Crops such as maize and cotton have already been made resistant to chewing insects by adding a gene for the bacterial toxin Bt. But Bt does not deter sap-suckers like aphids, so genetic engineers are looking at other natural substances to keep insects at bay, such as the lectin proteins found in many plants and seeds.

Lectins have a controversial history. It was lectin-transformed potatoes created by Arpad Pusztai that set off a storm in Britain about the safety of GM food. Now Nick Birch’s team at the Scottish Crop Research Institute near Dundee has found that potato plants transformed with lectin genes have lower levels of bitter-tasting chemicals called glycoalkaloids that make plants unpalatable to many mammals and insects.

Glycoalkaloid levels in the leaves of the lectin-transformed potatoes dropped by up to 44 per cent. This seems to be due to the genetic engineering technique itself, because introducing another type of gene, for another potential insect deterrent called cowpea trypsin inhibitor, also caused glycoalkaloid levels in the plants to drop by 70 per cent.

The team warns that plants with lower glycoalkaloid levels could be more vulnerable to a range of insect pests, including the potato leafhopper. And reduced levels of the glycoalkaloid alpha-chaconine actually stimulates the potato aphid to feed.

Cracked stems

The results are surprising, says Angelika Hilbeck, an ecologist at the Swiss Federal Institute of Technology in Zurich who studies the risks posed by GM crops. “We need to learn a lot more about the unintended side effects of the various transformation techniques.”

While the potatoes were only experimental varieties, unexpected side effects have also turned up in commercial GM crops. The stems of a herbicide-resistant soya bean created by Monsanto were found to crack open in hot climates, for instance (91av, 20 November 1999, p 25).

Unintended effects also occur in traditional breeding programmes, points out Howard Davies of the Scottish Crop Research Institute. But he says new techniques should help us to get a grip on the problem.

“Technologies are now being developed to measure several hundreds, if not thousands, of metabolites in plants using metabolic profiling procedures,” he says. These approaches, along with techniques that can profile thousands of genes or proteins simultaneously, should help reveal any possible unintended effects caused by genetic transformation, he says.

Journal reference: Annals of Applied Biology (vol 140, p 143)

Agricultural botanists in France have now shown that genes for herbicide
resistance engineered into oilseed rape can persist for several generations in
hybrids between the transgenic rape and wild radishes. Meanwhile, British
researchers have found that potatoes engineered to resist attack by aphids can
also harm ladybirds, the pests’ natural predators.

While experts stress that neither finding poses a major environmental threat,
industry sources fear that the new results will further undermine public
acceptance of genetically engineered crops.

The escape of genes into wild plants has always been the main worry
surrounding transgenic crops. To study this, Anne-Marie Chèvre and her
colleagues at INRA, France’s national agricultural research agency, based near
Rennes, planted plots of wild radish, Raphanus raphanistrum, next to
transgenic oilseed rape, Brassica napus. The rape was engineered to
carry a gene for resistance to the herbicide glufosinate ammonium and did not
produce pollen.

The researchers had found previously that the rape produced hybrids carrying
28 chromosomes, 19 from the rape, 9 from the radish. In this week’s
Nature (vol 389, p 924), they describe experiments in which the hybrids
were planted surrounded by wild radishes, and followed through four
generations.

Subsequent generations of the hybrids had variable numbers of
chromosomes—anywhere between 20 and 60. “We never found a stable variety,”
says Frédérique Eber, the team’s chromosome specialist. Even in
the fourth generation, however, 20 per cent of the hybrids retained the gene for
herbicide resistance.

Although the results suggest that the gene might be lost eventually,
botanists note that the hybrids studied by the French team are more persistent
than many crosses between different species, which frequently don’t survive
beyond the first generation. “Often things will die out at that stage,” says
Philip Dale of the John Innes Centre in Norwich.

John Beringer of the University of Bristol, who chairs Britain’s Advisory
Committee on Releases to the Environment, believes that there is no cause for
public alarm. But hybrid weeds could remain in fields and sprout despite
herbicide spraying. “It is more of a problem for farmers than an environmental
����Dz�����.”

There is already a climate of public opposition in Europe to imports of
American soya beans and maize engineered to produce the bacterial insecticide
Bt. Industry sources fear the French findings could delay approval for
transgenic crops currently awaiting the green light in Europe, which include
five separate strains of herbicide-resistant oilseed rape. “We’ve had a whole
spate of bad news recently,” says David Bennett of the European Federation of
Biotechnology, based in The Hague. “I can only assume the European Commission
will react badly.”

More bad news for plant biotechnologists comes from Nick Birch of the
Scottish Crop Research Institute in Dundee and Mike Majerus of the University of
Cambridge. They fed two-spot ladybirds, Adalia bipunctata, for two
weeks on peach-potato aphids, Myzus persicae, that had fed on sap from
potatoes engineered to carry a lectin from snowdrops—a protein that
interferes with insect digestion.

The engineered potatoes were made by John and Angharad Gatehouse of the
University of Durham, and in greenhouse tests they killed off significant
numbers of the aphid pests. But in experiments to be reported in a future issue
of Molecular Breeding, Birch and Majerus found that female ladybirds
fed with aphids from the engineered potatoes lived half as long as those fed on
aphids from normal potatoes. Males given lectin-containing aphids lived for an
average of 46 days, 5 days less than those in the control group.

In mating studies, up to 30 per cent fewer viable eggs were laid when one of
the parent ladybirds was fed aphids from lectin-transformed potatoes. “But these
effects on ladybird reproduction wear off after three to four weeks,” says
Birch.

This is the first time that such a knock-on effect on a beneficial predator
species has been seen. While it adds to the concerns about the safety of
transgenic crops, Birch notes that the engineered potatoes should require less
insecticides. “It may become a question of balancing the risks of transgenic
plants with the risks of chemical applications,” he says.

Previously, two related but distinct species of the virus, which is
transmitted by whitefly, were found in Africa: African cassava mosaic virus in
the west, and East African cassava mosaic virus east of the Rift Valley. Now
scientists at the Scottish Crop Research Institute in Dundee report a new strain
that contains DNA from both.

Bryan Harrison and his colleagues report in this month’s Journal of
General Virology(vol 78, p 2101) that the new strain is almost identical to
the East African virus, except for one key difference: the central section of
the coat protein gene comes from the western version.

“This is the first example of interspecific recombination between identified
plant viruses that has gone on to cause a major new epidemic,” says Harrison. He
adds that the new species probably arose in the late 1980s in a plant infected
by both the eastern and western strains.

The report follows recent experimental evidence that viruses can recombine in
plants (This Week, 16 August 1997, p 4),
and may provide ammunition to opponents of the use of viral genes in engineered crops.

In Uganda, about 5 per cent of the cassava crop has been bred to resist the
old strains of mosaic virus. Mike Thresh of the UK’s Natural Resources Institute
in Chatham, Kent, says these varieties have genes for resistance to both of the
old strains and so he expects resistance to hold up against the recombinant
virus.