Neural Interfaces to the Odour World of Scarab Beetles

Sammanfattning: Antennal detection and central nervous integration of behaviourally relevant odours in scarab beetles (Coleoptera: Scarabaeidae) was investigated, using the two Japanese scarabs Anomala cuprea and Phyllopertha diversa (Rutelinae), and the Central African fruit chafer Pachnoda marginata (Cetoniinae) as model species. Electrophysiological single cell recordings in A. cuprea and P. diversa characterized odour detection by antennal olfactory receptor neurons tuned to plant odours (flower odours, green leaf volatiles) and sex pheromones. In both species, dose-response tests showed that plant compounds as well as sex pheromone components were detected by specialized types of receptor neurons, each type responding preferentially to a single compound. Responses of antennal receptor neurons in P. marginata to single compounds in fruit extracts were investigated using single cell recordings, while stimulating with fruit compounds eluting one by one from a gas chromatograph. Olfactory receptor neurons in this species also responded to single compounds or a few structurally related compounds, in spite of generally high stimulus concentrations. The receptor neurons responding to the fruit volatiles could be grouped into 28 types, responding to a total of more than 50 compounds. The attractiveness of 35 electrophysiologically active fruit compounds was evaluated in a 2-choice olfactometer. Half of these compounds were attractive to P. marginata, while the others showed no attractiveness as single compounds. A mixture of three compounds not attractive by themselves was attractive to P. marginata males, showing that some compounds have behavioural relevance only as components of a blend. In P. marginata, 27 fruit compounds detected by antennal receptor neurons were used as stimuli in intracellular studies of odour integration by interneurons in the antennal lobe of the brain. Lucifer Yellow-filled interneurons consisted of uniglomerular and multiglomerular projection neurons, plus some local neurons, which all had multiglomerular arborizations and a dense focus in a single glomerulus. Most interneurons had narrow response spectra, responding to single or up to a handful of compounds, while the multiglomerular projection neuron responded to almost all stimuli. Almost all interneurons responded to attractive single compounds, while non-attractive compounds generally eliciting no or very weak responses.