Beauty in the eye of the beholder: the two blue opsins of lycaenid butterflies and the opsin gene-driven evolution of sexually dimorphic eyes

Although previous investigations have shown that wing coloration is an important component of social signaling in butterflies, the contribution of opsin evolution to sexual wing color dichromatism and interspecific divergence remains largely unexplored. Here we report that the butterfly Lycaena rubidus has evolved sexually dimorphic eyes due to changes in the regulation of opsin expression patterns to match the contrasting life histories of males and females. The L. rubidus eye contains four visual pigments with peak sensitivities in the ultraviolet (UV; lambda(max)= 360 nm), blue (B; lambda(max)= 437 nm and 500 nm, respectively) and long (LW; lambda(max)= 568 nm) wavelength range. By combining in situ hybridization of cloned opsin-encoding cDNAs with epi-microspectrophotometry, we found that all four opsin mRNAs and visual pigments are expressed in the eyes in a sex-specific manner. The male dorsal eye, which contains only UV and B (lambda(max)= 437 nm) visual pigments, indeed expresses two short wavelength opsin mRNAs, UVRh and BRh1. The female dorsal eye, which also has the UV and B (lambda(max)= 437 nm) visual pigments, also contains the LW visual pigment, and likewise expresses UVRh, BRh1 and LWRh mRNAs. Unexpectedly, in the female dorsal eye, we also found BRh1 co-expressed with LWRh in the R3-8 photoreceptor cells. The ventral eye of both sexes, on the other hand, contains all four visual pigments and expresses all four opsin mRNAs in a non-overlapping fashion. Surprisingly, we found that the 500 nm visual pigment is encoded by a duplicate blue opsin gene, BRh2. Further, using molecular phylogenetic methods we trace this novel blue opsin gene to a duplication event at the base of the Polyommatine+Thecline+Lycaenine radiation. The blue opsin gene duplication may help explain the blueness of blue lycaenid butterflies.