A combined genetic-morphometric analysis unravels the complex biogeographical history of Polyommatus icarus and Polyommatus celina common blue butterflies

Widespread species have the potential to reveal large-scale biogeographical patterns, as well as responses to environmental changes possibly unique to habitat generalists. This study presents a continental-scale phylogeographical analysis of Polyommatus icarus, one of the most common Palaearctic butterflies, and the morphologically and ecologically similar Polyommatus celina, a recently discovered cryptic species. By combining data from mitochondrial [cytochrome c oxidase subunit I (COI)] and nuclear [internal transcribed spacer (ITS2)] molecular markers with geometric morphometrics, we document a complex phylogeographical history for the two species. Despite morphological similarities, the genetic divergence between these two species is high (more than 5% at COI) and they are not sister species. For the first time, we show that P. celina occurs not only in North Africa but also in Europe, where it inhabits several west Mediterranean islands, as well as large parts of Iberia, where it occurs in parapatry with P. icarus. The two species appear to completely exclude each other on islands, but we provide morphological and molecular evidence that introgression occurred in the Iberian Peninsula. We discovered strongly diverged lineages that seem to represent relict populations produced by past range expansions and contractions: Crete and Iberian isolates for P. icarus, Balearics-Sardinia and Sicily-Lipari for P. celina. This study shows that a combined genetic-morphometric approach can shed light on cryptic diversity while providing the necessary resolution to reconstruct a fine-scale phylogeographical history of species at both spatial and temporal levels.