Biology Reference
In-Depth Information
Our knowledge of HT and TEs only began in the 1950s when Barbara McClintock
discovered TEs in maize. HT could occur from one population to another of the
same species, from one species to another, or to other organisms in the envi-
ronment. It is difficult to quantify this risk because we lack fundamental infor-
mation on the frequencies and mechanisms of HT. Because HT is rare, effective
sampling and statistical methods are especially important. The whole topic of HT
has received limited scientific attention until relatively recently.
HT across species of
Drosophila
by
P
and
mariner
elements provides some
data. HT of genes does occur between insect species by movement of naturally
occurring TEs (
Plasterk 1993
). HT is thought to be rare, yet we have observed
more than one such transfer within historical times in
D. melanogaster
and may
have missed other examples. The
P
element invaded
D. melanogaster
popula-
tions within the past 60 years, perhaps from a species in the
D. willistoni
group.
P
elements might have been transferred between these
Drosophila
species by a
semiparasitic mite (
Houck et al. 1991
). Another TE,
hobo
, also appears to have
invaded natural populations of
D. melanogaster
around the 1960s (
Bonnivard
et al. 2000
), the second invasion of this well-studied insect in the past 60 years.
Transfer of TE vectors from transgenic arthropods to other organisms is poten-
tially feasible, although these transfers should occur very rarely. Recall that risk
is determined by frequency of occurrence and the damage that might occur. In
this case, the frequency is expected to be very low if the natural invasions rep-
resent a realistic estimate of frequency. If active TEs are purposefully released as
drive mechanisms and if conversion of inactive TE vectors into active ones can
occur, then the frequency could be higher than the natural rate.
It is difficult to estimate the potential damage that invasions of TEs could
have on nontarget species. For example,
mariner
is widespread and the data
suggest that 1)
mariner
elements have been present in arthropods for a long
time, although some lineages have lost them; and 2) HT has occurred between
different arthropod families and orders, although some transfers occurred so
long ago that the
mariners
are degraded and inactive, probably due to a suc-
cessful defense against the damage they cause to the genome (
Lampe et al.
2000
).
Lampe et al. (2000)
noted the most recent events occurred at least
100,000 years ago. The two
mariner
s in the human genome probably invaded in
the “past 100 million years” (
Lampe et al. 2000
).
We are still discovering new aspects of the evolutionary role of TEs
(
(Brookfield 1996, Capy et al. 1996, Kidwell and Lisch 2001
), which makes it dif-
ficult to predict what would happen if arthropods were released that contained
