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▼a Insects and other animals rely on their sense of smell to successfully navigate the environment in search of food and mates. In insects, the primary organ responsible for detecting odors is the antenna, which is covered in sensory hairs called sensilla. Two broad classes of antennal proteins are believed essential for the sensillar response to odorants. One class, the odor receptors (Ors), have been mapped to specific types of sensilla and to individual neurons within those sensilla, and their functional specificities have been characterized. The second class, called odorant binding proteins (Obps), have been the subject of intensive investigation in insects but remain poorly understood. Obps are a family of insect proteins that are exceptional in number, abundance, and diversity. They are widely thought to be required for transporting hydrophobic odorants through an aqueous lymph to odorant receptors. However, an increasing body of evidence suggests that Obps play a broader role than simply in olfaction/odorant binding.The roles of Obps have remained mysterious due to a lack of understanding of their molecular organization, which has precluded precise manipulation of Obps to probe their function. We thus defined Obp organization using the model Drosophila melanogaster antenna system. We mapped the abundant Obps to olfactory sensilla in the antenna and discovered that abundant Obp genes are expressed in diverse antennal regions and in different morphological classes of sensilla. Within a morphological class, distinct Obp genes are expressed within particular cell types. These results laid the foundation for analysis of Obp function. This map identified a sensillum type containing a single abundant Obp, Obp28a. Surprisingly, deletion of Obp28a did not reduce the magnitude of olfactory responses, suggesting that this sensillum does not require an abundant Obp for odorant transport. Instead, increased responses to odor pulses occurred in the absence of Obp28a, suggesting that Obp28a binds odorants to reduce the amount available for receptor activation. Following stimulus termination, odorant released from the Obp-odorant complex could increase the level available for receptor binding. These results define a novel role for this Obp in buffering changes in the odor environment, perhaps providing a molecular form of gain control.Our map also revealed that one abundant Obp, Obp59a, is expressed only in one region of the antenna, the sacculus, which was recently defined to have a role in hygrosensing but by an unknown mechanism. Hygrosensation is an essential sensory modality that insects use to locate sources of moisture and to avoid desiccation, an increasing threat with climate change. Responses to changes in humidity, however, remain poorly understood. We unexpectedly discovered that humidity-detecting sensilla in the sacculus produce and rely on Obp59a. Mutants lacking Obp59a are defective in hygrosensory behavior, and remarkably increase their desiccation resistance. Obp59a is exceptionally well-conserved among insects, highly localized, and an abundantly-produced member of the Obp family. This conservation and specificity suggest that the role of Obp59a in humidity detection extends to other insect species that rely on humidity (e.g. to find human hosts and oviposition sites). Taken together, our results enhanced the understanding of hygroreception, Obp function, and desiccation resistance, a process that is critical to insect survival.Olfaction has not been extensively studied in disease-carrying insect vectors. Thus, analysis of olfaction in the tsetse fly Glossina morsitans, a vector of trypanosomiasis that tragically cripples the health and economy of sub-Saharan Africa, could enable an understanding of the mechanism by which it locates hosts. We thus examined the anatomical and molecular basis of olfactory function in G. morsitans. We discovered that the G. morsitans antenna contains morphologically similar sensilla to Drosophila, and that tsetse Obp and Or orthologs are expressed in the same morphological class of sensillum in G. morsitans. We identified three receptors with robust expression in OR neurons located in discrete antennal regions, and developed a new version of the "empty neuron" heterologous expression system to express functional versions of these G. morsitans Ors in the otherwise empty neuron in Drosophila. We discovered that Or35 responds to 1-hexen-3-ol, an odor present in human emanations. Another receptor, Or9, responds to attractive bovine pheromones acetone, 2-butanone, and 2-propanol. We also found that 2-propanol is an attractant for both G. morsitans and the related species Glossina fuscipes, a major vector of African sleeping sickness. Our results identified 2-propanol as a candidate for an environmentally-friendly and practical tsetse fly attractant. Overall, this work characterized the distinct olfactory system of a fly, Glossina, and provided an approach to identify new agents for controlling Glossina and the devastating diseases it carries. |