Projections of male-specific receptor neurons in the antennal lobe of the Oriental tobacco budworm moth, Helicoverpa assulta: a unique glomerular organization among related species.

The macroglomerular complex in the primary olfactory center of male moths receives information from numerous pheromone-detecting receptor neurons housed in specific sensilla located on the antennae. We investigated the functional organization of the three glomeruli constituting this complex in Helicoverpa assulta, a unique species among heliothine moths as concerns the composition of the pheromone blend. By tip recordings from the male-specific receptor neurons combined with cobalt-lysine stainings, the axon terminals in the brain were traced and subsequently reconstructed by camera lucida drawings. Some were also reconstructed in a digital form. The results showed that the sensilla could be classified into two functional types. A major category housed two colocalized receptor neurons, one responding to the primary pheromone component cis-9-hexadecenal and the other to the behavioral antagonists cis-9-tetradecenal and cis-9-hexadecenol. Cobalt-lysine applied to this sensillum type consistently resulted in two stained axons, each terminating in one of the two large subunits of the macroglomerular complex: the cumulus or the dorsomedial glomerulus. The second, less frequently appearing sensillum type contained a receptor neuron responding to the second pheromone component, cis-11-hexadecenal. Dye applied to this type resulted in stained axon projections in the ventral glomerulus. In an evolutionary context it is particularly interesting that differences of related heliothine species are reflected in the functional organization of the MGC compartments.