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Horizontal gene transfer - new evidence 5.12.98
Dr. Mae-Wan Ho
A group of researchers in Indiana University of the United States have
just reported1 that
a genetic parasite belonging to yeast has suddenly jumped into many
unrelated species of higher plants recently.
This parasite is a piece of DNA called a "group I intron" that
can splice itself in and out of a particular gene in the genome of
mitochondria. Mitochondria are little power houses of the cell that
oxidize food in order to turn it into a form of energy that can be used
for all living processes. Until 1995, this parasite was thought to be
largely confined to yeast and only one genus of higher plants out of the
25 surveyed had the parasite.
But in a new survey of species from 335 genera of higher plants, 48 were
found to have the parasite. These 48 genera were in five different
families: Asterids, Rosids, Monocots, Piperales, and Magnoliales.
Moreover, all the higher plants that have gained the group I intron had
the same one, as the DNA base sequence is more than 92% identical.
When this intron jumps into a genome, it also adds to its tail end an
extra stretch of DNA that does not belong to the host. By comparing this
extra tail, the researchers are able to conclude that almost all of the
horizontal gene transfer events were independent and occurred very
recently. "This massive wave of lateral transfers is of entirely
recent occurrence, perhaps triggered by some key shift in the intron's
invasiveness within angiosperms [i.e., higher plants]." Two possible
scenarios presented themselves, either an original yeast group I intron
jumped into a higher plant, and from there, infected all other genera
independently, or the same yeast intron has jumped independently to all
the plants. The present data cannot distinguish between the two
possibilities
So, what triggered the recent "explosive invasion" of the
higher plants by this genetic parasite? It could have got into the plant
cells by being carried in viruses, insects or bacteria. In order to get
into the genome, however, it has to overcome genetic barriers that keep
species distinct. For example, the genome has to have a specific site of
about 20 base pairs that is recognized by the parasite. Furthermore, in
order for the splicing gene carried by the parasite to become expressed,
it has to have a signal that is recognized by the host.2
The researchers themselves raise concerns about releasing transgenic
crops into the environment, given that horizontal gene transfer is now
found to be so widespread.
Additional comments:
• Only two months ago, it was reported in the Journal Nature
that genes transferred into transgenic plants can be up to 30 times more
likely to escape than the plant’s own genes.3
• Is it possible that the recent massive horizontal gene transfer
from yeast to higher plants was triggered by commercial genetic
engineering biotechnology?
• Genetic engineering makes use of artificial genetic parasites as
gene carriers, to transfer genes horizontally between unrelated species.
These artificial parasites are made from parts of the most aggressive
naturally occurring parasites, like the group 1 intron discussed here.
• The same kinds of explosive horizontal gene transfer have already
been documented among viruses and bacteria which are responsible for the
recent resurgence of drug and antibiotic resistant diseases.4
1. Cho, Y., Qiu, Y.-L., Kuhlman, P. and Palmer, J.D. (1998). Explosive
invasion of plant mitochondria by a group I intron. Proc. Natl. Acad.
Sci. USA 95, 14244-9.
2. Gray, M.W. (1998). Mass migration of a group I intron: Promiscuity on
a grand scale. Proc. Natl. Acad. Sci. USA 95, 14003-5.
3. Bergelson, J., Purrington,c.B. and Wichmann, G. (1998). Promiscuity
in transgenic plants. Nature 395, 25.
4. Ho, M.W., Traavik, T., Olsvik, O., Tappeser, B., Howard, C.V., von
Weizsacker, C. and McGavin, G. (1998). Gene technology and gene ecology of
infectious diseases. Microbial Ecology in Health and Disease 10,
33-59.
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