From Africa to Europe and back: refugia and range shifts cause high genetic differentiation in the Marbled White butterfly Melanargia galathea.

BACKGROUND: The glacial-interglacial oscillations caused severe range modifications of biota. Thermophilic species became extinct in the North and survived in southern retreats, e.g. the Mediterranean Basin. These repeated extinction and (re)colonisation events led to long-term isolation and intermixing of populations and thus resulted in strong genetic imprints in many European species therefore being composed of several genetic lineages. To better understand these cycles of repeated expansion and retraction, we selected the Marbled White butterfly Melanargia galathea. Fourty-one populations scattered over Europe and the Maghreb and one population of the sibling taxon M. lachesis were analysed using allozyme electrophoresis. RESULTS: We obtained seven distinct lineages applying neighbour joining and STRUCTURE analyses: (i) Morocco, (ii) Tunisia, (iii) Sicily, (iv) Italy and southern France, (v) eastern Balkans extending to Central Europe, (vi) western Balkans with western Carpathian Basin as well as (vii) south-western Alps. The hierarchy of these splits is well matching the chronology of glacial and interglacial cycles since the Günz ice age starting with an initial split between the galathea group in North Africa and the lachesis group in Iberia. These genetic structures were compared with past distribution patterns during the last glacial stage calculated with distribution models. CONCLUSIONS: Both methods suggest climatically suitable areas in the Maghreb and the southern European peninsulas with distinct refugia during the last glacial period and underpin strong range expansions to the North during the Postglacial. However, the allozyme patterns reveal biogeographical structures not detected by distribution modelling as two distinct refugia in the Maghreb, two or more distinct refugia at the Balkans and a close link between the eastern Maghreb and Sicily. Furthermore, the genetically highly diverse western Maghreb might have acted as source or speciation centre of this taxon, while the eastern, genetically impoverished Maghreb population might result from a relatively recent recolonisation from Europe via Sicily.