From the Cover: A genetic switch for male UV iridescence in an incipient species pair of sulphur butterflies

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.

Premating signals such as pheromones, calls, and displays often differ between sexes and species and, by helping animals to tell one another apart, they are integral to the formation of reproductive barriers during speciation itself (1, 2). Mating factors can diverge early in the speciation process due to local adaptation or later due to sexual selection that prevents the generation of unfit hybrids (3). While a coupling of premating and postmating isolation mechanisms is thought to be required for the completion of speciation (4), how mating cue variation actually coincides with other barrier loci to split lineages remains elusive in the empirical literature (5–7).Previous work on the genetics of hybridization between the sulphur butterflies Colias eurytheme and Colias philodice highlights their potential for the study of intermediate phases of speciation with gene flow. Initially restricted to the western United States, the range of C. eurytheme expanded following both the spread of agricultural alfalfa and the reduction in forest cover in the past 200 y into regions once limited to C. philodice (8). As a result, the two species occur in secondary sympatry throughout an anthropogenic contact zone that includes the eastern United States and southern Canada. Both pre- and postzygotic reproductive barriers maintain species status in this system. However, heterospecific matings happen at increased frequency in dense populations (9, 10), partly because males can locate newly emerged females incapable of performing mate rejection behaviors [teneral mating (11)]. Hybrid female sterility forms an intrinsic postzygotic barrier that affects one of the two heterospecific crosses: oogenesis fails in female offspring that inherit a C. eurytheme W chromosome and a C. philodice Z chromosome (12, 13). This incompatibility is sex-linked and implies that to produce fully fertile progeny, C. eurytheme females must select males that are homozygous for a conspecific Z chromosome. An iridescent ultraviolet (UV) pattern acts as a visual mating cue in males and accurately displays their Z-chromosome status to females (9, 14) (Fig. 1A and SI Appendix, Fig. S1). UV occurs on the dorsal wing surfaces of C. eurytheme males only. The Mendelian U locus controls this interspecific variation and was previously mapped to the Z chromosome (15): C. eurytheme homozygous recessive males are UV-iridescent (u/u), advertising two compatible Z chromosomes to C. eurytheme females. Incompatible mates such as C. philodice males (U/U) and heterozygous hybrids (U/u) bear the dominant allele and lack UV. Finally, the female preference trait itself is also linked to the Z chromosome (14). This Z-linked inheritance of genetic incompatibility, mating signal, and mating preference supports an “indicator” model of speciation, which was previously theorized as a system where the mating cue can signal species identity and enable selection against hybrids (3, 16). In this study, we examine the genomic footprint of sex-linked reproductive barriers, and fine-map the allelic variation that switches on the male UV signal in C. eurytheme.Open in a separate windowFig. 1.Large-Z architecture of species differentiation includes the U-locus candidate gene bab. (A) UV iridescence differentiates males from two incipient species. (B and B′) PCA (B) and distance-based phylogenetic network (B′) of 22 male whole-genome SNPs from the admixed Maryland population. (C and D) F_ST values for C. philodice vs. C. eurytheme plotted against recombination rate (C), and Manhattan plot (D). Red indicates windows with above-median recombination rate and in the 95^th percentile of F_ST, including on the Z chromosome (asterisk). (E) Quantitative trait locus (QTL) analysis of the presence/absence of UV in 252 male offspring from F₂ and BCs. (F) Genotype plot for the whole Z chromosome with resequencing data from 23 individuals. Each row is an individual, and each column is a color-coded SNP. The red bracket indicates a 2.5-Mb interval with high F_ST and above-median recombination rate. (G) Annotation of the U-locus zero-recombinant interval (box) and surrounding region.