Mating cues evolve rapidly and can contribute to species formation and maintenance. 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 courtship signal that differentiates the males of Colias eurytheme butterflies from a sister species, allowing females to avoid costly heterospecific matings. Anthropogenic range expansion of the two incipient species established a large zone of secondary contact across the eastern United States with strong signatures of genomic admixtures spanning all autosomes. In contrast, Z chromosomes are highly differentiated between the two species, supporting a disproportionate role of sex chromosomes in speciation known as the large-X (or large-Z) effect. Within this chromosome-wide reproductive barrier, linkage mapping indicates that cis-regulatory variation of bric a brac (bab) underlies the male UV-iridescence polymorphism between the two species. Bab is expressed in all non-UV scales, and butterflies of either species or sex acquire widespread ectopic iridescence following its CRISPR knockout, demonstrating that Bab functions as a suppressor of UV-scale differentiation that potentiates mating cue divergence. These results highlight how a genetic switch can regulate a premating signal and integrate with other reproductive barriers during intermediate phases of speciation.

Classical Drosophila eye color mutations have unearthed a toolkit of genes that have permitted candidate gene studies of the outstanding diversity of coloration patterns in other insects. The gene underlying the eye color phenotypes of the red Malphigian tubules (red) fly mutant was mapped to a LysM domain gene of unknown molecular function.

Mating cues evolve rapidly and can contribute to species formation and maintenance. 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 UV iridescence, a courtship signal that differentiates the males of Colias eurytheme butterflies from a sister species, allowing females to avoid costly heterospecific matings. Anthropogenic range expansion of the two incipient species established a large zone of secondary contact across the eastern US with strong signatures of genomic admixtures spanning all autosomes. In contrast, 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, cis-regulatory variation of bric a brac (bab) drives the male UV-iridescence polymorphism between the two species. Bab is expressed in all non-UV scales, and butterflies of either species or sex acquire widespread ectopic iridescence following its CRISPR knock-out, demonstrating that Bab functions as a suppressor of UV-scale differentiation that potentiates mating cue divergence. These results provide new insights into the diversification of sexual signals and the species concept.

No abstract

Continuous colour polymorphisms can serve as a tractable model for the genetic and developmental architecture of traits, but identification of the causative genetic loci is complex due to the number of individuals needed, and the challenges of scoring continuously varying traits. Here we investigated continuous colour variation in Colias eurytheme and C. philodice, two sister species of sulphur butterflies that hybridise in sympatry. Using Quantitative Trait Locus (QTL) analysis of 483 individuals from interspecific crosses and an high-throughput method of colour quantification, we found that two interacting large effect loci explain around 70% of the heritable variation in orange-to-yellow chromaticity. Knockouts of red Malphighian tubules (red), a candidate gene at the primary QTL likely involved in endosomal maturation, resulted in depigmented wing scales showing disorganised pterin granules. The Z sex chromosome contains a large secondary colour QTL that includes the transcription factor bric-a-brac (bab), which we show can act as a modulator of orange pigmentation in addition to its previously-described role in specifying UV-iridescence. We also describe the QTL architecture of other continuously varying traits, and that wing size maps to the Z chromosome, supporting a Large-X effect model where the genetic control of species-defining traits is enriched on sex chromosomes. This study sheds light on the genetic architecture of a continuously varying trait, and illustrates the power of using automated measurement to score phenotypes that are not always conspicuous to the human eye.