Conflicting mitochondrial and nuclear phylogeographic signals and evolution of host-plant shifts in the boreo-montane leaf beetle Chrysomela lapponica

► DNA sequence variation was explored at three loci in the leaf beetle Chrysomela lapponica. ► A strong phylogeographic break is observed only with the mitochondrial locus. ► Simulations in a coalescent framework reject a neutral model of sequence evolution. ► Introgression is the most likely cause of the observed patterns of sequence variation. ► A host-plant shift from willow to birch has occurred independently in three populations.