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Chew fly, don't bother me

August 30, 2001

Genetic engineering may offer new hope for controlling malaria, entomologists will tell the Royal Entomological Society's meeting Entomology 2001: "Insects and disease", to be held at the University of Aberdeen on 10-12 September 2001. Alex Schwartz of the University of Aarhus in Denmark, and Professor Jacob Koella of the Université Curie in Paris, will tell the meeting that genetic engineering could allow entomologists to make mosquitoes immune to the malaria parasite, so that they no longer pass the disease on to humans.

Entomologists are working on identifying the gene responsible for immunity to malaria - termed "refractoriness" - in Anopheles gambiae, which they could then incorporate into mosquito populations. However, the spread of the gene through the mosquito population depends on whether there is a biological "cost" to the mosquito. According to Schwartz: "We are looking at what the mosquito gains and loses by being immune to malaria. Immunity will spread through the population only if the benefits outweigh the costs."

For example, one benefit of immunity is that the mosquito would no longer be damaged by the malaria parasite. Another is that when infected by malaria, the parasite manipulates the mosquito to make it feed most only when the parasite is sufficiently developed to infect the human or animal that the mosquito bites. Schwartz says: "Our research in Tanzania indicates that simply harbouring the parasite can increase mortality of the mosquito, and our laboratory work shows that the parasite can manipulate the biting behaviour of the mosquito. The oocyst - or non-infective stage - of the malaria parasite prevents mosquitoes from blood feeding as much as they would if they were not infected. Thus, what can be seen as suicidal, kamikaze-like behaviour of the mosquito is beneficial to the parasite."

One way that mosquitoes achieve immunity against malaria is to encapsulate - or "melanise" - the parasite. But Schwartz has found that melanisation can be costly to mosquitoes, as those with strong melanisation lay fewer eggs and develop more slowly: "Our results show that the mosquito's melanisation response is associated with benefits and costs that might limit the spread of immunity in malaria-control programmes based on genetic engineering," he says.

Schwartz adds: "Our results emphasise the complexity involved in the evolutionary ecology of refractoriness. These aspects must be extensively studied, from the molecular level to natural populations, to understand and predict conditions under which genetic engineering can help reduce malaria transmission."

A. gambiae's genome is currently being sequenced, and a draft copy of its genetic code will be complete early in 2002. This will make A. gambiae only the second insect (after Drosophila melanogaster) whose genome has been sequenced. Professor Frank Collins of Notre Dame University in the USA will be giving a progress report on the A. gambiae genome project to Entomology 2001 and will outline future research that will become possible once the mosquito's genome has been sequenced.

New ways of controlling malaria are desperately needed as the malaria parasite becomes resistant to the drugs used to treat the disease, and A. gambiae (the major vector of malaria in Africa) develops resistance to insecticides. Half the world's population lives in malarious areas, and malaria kills two million people each year, most of them children living in sub-Saharan Africa.

Professor Jacob Koella and Alex Schwartz will present their full findings at 15:40 on Tuesday 11 September and 16:00 on Wednesday 12 September 2001, respectively. Professor Frank Collins will present his paper at 13:00 on Monday 10 September 2001.

Royal Entomological Society (RES)