Mimetic butterfly wings through mimetic butterfly eyes: Evidence that brightness vision helps Adelpha fessonia identify potential mates

Müllerian mimicry arises when conspicuous unprofitable species share a color pattern, gaining protection because predators learn and avoid the warning colors of a bad meal. Coloration also has other functions. For example, how does one butterfly recognize another of the same species to mate? Color vision is thought to play a key role in driving the evolution of animal color patterns via natural or sexual selection, while achromatic (brightness) vision is often ignored as a possible mechanism for species recognition. Here we find evidence that brightness vision rather than color vision helps some mimetic Adelpha butterflies identify potential mates while their co-mimetic wing coloration is indiscriminable to avian predators. To do so, we examined the visual system of the butterfly Adelpha fessonia , a member of a diverse genus of butterflies comprising over 200 taxa with multiple mimicry complexes. We characterized the photoreceptors of A. fessonia using RNA-seq, eyeshine, epi-microspectrophotometry, optophysiology and comparative sequence analysis. We used these data to model the discriminability of wing color patches of A. fessonia in relation to those of its sympatric co-mimic, A. basiloides , through A. fessonia and avian visual systems. Adult A. fessonia eyes express three visual opsin mRNAs encoding long wavelength-, blue-, and ultraviolet-sensitive rhodopsins with peak sensitivities (λmax values) at 530 nm, ~431 nm and 355 nm, respectively. Red-reflecting ommatidia, found in other nymphalid butterflies such as monarchs and Heliconius butterflies, are absent from the eyeshine of A. fessonia , indicating A. fessonia eyes lack heterogeneously expressed red filtering pigments and red-sensitive photoreceptors. Visual models of Adelpha wing coloration suggests that A. fessonia can distinguish conspecifics from co-mimics using achromatic vision. Avian predators, on the other hand, cannot distinguish between co-mimic wing color using either chromatic or achromatic cues. Taken together, these results suggest that mimetic wing color patterns and visual systems have evolved in tandem to maintain mimicry while avoiding mating between look-alike species. ### Competing Interest Statement The authors have declared no competing interest.