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Making mosquito sperm glow bright green could help fight malaria, say researchers.

The team says this is a reliable way of identifying males, which is crucial to efforts to eradicate the insects.

Previously, scientists have successfully controlled pest insects by releasing large numbers of both sexes that have been sterilised by irradiation, to mate with the wild population.

Because the females of many insect pest species mate only once, this method reduces the number of offspring. It also means that dangerous chemical sprays are unnecessary.

But, unlike male mosquitoes, which never feed as adults, sterile females can still transmit malaria when they feed on human blood.

So, to control malaria, only sterile males should be released, say researchers at Imperial College London, United Kingdom.

Until now it has been virtually impossible to identify the sex of mosquito larvae, meaning it was impractical to separate the sexes and sterilise only males.

In research published online today (10 October), the Imperial College team modified mosquitoes to produce a fluorescent chemical by inserting a jellyfish gene into their DNA.

The inserted gene works only in sperm — meaning that female offspring of the genetically modified mosquitoes do not glow, but males do.

Using a laboratory 'sorting machine' that can detect fluorescence, the team accurately separated male and female mosquito larvae.

Crucially, the researchers found that the glowing sperm could be seen in the females after they had mated with the males — proof that modifying the mosquitoes did not affect their mating.

Lead researcher Andrea Crisanti told SciDev.Net that the next stage in their research is to test whether populations of captive mosquitoes could be wiped out by flooding the colonies with large numbers of sterile males that had been identified using fluorescence.

Eventually, he hopes to test his theory outdoors with wild mosquitoes.

Crisanti says the fact that the insects would be sterile and unable to pass on their genes should ease concerns about releasing genetically modified insects into the wild.

Peter Atkinson at the University of California at Riverside, United States, told SciDev.Net that the research was significant, simple and elegant.

"If it is only the males — since they mate multiple times — that are the 'key' to the eradication programme, then why waste money and resources rearing, irradiating and releasing females?" he adds. 

Atkinson points out that Crisanti's method paves the way for getting other 'foreign' genes to work only in male insects.

"These might be lethal genes which, in different genetic control programmes, kill males."

Crisanti's study will be published in the November 2005 issue of Nature Biotechnology.

Male and females mosquito larvae are hard to tell apart, but
with Crisanti's method, the male's gonads glow green

Reference: Nature Biotechnology doi: 10.1038/nbt1152 (2005)

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