The evolution of red colour vision is linked to coordinated rhodopsin tuning in lycaenid butterflies
Authors: Liénard Marjorie A. , Bernard Gary D. , Allen Andrew A. , Lassance Jean-Marc , Song Siliang , Childers Richard Rabideau , Yu Nanfang , Ye Dajia , Stephenson Adriana , Valencia-Montoya Wendy A. , Salzman Shayla , Whitaker Melissa R.L. , Calonje Michael , Zhang Feng , Pierce Naomi E. 
Published: April 2020

Colour vision is largely mediated by changes in number, expression, and spectral properties of rhodopsins, but the genetic mechanisms underlying adaptive shifts in spectral sensitivity remain largely unexplored. Usingin vivophotochemistry, optophysiology, andin vitrofunctional assays, we link variation in eye spectral sensitivity at long wavelengths to species-specific absorbance spectra for LW opsins in lycaenid butterflies. In addition to loci specifying an ancestral green-absorbing rhodopsin with maximum spectral sensitivity (λmax) at 520-530 nm inCallophrys sheridaniiandCelastrina ladon, we find a novel form of red-shifted LW rhodopsin at λmax= 565-570 nm inArhopala japonicaandEumaeus atala. Furthermore, we show thatCa. sheridaniiandCe. ladonexhibit a smaller bathochromic shift at BRh2 (480-489 nm), and with the ancestral LW rhodopsin, cannot perceive visible red light beyond 600 nm. In contrast, molecular variation at the LW opsin inA. japonicaandE. atalais coordinated with tuning of the blue opsin that also shifts sensitivity to longer wavelengths enabling colour discrimination up to 617 nm. We then useE. atalaas a model to examine the interplay between red and blue spectral sensitivity. Owing to blue duplicate expression, the spatial distribution of opsin mRNAs within an ommatidium defines an expanded retinal stochastic mosaic of at least six opsin-based photoreceptor classes. Our mutagenesisin vitroassays with BRh1 (λmax= 435 nm) chimeric blue rhodopsins reveal four main residues contributing to the 65 nm bathochromic shift towards BRh2 (λmax= 500 nm). Adaptations in this four-opsin visual system are relevant for discrimination of conspecific reflectance spectra inE. atala. Together, these findings illustrate how functional changes at multiple rhodopsins contribute to the evolution of a broader spectral sensitivity and adaptation in visual performance.

Significance Statement

Rhodopsins are photosensitive protein molecules that absorb specific wavelengths of incoming light and convey colour information in the visual system. We show that molecular evolution in a green insect opsin gene resulted in a shift in its maximal absorbance peak, enabling some lycaenid butterflies to use spectral energy of longer wavelengths (LW) to discriminate colours in the red spectrum better than relatives bearing ancestral green LW rhodopsins. Lycaenids also evolved a duplicate blue opsin gene, and we illustrate an example where species equipped with red LW rhodopsins shifted their blue sensitivity peak to longer wavelengths due to changes in several blue-tuning residues that have evolved repeatedly in different insect lineages. We demonstrate how changes at multiple vision genes in the insect eye effectively create a coordinated mechanism expanding spectral sensitivity for visually guided behaviours such as selecting host plants and mates.