"Horizontal Gene Transfer - New Evidence"
by Dr. Mae Wan Ho
December 4, 1998

A group of researchers in Indiana University of the United States, headed by
Dr. Jeffrey Palmer, have just reported in the current issue of the
Proceedings of the National Academy of Sciences USA 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 oxdize 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 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, 48 were found to have the parasite.

Moreover, all the higher plants that have gained the group I intron has 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 must have 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]"

So, what triggered this recent explosive invasion of the higher plants by
the particular 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 species barriers. 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.

The researchers themselves raise concerns about releasing transgenic crops
into the environment, if horizontal gene transfer is so widespread.

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.

· Is it possible that the recent massive horizontal gene transfer from yeast
to higher plants was triggered by commercial genetic engineering
biotechnology itself?

· 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 infectious diseases (reviewed by
Ho et al, Microbial Ecology in Health and Disease vol 10, 33-39m 1998).

· We should take this new evidence very, very seriously. There should be an
immediate moratorium on further releases of transgenic plants, in particular
those carrying antibiotic resistance genes like the Novartis maize.