurred, as well. More than
one subfamily of
P
elements may exist within a group, with sequences differing
by as much as 36%. In fact, horizontal transfer may be essential to the long-term
survival of TEs (
Clark and Kidwell 1997
).
P
probably invaded
D. melanogaster
within the past 60 years. The donor
species that provided a
P
to
D. melanogaster
is thought to be in the
willistoni
group, which is not closely related to
D. melanogaster
(
Daniels et al. 1984,
Lansman et al. 1985, Daniels and Strausbaugh 1986, Engels 1997
). Because these
species diverged from each other
≈
60 million years ago, there should have been
sufficient time for considerable sequence divergence in the
P
elements if they
had been present in both genomes before divergence (and subsequently trans-
mitted vertically). However,
P
-element sequences from
melanogaster
and
wil-
listoni
are nearly identical, supporting the hypothesis of horizontal transfer.
Engels (1997)
speculated that the invasion of
D. melanogaster
by
P
occurred
after
D. melanogaster
was introduced into the Americas and that invasion
by TEs could be a “general hazard associated with the expansion of any spe-
cies into a new ecosystem.” Such TE invasions potentially could provide genetic
variation that contributes to postrelease adaptations that occurs in some species
subsequent to their invasion into new environments.
Two mechanisms were proposed to explain how
P
could have infected
D. melanogaster
. One mechanism involves horizontal transfer and the other
mechanism involves interspecific crosses. Both
D. melanogaster
and
D. willistoni
now overlap in their geographical ranges in Florida and in Central and South
America, but they apparently are unable to interbreed. Horizontal transfer
could have been achieved by a viral, bacterial, fungal, protozoan, spiroplasmal,
mycoplasmal, or small-arthropod vector (Hymenoptera or Acari [mites]). One
candidate for horizontal vector may be a semiparasitic mite,
Proctolaelaps rega-
lis
, which is associated with both
Drosophila
species (
Houck et al. 1991, Kidwell
1992, Engels 1997
). This mite has been found in laboratory colonies and in the
field associated with fallen or rotting fruit, which is the natural habitat for these
species. Laboratory observations indicate that
P. regalis
feeds on fly eggs, larvae,
and pupae and can make rapid thrusts of its mouthparts into a series of adja-
cent hosts. This brief feeding on multiple hosts might allow it to pick up DNA
from one egg and inject it into another. Mites obtained from
Drosophila
colo-
nies with
P
elements in their genome were analyzed by the PCR and Southern
