The acquisition of novel sexually dimorphic traits poses an evolutionary puzzle: How do new traits arise and become sex-limited? Recently acquired color vision, sexually dimorphic in animals like primates and butterflies, presents a compelling model for understanding how traits become sex-biased. For example, some Heliconius butterflies uniquely possess UV (ultraviolet) color vision, which correlates with the expression of two differentially tuned UV-sensitive rhodopsins, UVRh1 and UVRh2. To discover how such traits become sexually dimorphic, we studied Heliconius charithonia, which exhibits female-specific UVRh1 expression. We demonstrate that females, but not males, discriminate different UV wavelengths. Through whole-genome shotgun sequencing and assembly of the H. charithonia genome, we discovered that UVRh1 is present on the W chromosome, making it obligately female-specific. By knocking out UVRh1, we show that UVRh1 protein expression is absent in mutant female eye tissue, as in wild-type male eyes. A PCR survey of UVRh1 sex-linkage across the genus shows that species with female-specific UVRh1 expression lack UVRh1 gDNA in males. Thus, acquisition of sex linkage is sufficient to achieve female-specific expression of UVRh1, though this does not preclude other mechanisms, like cis-regulatory evolution from also contributing. Moreover, both this event, and mutations leading to differential UV opsin sensitivity, occurred early in the history of Heliconius. These results suggest a path for acquiring sexual dimorphism distinct from existing mechanistic models. We propose a model where gene traffic to heterosomes (the W or the Y) genetically partitions a trait by sex before a phenotype shifts (spectral tuning of UV sensitivity).

Transcriptome fasta files were produced using Trinity and stringtie assemblies of RNA-seq data derived from antennae, heads, legs, and mouthparts of adult male and female Heliconius charithonia butterflies. Reference genome fasta file of DNA extracted from a single H. charithonia female pupae was produced using Falcon and Canu (v1.6) assemblies that were combined into a single fasta file with Quickmerge.

Behavioral data was collected by first training adult Heliconius charithonia butterflies to associate a sugar reward with 390 nm light. Butterflies were then given a choice between 380 nm and 390 nm lights at intensity ratios of 5:1, 1:1, and 1:5 where the first number indicates the rewarded light.

A variety of text file programs can be used to open the fasta (e.g. Windows Editor, Nano for Linux, and TextEdit for macOS) and gff files (IGV). Quicktime (and other programs) can be used to view the behavioral videos.