Hannah Hoag
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Mosquitoes
engineered to not transmit malaria fare better than their unaltered
siblings, according to new research. The work rekindles hope that
transgenic mosquitoes could one day be used to wipe out natural insects
in the wild, helping to control the spread of malaria.
A
team from the Bloomberg School of Public Health and the Malaria
Research Institute at Johns Hopkins University in Baltimore, Maryland,
let caged wild-type and transgenic mosquitoes dine on mice that had
been infected with malaria. Not only did the altered mosquitoes not
become infected by the parasite, but over time they outnumbered their
normal siblings, the researchers report in Proceedings of the National Academy of Sciences1. The population of malaria-resistant mosquitoes grew because they produced more eggs and were less likely to die.
When transgenic and non-transgenic mosquitoes were fed non-infected blood, their numbers remained approximately equal.
"What this study shows is that the transgenic mosquitoes do have an advantage," says team leader Marcelo Jacobs-Lorena.
"This paper is the experimental proof we needed," says Andrea Crisanti of Imperial College in London. "This is good news."
Fitness fad
For
some time, researchers have been trying to create transgenic mosquitoes
that are somehow resistant to the Plasmodium parasite that causes
malaria, making it impossible for the insects to spread the disease.
But whether the transgenic insects would be able to out-compete their
normal mates and spread their genes, or whether they would simply die
out soon after being released has remained a mystery.
A 2003 study by Crisanti and colleagues suggested that at least one type of transgenic mosquito was less fit than normal ones2,
temporarily dashing hopes that the designer mosquitoes could be used to
fight malaria. But Crisanti did not use malaria-resistance genes in his
study.
In
2006, researchers found that many mosquitoes are in fact naturally
resistant to the parasite, boosting hopes that there wasn't an
insurmountable cost to being resistant3.
The
new study looks at mosquitoes intentionally modified to beat back the
malaria parasite, and found much more encouraging results than the 2003
work. But "it does not solve all of the problems," says Jacobs-Lorena.
Most researchers think that for the gene to spread effectively during
mating, another technique would need to be used to encourage transfer -
such as adding a genetic element called a transposon to the genome, or
dosing mosquitoes with bacteria called Wolbachia, which
encourages the selective survival of offspring infected with the
bacterium. These extra elements could affect the survival of the
transgenic insects in as-yet-unknown ways, he says.
"An
individual mosquito can be transformed in the lab, but now the
challenge is to transform an entire population," says Crisanti.
Control measures
Others
worry that intentionally trying to replace a natural population with a
genetically altered one could have a whole host of unintended
consequences on the environment.
The designer mosquito was constructed from Anopheles stephensi,
a mosquito that spreads malaria in Asia, and a synthetic gene for a
peptide called SM1. The genetically modified mosquitoes killed off Plasmodium berghei
that they ingested - the parasite responsible for the mouse version of
malaria. The ultimate target would be to manipulate the genome of A. gambiae,
the mosquito most responsible for malaria worldwide, to be resistant
against one or more of the four parasites that infect humans.
The
strategy is one of many tools that researchers are working on to fight
malaria. Other ideas include creating transgenic mosquitoes with
altered immune systems, shielding mosquitoes against the bug by
vaccinating humans and waiting for the pests to drink up the
antibodies, or immunizing humans against the disease. Bed-nets and
insect sprays are also still crucial to malaria control programmes.
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