Parallel clines in a quantitative trait in two mimetic butterfly species despite different levels of genomic divergence and selection

Hybrid zones, where phenotypically distinct populations meet and interbreed, give insight into the process of divergence and how differences between populations are maintained despite gene flow. Divergence in quantitative traits, which are controlled by multiple loci, may require stronger barriers to gene flow than traits controlled by few, major effect loci. The butterflies Heliconius erato and Heliconius melpomene are distantly related Müllerian mimics that show parallel divergence in wing colour patterns between geographical races across South America. Here we investigated intraspecific hybrid zones in both of these species. In these zones, a colour pattern change is controlled by a single locus but there is also previously unstudied variation in a structurally produced iridescent blue, which segregates as a quantitative trait. The centres of the iridescence cline did not differ between species, consistent with selection for mimicry, but the cline widths were different between species, indicating differences in the strength of selection. Despite the different genetic architectures of the colour traits, we found that, within species, cline widths for these traits were not significantly different, suggesting that the quantitative trait cline was not more prone to erosion by gene flow. Genotyping-by-sequencing revealed strikingly different levels of genetic differentiation in the two species, indicating different evolutionary histories. In both species, iridescence clines appeared to be somewhat independent of clines in genome-wide ancestry, suggesting they are not maintained by genome-wide barriers to gene flow.