Key players : selected chemosensory sensilla in the tobacco hawkmoth Manduca sexta
The natural environment of insects comprises a variety of volatile chemical signals that provide essential information about intraspecific communication, host plant location, suitability of oviposition sites or presence of natural enemies. The detection of volatile cues by sensory neurons on chemosensory organs is thus a fundamental process to ensure the survival of an insect. The aim of this thesis was to investigate the peripheral chemosensory system on a molecular and functional level. For this approach we employed the tobacco hawkmoth Manduca sexta as a valuable model organism, which has been studied intensively from various aspects like morphology, physiology, ecology and olfactory guided-behaviors. The main chemosensory organ is the antenna, which bears several types of olfactory sensilla. These sensilla contain dendrites of olfactory sensory neurons (OSNs) in which the chemosensory receptors are expressed. Three major families of chemosensory receptors have been described in insects: olfactory receptors (ORs), ionotropic receptors (IRs) and gustatory receptors (GRs), which include CO2 receptors. By generating a high-quality genomic data set for M. sexta (manuscript I), accompanied by RNAseq data from several chemosensory tissues we could establish an excellent chemosensory receptor reference gene set (manuscript II). In this process we improved and corrected gene models, and verified receptor genes by cloning. In total, we identified 73 ORs, 21 IRs and 2 CO2 receptors in M. sexta. The reference gene set provides a base for further comparative genomic approaches and may support the identification of new chemosensory receptor genes in other species. Our analysis revealed that M. sexta has one of the largest antennal IR repertoires found in lepidopteran species. In contrast to the divergent ORs, IRs are far more conserved, which suggests that the functions of IRs seem to be conserved as well. Olfactory IRs are usually expressed by sensory neurons housed in coeloconic sensilla. Since no previous study was conducted on functional characterization of coeloconic sensilla on the antenna of M. sexta, we determined ligand profiles for this type of olfactory sensillum (manuscript III). By using electrophysiological measurements we could demonstrate that OSNs of coeloconic sensilla on the antenna of M. sexta respond predominantly to acids, amines and aldehydes, highly matching the ligand profiles of coeloconic sensilla of D. melanogaster, but not to its other olfactory sensillum types. Similar results were obtained in electrophysiological studies on other lepidopteran species. This indicates that the function of coeloconic sensilla in the detection of such substances is conserved.
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