The compound eye of Colias erate is composed of about 6,500 ommatidia, each
containing nine photoreceptors (R1-9), forming a tiered rhabdom, surrounded by four
reddish pigments. The ommatidia are divided into three types (I-III) due to the
arrangement of perirhabdormal pigment spots and expression pattern of four opsin
mRNAs. The opsins are already identified: they are short (CeUV), two middle (CeV1,
CeV2) and L (CeL)-absorbing types. In the thesis research, I studied the physiological
basis of color vision in the Eastern Pale Clouded yellow butterfly, Colias erate Esper, in
detail focusing on the production mechanism of the variety of spectral photoreceptors.
Following General Introduction, I describe a few basic properties of the Colias eye in
Chapter 1. The basic properties are the eyeshine, which is a reflection from the tapetum at
the bottom of the ommatidia, and the spectral sensitivity of the entire eye. The eyeshine of
the dorsal region has identical reflectance spectrum in both sexes. In the ventral region of
males, type I ommatidia reflect 660 nm and type II and III ommatidia reflect 730 nm. In
females, type II ommatidia have broadband reflection from 620 to 730 nm, while type I
and III respectively reflect 660 nm and 730 nm as in males. Remarkable sexual difference
of eyeshine spectrum in type II ommatidia is attributable to the female-specific orange
pigment. Regionalization was also found in the eye spectral sensitivity determined by
electroretinography. Dorsal region of the retina is relatively more sensitive below 420 nm
range, whereas the ventral region is more sensitive in the wavelength range of 420 nm to
550 nm. This is probably because of the differential distribution of different ommatidial
types.
Four of nine photoreceptor cells contribute their rhabdomeral microvilli to the distal
tier of the tired Colias rhabdom. In chapter 2, I analyzed the visual pigments and spectral
sensitivities of individual photoreceptors in the distal tier in both sexes of Colias erate. A
subset of photoreceptor cells expresses a newly discovered middle wavelength-absorbing
opsin, Colias erate Blue (CeB), in addition to two previously described middle
wavelength-absorbing opsins, CeV1 and CeV2. The other photoreceptors either coexpress
CeV1 and CeV2, or exclusively express a short wavelength-absorbing opsin, CeUV, or a
long wavelength-absorbing opsin, CeL. Males and females have the same visual
pigment expression patterns, but the photoreceptor spectral sensitivities are sexually
dimorphic. The photoreceptors coexpressing three middle wavelength-absorbing opsins
are broad-blue receptors in males, but in females they are narrow-blue receptors. Those
with CeV1 and CeV2 are violet receptors in females, while they are shouldered-blue
receptors in males. The sexual dimorphism in spectral sensitivity is caused by a
sex-specific distribution of fluorescent pigment that functions as a spectral filter.
In chapter 3, I demonstrated by intracellular recording that the spectral
sensitivities of the proximal photoreceptors (R5-8) of all ommatidia in both sexes are
strongly tuned by the perirhabdomal pigments. These act as long-pass filters, shifting the
peak sensitivities into the wavelength range above 600 nm. Due to the sex-specific
pigments in type II ommatidia, the spectral sensitivities of the R5-8 photoreceptors of
females peaked at 620 nm while those in males peaked at 660 nm. The measured
spectral sensitivities could be well reproduced by an optical model assuming a long
wavelength-absorbing visual pigment with peak absorbance at 565 nm. Whereas the
sexual dimorphism was unequivocally demonstrated for the ventral eye region,
dimorphism in the dorsal region was not found. Presumably the ventral region is
strongly related to sexual behaviors such as courtship and oviposition.
I demonstrated in this thesis that males and females of Colias have different set
of eight classes of spectral photoreceptor in the ventral region of the eye. On the other
hand, I did not detect any sexual dimorphism in the dorsal retina. The eight types of
photoreceptor identified in the Colias ventral retina are generated by 1) duplication of
opsin genes, 2) expression of multiple opsins in single photoreceptors and 3) the filtering
effect of the fluorescent and perirhabdomal pigment. Together with previous studies,
especially on the small white, Pieris rapae, and the Japanese yellow swallowtail Papilio
xuthus, these mechanisms underlying various spectral receptors appear to be widespread
among butterflies.
The physiological basis for color vision in the Eastern Pale Clouded yellow butterfly, Colias erate
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