Odor Coding of Social Behavior in Eusocial Ants
Ferguson, Stephen Thomas
Ants coordinate social behaviors by relying on a well-developed chemical communication system. Indeed, one of the most distinctive features of the ant genome is a highly expanded repertoire of odorant receptors (ORs) whose evolution likely facilitated more complex social behaviors. The research constituting this dissertation sought to identify the chemoreceptors involved in social pheromone detection, determine the role of ORs in social behaviors, and characterize olfactory changes associated with age polyethism. Cuticular hydrocarbons (CHCs) are an important source of social information in ant colonies. Subtle differences in the ratio of CHCs communicate information such as worker task, fertility, and colony membership. Using a heterologous expression system, a wide range of Harpegnathos saltator ORs were screened for their responses to chemical cues. We discovered that ORs detect CHCs in addition to other general odorants. This represented an important step towards understanding the sensory pathways involved in pheromone signaling. Extending from this seminal work on hydrocarbon detection, I next led a series of studies to test the hypothesis that ORs are necessary for nestmate recognition in the ant Camponotus floridanus. I established an aggression bioassay incorporating a set of pharmacological compounds that selectively modulate OR activity to either inhibit (antagonist) or activate (agonist) signaling. Neither the absence of friendly cues (antagonist) nor the presence of an ambiguous odor cue (agonist) were sufficient to elicit aggression. This data demonstrated that ORs are necessary and sufficient for nestmate recognition and supported the hypothesis that ants recognize foes but not friends through a lock-and-key odor coding model. Focus then turned to olfactory sensitivity and task allocation in C. floridanus. Callow workers typically begin their life as nurses before an age-associated transition to other tasks such as foraging. However, it is unclear how changes in olfactory signaling are associated with age polyethism. To address this, we conducted a comprehensive electrophysiological screen to assess olfaction in aging workers. These studies revealed significant age- and task-associated changes in olfactory sensitivity that may represent an important feature in the coordination of social behaviors in ant colonies.