NSF-BSF: Exploiting the evolution of odorant discrimination in ants to decipher the olfactory code

Project: Research project

Project Details

Description

Overview: Understanding olfaction is impeded by the multidimensionality of the olfactory space, with discontinuous stimuli from vastly different types of odorant molecules. A major determinant of olfactory space is the specificity of odorant receptors (ORs). Our knowledge of how specificity is achieved by the binding properties of OR proteins lags behind our understanding of receptors in vision and audition. We will use the evolution of olfactory systems in social insects, in particular the expansion of the OR repertoire, to address this problem while also clarifying if the benefits of increased pheromonal communication and discrimination drive OR expansion in ants, a hypothesis that is missing support through functional studies.
Discrimination of colony member and foreigner in ants is achieved through colony-specific cuticular hydrocarbon (CHC) profiles on the insect surface. Using this system has big advantages: (i) The use of structurally similar, but highly variable CHCs simplifies the complexity of the olfactory space. (ii) CHC discrimination is well established in ants (iii) many CHC-sensitive ORs are available in ants to study OR evolution and to assess the association between OR sensitivity and OR structure.
We will study these topics by (i) using established conditioning bioassays to assess discrimination abilities, (ii) expressing single ant ORs in Drosophila fly lines to electrophysiologically characterize individual ORs sensitivity to a spectrum of CHCs, (iii) creating a high resolution OR phylogeny, identifying closely related ORs and analyzing selective pressures, and (iv) performing a 3D reconstruction of closely related ORs and relating receptor structural differences to changes in their specificity for CHC ligands with verification through directed OR mutation, in order to address the following objectives where we assess whether
1. ants better discriminate between socially relevant than between socially irrelevant CHCs,
2. CHC-sensitive ORs have undergone extensive gene duplications in the OR phylogeny and fine tuning of their ligand specificity,
3. there is positive selection on CHC-sensitive ORs in areas of the phylogeny with OR expansion,
4. the association between changes in CHC sensitivity of ORs and their molecular structure identifies receptor regions that determine OR ligand specificity.
Intellectual Merit: Our system is an excellent model to investigate the adaptive evolution of olfaction. Our behavioral, electrophysiological, and phylogenetic analyses will determine if the benefits of increased CHC profile discrimination are driving the evolution and expansion of ORs in ants and other social insects. This result will be supported by our analysis of positive selection on CHC-sensitive ORs that extend the olfactory space of CHCs. Our approach will clarify if benefits of discrimination of complex odorant mixtures can drive OR expansion in general. The combination of our functional analyses of ligand specificity of closely related ORs with the 3D reconstruction of the OR molecules will establish how molecular sequence changes lead to the expansion of the olfactory space. Our high-resolution approach, through the inclusion of many closely related ORs with their closely related CHC ligands, will address the structure/function analysis of ORs at a level of detail that to our knowledge has not been achieved in any animal, and therefore will bring us substantially closer to cracking the olfactory code.
Broader Impacts: We will develop an educational game for ASUs world-renowned free Ask A Biologist program that had over 15 million visits in 2018, and that has been used in almost every country across the globe. The spy game will be based on ant colonies efforts to reject potential intruders that will steal resources from the colony, and will introduce users to important concepts such as the interplay between olfactory receptors and precision of the olfactory system, and an evolutionary arms race between host and intruder. We will also collaborate with local high schools and recruit 10th-12th-grade students from underrepresented groups in the community. These students will participate in ongoing discrimination experiments, and will produce short videos on their application of the scientific method. Undergraduate and graduate students will be trained in modern approaches of behavioral analysis and experimentation, and major results of the study will be broadly disseminated to the mass media.
StatusActive
Effective start/end date8/15/207/31/23

Funding

  • National Science Foundation (NSF): $630,927.00

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