Class-wide sequence, structural, and docking-profile diversity of insect odorant receptors and structural specialization of Orco

Abstract Insect olfaction is facilitated by a heterotetrameric odorant receptor–odorant receptor co-receptor (OR-Orco) complex, which is distinct from that of vertebrate ORs. However, extensive sequence divergence and lineage-specific expansion have complicated large-scale comparisons of OR repertoires across insects. Here, we integrated sequence similarity, structural similarity, and molecular docking to analyze ORs from 115 insect species. Across this class-wide sampling, OR repertoire size was associated with adult diet, larval diet, and habitat, but not with circadian rhythm. We developed a “trunk–branch” network framework to classify ORs according to sequence and structural similarity and used large-scale molecular docking to derive comparative docking profiles. Despite marked differences in sequence-and structural-community composition, almost all sampled orders contained ORs assigned to all six docking-profile communities. Holometabolous insects showed higher OR sequence-community diversity than non-holometabolous insects, and docking-profile composition differed between extant lineages whose order-level origins predate or postdate the end-Permian mass extinction. Most notably, we identified a highly conserved extracellular β-sheet in Orco that restricts ligand access to the binding pocket and may contribute to its reduced capacity to bind natural odorants. The emergence of Orco represents a key evolutionary transition from homomeric receptors to heteromeric OR–Orco complexes, and our results suggest that this transition may have been accompanied by pronounced specialization of the extracellular domain and binding pocket. Together, these analyses provide a class-wide framework for comparing insect OR diversity and generate testable hypotheses for future experimental and evolutionary studies.