Abstract
BackgroundInsects rely more on chemical signals (semiochemicals) than on any other sensory modality to find, identify, and choose mates. In most insects, pheromone production is typically regulated through biosynthetic pathways, whereas pheromone sensory detection is controlled by the olfactory system. Orchid bees are exceptional in that their semiochemicals are not produced metabolically, but instead male bees collect odoriferous compounds (perfumes) from the environment and store them in specialized hind-leg pockets to subsequently expose during courtship display. Thus, the olfactory sensory system of orchid bees simultaneously controls male perfume traits (sender components) and female preferences (receiver components). This functional linkage increases the opportunities for parallel evolution of male traits and female preferences, particularly in response to genetic changes of chemosensory detection (e.g. Odorant Receptor genes). To identify whether shifts in pheromone composition among related lineages of orchid bees are associated with divergence in chemosensory genes of the olfactory periphery, we searched for patterns of divergent selection across the antennal transcriptomes of two recently diverged sibling species Euglossa dilemma and E. viridissima.ResultsWe identified 3185 orthologous genes including 94 chemosensory loci from five different gene families (Odorant Receptors, Ionotropic Receptors, Gustatory Receptors, Odorant Binding Proteins, and Chemosensory Proteins). Our results revealed that orthologs with signatures of divergent selection between E. dilemma and E. viridissima were significantly enriched for chemosensory genes. Notably, elevated signals of divergent selection were almost exclusively observed among chemosensory receptors (i.e. Odorant Receptors).ConclusionsOur results suggest that rapid changes in the chemosensory gene family occurred among closely related species of orchid bees. These findings are consistent with the hypothesis that strong divergent selection acting on chemosensory receptor genes plays an important role in the evolution and diversification of insect pheromone systems.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-015-0451-9) contains supplementary material, which is available to authorized users.
Highlights
IntroductionInsects rely more on chemical signals (semiochemicals) than on any other sensory modality to find, identify, and choose mates
Insects rely more on chemical signals than on any other sensory modality to find, identify, and choose mates
Candidate gene detection To identify chemosensory genes of the Odorant Receptor (OR), Ionotropic receptor (IR), Gustatory Receptor (GR), Odorant-binding protein (OBP), and Chemosensory protein (CSP) gene families in Euglossa dilemma and E. viridissima, we reconstructed the antennal transcriptomes for each species using a conservative meta-assembly approach
Summary
Insects rely more on chemical signals (semiochemicals) than on any other sensory modality to find, identify, and choose mates. This study lacked a direct test on candidate genes, it suggests that molecular divergence of chemosensory genes could have promoted the early differentiation of pheromone sensory tuning in these two species of moth This mechanism has been put forward to explain the rapid evolution of pheromone communication systems in other sympatric sibling species of Lepidoptera [10, 11]. A single receptor mutation was shown to drive the early divergence in pheromone detection in Ostrinia furnacalis, a close relative to O. nubilalis [12] It remains unclear whether these findings are broadly applicable to other insect taxa that rely on pheromone reception for sexual communication, mainly because few studies have examined the evolution of peripheral olfactory systems in closely related species. The relative impact of these two mechanisms on pheromone recognition of closely related species is less well understood
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