Polymorphism of red receptors: sensitivity spectra of proximal photoreceptors in the small white butterfly Pieris rapae crucivora.

The compound eye of the small white butterfly Pieris rapae crucivora contains three anatomically distinct types of ommatidia. They differ in pigmentation around the rhabdom, colour of tapetal reflection and violet light-induced autofluorescence, indicating physiological differences between them. We recently reported that the ommatidia are in fact spectrally heterogeneous: in the distal part of the tiered retina they contain different sets of the spectral receptors R1-4. This study examines whether the ommatidia in the proximal retinal tier also show the spectral heterogeneity for the receptors R5-8. We recorded the sensitivity spectra of the proximal photoreceptors, and subsequently injected the dye Alexafluor 568 into proximal photoreceptors, to localize the cell and identify the ommatidial type to which it belonged. We analysed 13 successfully labeled proximal photoreceptors, and found that the sensitivity spectrum of the proximal photoreceptors in types I and III ommatidia peaks at 620 nm, whereas that of type II ommatidia peaks at 640 nm. The difference in the sensitivity spectra can be explained by the anatomical characteristics of each ommatidial type. This is the first demonstration of red receptor polymorphism in insects. The polymorphic red receptor system most probably enhances contrast sensitivity and/or color discrimination in the long wavelength spectral region.