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

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(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