The molecular evolution of four anti-malarial immune genes in the Anopheles gambiae species complex

Abstract

Background: If the insect innate immune system is to be used as a potential blocking step in transmission of malaria, then it will require targeting one or a few genes with highest relevance and ease of manipulation. The problem is to identify and manipulate those of most importance to malaria infection without the risk of decreasing the mosquito's ability to stave off infections by microbes in general. Molecular evolution methodologies and concepts can help identify such genes. Within the setting of a comparative molecular population genetic and phylogenetic framework, involving six species of the Anopheles gambiae complex, we investigated whether a set of four pre-selected immunity genes (gambicin, NOS, Rel2 and FBN9) might have evolved under selection pressure imposed by the malaria parasite.

Results: We document varying levels of polymorphism within and divergence between the species, in all four genes. Introgression and the sharing of ancestral polymorphisms, two processes that have been documented in the past, were verified in this study in all four studied genes. These processes appear to affect each gene in different ways and to different degrees. However, there is no evidence of positive selection acting on these genes.

Conclusion: Considering the results presented here in concert with previous studies, genes that interact directly with the Plasmodium parasite, and play little or no role in defense against other microbes, are probably the most likely candidates for a specific adaptive response against P. falciparum. Furthermore, since it is hard to establish direct evidence linking the adaptation of any candidate gene to P. falciparum infection, a comparative framework allowing at least an indirect link should be provided. Such a framework could be achieved, if a similar approach like the one involved here, was applied to all other anopheline complexes that transmit P. falciparum malaria.

Figure 1

Rel2 Bayesian Inference Tree. 50% majority-rule consensus Bayesian (unrooted) tree of Rel2. Numbers on branches are the posterior probabilities of clades, only values above 0.5 are presented. Species names have been abbreviated as follows: ARA: An. arabiensis, BWA: An. bwambae, GAM: An. gambiae, MEL: An. melas, MER: An. merus, and QUA: An. quadriannulatus. The number following the species abbreviation refers to the individual specimen code, whereas the letters A and B differentiate between the two alleles of a single individual specimen. Details of the Bayesian analysis can be provided upon request.

Figure 2

NOS Bayesian Inference Tree. 50% majority-rule consensus Bayesian (unrooted) tree of NOS. Numbers on branches are the posterior probabilities of clades, only values above 0.5 are presented. Species names have been abbreviated as follows: ARA: An. arabiensis, BWA: An. bwambae, GAM: An. gambiae, MEL: An. melas, MER: An. merus, and QUA: An. quadriannulatus. The number following the species abbreviation refers to the individual specimen code, whereas the letters A and B differentiate between the two alleles of a single individual specimen. Details of the Bayesian analysis can be provided upon request.