These Results provide the first description of an olfactory sensillum present on a male insect sensitive to intra-sexual odor cues. H. virescens male hairpencil odors may function as an anti-aphrodisiac during courtship, repelling competing conspecific males, as upwind flight to synthetic female sex pheromone is inhibited by coincident presentation of 16:OAc and 18:OAc. The presence of uniglomerular projections and uniform spike amplitudes also suggests that this sensillum houses a single 18:OAc/16:OAc/Z11-16:OAc sensitive olfactory receptor neuron that principally regulates this behavioral effect. It is possible that additional rare sensillar types housing additional olfactory receptor neurons (such as the single 16:OH sensitive sensillum) are also present on the antenna (sensitive to the hairpencil odorants tested) but no others were noted in this study.
When comparing the sensitivity of this olfactory receptor neuron to each of the tested odorants (), it is important to consider their relative volatilities. Filter paper cartridges were loaded at similar concentrations, however the relative molecular weights and vapor pressures will affect the emission rates and volume of odorant delivered in each stimulus ‘puff’ (Cossé et al. 1998
). 18:OAc has a considerably lower vapor pressure (6.1×10-5
mm Hg) than either 16:OAc (2.7×10-4
mm Hg) or Z11-16:OAc (3.9×10-4
mm Hg), and therefore was delivered at a much lower concentration relative to it loading in the stimulus cartridge (Koutek et al. 1992
; PhysProp Online™ 2006
). Therefore, this olfactory receptor neuron is most likely primarily sensitive to 18: OAc, and secondarily to 16:OAc/Z11-16:OAc, based on dosage response curves (). This is further supported by the wind tunnel assay wherein antagonism of upwind flight was more pronounced in conjunction with 18:OAc presentation (). It remains unclear, however, whether 18:OAc or 16:OAc is more important in mediating male-male interactions, as the concentration of 16: OAc is much higher in male H. virescens
hairpencil gland extracts (the natural concentration of each in airborne effluvia is unknown; Teal and Tumlinson, 1989
The proximity of this glomerulus to the macroglomerular complex may enable integration and cross-glomerular interactions between glomeruli during processing of attractive odorants, such as Z11-16:Ald or Z9-14:Ald, and antagonistic cues such as those in the hairpencil blend. Previous studies have found that Z11-16:OAc, an antagonistic odorant released in the pheromone blends of closely related species, is processed in an anteromedial glomerulus of the macroglomerular complex (; Vickers et al. 1998
). Evidence from the current study now indicates that at least two glomeruli in relatively close proximity are activated by the presence of Z11-16:OAc. This glomerular organization may represent an important facet in the general combinatorial coding of Z11-16:OAc through activation of both glomeruli simultaneously. Alternatively, Z11-16:OAc may be important to male H. virescens
under different behavioral contexts, and hence is represented within two distinct locations in the antennal lobe.
This study also presents the possibility that in this species a functional division of the antennal lobe exists, wherein glomeruli near the base of the antennal nerve serve to process odors produced by both males and females. The functional role of the sexually dimorphic macroglomerular complex structure in all moth species appears to be for processing female sex pheromones and odorants present in the blends of other sympatric species (Christensen and Hildebrand 2002
; Hansson and Christensen 1999
). Our current Results suggest that a subgroup of glomeruli at the base of the antennal nerve, including the macroglomerular complex, may be responsible for processing both male and female produced pheromones. Whereas the macroglomerular complex is likely a derived structure, regional specialization of isomorphic glomeruli near the base of the antennal nerve may be the ancestral state for macroglomerular complex development (Hansson and Christensen 1999
). Furthermore, this provides an indication of specialization of glomerular clusters throughout the antennal lobe, wherein glomeruli receiving input from odors of a similar behavioral meaning (e.g. male and female pheromones, host plant odors), might be localized in close proximity within the antennal lobe.
Physiological and anatomical similarities were found between males and females in the odorant response and axonal projection patterns of olfactory receptor neurons within this sensillum type. We have previously identified a sensillar type on female H. virescens
which contained an olfactory receptor neuron responsive to 18:OAc, 16:OAc and Z11-16:OAc (type 1) and projected to a glomerulus near the base of the antennal nerve (; Hillier et al. 2006
). This suggests a degree of homology in peripheral and central processing of these hairpencil odors between the sexes. This result concurs with the overall similarity in antennal lobe structure observed between sexes in this species, and with results from imaging studies that have documented similar odor-evoked activation patterns in the antennal lobe of male and female moths (Berg et al. 2002
; Skiri et al. 2004
; Galizia et al. 2000
). Similar glomerular topology between males and females may provide an anatomical ‘baseline’ for the development of sexually dimorphic antennal lobe structures, such as the macroglomerular complex.
It is noteworthy that the behavioral outcome of detecting these odorants might be relatively similar between the sexes — an overall suppression in activity. In female H. virescens,
exposure to hairpencil pheromone results in quiescence that facilitates mating, in males, it inhibits upwind flight toward a calling female (Hillier and Vickers 2004
). Further work will be required, however, to clarify how similar hairpencil odor processing is between the sexes, despite the behavioral outcome.
For the wind tunnel assays, aliquots of 16:OAc and 18: OAc were applied, along with the agonistic pheromone blend, directly to the filter paper odor source resulting in co-emission of pheromone blend and test odorant. This, however, may not precisely represent the natural circumstance, wherein a courting male would expose hairpencils adjacent (2–10 mm) to the calling female. Previous work on Helicoverpa zea
has indicated that male moths exhibited weaker antagonism when an antagonist was presented on an adjacent odor source (1 mm upwind or downwind), compared with application of agonistic and antagonistic odorants on the same odor substrate (Baker et al. 1998
; Fadamiro and Baker 1997
). Male-male behavioral responses to 16: OAc and 18: OAc therefore might also be affected by such fine scale integration of agonistic and antagonistic odorant plumes. Consequently, male H. virescens
hairpencil components released during normal courtship may result in differing behavioral outcomes with coincident odor delivery, than those observed herein.
This discovery provides an insight into olfactory processing of a male sex pheromone by conspecific males, as well as a potential new prospect for management of H. virescens pest populations.