Moth olfactory trichoid sensilla exhibit nanoscale-level heterogeneity in surface lipid properties

Chemical force microscopy (CFM) based on tapping mode Atomic force microscopy (AFM) utilized with topographic and phase-shift analyses was used to investigate the topography and surface chemical properties, respectively, of the long trichoid sensilla on the antennae of male Helicoverpa zea. AFM topographic imaging revealed regular series of step-ridges along nearly the entire length of each sensillum, except for the basal ca. 1/3 portions, which were devoid of such ridges. Inter-ridge regions were flat, with regularly spaced pores, ca. 30 nm in diameter populating these planar areas. Many pores exhibited a raised dome that often nearly completely spanned the depression, with only the edges of the depressed portion of the pore still visible. Some pores were observed also along the bases of the ridges. CFM probing of the surface for chemical interactions with the SiO(2) hydrophilic tip revealed consistently diminished hydrogen bonding of the ridge edge areas with the tip than along the flat planar inter-ridge regions. Surfaces of domes over the pores also tended to have less hydrogen bonding with the tip than the planar surfaces. Functionalizing the CFM tip by bonding octadecyl-hydrocarbon to it eliminated these surface chemical-CFM tip interactions and no differences in tip interaction with the sensillar surfaces were observed. Trichoid sensilla from the male antennae of a second species, Utethesia ornatrix, did not exhibit similar heterogeneity between ridge edges versus planar areas with regard to hydrogen bonding with the SiO(2) hydrophilic tip. Pores on U. ornatrix sensilla occurred only along the bases of ridges on their trichoid sensilla. We suggest that the surface lipids of the H. zea sensilla are distributed in a chemically heterogeneous fashion to aid adsorption and transport of aldehyde pheromone component molecules through the pores into the sensillum lumen, possibly through solubilization in an epicuticular lipid layer. The trichoid sensilla of U. ornatrix do not exhibit such surface chemical heterogeneity, and this species-difference may be due to the usage by U. ornatrix of hydrocarbon molecules rather than aldehydes for their sex pheromone components.