Sequencing the genomes of numerous Anopheles mosquito species from locations around the world has shown why some can carry and spread the deadly disease malaria while others don't, researchers say.

Evolution among more than 500 different Anopheles species has left certain varieties genetically better equipped for transmitting the disease than even some close cousins, they say.

Only about 60 of those species can transmit the malaria parasite Plasmodium and infect humans, an international research team reports in the current issue of Science.

Those parasites cause an estimated 200 million cases of malaria each year, resulting in than 600,000 deaths annually.

Most of those deaths happen in sub-Saharan Africa, the home of Anopheles gambiae, the major vector species of mosquito.

Genome studies have provided insights into the way this particular mosquito species evolved, becoming highly specialized for living among and feeding on humans, while other species evolved in different ways.

Understanding the evolution behind the differences among species might lead to improvements in way to control mosquitoes to stop malaria transmission, the researchers say.

The international research team, in an attempt to understand the evolutionary differences, mounted an effort to sequence the genomes of 16 Anopheles species, all of which evolved from a common ancestor around 100 million years ago.

"We selected species from Africa, Asia, Europe, and Latin America that represent a range of evolutionary distances from Anopheles gambiae, a variety of ecological conditions, and varying degrees of vectorial capacity," says Nora Besansky of the University of Notre Dame, one of the study leaders.

The results allowed researchers to examine different features of mosquito biology including their reproductive processes, insecticide resistance, immune response, and chemosensory systems.

"With the availability of genome sequences from Anopheles mosquitoes of divergent lineages, variable adaptations, and differing disease-transmission abilities, we now have the exciting opportunity to significantly improve our understanding of these important malaria vectors and develop new strategies to combat malaria and other mosquito-borne diseases," says researcher Zhijian Tu, a biochemistry professor at Virginia Tech.

One key finding of the researchers is that A. Gambiae, the most deadly species, has increased its transmission capabilities by exchanging genes at the chromosome level, greatly enhancing its capacity to carry and pass on the malaria parasite.

That suggests an inherent ability to evolve quickly that has given A. gambiae the flexibility to adapt to new environments and to seek out human blood to become a disease scourge to all humanity, the researchers said.

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