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Blood-sucking killers

August 9, 2011

Genetic modification could hold the key to fighting one of the major carriers of malaria. Researchers have succeeded in 'silencing' a sperm gene in one species of mosquito - without diminishing its appeal to females.

The Anopheles gambiae mosquito, the main vector for the malaria parasite
Nearly one million people died from malaria in 2008, the World Health Organization saidImage: picture alliance/dpa

Anopheles gambiae sensu stricto, the scientific name for the deadliest malaria carrier in the world, is the target of a new attempt at population control.

With the help of genetic engineering, scientists in Britain have a developed a group of male mosquitoes without sperm - a breakthrough that could curb the species' numbers and with it, the spread of a disease that claims the lives of thousands of people each year.

The team's study, published online Monday in the journal Proceedings of the National Academy of Sciences, said female mosquitoes, which feed on human blood to aid in egg production, cannot tell the difference between fertile and sterile males.

This work builds on previous results published in April 2011.

A patient inside a mosquito net in India
The study's authors said new approaches were needed to stop the spread of malariaImage: DW

As a result, spermless males - despite their reproductive shortcomings - are able to attract females to mate. And though the sterile males have no sperm, they do have seminal fluid.

The researchers, from Imperial College London and the University of Oxford, found that females that mated with spermless males displayed the same behavior as those that mated with normal partners, and both groups laid large numbers of eggs.

But there was one key difference: None of the eggs laid by female An. gambiae mosquitoes that had mated with sterile males hatched - and that meant no baby mosquitoes.

The mating game

The mating process dramatically reduces the female An. gambiae's desire to copulate again.

To keep the population control experiment on course, females that partnered with spermless specimens would also have to exhibit no desire to re-mate.

According to the study, the presence of sperm played no role: The findings showed that these females still were not receptive to new mates, despite the fact that their first batch of eggs had gone unfertilized.

An Anopheles stephensi mosquito
Drakeley said there were up to 20 different vectors for malaria in parts of southeast AsiaImage: Universitätsklinikum Heidelberg/Frischknecht

And since An. Gambiae females do not live long after laying their eggs, a sterile mating would "completely eliminate reproduction for the duration of their lives," the study said.

Scientists used RNA interference to create sterile males by targeting a gene that regulates germ cell development.

"The males exhibited very small testes," the researchers wrote. "And microscopic analysis confirmed their gonads did not contain any sperm cells." The team used a similar number of fertile mosquitoes as a control group.

Despite their inability to produce offspring, the spermless mosquitoes were not ignored by females during the mating process. The scientists even cited other research suggesting that it is "metabolically costly" to make sperm - meaning sterile male mosquitos could even be at an advantage compared to their sperm-carrying counterparts.

Nearly three-quarters of females that mated with spermless males laid eggs, compared to 83 percent for the control group - but the study found no significant difference in the numbers.

Practical applications

But how might the study transfer from the lab to the outside world?

Scientists acknowledged the need to compare how competitive sterilized versus fertile males would be in a field environment - particularly to determine the number of insects needed to effectively control the mosquito population.

Chris Drakeley, who was not involved in the study and serves as director of the Malaria Centre at the London School of Hygiene and Tropical Medicine, said he was very supportive of the latest research - but said the work needed to be taken beyond a controlled environment.

A mother in western Kenya watches over her child, who is sick with malaria
The Anopheles gambiae mosquito is the main vector for malaria in AfricaImage: AP

"As a scientist, it's something that someone has to see operate in the field, and I think that part of the big hill these genetic modifications have to overcome is translating these genetic modifications into field studies," Drakeley told Deutsche Welle.

Moreover, the composition of the vectors responsible for malaria is very complex; Drakeley said there were up to 20 in some areas of southeast Asia.

"The vector that they talk about - Anopheles gambiae - is widespread all over Africa," he added. "I know of very, very few situations where it is the only vector." That raises the question of whether curbing one mosquito species' numbers would influence the populations of others.

Drakeley said that the advantage to some malaria prevention measures, such as insecticide-treated nets and the use of larvicides, was the fact that those approaches were indiscriminate - unlike genetic modification targeting a certain species.

"With this particular approach, and anything else that is species-specific, you have to consider what other vectors are there and how one deals with those," he said.

But the study said the "constant spread" of the disease nevertheless underscored the need for new approaches.

Author: Amanda Price
Editor: Cyrus Farivar

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