Odour-mediated host preference in mosquitoes

The mechanisms underlying host preference in mosquitoes are ultimately a result of optimal foraging on currently available hosts and historical patterns of host availability. The proximate mechanisms are regulated by available host cues and the nature of the mosquitoes’ responses to these cues. Although mosquito host preference has a genetic background and the response to specific host cues are adaptive, mosquitoes exhibit a high degree of plasticity in their host preference. The purpose of this study was to analyse the extent of this variability in the disease vectors Aedes aegypti, Anopheles gambiae and Culex quinquefasciatus, and if their behavioural response is constrained by qualitative and quantitative differences in host cues. I investigated the sensory mechanisms and constraints regulating mosquito behaviour in response to various carbon dioxide (CO2) concentrations, within the range emitted from vertebrate hosts. The temporal capacity of the CO2-ORNs in Ae. aegypti and An. gambiae is higher than in Cu. quinquefasciatus, which translated into behavioural constraints leads to an increased time to take off at high concentrations. The altered sensitivity of CO2-ORNs in transient elevated background levels of CO2 also constrain the host-seeking behaviour in Ae. aegypti. Take off flight was impeded, correlating with a behavioural threshold in the net CO2-ORN response. Hence, sensory limitations of CO2-ORNs induce behavioural constraints at higher CO2 concentrations, which may be partially responsible for species-specific host preferences.The specificity and sensitivity of host volatile blends play a key role in mosquito host recognition and non-host avoidance. I determined that Ae. aegypti and An. gambiae consider (R)-1-octen-3-ol as a host cue, whereas Cu. quinquefasciatus perceives it as a non-host cue, when not in the context of a preferred host. Hence, host recognition is regulated by the perception of specific volatiles in the context of a blend. This specificity appears to be consistent with host preference, as Ae. aegypti is attracted by a wide range of (R)-1-octen-3-ol concentrations, while in Cu. quinquefasciatus it elicits no attraction or even repels at higher concentrations. The behavioural and physiological specificity and sensitivity to (R)-1-octen-3-ol is the output of the sensitivities of the orthologous mosquito odorant receptors, the OR8s. The functional tuning of these receptors and their cognate neurons was reflected in minor differences in the receptors amino acid structure. These findings provide a substrate with which to examine how the sensory system of a mosquito interprets host and non-host odour cues and thus produces host recognition behaviour.