Wilant A. VAN GIESSEN1,2, D. Michael JACKSON-2 & Joop J. A. VAN LOON-1
1-Department of Entomology, Wageningen Agricultural University, P.O. Box 8031,
NL-6700 EH Wageningen, The Netherlands.
2-USDA-ARS, U.S. Vegetable Laboratory, 2875 Savannah Highway, Charleston, SC 294145334,
USA.
In most studies of olfactory responses of insects to volatile stimuli through electroantennography (EAG), the maximum voltage deflection following stimulation is used as a measure for the receptors' affinity for these compounds However, several authors have suggested that, apart from the amplitude, the shape of the electroantennogram also carries information with regard to the specificity of the antenna) receptors for particular volatile compounds (1). Hypothetically, the rate with which the signal returns to baseline is correlated with the speed with which stimulus molecules are removed from the receptor sites (2).
Surprisingly, analyses of EAG shape in general and EAG recovery rates in particular have so far been limited in scope. We recently developed new software for the detailed analysis of EAG shape. ELAN (=ELectroantennogram ANalysis) reads, converts, and analyzes output-files generated by 'EAG for Windows' (SYNTECH, Hilversum, The Netherlands). A major distinction between ELAN and similar software developed by other authors is that EAG shape parameters are based on the actual EAG signal and not on arbitrarily set intervals after the start of stimulation. ln addition' ELAN corrects for baseline drift, detects the actual start and end of depolarizatior~ (within 0.04 s), and standardizes the signal between 0 (start of stimulation) and 100 (end of stimulation). In preliminary experiments, we found that EAG shape to be independent of stimulus concentration. EAG standardization facilitates characterization of the EAG shape into the components depolarization and recovery through averaging over multiple EAGs.
To study the significance of EAG recovery rate, we used ELAN to compare the shape of EAGs recorded from five insect species responding to 1-hexanol, (Z)-3-hexen1-ol (both green leaf volatiles), mustard essential oil, and allyl-isothiocyanate, a compound characteristic of cruciferous plants (3).
The five species selected were the aphid species Lipaphis erysimi (Kaltenbach) (mustard aphid), Brevicolyne brassicae (L.) (cabbage aphid), and Acyrthosiphon pisum Harris (pea aphid) (Homoptera: Aphididae), the aphid parasitoid Diaeretiella rapae (M' Intosh) (Hymenoptera: Braconidae) , and Plodia interpunctella (Hübner) (Indian meal moth, Lepidoptera: Pyralidae). All insects tested were adult (winged) females.
EAG parameters calculated were the EAG response relative to the absolute response to 1-hexanol (stimulus A) and RECos, the percentage recovery to baseline 0.5 s aHer the end of depolarization.
The species that are specihc on cruciferous plants can be expected to respond more strongly to volatiles from these plants. In behavioral experiments, Read et al. (4) found that D. rapae is attracted to mustard oil and allyl-isothiocyanate. Indeed, L. erysimi and D. rapae showed high relative EAG responses to both mustard essential oil and allyl-isothiocyanate (Figure 1). The aphid species A. pisum and B. brassicae showed intermediate responses. As predicted, the stored product pest species P. interpunctella showed a low response to these two stimuli. All species showed similar responses to (Z)-3-hexen-1-ol.
The EAG recovery parameter REC0.5 did not follow the same pattern across the five species as the relative maximum response. Interestingly, while both species are specific on cruciferous plants, the fastest recovery rates to mustard oil and allylisothiocyanate were observed in B. brassicae, while the lowest RECos values were found for L. erysimi. Also, P. interpunctella shows faster recovery rates than D. rapae to these two compounds, even they are biologically unimportant to the first species. If relative EAG responses and EAG recovery rates would both be indicative for receptor affinity, some correlation would be expected.
This analysis was our first (limited) exploration of the relevance of EAG shape with regard to receptor specificity. We plan to expand this study by comparing species from as many as eight orders. responding to an array of up to 30 volatile compounds. Using ELAN, it may be possible to obtain 'EAG signatures' for each combination of species and stimulus.
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