Mosquito odor receptor gene targets to alter vector host choice

Abstract

Olfaction plays an important role in mosquito behaviors, including foraging for nectar and identifying potential hosts for a blood meal. Mosquitoes detect semiochemicals through odorant receptors, each of which interact with specific odorants. Differences in feeding behaviors have significant consequences for the vectorial capacity of Anopheles mosquitoes. Mosquitoes in the genus Anopheles vector the pathogens causing human malaria, which resulted in over 600,000 deaths in 2020 (WHO World malaria report 2021). Yet many species of Anopheles, including An. quadriannulatus, are competent for malaria but are not significant vectors (Takken & Verhulst, 2012). A defining characteristic of the most important vectors of malaria, which includes the significant urban vector An. stephensi, is a proclivity for feeding on human blood. Multiple sensory cues influence mosquito behaviors, and olfaction is particularly important to host preference. Among mosquito chemosensory gene families, odorant receptors (ORs) are receptive to volatile components of sweat, breath, and skin microbiota which mosquitoes use to identify hosts for blood feeding. The hypothesis that odor receptors with differences in transcriptional abundance between anthropophilic and zoophilic species, male and female mosquitoes, and blood fed and non-fed females mediate differences in mosquito host choice was tested. Using the Drosophila empty neuron system, we characterized odorant receptors from two species of Plasmodium competent mosquito, one anthropophilic (An. stephensi) and the other zoophilic (An. quadriannulatus). The odorant receptors were characterized with a panel of structurally diverse odorants, many of which are components of human and animal odors. Anopheles stephensi is an important urban malaria vector in the Middle-East, Asia and now Africa. The first functional characterizations of chemosensory genes in this species are presented in this dissertation. Using RNAseq, we measured chemoreceptor expression patterns in the An. stephensi mouthparts, which are important to mosquito host seeking behavior. An odor receptor, An. stephensi Or8, which is prominently expressed in the mouthparts was functionally characterized in the empty neuron system. Or8 was found to strongly interact with human sweat odorants, including 1-octen-3-ol and sulcatone. Moreover, capitate peg B neurons on the An. stephensi maxillary palps responded to odorants with the same activity pattern identified for Or8 expressing Drosophila empty neurons. This work is presented in Chapter 2 of this dissertation. We characterized several Anopheles odorant receptors with expression differences between anthropophilic and zoophilic species, which are upregulated in female and non-blood fed mosquitoes. The odor receptors characterized had diverse activities, which included inhibiting the empty neuron in response to odorants. The diverse activity patterns allow for comparisons with odor receptors previously characterized in other species, and provides new insights into the conservation of odor receptor functions between Anopheles which diverged ~50 MYA. One odor receptor from the zoophilic An. quadriannulatus, AqOr9, detected aromatic molecules from the original odorant panel. AqOr9 activity was further explored in a reiterative process, of selection for molecules with structural similarities to the most activating odorants, in order to identify environmental sources which activate olfactory receptor neurons expressing this receptor. It was found that the receptor interacts most strongly to guaiacols, which to humans have a smoky smell, and which are released upon lignin pyrolysis (Chapter 3). The final chapter explores a mechanism to alter mosquito host choice. We propose that host choice of vector mosquitoes might be altered by the knockout or substitution of odor receptors sensitive to predominantly human-emitted odors with receptors sensitive to odors emitted in greater abundance by other animals or environmental sources. We develop a construct to replace activity of An. stephensi capitate peg B neurons using Cas9 induced double-strand break repair. Ultimately, this research informs an approach to alter vector host choice using gene drives to break the human-mosquito malaria transmission cycle.