Date of Award
Spring 2023
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Neuroscience
First Advisor
Carlson, John
Abstract
Courtship is a critical and fascinating behavior. To propagate, animals must find suitable mating partners in a complex environment, and dedicate significant amounts of time and energy towards the pursuit of a mate. Multiple sensory cues play crucial roles in guiding this process, as evidenced by the elaborate song of a male zebra finch, the complex dance of a bird of paradise, and the exceptional pheromone-sensing ability of a moth. These behaviors rely on different sensory systems. Given its remarkable diversity and its importance in evolution and neuroethology, the neural basis of courtship behavior has long been a subject of great interest. In the first chapter of this thesis, we studied the Drosophila wing, an understudied taste organ, and its role in courtship behavior. We carried out differential RNA sequencing analysis of chemosensory sensilla on the anterior wing margin, and found expression of many genes associated with pheromone and chemical perception. One Ionotropic receptor (Ir) gene, Ir52a, caught our attention because of its sexually dimorphic expression. Mutation of Ir52a and optogenetic silencing of Ir52a+ neurons suppressed male sexual behavior. We found that activation of these neurons drove courtship behavior in males towards other males and towards females of another species. Surprisingly, unlike other pheromone receptors, Ir52a is also required for normal sexual behavior in females. Optogenetic activation of Ir52a+ neurons in mated females induced copulation, which normally occurs rarely among mated females. Circuit tracing using trans-Tango revealed sexually dimorphic morphology of the second-order projection neurons of Ir52a+ neurons, and optogenetic activation of Ir52a+ neurons in the wing activated these dimorphic neurons. Together, this study established a new role for Irs in regulating male and female sexual behavior, and provided new insights into the function of the wing in chemosensensation. In the second chapter, we further investigated the Drosophila melanogaster Ir52 cluster, which consists of five closely-related Ir genes, including Ir52a. The Ir52 genes are polymorphic in D. melanogaster, with the number of functional genes varying across strains collected from different regions of the world. Through BLAST searches, we identified 99 Ir52 genes among 24 species of Drosophilidae. These genes undergo significant expansion/contraction, with numbers of homologs ranging from one to nine in different species. Using RNA sequencing, we found that the Ir52 genes were unique among all Irs and Gustatory receptor (Gr) genes in that they are expressed in leg gustatory receptor neurons (GRNs), but not GRNs of the labellum, the main taste organ of the head. This expression pattern is conserved in D. suzukii, a species that diverged from D. melanogaster ~15 million years ago. Using an unconventional single-sensillum recording technique, we developed a new in vivo expression system for studying the response of taste receptors to hydrophobic ligands such as pheromones. Using this system we found that the misexpression of the Ir52a, b and d receptors can confer a pheromone response to sweet- and bitter-sensing neurons that do not normally respond to pheromones. Pheromone extracts made with male, virgin female or mated female D. melanogaster flies elicited equal responsiveness from bitter-sensing neurons misexpressing Ir52a, b or d. Extracts made from D. erecta, but not D. simulans, flies also activated neurons misexpressing Ir52a, b and d receptors. Furthermore, we tested 9 available pheromones among ~ 85 cuticular compounds identified in D. melanogaster, and found the Ir52a, Ir52b and Ir52d receptors were not responsive to them even at high concentrations. Interestingly, however, we observed both inhibitory responses as well as excitatory responses when these receptors were tested with certain medium-chain aliphatic derivatives, such as alcohols, aldehydes and acids. Finally, we utilized trans-Tango circuit tracing and found that the Ir52-expressing neurons are pre-synaptic to a group of sexually-dimorphic pheromone projection neurons. These neurons have similar morphologies to those that are post-synaptic to known pheromone-sensing neurons expressing ppk23 and ppk25. Together, our findings provide new insights into the function of an understudied family of sensory receptors. In the third chapter we identified the long-sought receptor that serves the evolutionarily ancient function of salt detection in flies. Using electrophysiological recordings, we identified gustatory neurons that are sensitive to sodium salts. These neurons have high specificity to Na+ but not to other metal ions including Li+, K+, Mg2+ and Ca2+. In Ir56b mutant animals, the responses to Na+ are eliminated, demonstrating the importance of Ir56b for sodium taste. Misexpression of Ir56b in bitter-sensing neurons could confer sodium sensitivity to these sodium-insensitive neurons. Ir56b is expressed in a subset of sugar-sensitive neurons, and is required for appetitive behavioral responses to sodium. Interestingly, the Ir56b receptor also has an atypical structure, lacking the N-terminal region proposed to contain ligand-binding domains in other Ir receptors. Moreover, Ir56b is a “pseudo-pseudogene”: its coding sequence contains a premature stop codon that can be replaced with a sense codon without loss of function. In the fourth chapter, we reviewed a ground-breaking paper on the structure of an insect Odorant receptor (Or). The Or family may be the largest family of ligand-gated ion channels in the animal kingdom. Since their discovery in 1999, many studies have focused on the function and response profiles of Ors. However, the molecular structure of Ors had remained unknown. We reviewed a study that used single-particle Cryogenic Electron Microscopy (Cryo-EM) to determine the structure of an insect Or.
Recommended Citation
Luo, Yichen, "The taste of love: detection of pheromones by the taste system" (2023). Yale Graduate School of Arts and Sciences Dissertations. 1045.
https://elischolar.library.yale.edu/gsas_dissertations/1045
