VIRUSES given a gene for a toxin from one of the world鈥檚 deadliest spiders
could replace chemical pesticides, say researchers in the US. They plan to carry
out field trials, although there are fears about the wisdom of releasing such
viruses.
Glenn King of the University of Connecticut Health Center in Farmington and
his colleagues recently identified a unique family of toxins in the venom of a
funnel-web spider. These neurotoxins are lethal when injected into insect
tissues, yet have no effect if eaten by insects or other animals (Nature
Structural Biology, vol 7, p 505).
King鈥檚 team is now engineering the gene for one of these toxins into
baculoviruses, common viruses that infect certain moths and butterflies, and
have long been used as 鈥渂iopesticides鈥�. When the modified baculovirus infects an
insect, the insect鈥檚 cells should start to produce the toxin, killing it faster
than wild viruses. Because the host dies quickly, before much virus can
replicate, the modified virus shouldn鈥檛 persist in the environment, say the
researchers.
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鈥淚 welcome a potentially environmentally friendly pest control but it鈥檚
abundantly clear we need to be more firm about risk issues,鈥� comments George
McGavin, an entomologist at Oxford University. 鈥淚f we are not 100 per cent sure,
it shouldn鈥檛 be in the field.鈥�
There have already been several field trials worldwide of baculoviruses given
a gene for a scorpion toxin (New 杏吧原创, 21 January 1995, p 6).
However, most of the scorpion toxin made in infected insects fails to fold into
the correct shape, says King. By contrast, tests in bacteria suggest that almost
100 per cent of the spider toxin should fold properly, making the virus
deadlier.
King thinks engineering toxin genes into viruses is preferable to adding them
to plants, such as Bt maize. Not only does it mean that people do not have to
eat plants that produce insecticidal toxins, but only target insects will be
affected, he says. 鈥淭hese viruses can be exquisitely specific, right down to
infecting individual species,鈥� King claims. 鈥淭his means that only the pest
insects will be killed whilst beneficial insects such as bees remain
耻苍补蹿蹿别肠迟别诲.鈥�
However, critics fear that the virus will spread into the environment and
affect other kinds of butterflies and moths. 鈥淎 containment environment could
not possibly hold a virus,鈥� says McGavin, who opposed trials of a scorpion toxin
virus in Oxfordshire in the 1990s. 鈥淚f you could get a specific baculovirus it
would be great, but baculoviruses do pass on [to other species].鈥�
There are also fears that the toxin gene might be transferred to other
viruses. 鈥淭here is no instance of a toxin gene jumping from virus A to virus B,鈥�
says Bruce Hammock of the University of California, Davis, who is also working
on modified baculoviruses. 鈥淏ut if it jumped, the new virus would become less
别蹿蹿别肠迟颈惫别.鈥�
Jenny Cory of the Centre for Ecology and Hydrology in Oxford agrees transfer
of the toxin gene is unlikely, but thinks further tests would be helpful. 鈥淚t鈥檚
a vicious circle,鈥� she says, 鈥測ou have to do a risk assessment before you do the
experiment but we don鈥檛 know all the risks without doing field experiments in
the first place.鈥�