Evidence for trichromacy in the green peach aphid, Myzus persicae ...

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1256 occidentalis (Pergande) (Matteson et al.,1992),and—as the only phytophagous Homopteran species—the glasshouse whitefly, Trialeurodes vaporariorum (Westwood) (Mellor et al.,1997) have been examined. Especially, there is no data regarding the spectral sensitivity of aphids yet (Prokopy and Owens,1983; Gao et al.,2000; Kelber et al.,2003). Therefore,the aim of this study was to apply the electroretinogram (ERG) technique (Goldsmith and Bernard,1974; Brown and Anderson,1996; Scharstein and Stommel,1999) toM. persicae in order to contribute to a better understanding of the spectral properties of the eye and host finding of this agriculturally most important aphid species. 2. Materials and methods 2.1. Electroretinogram recordings A 100 W xenon arc lamp (Osram XBO100W/2 OFR) was used as a light source in Xe-100 lamp housing device (UV-Gro¨ bel,Ettlingen,Germany). Narrow bandpass filters (10 nm half-width,from Ealing Davin Optronics, Watford,UK) provided nearly monochromatic light in the wavelength range 320–640 nm in 10 nm steps. Neutral density filters (Ealing) allowed the intensity of light passing through the bandpass filters to be varied over approx. 2.5 log units. A manual shutter (T70 camera,Canon) produced 0.5 s light flashes as test stimuli,which were transmitted through a 3 mm dia. liquid light guide (UV-Gro¨ bel) into a light-proof Faraday cage. The terminal end of the light quide was fixed to a micromanipulator (Prior,Cambridge,UK) at about 1.5 cm distance to the aphid’s eye. Summer migrants (winged virginoparae) were reared on chili pepper plants (Capsicum annuum L.) under crowded conditions in long days (16 h light,8 h dark at 20 and 16 1C,respectively) in an Environmental Incubator (Inculight 400,Schu¨ tt,Go¨ ttingen,Germany). The aphids were mounted on double-sided sticky tape on a 10 10 mm platform. Under a stereomicroscope (80 magnification,Leica S6E from Leica,Wetzlar, Germany),legs,wings and antennae were gently secured on the tape in order to immobilise the aphid. The recording and indifferent electrodes were made from electrolytically sharpened 0.2 mm diameter tungsten wire (Goodfellow,Cambridge,UK). The recording electrode was inserted into the left eye through the cornea and the grounded reference electrode was placed in the abdomen. Light stimuli were applied to the eye and the response was displayed on a Digital Storage Oscilloscope (HM205-3 from Hameg,Mainhausen, Germany) via an AC amplifier equipped with low-pass (300 Hz) filters and high-pass (0.1 Hz) filters (P-55 from Grass Telefactor,West Warwick,USA). All recordings took place at 20 1C. ARTICLE IN PRESS S.M. Kirchner et al. / Journal of Insect Physiology 51 (2005) 1255–1260 2.2. Experimental protocol The optical system was calibrated at the position of the aphid’s compound eye by measuring the emitted light for all possible combinations of our neutral density filters and bandpass filters with a spectrometer (RAMSES ACC,from TriOs,Osnabru¨ ck,Germany) in mW s 1 m 2 . These data were transformed to photon flux equivalents by multiplication with the respective wavelengths. Pretests at 350,450,and 550 nm showed that intensity–response curves followed log-linear functions. An appropriate criterion response was selected to ensure that over the whole wavelength range a constant response could be produced by adjusting the light intensity. For each wavelength,0.5 s flashes of 3–5 different light intensities were given to produce responses near the selected criterion response. The photon flux required to generate the exact criterion response was found by calculating linear regression of the response values as plotted against a log photon flux scale. Spectral sensitivity was calculated as 1/photon flux and values were normalised relative to the maximum sensitivity for each individual. A mean curve was produced from 7–9 individual aphids. Spectral sensitivity was investigated under three different adaptation conditions: (1) dark adapted,(2) adapted with white UV-free light,and (3) yellow adapted (590 nm). Adaptation light was produced by LEDs and was spectrally characterised with a spectrometer (TriOs RAMSES ARC) (Fig. 1). The adaptation light source was fixed to the terminal end of the light guide. Adaptation time prior to test flashes was 30 min. The test flashes were delivered under continuous presence of adaptation light. After pre-adaptation the duration of the test series for each individual was 40–50 min. Fig. 1. Normalised irradiance spectrum of LEDs for adaptation (arbitrary units); UV-free white (continuous line) and yellow (dotted line).
3. Results 3.1. Form of the electroretinogram Fig. 2 shows a typical dark adapted ERG recording elicited from M. persicae. It was found to be biphasic with a strong corneal negative on-response. The ERG reading was taken as the voltage difference between the pre-flash potential and the peak of the negative ERG component. 3.2. Spectral sensitivity The spectral sensitivity of the dark adapted compound eye of M. persicae showed one peak of sensitivity at 530 nm (Fig. 3). No further peak could be found in Fig. 2. A typical ERG recording from the green peach aphid at 530 nm light stimulus as measured by AC amplifier: A ¼ base-line potential, B ¼ receptor component of ERG,C ¼ off-response,D ¼ duration of stimulus. Fig. 3. Spectral sensitivity of the dark-adapted eye of alate M. persicae. Mean and standard error of 9 individuals (continuous line) and modelled sensitivity of a green receptor (lmax ¼ 527 nm,dotted line). For details of modelling see text. ARTICLE IN PRESS S.M. Kirchner et al. / Journal of Insect Physiology 51 (2005) 1255–1260 1257 Fig. 4. Spectral sensitivity of the white-adapted eye of alate M. persicae. Mean and standard error of 7 individuals. Fig. 5. Spectral sensitivity of the yellow-adapted eye of alate M. persicae. Mean and standard error of 7 individuals. the UV (320–400 nm). In the red region (620–640 nm) sensitivity was small (o8%). A distinct shoulder was observed in the sensitivity curve between 500 and 510 nm. When the eye was tested in the presence of adapting lights,the spectral sensitivity function showed two further peaks (Figs. 4 and 5). A UV-sensitive component with a maximum sensitivity in the region 330–340 nm appeared when the eye was white- or yellow-adapted. In addition,in presence of yellow adapting light a blue– green-sensitive component occurred with a maximum sensitivity at 490 nm (Fig. 5). 3.3. Fitting a nomogram The exponential function a ¼ A exp½ a0x 2 ð1 þ a1xÞŠ
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Evidence for trichromacy in the green peach aphid, Myzus persicae ...

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