Biology Reference
In-Depth Information
similar sequence to TEs from plants (
Activator
from corn and
Tam3
from snap-
dragon) (
Calvi et al. 1991
). Another element,
jockey
, identified from
D. mela-
nogaster
has been found in the distantly related
Drosophila funebris
, but not
in species closely related to
funebris
. This occurrence again suggests that
jockey
moved horizontally from
D. melanogaster
into the genome of
D. funebris
(
Mizrokhi and Mazo 1990
).
A possible superstar at horizontal transfer may be
mariner
. Originally,
mariner
was found in
Drosophila mauritiana
and several other species of
Drosophila
, as
well as in the moth
Hyalophora cecropia
. Subsequently,
Robertson (1993)
found
that several types (subfamilies) of
mariner
are widespread in insects. It is found
in other organisms as well. For example,
mariner
was found in the predatory
mite
Metaseiulus occidentalis
(Acari: Phytoseiidae) (
Jeyaprakash and Hoy 1995
).
It is likely that
mariner
has moved horizontally among diverse insect and mite
species, although the frequency of horizontal transfer is infrequent on a human
time scale (
Robertson and Lampe 1995
). Many
mariner
elements have degener-
ated and become inactive in the genomes of their hosts.
Relatively little information is available as to how TEs invade populations and
the mechanisms involved in the first step of the invasion (
Biemont et al. 1999
). One
of the best-known examples is the invasion of
D. melanogaster
by the
P
element
(for a review, see Chapter 9). Another evaluation of TE invasion was carried out by
Biemont et al. (1999)
in natural populations of
D. simulans
: populations around
the world are in the process of being invaded by a variety of TEs with whimsical
names. Populations differ in the number and type of TEs, with some TEs absent in
most populations, except for one or two populations that have high copy numbers.
Biemont et al. (1999)
suggest that the “initially selfish genes will surely appear more
and more as 'symbionts' that have played a major role in evolution and that may
still provide genomic flexibility and variability for population adaptation.”
If horizontal transmission of TEs occurs with some regularity, the implications
are dramatic for evolutionary theory, likely serving as a “major force propelling
genomic variation and biological innovation” (
Schaack et al. 2010
). Horizontal-
transfer risks also could influence regulations regarding the risks associated with
releases of transgenic arthropods into the environment (
Brosius 1991, Plasterk
1999, Hoy 2000
). As discussed in Chapter 9, studies of the transfer of
P
elements
by a mite vector provide an intriguing glimpse at one possible mechanism by
which TEs are able to move between species (
Houck et al. 1991
).
Other possible vectors of TEs are insect viruses. Insect viruses may carry DNA
from their hosts; a proportion of foreign DNA within insect viruses consists of
