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A Bug's Death
September 25, 2003
By OLIVIA JUDSON
LONDON - Should we send the malaria mosquito the way of the
dodo?
So far, genetic modification has been a tool of creation.
We've made crops that grow faster or that are resistant to
pests, we've made animals that produce useful hormones in
their milk, we've even - presto! - made white rabbits that
glow green under black light. But now another, more radical
use for genetic modification is in the offing: the
engineering of extinction.
The basic idea is simple. Specicide - the deliberate
extinction of an entire species - could be engineered by
exploiting the biology of selfish genetic elements. These
are segments of genetic material found in the genomes of
all organisms; they contribute nothing to the well-being of
their hosts, but simply proliferate themselves. And
proliferation is something they excel at. A feature of all
selfish genetic elements is that they cheat at Mendel's
rules of inheritance and so have better odds for getting
into eggs and sperm than regular genes do. As a result, a
selfish genetic element can spread through a population
extremely fast - far faster than a regular gene - even if
it is harmful to its host.
That is why these elements are attractive to genetic
engineers: attach a useful gene to a selfish genetic
element, release individuals modified to carry the element
and, within about a dozen generations, that gene should be
present in every individual in a population. Or, to
engineer extinction, devise an extinction gene - a selfish
genetic element that has a strongly detrimental effect. The
element could, for example, be designed to put itself into
the middle of an essential gene and thereby render it
useless, creating what geneticists call a "knockout." If
the knockout is recessive (with one copy of it you're alive
and well, but with two you're dead), it could spread
through, and then extinguish, a species in fewer than 20
generations.
Whether this can work in practice remains to be seen.
Designing an extinction gene won't be easy. A suite of
technical problems remains to be solved. But the idea is
moving from theory to experiment; suitable selfish genetic
elements are starting to be engineered to attack the genes
of laboratory fruit flies, the guinea pigs of the insect
world. And although field trials remain far in the future,
it is not too soon to ask ourselves whether we should even
think about using such a potentially powerful technology to
wipe out a creature.
Given how hard we try to prevent the demise of one species
or another - from the African elephant to the northern
hairy-nosed wombat - it may seem perverse to entertain the
notion of causing an extinction on purpose. Yet there's a
handful of species we've tried (and failed) to destroy - at
great expense, both to the environment and to our wallets.
Chief among these, and the most obvious candidate for
specicide, is the Anopheles mosquito, the mosquito that
spreads malaria.
Each year, malaria kills at least one million people and
causes more than 300 million cases of acute illness. For
children worldwide, it's one of the leading causes of
death. The economic burden is significant, too: malaria
costs Africa more than $12 billion in lost growth every
year. In the United States, hundreds of millions of dollars
are spent every year on mosquito control. What's more, a
malaria vaccine is still out of reach; the parasite's
resistance to drugs is a growing problem, as is the
mosquito's resistance to insecticide. The proposed
extinction technology could eradicate the malaria mosquito,
and malaria with it, within 10 years of the time mosquitoes
modified to carry an extinction gene are released into the
wild. Tempting stuff.
Or tempting fate? As with any new technology, the benefits
of using it must be measured against possible risks. Here,
the risks are two: ecological collapse and genetic escape.
Genetic escape is the idea that the extinction gene might
somehow get into a species other than the target and
inadvertently wipe it out as well. In principle, this could
happen in either of two ways. Anopheles mosquitoes might
not be fussy about whom they mate with; if they engage in
sex with mosquitoes of other species, the gene could spread
into those species and eliminate them, too. Most animals
avoid sex with members of different species, so a priori,
the likelihood of hybridization seems small; all the same,
this is something that should be investigated
experimentally before the technology is put in place.
Alternatively, the extinction gene itself might prove
unstable, and jump into a different species entirely.
Though such jumping is not unknown for wild selfish-genetic
elements, it is rare, and the chance of this being a
problem seems remote. (The risk to us from this technology
is negligible. Even supposing an extinction gene appeared
in humans - by accident or by malice - it would take
thousands of years for extinction to be effected. During
this time, it is inconceivable the gene's spread would go
unnoticed; once noticed, it could easily be stopped.)
What about the ecological impact of removing Anopheles
mosquitoes? Hard to predict. But several facts are worth
bearing in mind. First, our current methods of mosquito
control are crude and kill more than just mosquitoes. An
extinction gene at least has the benefit of being precise
and clean. Second, there's nothing sinister about
extinction; species go extinct all the time. The
disappearance of a few species, while a pity, does not
bring a whole ecosystem crashing down: we're not left with
a wasteland every time a species vanishes. Removing one
species sometimes causes shifts in the populations of other
species - but different need not mean worse.
Moreover, the earth is home to more than 2,500 species of
mosquito. Even if we were to eradicate the approximately 30
species that are regular carriers of malaria, and for good
measure, the Aedes mosquitoes that spread dengue and yellow
fever, we'd hardly be creating a mosquito-free world. It is
hard to argue that a targeted, genetic attempt to remove an
insect that is clearly harmful to us is worse than the
haphazard, expensive, destructive and largely unsuccessful
approach we're using now.
Nevertheless, friends of the mosquito will say that
eradicating the mosquito is akin to shooting the messenger.
Why not, they will surely ask, commit specicide on the
malaria parasite instead? Alas, this is probably impossible
to do by extinction gene: the parasite is prone to
self-fertilizing, a habit that would prevent the gene's
spread. Besides, to get the genetically modified parasite
into the wild, you'd have to infect people with it, which
would clearly be unethical. The obvious mosquito-saving
alternative - modifying the creature so that it cannot
spread the disease - is probably also unfeasible. This is
technically harder than the extinction approach and there
are, in any case, theoretical reasons to suppose the effort
would fail.
Ideally, malaria would be defeated in other ways. Uganda
has recently reported a 50 percent drop in death rates as a
result of handing out free malaria medicines; if the
program can be emulated and the trend sustained, perhaps by
the time the technology is ready, it will no longer be
needed. But if, by then, the situation is not much
improved, we should consider the ultimate swatting.
Olivia Judson, an evolutionary biologist at Imperial
College in London, is author of "Dr. Tatiana's Sex Advice
to All Creation: The Definitive Guide to the Evolutionary
Biology of Sex."
http://www.nytimes.com/2003/09/25/opinion/25JUDS.html?ex=1065519728&ei=1&en=f0648d7f197e7f31
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