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

Abstract

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.

Genes encoding candidate pheromone receptors in a moth ( Heliothis virescens )

“…2), but gave only a very faint band in PCR experiments with preparations from female antennae. Electrophysiological recordings from individual olfactory cells in male antennae of Heliothis have demonstrated that components of the female sex pheromone are perceived by means of specific sensory cells in sensilla trichodea types 1 and 2, whereas host-plant odors are perceived by means of sensory cells of other sensilla types (20,21). The observation that the novel receptor types HR13-HR16 were expressed in cells that could be confined to long sensilla trichodea supports the view that they are candidate pheromone receptors.…”

“…In many moth species, the antennae are sexually divergent; usually, only male antennae contain sex-pheromoneresponsive cells, which may comprise as many as 50% of all chemosensory antennal cells (29). However, in H. virescens also, females might possess a few sensory neurons responding to pheromones (20). Interestingly, HR13 was found to be highly expressed in male antennae (Fig.…”

“…The observation that the novel receptor types HR13-HR16 were expressed in cells that could be confined to long sensilla trichodea supports the view that they are candidate pheromone receptors. The different number of cells expressing the various receptor types may be of particular interest in view of previous electrophysiological findings indicating that a rather high number of sensilla trichoidea type 1, and also of type 2, contain neurons tuned to (Z)-11-hexadecenal, one of the major pheromone components, whereas the secondary pheromone component, (Z)-9-tetradecenal, activates significantly fewer cells (20,21,30). For other minor components of the complex female pheromone blend (31,32), how many cells are activated is unknown.…”

“…the opposite side comprises no sensilla, but rather, scales (21). Additional pheromone-responsive neurons have been found in a type of shorter sensory hairs, which are classified as sensilla trichoidea type 2; this type of sensilla is located on the leading edge of all Ϸ80 segments of the male antenna (20,21). To explore whether the characteristic receptor types may be expressed in neurons located in sensilla trichoidea, in situ hybridization experiments were performed to visualize cells expressing HR13-HR16 in cryosections of the proximal segments from male antenna.…”

“…Because pheromone detection is probably under different kinds of selective pressure than detection of food odors or host odors, one might speculate whether receptors for pheromones are more conserved than other olfactory receptor types. Furthermore, candidate receptors for sex pheromones released by female moths are expected to be expressed only (or preferentially) in antennae of male moths, and more specifically, in cells that can be assigned to malespecific pheromone-sensitive sensory hairs (20,21). Therefore, putative olfactory receptors from the moth Heliothis virescens were assessed for sequence similarities as well as for their tissueand cell-specific expression patterns in an attempt to identify candidate pheromone receptors.…”