| Understanding how insects detect chemical cues is important to society. Insects can both pollinate crops and spread diseases to humans. Insects rely on their olfactory systems to find plant and human hosts as well as for basic survival. Drosophila olfactory organs have three types of sensilla---basiconic, trichoid and coeloconic sensilla. Functional, molecular and genetic studies have characterized the odorant responses of basiconic and trichoid olfactory receptor neurons (ORNs), which express receptors from the Odorant receptor (Or) gene family. However, less is known about neurons in the antenna that do not express Or genes.;One class of neurons that do not rely on Or receptors are coeloconic sensillar neurons. Little is known about the function of these sensilla because of the difficulty in obtaining high-quality electrophysiological recordings. Here I describe a functional characterization of coeloconic sensilla. Coeloconic ORNs respond to odorants with diverse response properties and dynamics. Furthermore, unlike basiconic and trichoid ORNs, coeloconic ORNs respond to amines. Amines such as ammonia are present in human sweat and highly attractive to blood-feeding mosquitoes. I also identify two coeloconic ORNs, ac1B and ac2B, that respond to humidity. The humidity response of ac2B can be inhibited by odorants, suggesting that this neuron may be bi-modal. The identification of amine- and humidity-sensitive neurons lays the groundwork for future work on the molecular bases for these responses.;ab1C neurons, which are dedicated CO2 sensors, also do not express Or genes. Recent work on signal transduction in Or-expressing ORNs resulted in several conflicting models. However, signaling in ab1C, which express receptors from the Gustatory receptor (Gr) gene family, had not been investigated. Using complementary genetic approaches and electrophysiology, I found that ab1C requires Galphaq, but not any other Galpha, for the CO2 response. Furthermore, I found that the dependence on Galphaq is unique to ab1C. An Or-expressing basiconic ORN, ab3A, responds normally to odorants when null for Galphaq. In addition, mosaic and genetic analysis show that ab3A does not require Galphas or Galphao for its odorant responses. This work extends our understanding of Gr receptor signaling and contributes to the debate over Or receptor signaling. |
