Evolutionary lability of odour-mediated host preference by the malaria vector Anopheles gambiae

Many species of disease-vector mosquitoes display vertebrate host specificity. Despite considerable progress in recent years in understanding the proximate and ultimate factors related to non-random host selection at the interspecific level, the basis of this selection remains only partially understood. Anopheles gambiae sensu stricto, the main malaria vector in Africa, is considered a highly anthropophilic mosquito, and host odours have been shown to play a major role in the host-seeking process of this species. Studies on host preference of An. gambiae have been either conducted in controlled conditions using laboratory reared mosquitoes and worn stockings as host-related stimuli, or have been done in the field with methods that do not account for internal (e.g. age of sampled mosquitoes) and/or environmental effects. We explored differential behavioural responses to host odours between two populations of the same sibling species, An. gambiae in semi-field conditions in Burkina Faso. The behavioural responses (i.e. degree of activation and strength of anemotaxis) were investigated using a Y-olfactometer designed to accommodate whole hosts as a source of odour stimuli. Two strains of An. gambiae (3 to 4-day-old female) from laboratory Kisumu strain, and from field-collected individuals were confronted to combinations of stimuli comprising calf odour, human odour and outdoor air. In dual-choice tests, field mosquitoes chose human odour over calf odour, outdoor air over calf odour and responded equally to human and outdoor air, while laboratory mosquitoes responded equally to human and calf odour, human odour over outdoor air and calf odour over outdoor air. Overall, no effect of CO(2) exhaled by humans and calves neither on the proportion of activated mosquitoes nor on the relative attractiveness to odour stimuli was found. We report for the first time an intraspecific variation in host-odour responses. This study clearly suggests that there may be genetic polymorphism underlying host preference and emphasizes that the highly anthropophilic label given to An. gambiae s.s. must be carefully interpreted and refer to populations rather than the whole sibling species.