Malaria mosquitoes split (genetic) ways

CORNELL (US) —Neighboring malaria mosquito groups in Sub-Saharan Africa have taken two different evolutionary approaches to fight pathogens, new research shows.

Genes in animal immune systems may evolve in one of two main ways in the constant fight against pathogens: They may evolve diverse forms of genes (alleles) to fight a wide variety of pathogens, or when only a few pathogens dominate, they may evolve one or a few alleles that specialize against common infections.

Researchers have found evidence of both of these adaptive strategies occurring in the same immune-defense genes in different subpopulations of the human malaria vector mosquito, Anopheles gambiae.

The research, appearing in the journal PLoS Biology, focuses on a cluster of genes called APL1, which are part of the mosquitoes’ immune defense against malaria parasites and other pathogens.

Malaria infects humans, but it also sickens the mosquitoes that transmit it.

“From a purely evolutionary biology perspective, seeing both of those patterns occur in a single gene is very unusual; it validates both models,” says Brian Lazzaro,  associate professor of evolutionary genetics at Cornell University.

The APL1 genes of one A. gambiae subpopulation (the “S” population) carried 10 times more genetic diversity than typical A. gambiae genes.

Meanwhile, a second mosquito subpopulation (the “M” population)—which lives in the same geographic area as the “S” population—showed little genetic diversity among APL1 alleles, indicative of recent, strong natural selection.

“In the ‘M’ population, we see this recent selection targeting a single or restricted set of pathogens,” Lazzaro says.

There is no difference in transmission of malaria to humans between the two populations, which leads to the conclusion that a pathogen other than malaria has placed a strong natural selection on the “M” population to specialize its defense system.

“The two populations differ in where they lay their eggs, suggesting that there could be a pathogen in the water that infects the larvae of the ‘M’ population,” Lazzaro says. “Malaria parasites may be incidental here.”

An independently published report finds a related immune-defense gene, called Tep1, exhibited the same pattern of high genetic diversity in the “S” population and just a few alleles in the “M” population. Tep1 genes, which reside on a separate chromosome, express proteins that physically bind to APL1 proteins to form a disease-fighting protein complex.

“Finding this same pattern of evolution in different parts of the genome is unusual and suggests that the genes may be evolving in concert,” Lazzaro says.

To vector biologists and public health researchers, these genes could be important in fighting malaria.

“It’s possible that the differences in genetic diversity could change disease transmission to humans by mosquitoes from these subpopulations, but there is no evidence of this so far,” Lazzaro adds.  “It could also change the susceptibility of these mosquitoes to other pathogens.”

Researchers from the University of Minnesota, the Institut Pasteur in Paris, the University of Bamako in Mali and Institut de Recherche pour le Developpement in Cameroon contributed to the study, funded by the National Institutes of Health.

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