insects Article Characterization of Anopheles stephensi Odorant Receptor 8, an Abundant Component of the Mouthpart Chemosensory Transcriptome Zachary Speth 1,2 , Gurlaz Kaur 1,2 , Devin Mazolewski 1,2 , Rayden Sisomphou 2 , Danielle Denise C. Siao 2 , Rana Pooraiiouby 2,† , Hans Vasquez-Gross 3 , Juli Petereit 3 , Monika Gulia-Nuss 4 , Dennis Mathew 5 and Andrew B. Nuss 2,4, *   Citation: Speth, Z.; Kaur, G.; Mazolewski, D.; Sisomphou, R.; Siao, D.D.C.; Pooraiiouby, R.; Vasquez-Gross, H.; Petereit, J.; Gulia-Nuss, M.; Mathew, D.; et al. Characterization of Anopheles stephensi Odorant Receptor 8, an Abundant Component of the Mouthpart Chemosensory Transcriptome. Insects 2021, 12, 593. https://doi.org/10.3390/ insects12070593 Academic Editors: Paul T. Leisnham and Bruce Noden Received: 25 May 2021 Accepted: 26 June 2021 Published: 30 June 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Cell and Molecular Biology Graduate Program, University of Nevada, Reno, NV 89557, USA; zspeth@nevada.unr.edu (Z.S.); gurlazk@nevada.unr.edu (G.K.); dmazolewski@nevada.unr.edu (D.M.) 2 Department of Agriculture, Veterinary and Rangeland Sciences, University of Nevada, Reno, NV 89557, USA; raydensisomphou1@gmail.com (R.S.); danielle.siao@gmail.com (D.D.C.S.); rpooraiiouby@med.unr.edu (R.P.) 3 Nevada Bioinformatics Center, University of Nevada, Reno, NV 89557, USA; hvasquezgross@unr.edu (H.V.-G.); jpetereit@unr.edu (J.P.) 4 Department of Biochemistry and Molecular Biology, University of Nevada, Reno, NV 89557, USA; mgulianuss@unr.edu 5 Department of Biology, University of Nevada, Reno, NV 89557, USA; dennismathew@unr.edu * Correspondence: nussab@unr.edu; Tel.: +1-775-327-5096 † Current Address: University of Nevada Reno School of Medicine, Reno, NV 89557, USA. Simple Summary: Human malaria is transmitted by mosquitoes in the genus Anopheles—particularly species that prefer to feed on humans. The mosquito’s sense of smell drives this preference; however, most studies have focused on species native to Sub-Saharan Africa. Malaria vectors in other parts of the world may use different odorants to choose hosts. We, therefore, focused on Anopheles stephensi, the south Asian malaria mosquito, in this study. Mosquitoes have different organs specialized for odor perception, such as the antennae; however, we focused on the mouthparts (primarily the maxillary palp and labella) in this study. We used the RNAseq technique to determine which odor receptors are present in the mouthparts and then focused on one of these receptors: Or8. Using a technique known as the Drosophila empty neuron system, we tested this receptor’s ability to detect different odorants, particularly chemicals emitted by humans. This receptor in An. stephensi detected similar odors to a homologous receptor in an African species, Anopheles gambiae, with the exception of the chemical sulcatone. This chemical is an important attractant in other disease-transmitting mosquitoes and suggests that different mosquito species may be using odors differently to find hosts. Abstract: Several mosquito species within the genus Anopheles are vectors for human malaria, and the spread of this disease is driven by the propensity of certain species to feed preferentially on humans. The study of olfaction in mosquitoes is important to understand dynamics of host-seeking and host-selection; however, the majority of these studies focus on Anopheles gambiae or An. coluzzii, both vectors of malaria in Sub-Saharan Africa. Other malaria vectors may recognize different chemical cues from potential hosts; therefore, in this study, we investigated An. stephensi, the south Asian malaria mosquito. We specifically focused on the mouthparts (primarily the maxillary palp and labella) that have been much less investigated compared to the antennae but are also important for host-seeking. To provide a broad view of chemoreceptor expression, RNAseq was used to examine the transcriptomes from the mouthparts of host-seeking females, blood-fed females, and males. Notably, AsOr8 had a high transcript abundance in all transcriptomes and was, therefore, cloned and expressed in the Drosophila empty neuron system. This permitted characterization with a panel of odorants that were selected, in part, for their presence in the human odor profile. The responsiveness of AsOr8 to odorants was highly similar to An. gambiae Or8 (AgOr8), except for sulcatone, which was detected by AsOr8 but not AgOr8. Subtle differences in the receptor sensitivity to specific odorants may provide clues to species- or strain-specific approaches to host-seeking and host selection. Further Insects 2021, 12, 593. https://doi.org/10.3390/insects12070593 https://www.mdpi.com/journal/insects