Abstract
BackgroundMosquitoes of the Anopheles gambiae species complex are the primary vectors of human malaria in sub-Saharan Africa. Many host genes have been shown to affect Plasmodium development in the mosquito, and so are expected to engage in an evolutionary arms race with the pathogen. However, there is little conclusive evidence that any of these mosquito genes evolve rapidly, or show other signatures of adaptive evolution.MethodsThree serine protease inhibitors have previously been identified as candidate immune system genes mediating mosquito-Plasmodium interaction, and serine protease inhibitors have been identified as hot-spots of adaptive evolution in other taxa. Population-genetic tests for selection, including a recent multi-gene extension of the McDonald-Kreitman test, were applied to 16 serine protease inhibitors and 16 other genes sampled from the An. gambiae species complex in both East and West Africa.ResultsSerine protease inhibitors were found to show a marginally significant trend towards higher levels of amino acid diversity than other genes, and display extensive genetic structuring associated with the 2La chromosomal inversion. However, although serpins are candidate targets for strong parasite-mediated selection, no evidence was found for rapid adaptive evolution in these genes.ConclusionIt is well known that phylogenetic and population history in the An. gambiae complex can present special problems for the application of standard population-genetic tests for selection, and this may explain the failure of this study to detect selection acting on serine protease inhibitors. The pitfalls of uncritically applying these tests in this species complex are highlighted, and the future prospects for detecting selection acting on the An. gambiae genome are discussed.
Highlights
Mosquitoes of the Anopheles gambiae species complex are the primary vectors of human malaria in sub-Saharan Africa
There has been a substantial effort to identify the genes involved in the mosquito immune response against Plasmodium, including studies to identify genes associated with variation in vector competence [1,2,3,4]
The results show how standard population-genetic tests for selection may be difficult to apply in the An. gambiae species complex; this is due to for both demographic and phylogenetic factors that are already widely known, and further supported by the present data
Summary
It has been widely hypothesized that these immune response genes may be subject to strong parasite-mediated selection, such as that which occurs in a coevolutionary 'arms-race' [5,6]. Such arms-races involve strong reciprocally-antagonistic selection, leading to the frequent and rapid fixation of (page number not for citation purposes). Malaria Journal 2009, 8:117 http://www.malariajournal.com/content/8/1/117 new alleles. This reduces within-species diversity, while driving between-species protein divergence, and leaves a genomic signature of past selection that can be identified through DNA sequence analysis [7,8]. DNA sequence analysis and the tools of population genetics can augment understanding of immune gene function in host-parasite interaction by identifying genes that are the target of parasite adaptation, and even distinguish between forms of parasite-mediated selection [5,6,9]
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