Abstract
Olfactory receptor repertoires exhibit remarkable functional diversity, but how these proteins have evolved is poorly understood. Through analysis of extant and ancestrally reconstructed drosophilid olfactory receptors from the Ionotropic receptor (Ir) family, we investigated evolution of two organic acid-sensing receptors, Ir75a and Ir75b. Despite their low amino acid identity, we identify a common 'hotspot' in their ligand-binding pocket that has a major effect on changing the specificity of both Irs, as well as at least two distinct functional transitions in Ir75a during evolution. Moreover, we show that odor specificity is refined by changes in additional, receptor-specific sites, including those outside the ligand-binding pocket. Our work reveals how a core, common determinant of ligand-tuning acts within epistatic and allosteric networks of substitutions to lead to functional evolution of olfactory receptors.
Highlights
Amongst the senses, olfaction is flexible over evolutionary time, enabling animals to adapt their recognition of the vast, ever-changing universe of volatile chemicals in the environment (Bargmann, 2006; Ramdya and Benton, 2010)
The distinct responses of D. sechellia Ir75a (DsecIr75a) compared to orthologs in its two generalist cousins (Prieto-Godino et al, 2016) suggested that acetic acidsensing was the ancestral function of Ir75a
Population genetic analyses of D. melanogaster strains isolated from diverse global habitats reveal that olfactory receptors display some of the strongest genomic signatures of recent selection (Arguello et al, 2016), suggesting these proteins act as “first responders” in local adaptation to new environments
Summary
Olfaction is flexible over evolutionary time, enabling animals to adapt their recognition of the vast, ever-changing universe of volatile chemicals in the environment (Bargmann, 2006; Ramdya and Benton, 2010). This flexibility is reflected in the evolution of large, divergent families of olfactory receptors with different odor tuning properties. Comparative sequence and functional analyses of orthologous receptors across species have started to identify amino acid differences that can explain speciesspecific receptor tuning properties (Adipietro et al, 2012; Auer et al, 2020; del Mármol et al, 2021; Leary et al, 2012; Mainland et al, 2013; Prieto-Godino et al., 2017; Yang et al, 2017).
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