Subtle Introgression Footprints at the End of the Speciation Continuum in a Clade of Heliconius Butterflies

Quantifying gene flow between lineages at different stages of the speciation continuum is central to understanding speciation. Heliconius butterflies have undergone an adaptive radiation in wing color patterns driven partly by natural selection for local mimicry. Color patterns are also known to be used as assortative mating cues. Therefore, wing pattern divergence is considered to play a role in speciation. A corollary is that mimicry between closely related species may be associated with hybridization and interfere with reproductive isolation. Here, we take a multifaceted approach to explore speciation history, species boundaries, and traits involved in species differentiation between the two closely related species, Heliconius hecale and Heliconius ismenius. We focus on geographic regions where the two species mimic each other and contrast this with geographic regions where they do not mimic each other. To examine population history and patterns of gene flow, we tested and compared a four-population model accounting for linked selection. This model suggests that the two species have remained isolated for a large part of their history, yet with a small amount of gene exchange. Accordingly, signatures of genomic introgression were small except at a major wing pattern allele and chemosensing genes and stronger in the mimetic populations compared with nonmimetic populations. Behavioral assays confirm that visual confusion exists but that short-range cues determine strong sexual isolation. Tests for chemical differentiation between species identified major differences in putative pheromones which likely mediate mate choice and the maintenance of species differences.