RESUMO
The Paranoplocephla arctica complex (Cyclophyllidea, Anoplocephalidae), host-specific cestodes of collared lemmings Dicrostonyx, include two morphospecies P. arctica and P. alternata, whose taxonomical status now must be considered ambiguous. The genetic population structure and phylogeography of the P. arctica complex was studied from 83 individuals sampled throughout the Holarctic distribution range using 600 bp of the mitochondrial cytochrome c oxidase subunit I gene (COI). The mitochondrial DNA (mtDNA) phylogeny divides the species complex into one main Nearctic and one main Palaearctic phylogroup, corresponding to the main phylogenetic division of the hosts. In the Palearctic phylogroup, the parasite clades correspond to the host clades although the parasites from Wrangel Island form an exception as the host on this island, D. groenlandicus, belongs to the Nearctic phylogroup. In the Nearctic, northern refugia beyond the ice limit of the Pleistocene glaciations are proposed for the hosts. All reconstructions of parasite phylogeny show a genetically differentiated population structure that in the Canadian Arctic lacks strict congruence between phylogeny and geography. The parasite phylogeny does not show complete congruence with host relationships, suggesting a history of colonization and secondary patterns of dispersal from Beringia into the Canadian Arctic, an event not proposed by the host phylogenies alone.