Genomics Reveal Admixture and Unexpected Patterns of Diversity in a Parapatric Pair of Butterflies.

We studied the evolutionary relationship of two widely distributed parapatric butterfly species, and , using the ddRAD sequencing approach, as well as genital morphology and mtDNA data. was retrieved as paraphyletic with respect to . Several cases of mito-nuclear discordance and morpho-genetic mismatch were found in the contact zone. A strongly diverged and marginally sympatric clade of from the Balkans was revealed. An in-depth analysis of genomic structure detected high levels of admixture between and at the contact zone, though not reaching the Balkan clade. The demographic modelling of populations supported the intermediate genetic make-up of European populations with regards to and the Balkan clade. However, the dissimilarity matrix of genotype data (PCoA) suggested the Balkan lineage having a genetic component that is unrelated to the - group. Although narrowly sympatric, almost no signs of gene flow were found between the main group and the Balkan clade. We propose two possible scenarios on the historical evolution of our model taxa and the role of the last glacial maximum in shaping their current distribution. Finally, we discuss the complexities regarding the taxonomic delimitation of parapatric taxa.