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Selfish genes are key to driving populations of mosquitoes resistant to malaria and dengue fever into wild populations, say researchers.

Scientists have managed to create genetically modified (GM) mosquitoes resistant to malaria and dengue fever, but they need to ensure that the GM populations dominate the natural ones to achieve significant disease control (see Scientists create GM mosquitoes to fight malaria and GM mosquitoes stop dengue virus replicating).

To do this, genes conferring resistance need to be combined with genes that ensure the dominance of resistant mosquitoes over the natural population.

Research published in Science last week (29 March) identifies a class of selfish genes known as "Medea" elements that could be used to this effect.

Medea gene manipulation induces the death of all offspring that do not inherit the required genetic code. It is thought to work by encoding both a toxin and an antidote — if the toxin is present in the mother's eggs without the antidote, offspring die.

The researchers ― Bruce Hay, Chun-Hong Chen and colleagues from the US-based California Institute of Technology ― created a synthetic Medea-like genetic element in the frutifly Drosophila.

Instead of making a genetic element that coded for a toxin, they made elements that silenced a gene called Myd88. Myd88 is crucial to the early development of  fruitfly embryos.

Embryos resulting from these eggs died. But if the embryos carried the team's Medea element — an antidote in the form of an extra copy of the Myd88 gene switched on after fertilisation ― development was normal.

The next step, say the researchers, is to add a resistance gene into the element to ensure that the flies are resistant to disease.

They point out that to successfully decrease malaria and dengue fever infection rates, the genes that convey dominance and those that provide resistance must be tightly linked.

The technology should be transferable to mosquitoes once researchers know more about the genes involved in mosquito development.

Kenneth Olson of North Carolina State University, United States, says the new study is a big step forward in making the notion of transgenic mosquitoes 'fly', according to Science.

Dengue fever kills about 20,000 of the 50 million people it infects annually worldwide. Malaria kills more than a million people globally, mostly young children in Africa, according to the World Health Organization.

Link to full paper in Science 

Reference: Science doi: 10.1126/science.1138595 (2007)