Sison-Mangus, B., Briscoe, A., Zaccardi, G., Knuettel, H., and Kilber, A. (2008) “The lycaenid butterfly Polyommatus icarus uses a duplicated blue opsin to see green.” The Journal of Experimental Biology 211, 361-369.

In butterflies, there is an unusual diversity in visual perception due to duplication of the opsin gene in some lineages and to filtering pigments that are expressed heterogeneously within the eye. The authors of this study wanted to know whether P.icarus can see iin the red range of the color spectrum and whether the duplicate copy of the blue opsin gene codes for pigments that are used for color vision (in a feeding context). From previous studies on a related butterfly, L. rubidus, it was known that there are four opsin genes in this system: one UV gene, two B (blue-range) genes, one of which is a duplicate, and one LW (long-wavelength) gene, which produce pigments tuned to the wavelengths 360 nm, 437 nm, 500 nm, and 568 nm, respectively.

Also, the eyes of this species have a red filtering pigment found only on the ventral side of the retina in eyes that also contain the duplicate B opsin. The researchers therefore believed it was possible that there were five, instead of four, opsin classes, one each for UV, B1, B2, and LW, and an additional one for the red pigment and LW together (with theB2 opsin). They hypothesized that this last opsin class could extend the organism’s vision into the red range.

The authors used light microscopy to examine cross-sections of P. icarus eyes, took photographs of eye-shine (light that was not absorbed by the eye upon first passing) from P. icarus eyes, sequenced of opsin genes from P. icarus in order to create an opsin gene tree with sequences from other lepidopterans, and performed two color discrimination experiments.

They did find a red filtering pigment in the eye of P. icarus. Their photographs of eye-shine of different sections of the eye indicated that P. icarus might have red-sensitive photoreceptors in the ventral portion of the retina, similar to L. rubidus. The authors found that P. icarus failed the red discrimination test between yellow and red, and used brightness instead of color to make their choices, so the authors concluded P. icarus does not have vision in the red part of the spectrum. However, P. icarus could discriminate between yellow and green, which the authors attributed to B2 (which is shifted downward 63 nm compared to B1, which is tuned to 437 nm).

This study supports what we learned in class about the number of photoreceptor types animals can have and how pigments can be heterogeneously distributed across the retina. These butterflies have four opsin genes, one of which is a duplicate of another, the pigments of which are unevenly distributed in the butterfly eye (the red pigments are concentrated on the ventral side of the retina—not much is said about the other pigments). This study is also interesting because it demonstrates how color vision and diversity in visual systems can evolve through functional shifts of a duplicate gene.