(-)-Germacrene D receptor neurones in three species of heliothine moths: structure-activity relationships.

Specificity of olfactory receptor neurones plays an important role in food and host preferences of a species, and may have become conserved or changed in the evolution of polyphagy and oligophagy. We have identified a major type of plant odour receptor neurones responding to the sesquiterpene germacrene D in three species of heliothine moths, the polyphagous Heliothis virescens and Helicoverpa armigera and the oligophagous Helicoverpa assulta. The neurones respond with high sensitivity and selectivity to (-)-germacrene D, as demonstrated by screening via gas chromatography with numerous mixtures of plant volatiles. Germacrene D was present in both host and non-host plants, but only in half of the tested species. The specificity of the neurones was similar in the three species, as shown by the "secondary" responses to a few other sesquiterpenes. The effect of (-)-germacrene D was about ten times stronger than that of the (+)-enantiomer, which again was about ten times stronger than that of (-)-alpha-ylangene. Weaker effects were obtained for (+)-beta-ylangene, (+)-alpha-copaene, beta-copaene and two unidentified sesquiterpenes. The structure-activity relationship shows that the important properties of (-)-germacrene D in activating the neurones are the ten-membered ring system and the three double bonds acting as electron-rich centres, in addition to the direction of the isopropyl-group responsible for the different effects of the germacrene D enantiomers.

Heliothis virescens: Response characteristics of receptor neurons in Sensilla Trichodea type 1 and type 2.

Partial electroantennograms (EAGs) and single cell recordings fromHeliothis virescens males have demonstrated the presence of pheromones receptor neurons in sensilla trichodea type 2 as well as in type 1. This is supported by cobalt tracing experiments, showing that primary axons from the distal flagellum, containing only s. trichodea type 2, project into the macrogiomerulus complex in the male antennal lobes. Four types of finely tuned pheromone receptor neurons were found in males, whereas in females the corresponding neurons responded mainly to host odors. In males the majority (75 and 18%, respectively) were tuned to the majorHeliothis virescens pheromone components (Z)-11-hexadecenal (Z11-16∶A1) and (Z)-9-tetradecenal (Z9-14∶A1). The others (5 and 2%, respectively) responded specifically to (Z)-1 1-hexadecen-1-ol (Z1 1-16∶OH) and (Z)-1 1-hexadecen-1-ol acetate (Z1 1-16∶Ac). No neurons responding selectively to the minor pheromone components were found. The Z11-16∶A1 neurons of both sensilla types possessed similar specificity. However, the sensitivity decreased toward the medial and distal part of the flagellum, where s. trichodea type 2 are located. This suggests that the pheromone concentrations can be detected peripherally by a spatial as well as a temporal mechanism. Differences in temporal response patterns (pronounced phasic vs. tonic component) were found within the same type of neurons, suggesting different ability to encode intermittency of the pheromone plume as well as to mediate maintenance of flight.

Pheromone reception in tobacco budworm moth,heliothis virescens.

Eleclrophysiological recordings from single olfactory receptor cells were carried out in the male tobacco budworm moth,Heliothis virescens. Recordings were made primarily from the sensilla trichodea type 1, which are located in the characteristic circumferential rows on the antennae. They possess the longest sensilla hairs as revealed by scanning electron microscopy (SEM). The sensory cells of these sensilla responded specifically to pheromones. Only three types of receptor neurons were found, each tuned to one of the female-produced components. The majority (58%) of the neurons were tuned to the major component (Z)-11-hexadecenal (Z11-16:A1). Another large group (27%) responded specifically to stimulation with (Z)-9-tetradecenal (Z9-14: Al). These two compounds are the most important components of the pheromones as judged by their influence on the behavioral responses of the males. The third type of neurons responded specifically to (Z)-11-hexadecen-1-ol (Z11-16: OH), which may act either as a pheromone component or as an interspecific cue. None of the receptor neurons in the long sensilla trichodea responded specifically to the minor aldehyde components of the pheromone, which have subtle effects on behavior. Mixture experiments provided no evidence that minor components influence the receptor responses to the major components. Olfactory sensilla outside the crosswise rows were also characterized morphologically by SEM. Included in these were sensilla of different lengths, corresponding to a classification as s. basiconica and s. trichodea type 2. Electrophysiological recordings from these sensilla showed that they are involved primarily in host odor reception. However, a few of these neurons responded to pheromones.