Colour and symbionts of aphids.
The colour of the green and brown forms of Silobion avenae
result from qualitative and quantitative differences in their
carotene pigments. The green form contained mainly a-carotene
(bicyclic), and the brown form mainly y-carotene (monocyclic),
lycopene, torulene and 3', 4'-didehydrolycopene (acyclic). The
total carotene content of the brown form was approximately
three times higher than the green form. The striking pink colour
of male Metopolophium dirhodum was the result of qualitative
differences in carotenes present in the green vir'ginoparae of this
To investigate the possibility that *brown clones had a selective
advantage over green clones at long " daylengths, six clones of S.
avenae (three green and three dark), were reared on artificial
diet, at photoperiods of either 8,16 or 24 h. The fecundity of five
of the clones was highest at 16 h and lowest at 8h. One of the
brown clones had its highest fecundity at 24 h and lowest at 8h.
These results do not support the hypothesis that brown clones
have a selective advantage at long daylengths.
When a green and a brown clone were exposed to UV light (58
tmol m-2), the brown clone suffered the least mortality and had a
significantly lower reduction in fecundity. This result suggests
that at high light intensities brown clones have a selective
advantage over green clones.
Another possible source of selective advantage investigated
was the number of bacteriacytes individuals of the above clones
possessed. This was estimated by histological examination and no
significant difference was foundThe carotenes of S. avenae are not derived from an exogenous
source, as three clones were reared on a diet lacking carotenoids
for up to three generations, with no loss of pigmentation. Adding
chlorotetracycline to the diet did not inhibit the production of
carotenes. This result suggests that the symbionts are not
synthesising the carotenes. The colour of the offspring of
virginoparae, reared on artificial diet at a number of different
daylengths, was not affected. When sexual morphs were induced,
the oviparae of the brown forms were green.
Individuals of an insecticide resistant strain of Myzus
persicae, whose symbionts were disrupted by feeding them on a
diet containing antibiotic did not show a reduction in the base
level of esterase E4 activity, the enzyme conferring resistance in
this species. This demonstrates that the bacteriasome is probably
not the major site of synthesis of this enzyme