PLANTS that don鈥檛 interbreed can still exchange DNA using go-betweens such as
fungi, viruses or aphids, new research suggests. The findings are likely to be
seized on by opponents of genetic engineering, who fear the spread of modified
genes from crops to wild plants. But the scientists behind the research stress
that such DNA transfers are very rare events.
Jeff Palmer and his colleagues at Indiana University in Bloomington have
discovered a stowaway gene segment in scores of unrelated flowering plants,
including coffee, bananas, cucumbers, periwinkles and foxgloves. They speculate
that the segment originated in fungi, as fungal species including baker鈥檚 yeast
(Saccharomyces cerevisiae) are known to carry it. 鈥淲e think there was
at least one original donation from a fungus to a plant,鈥� says Palmer. Since
then, it may have been shuttled from plant to plant by aphids or viruses.
Palmer鈥檚 group screened 335 families of flowering plants for the segment, a
chunk of DNA called an intron. Introns are junk DNA that is clipped out of genes
before they are transcribed into proteins. The stowaway intron, which buries
itself in the same gene whichever plant it invades, appeared in 48 of the
families, the researchers report in the latest Proceedings of the National
Academy of Sciences (vol 95, p 14 244).
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Through analysis of plant family trees, Palmer and his colleagues have found
that in 32 of these 48 cases, the intron spread 鈥渓aterally鈥� between unrelated
species rather than 鈥渧ertically鈥� through inheritance. 鈥淲e normally think of
lateral transfer as a very rare event between plants, especially those that lost
the ability to mate with one another millions of years ago,鈥� says Palmer.
鈥淲hat鈥檚 novel is finding this massive wave of intron movements.鈥� Scaling up to
the 13 000 known families of flowering plants, Palmer says the intron must have
jumped species at least 1000 times.
The promiscuous intron studied by Palmer carries a molecular tool for
jemmying itself into DNA. This tool, the gene for an enzyme called an
endonuclease, always wedges the intron into cox1, a gene vital for
energy metabolism in plants.
Sequences lacking endonuclease genes are probably much less mobile, says
Palmer. 鈥淚t makes it difficult to say how often other genes without these
elements move around,鈥� he says. And even with its endonuclease, the stowaway
intron has probably jumped species perhaps just once every 5 million years.
Given the limits to lateral DNA transfer, Palmer doesn鈥檛 think that the risks
posed by genetically engineered plants need to be reassessed in the light of his
research. 鈥淚 doubt this lateral transfer occurs often enough to cause concern on
a human timescale,鈥� he says. Nevertheless, Phil Dale of the John Innes Centre in
Norwich, a member of Britain鈥檚 Advisory Committee on Releases to the
Environment, says: 鈥淚t鈥檚 sufficiently important to warrant a careful look to see
if it changes our assumptions.鈥�