Graduate Student The George Washington University Washington, District of Columbia
Mating cues can evolve rapidly and contribute to the formation or maintenance of species. However, little is known about how sexual signals diverge and how this variation integrates with other barrier loci to shape the genomic landscape of reproductive isolation. Here, we elucidate the genetic basis of ultraviolet (UV) iridescence, a sexual ornament courtship signal that differentiates the males of Colias eurytheme butterflies from sister species Colias philodice thereby allowing females to avoid costly heterospecific matings. The eastern US is a sympatric zone with strong signatures of admixtures spanning all autosomes while Z chromosomes are highly differentiated between the two species, supporting a disproportionate role of sex chromosomes in speciation known as the large-X effect. Within this chromosome-wide reproductive barrier variation in bric a brac (bab) expression drives the male UV-iridescent trait between the two species. Linkage mapping on lab-reared, back-crossed broods of the two species resulted in finding a zero-recombination window where bab was identified. Bab is expressed in all non-UV scales, and butterflies of both species and sex acquire widespread iridescence following its CRISPR knock-out, demonstrating that Bab functions as a suppressor of UV-scale differentiation significantly contributing to mating cue divergence. These results provide new insights into the diversification of sexual signals and the process of speciation.
