Graphical Abstract Highlights d Ae. aegypti Ir8a mutant mosquitoes cannot sense lactic acid, a human sweat component d Attraction to humans and human odor is reduced in Ir8a mutant mosquitoes d Ae. aegypti IR8a pathway responds to human-odor cues during blood feeding d The Ir8a mutant host-seeking defect cannot be rescued by other olfactory receptors SUMMARY Mosquitoes use olfaction as a primary means of detecting their hosts. Previously, the functional ablation of a family of Aedes aegypti olfactory receptors, the odorant receptors (ORs), was not sufficient to reduce host seeking in the presence of carbon dioxide (CO 2 ).This suggests the olfactory receptors that remain, such as the ionotropic receptors (IRs), could play a significant role in host detection. To test this, we disrupted the Ir8a co-receptor in Ae. aegypti using CRISPR/Cas9. We found that Ir8a mutant female mosquitoes are not attracted to lactic acid, a behaviorally active component of human sweat, and they lack odor-evoked responses to acidic volatiles. The loss of Ir8a reduces mosquito attraction to humans and their odor. We show that the CO 2 -detection pathway is necessary but not sufficient for IR8a to detect human odor. Our study reveals that the IR8a pathway is crucial for an anthropophilic vector mosquito to effectively seek hosts.

The mosquito, Aedes aegypti, is highly anthropophilic and transmits debilitating arboviruses within human populations and between humans and non-human primates. Female mosquitoes are attracted to sources of blood by responding to odor plumes that are emitted by their preferred hosts. Acidic volatile compounds, including carboxylic acids, represent particularly salient odors driving this attraction. Importantly, carboxylic acids are major constituents of human sweat and volatiles generated by skin microbes. As such, they are likely to impact human host preference, a dominant factor in disease transmission cycles. A more complete understanding of mosquito host attraction will necessitate the elucidation of molecular mechanisms of volatile odor detection that function in peripheral sensory neurons. Recent studies have shown that members of the variant ionotropic glutamate receptor gene family are necessary for physiological and behavioral responses to acidic volatiles in Aedes. In this study, we have identified a subfamily of variant ionotropic receptors that share sequence homology across several important vector species and are likely to be activated by carboxylic acids. Moreover, we demonstrate that selected members of this subfamily are activated by short-chain carboxylic acids in a heterologous cell expression system. Our results are consistent with the hypothesis that members of this receptor class underlie acidic volatile sensitivity in vector mosquitoes and provide a frame of reference for future development of novel mosquito attractant and repellent technologies.

No abstract

The mosquito,Aedes aegypti, is highly anthropophilic and continues to transmit debilitating arboviruses within human populations. Female mosquitoes are attracted to sources of blood by responding to odor plumes that are emitted by their preferred hosts. Acidic volatile compounds, including carboxylic acids, represent particularly salient odors driving this attraction. Importantly, carboxylic acids are major constituents of human sweat and volatiles generated by skin microbes. As such, they are likely to impact human host preference, which is a dominant factor in disease transmission cycles. A more complete understanding of mosquito host attraction will necessitate the elucidation of molecular mechanisms of volatile odor detection that function in peripheral sensory neurons. Recent studies have shown that members of the variant ionotropic glutamate receptor gene family are necessary for physiological and behavioral responses to acidic volatiles inAedes. In this study, we have identified a subfamily of variant ionotropic receptors that share sequence homology across several important vector species and are likely to be activated by carboxylic acids. Moreover, we demonstrate that selected members of this subfamily are activated by short chain carboxylic acids in a heterologous cell expression system. Our results are consistent with the hypothesis that members of this receptor class underlie acidic volatile sensitivity in vector mosquitoes and provide a frame of reference for future development of novel mosquito attractant and repellent technologies.

The human skin bacteria play an important role in the production of volatiles that attract mosquitoes. Using some of the most abundant human skin bacterial species, we created in vitro community models to assess whether increased microbial biodiversity could reduce human attractiveness to females of the dengue fever mosquito Aedes aegypti and whether co-culturing bacterial commensals affects overall attraction. More complex bacterial models were less attractive to female mosquitoes than the simplest models. For instance, the triple bacterial community model was approximately three times less attractive than Staphylococcus epidermidis alone. Our data show, for instance, that an in vitro community model mimicking the skin composition of a highly attractive individual to the anthropophilic Anopheles gambiae was also more attractive to anthropophilic Ae. aegypti than a community model mimicking the skin composition of a poorly attractive individual to An. gambiae. In line with these results, volatile analyses of the blends emitted by the different in vitro community models showed that the more complex models had lower emission overall. Effects on mosquito responses differed sharply when the different bacteria species were sharing the same resources used for growth, showing that either competition or commensalism may influence their relative growth, and that this consequently can influence mosquito responses. We conclude that studies on mosquito responses to skin volatiles need to take the microbial community into account.