Coexpression of three middle wavelength-absorbing visual pigments in sexually dimorphic photoreceptors of the butterfly Colias erate.

The tiered ommatidia of the Eastern Pale Clouded yellow butterfly, Colias erate, contain nine photoreceptor cells, four of which contribute their rhabdomeral microvilli to the distal tier of the rhabdom. We analyzed the visual pigments and spectral sensitivities of these distal photoreceptors 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.