On dengue trail

DISEASES like dengue and yellow fever are transmitted by the mosquito Aedes aegypti. The mosquito originated in sub-Saharan Africa, where some of its populations still breed in forests and feed on animals. Many of its populations, however, have evolved to thrive in human habitats in and outside Africa. It is unclear whether the global populations of Aedes aegypti are a result of a single or multiple domestications, that is, acclimatisation of the mosquito in human surroundings. But a group of scientists seem to have found the answer.

Scientists from the Department of Ecology and Evolutionary Biology, Yale University in the US examined 1,152 mosquitoes from 24 locations spanning five continents using genetic markers called microsatellites. Used in forensics, microsatellites are repeating sections of DNA. Aedes aegypti individuals were divided into groups based on the length of each microsatellite. Through a statistical computer programme, scientists found the populations were genetically different.

The scientists identified two distinct genetic clusters: one included domestic populations outside Africa and, the other, both domestic and forest populations within Africa. “Most populations of Aedes aegypti in Africa are different from those across the world,” said lead author Julia Brown. She continued, “Even within Africa, the populations have evolved multiple times living alongside humans, implying they have the ability to continue invading human habitats.” Brown warned, “this may continue happening as people turn natural habitats into cities, raising risks of an epidemic.”

Understanding worldwide patterns of genetic differentiation is vital as they help understand the disease transmission capabilities of Aedes aegypti populations. For example, subspecies Aedes aegypti formosus is less competent in transmitting dengue than Aedes aegypti aegypti. Studies on the former cannot be used to understand the threat posed by Aedes aegypti aegypti, claimed the researchers of the study, published in the January 2011 issue of Proceedings of the Royal Society B.

Care must be taken during eradication campaigns against Aedes aegypti formosus to avoid opening niches for the more competent aegypti subspecies to invade, the scientists added.

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