A genetic switch for male UV iridescence in an incipient species pair of sulphur butterflies

Significance Incipient species are at an intermediate stage of speciation where reproductive isolation is counteracted by the homogenizing effects of gene flow. Human activity sometimes leads such species to reunite, as seen in the Orange Sulphur butterfly, which forms large hybridizing populations with the Clouded Sulphur in alfalfa fields. Here we show that sex chromosomes maintain these species as distinct, while the rest of their genome is admixed. Sex chromosomes notably determine which males display to females a bright, iridescent UV signal on their wings. Genetic mapping, antibody stainings, and CRISPR knockouts collectively indicate that the gene bric a brac controls whether UV-iridescent nanostructures develop in each species, illustrating how a master switch gene modulates a male courtship signal.