Studies on the ecology and adaptation of Anopheles gambiae in Mali and their impacts on malaria transmission and control

Anopheles gambiae ecology leads to particular patterns of population density and structure in Mali which have significant impacts on species evolution and prospects of malaria control through genetically modified vectors. How density affects transmission was studied in Niono, Mali, where villages in irrigated areas usually have more anopheline vectors than adjacent non-irrigated villages, but overall malaria prevalence is lower. An analysis of correlations between adult mosquito density and survival, anthropophily and vectorial capacity using field collected adults within the irrigated area showed a decrease in anthropophily and survival with adult density, supporting the "competition hypothesis". This hypothesis holds that high larval densities lead to smaller mosquitoes, which suffer elevated mortality, leading them to be less efficient vectors. Further field experiments showed a modest positive relationship between densities of immatures and larval size and a strong relationship between larval and adult size. Adult survivorship was found to be higher in non-irrigated areas; However, there was no effect of size on survivorship between comparable samples, rejecting the competition hypothesis. Studies in Banambani, Mali, focused on the population structure of the An. gambiae complex. An analysis of bi-weekly adult samples through molecular and demographic techniques revealed a succession of species with changes in density due to climate, and a severe reduction in population size for some types. The bi-weekly samples point to an important role of annual extinction and metapopulation dynamics to malaria, control. Simulation models for studying this question indicated that structure can qualitatively affect the outcome of malaria control schemes using a transposable element to drive a gene for refractoriness into the vector population. Finally, a mechanism of adaptation and speciation dependent on structure, called "ecotypification", was examined through simulations. Results indicated that ecotypification is a viable explanation for chromosomal inversion polymorphism, a precursor to speciation in this taxon. Inversion polymorphism was found to be dependent on population size, structure and niche differentiation. Overall, simulated and empirical data presented strongly suggest that the density and population structure of Anopheles gambiae are of central importance to its evolution and to malaria control.