Mosquitoes Have Different Evolutionary Strategies To Avoid Malaria

Academic Journal
Main Category: Tropical Diseases
Article Date: 09 Mar 2011 - 4:00 PDT


The immune system of animals is constantly evolving under pressure from pathogens. Immune genes can have high genetic diversity in order to defend against a wide array of pathogens, or low genetic diversity to specialize against one or a few common infections. A team of researchers based at Cornell University and Institut Pasteur in Paris, show in a paper publishing today in the open access journal PLoS Biology, that both modes of evolution occur in critical anti-malaria genes of the vector mosquito Anopheles gambiae.

The APL1 genes, which form an essential part of the mosquito's immune defense against malaria parasites and other pathogens, carry 10 times more genetic diversity than standard genes in one mosquito population (the 'S' population). In a second mosquito population (the 'M' population), strong natural selection has eliminated this diversity in favor of single major variants at the same genes, even though the two populations overlap in geographic range. Both M and S mosquito populations efficiently transmit human malaria despite the evolutionary differences in their immune systems. It is extremely unusual for insect defense genes to display the high diversity that is seen in the S form of A. gambiae. The APL1 genes in M form mosquitoes have lowered genetic diversity and other features that are characteristic of recent, strong natural selection acting on the genes.

"The adaptive evolution we see in the 'M form' population suggests that these mosquitoes may have recently been exposed to a novel, severe pathogen," said Brian Lazzaro of Cornell University, the principal investigator on the study. "We have no idea what this pathogen is, although there is no reason to believe that the human malaria parasite is responsible. In fact, we think it is more likely to be a pathogen of the mosquito larvae." Although adult A. gambiae mosquitoes of both the M and S populations can be found in the same villages, the aquatic larvae of the two forms are found in different environments that may contain different pathogens.

The pattern of genetic diversity observed in the APL1 genes is mirrored in another anti-malaria gene, Tep1, whose protein product physically binds to the APL1C protein. The Tep1 gene is also highly diverse in the S form and has been the target of strong recent selection in the M form, according to a recent independently published report. "The similarity in patterns of genetic diversity suggest that the APL1 and Tep1 defense genes evolve in concert despite residing on separate chromosomes," adds Lazzaro. "It is very unusual to see this kind of coordinate evolution in physically separate genes."

Lazzaro and Vernick agree that future experiments will be required to determine how the diverse APL1 alleles differ in function, and how this diversity protects mosquitoes from infection by varied pathogens, including malaria parasites.

Funding: This work was supported by National Institutes of Health grant R01 AI062995. The funding agency had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests statement: The authors declare that no competing interests exist.

Citation:

"Exceptional Diversity, Maintenance of Polymorphism, and Recent Directional Selection on the APL1 Malaria Resistance Genes of Anopheles gambiae"
Rottschaefer SM, Riehle MM, Coulibaly B, Sacko M, Niare´ O, et al. (2011).
PLoS Biol 9(3): e1000600. doi:10.1371/journal.pbio.1000600

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