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

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1258 (Stavenga et al.,1993,Eq. (1)),with a the absorbance at wavelength l, a the sensitivity/100, x¼ 10 logðl=lmaxÞ, with lmax the wavelength of maximum absorbance,and A ¼ 1 (as a restriction),was fitted to the long wavelength tail (580–640 nm) of the sensitivity curves of the 9 dark adapted aphids by multiple regression analysis of the linearised form of Eq. (1) with the statistical programme SAS 6.12 (SAS,1989). The coefficient of determination was greatest (adjusted R 2 ¼ 0:971) at lmax ¼ 527 nm,with a0 ¼ 365:31 and a1 ¼ 4:994. Since the observed dark adapted sensitivity curve must be seen as the sum of the responses of all photoreceptors,the model was fitted to the long wavelength tail of ERG spectral sensitivity. This is because there,the sensitivity of a potential blue receptor is negligible in comparison to that of the green receptor. Therefore,the modelled function can be seen as the preliminary sensitivity function of the green receptor (a-band). 4. Discussion Under the chosen adaptation conditions,the green peach aphid showed three peaks of spectral sensitivity, one in the green region around 530 nm,a secondary blue–green peak (490 nm),and a third in the near UV (330–340 nm). From this we conclude that M. persicae possesses three types of photoreceptors. The existence of a green receptor is evident from the clear peak at 530 nm in the sensitivity function of the dark and white adapted animals. With white adapting light,which reduces the sensitivity of any receptors in the visible range,the sensitivity function peaks at 330 nm. This indicates the existence of a UV receptor. Adaptation with 590 nm reduces the sensitivity of the green receptor more than of any other possible receptors that have their sensitivity maximum at shorter wavelengths than 530 nm. So,by yellow adaptation the maximum sensitivity was shifted from 530 to 490 nm. This can only be explained by assuming the presence of a third receptor type sensitive in the blue region. In addition,at wavelengths below 500 nm,the modelled green receptor sensitivity clearly differs from the observed sensitivity function of dark adapted animals. This is further evidence for the existence of a blue sensitive photoreceptor in M. persicae (Fig. 3). From the spectral sensitivity function of the ERG,only the a-band of the green receptor can be modelled. Because the b-band cannot be predicted from our extracellular recordings but also contributes to the ERG,we could not determine the precise sensitivity peak of the blue receptor. So,for a validation and a more precise characterisation of this putative blue receptor intracellular electrophysiological measurements are required. ARTICLE IN PRESS S.M. Kirchner et al. / Journal of Insect Physiology 51 (2005) 1255–1260 The evidence for three types of photoreceptors in M. persicae is surprising as it was assumed earlier from behavioural observations tests,when spectral sensitivity data of aphids were not available,‘‘that there are two receptors in the eye of the aphid,one having a shortwave peak and the other a yellow peak’’ (Kring,1972, p. 472). In addition, Gao et al. (2000) found only 2 retinal opsins in the vetch aphid (Megoura viciae Buckt.),using reverse transcription-PCR of cDNA; these opsins were classified to be sensitive to UV and long wavelengths. Matteson et al. (1992) have concluded from their research that the phytophagous western flower thrips (Frankliniella occidentalis) has 1 spectral sensitivity peak in the ultraviolet and 1 in the visible with a maximum at around 540 nm. However,the ‘‘double peak’’ they observed in the visible region of the spectral sensitivity curve could imply the presence of 2 distinct types of photoreceptors in this region. In our own ERG measurements of dark adapted M. persicae,some individual sensitivity curves also showed a ‘‘double peak’’. This also suggests the presence of 2 photoreceptor types with the sensitivity peaks close together,as found when the animals where yellow adapted. With M. persicae having 3 types of photoreceptors,it is remarkable that the ERG of the whitefly (Trialeurodes vaporariorum) was found to have just 2 spectral peaks in the ERG (Mellor et al.,1997),since both species share a number of traits: both are members of the Sternorrhyncha,are attracted to yellow,and feed on plant sap. Further investigations are required,using higher wavelength resolution,adaptation light or intracellular techniques,in order to specify if the observed differences between the 2 species are based on their actual physiology. Although true colour vision was shown for the probing behaviour of M. persicae already by Moericke (1950) (Kelber et al.,2003),wavelength-dependent behaviour (Goldsmith,1994) may also be involved in other behaviours of this species. While the experiments conducted by Moericke (1950) prove colour vision,they do not allow to infer a specific wavelength of peak response,as probing was induced in a rather broad wavelength range of 500–600 nm. Flying black bean aphids (Aphis fabae Scopoli) were shown to be maximally responsive to wavelengths in the green region (530–560 nm) and ultraviolet (360 nm) (Hardie,1989). Furthermore,the bird cherry aphid (Rhopalosiphum padi L.) was maximally responsive to targets illuminated with green (555 nm) and ultraviolet (360 nm) light in a flight chamber (Nottingham et al.,1991). Wavelengths 4700 nm were not attractive to any of the species. While these findings suggest that the maximal behavioural response of aphids more or less coincides with the maximal reflection of green leaves,at present it is not possible to speculate how the input from the receptors is
evaluated and translated into behaviour. As the wavelength discrimination is best where sensitivities of receptors overlap (Kelber et al.,2003),it might be of special value to a herbivore to have 2 receptors with maximal sensitivities laying close together in the blue– green region. However,in order to back this suggestion, leaf reflection spectra,as well as further behavioural experiments and the sensitivities of the single receptors are needed. This would possibly reveal which role this combination of photoreceptor types plays in the evaluation of the physiological state of a plant or in the visual discrimination of host or non-host plant species. Acknowledgements We would like to thank M. Egelhaaf and R. Wodjinsky for kindly providing parts of the equipment, and L. Chittka,J. Spaehte,and T. Spehr for continual help and stimulating discussions. We especially thank T. Labhart for valuable suggestions regarding the experimental set-up and corrections on previous versions of the manuscript. We are in debt to C. Merx who helped in the aphid rearing process and Sophie von Lilienfeld- Toal for their assistance during the experiment. We are also grateful to R. Heuermann for important comments concerning the calibration of the ERG apparatus. This work would not have been possible without benefits from M.R. Finckh and financial support by the University of Kassel (ZFF). References Bernays,E.A.,Chapman,R.F.,1994. Host-plant Selection by Phytophagous Insects. Chapman & Hall,London,New York. Briscoe,A.D.,Chittka,L.,2001. The evolution of color vision in insects. Annual Review of Entomology 46,471–510. Brown,P.E.,Anderson,M.,1996. Spectral sensitivity of the compound eye of the cabbage root fly, Delia radicum (Diptera: Anthomyiidae). Bulletin of Entomological Research 86,337–342. Chapman,R.F.,Bernays,E.A.,Simpson,S.J.,1981. 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Page 3: 3. Results 3.1. Form of the electro

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