Mosquito heat and humidity detection in host seeking and oviposition behavior.

The host seeking and oviposition behavior of female mosquitoes makes important contributions to the spread of devastating mosquito-borne diseases. Among the sensory cues mosquitoes exploit to perform these behaviors, the physical cues of heat and humidity serve as key indicators of target presence and proximity. Recent evidence suggests that mosquitoes initially sense heat from a human host in the form of infrared radiation, which they find attractive when combined with other host-associated cues. Infrared detection relies on the cation channel TRPA1, its function potentiated by two opsins. Closer in, mosquito heat seeking is also driven by heat transmitted by convection and conduction. Ionotropic receptor family members have key roles in these processes: Ir21a and Ir93a are required by cooling-activated thermosensors, which are major drivers of heat seeking, while the broadly expressed coreceptor Ir25a is absolutely required for heat seeking. Humans also produce short-range humidity gradients, which appear to act redundantly with heat to drive host seeking. Ionotropic receptors also mediate humidity sensing (hygrosensation): Ir40a supports dry-activated hygrosensors (dry cells), Ir68a supports moist-activated hygrosensors (moist cells) and Ir93a supports both. Dry cells and moist cells function in a redundant manner to promote blood feeding, while moist cells are key drivers of oviposition site seeking. Both Ir68a and Ir93a mutants are unable to find water to lay eggs. Despite a growing parts list of sensory receptor molecules and neurons driving host seeking and oviposition, understanding the mechanisms by which these cells and molecules detect sensory stimuli remains a major challenge.