Blue and double-peaked green receptors depend on ommatidial type in the eye of the Japanese yellow swallowtail Papilio xuthus

The compound eye of the butterfly Papilio xuthus is composed of three spectrally distinct types of ommatidia. We investigated the blue and double-peaked green receptors that are encountered distally in type I and III ommatidia, by means of intracellular recordings, in vivo fluorescence microscopy, and histology. The blue receptors are R1 and/or R2 photoreceptors; they contain the same mRNA encoding the opsin of the blue-absorbing visual pigment. However, here we found that the sensitivity in the UV wavelength region strongly depends on the ommatidial type; the blue receptors in type I ommatidia have a distinctly depressed UV sensitivity, which is attributed to lateral filtering in the fused rhabdom. In the main, fronto-ventral part of the eye, the R3 and R4 photoreceptors of all ommatidia contain the same set of two mRNAs encoding the opsins of green-absorbing visual pigments, PxL1 and PxL2. The spectral sensitivities are double-peaked, but the UV sensitivity of the R3 and R4 photoreceptors in type I ommatidia appears to be reduced, similar to that of the co-localized blue receptors.